Corn Rootworm (Coleoptera: Chrysomelidae) Larval Injury and Root Compensation of 12 Maize Hybrids: an Assessment of the Economic Injury Index

Transcription

1 FIELD AND FORAGE CROPS Corn Rootworm (Coleoptera: Chrysomelidae) Larval Injury and Root Compensation of 12 Maize Hybrids: an Assessment of the Economic Injury Index MICHAEL E. GRAY AND KEVIN L. STEFFEY Department of Crop Sciences, University of Illinois, Urbana, IL J. Econ. Entomol. 91(3): 723Ð740 (1998) ABSTRACT A 4-yr Þeld investigation (1993Ð1996) to examine the compensatory root regrowth of 12 commonly grown maize hybrids after larval injury by corn rootworms, Diabrotica spp., was conducted at 2 locations in Illinois. Root injury ratings, root volume measurements taken in July and August, and root regrowth parameters were evaluated for their usefulness in predicting yield. Root ratings were as useful as root volumes and root regrowth measurements in predicting yield. Large root systems in July and August generally were positive factors contributing to yield; however, compensatory root regrowth, particularly when soil moisture was adequate, negatively affected yield. Root regrowth after larval injury typically had a positive effect on yield when soil moisture was inadequate. Regression equations described the very dynamic nature of root injury, root volume, and root regrowth and their impact on yield in different growing seasons and at different locations. In addition, proþt margins were estimated using a Þxed insecticide cost, actual root injury data, and 4 market prices of maize. Results from different growing seasons and locations indicate that root ratings well below 4.0 can contribute to economic losses. KEY WORDS Diabrotica spp., maize, root injury, root compensation, injury index THE WESTERN CORN rootworm, Diabrotica virgifera virgifera LeConte, can cause economic losses throughout the Midwest and in certain eastern and northeastern states where maize is produced. The biology, ecology, sampling methods, and management approaches have been reviewed extensively (Chiang 1973, Krysan and Miller 1986, Levine and Oloumi-Sadeghi 1991). The cost of managing western and northern, Diabrotica barberi Smith & Lawrence, corn rootworms is the largest expenditure for insect management in maize production systems (Pike et al. 1995). Most recent estimates for Illinois indicate that 88% (997,167 ha) of nonrotated maize hectares (1,133,144 ha) are treated annually with a soil insecticide applied at planting (Pike et al. 1991). Research conducted in Indiana during the early 1970s (Turpin and Thieme 1978) and more recently in Illinois (Gray et al. 1993) indicates that as many as one-half to two-thirds of maize producers did not beneþt economically from applying a soil insecticide. The treatment of 1st-yr maize (corn rotated annually with another crop) with a soil insecticide (13% of hectares) has declined in Illinois since the late 1970s. In 1995, however, maize producers in east-central Illinois and northwestern Indiana suffered severe economic losses caused by western corn rootworm larval injury to their rotated maize (Gray et al. 1996, OÕNeal et al. 1996, Spencer et al. 1996). In response, producers throughout eastern Illinois and in some areas of Indiana increased their use of soil insecticides during the 1996 growing season. Progress made since the late 1970s in reducing soil insecticide usage for corn rootworm control is eroding quickly in response to the threat posed by western corn rootworms to rotated maize in certain areas of the eastern Corn Belt. The current focus on nonchemical alternatives for pest management and in developing transgenic crops for insect control has generated renewed interest in the development of maize hybrids that are resistant or tolerant to corn rootworms. The relationship between corn rootworm larval injury and corn plant response has been examined by several entomologists. Turpin et al. (1972) established an initial relationship between root injury and yield loss, and others (Stamm et al. 1985, Mayo 1986, Sutter et al. 1990) have offered modiþcations to the original economic injury index. In the late 1980s and early 1990s, researchers began to quantify the relationship between rootworm larval injury and reductions in vegetative biomass (Spike and Tollefson 1991), stalk lodging (Sutter et al. 1990), corn nutrient content (Kahler et al. 1985), above-ground dry weights (Gibb and Higgins 1991), gas exchange parameters (Godfrey et al. 1993a, Riedell 1990), vegetative and reproductive biomass accumulations (Godfrey et al. 1993b), and nitrogen deþciencies (Riedell et al. 1996, Spike and Tollefson 1989a). This information as well as new research on the response of maize to corn rootworm larval injury may aid in the development of new resistant, or more tolerant, maize hybrids and improve our ability to predict yield losses based upon larval injury /98/0723Ð0740$02.00/ Entomological Society of America

2 724 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 91, no. 3 Plant breeders and entomologists have long searched for sources of resistance in maize against corn rootworm larvae (Branson 1986). Commercial hybrids that have antixenotic or antibiotic properties have not been marketed. However, some maize hybrids exhibit tolerance to larval injury. Entomologists have developed subjective rating systems or have measured the amount of plant lodging, size of root systems, amount of secondary root system development, or the vertical pulling resistance of roots to evaluate maize for tolerance to corn rootworms. Branson (1986) describes the origin of these different measurements for tolerance. Research to date suggests that the larger the root system, the more tolerant the plant is to rootworm larval injury. Tolerance also has been associated with compensatory abilities of the plant to grow new roots after injury has occurred. Rogers et al. (1975) evaluated 25 commercial maize hybrids for tolerance to rootworm larval injury and reported generally low levels of this trait among the hybrids tested. Riedell and Evenson (1993) compared the tolerance of 11 single-cross maize hybrids from the 1960s (3), 1970s (3), and 1980s (5) eras. They reported (p. 953) that Analysis of root pull resistance and lodging data using orthogonal comparisons showed increased root system size and decreased lodging percentage in the newer compared with the older genotypes. This indicates that tolerance to corn rootworms occurs in the form of increased root system size and decreased lodging in the and 1970-era genotypes compared with the 1960 era and as decreased lodging percentage only in the 1980-era genotypes compared with the 1970-era genotypes. Riedell and Evenson (1993, p. 955) further concluded, Our results suggest that large root system size is the rootworm-tolerance trait present in maize hybrids currently used in the northern USA. Besides the publications of Riedell and Evenson (1993) and Riedell (1994), little information is currently available regarding the degree to which popularly grown commercial maize hybrids tolerate or compensate for corn rootworm larval injury. Particularly lacking is research that involves large, Þeld-scale trials conducted in different locations, the use of multiple hybrids, actual volumetric root measurements, replicated over many years. Riedell and Evenson (1993) relied upon artiþcial infestations of western corn rootworm eggs, as have other investigators (Spike and Tollefson 1989a, b; Sutter et al. 1990; Spike and Tollefson 1991; Godfrey et al. 1993a, b) who have examined the relationship between rootworm larval injury and other plant parameters. In addition, investigators such as Riedell and Evenson (1993) and Riedell (1994) relied upon root-pull resistance data instead of measuring root volumes in evaluating the compensatory responses of maize to root injury. This article reports the results of a 4-yr (1993Ð1996) Þeld investigation conducted in 2 Illinois locations. The objectives of our research were to evaluate the root regrowth responses of 12 commercial maize hybrids by measuring root volumes (water displacement) soon after peak root injury and again 1 mo later; to evaluate the currently accepted economic injury index for rootworm larval injury at different market prices of corn, different root injury levels, and at a Þxed soil insecticide cost; and to compare root injury ratings with root volume measurements and root regrowth parameters as potential predictors of yield. Materials and Methods Agronomic and Experimental Design Information, Our investigation was conducted at the Northern Illinois Agronomy Research Center (De- Kalb, IL) and the Shaw Entomology Research Farm (Urbana, IL). To ensure an infestation of rootworm larvae, a trap crop (late planted maize) was planted in the 1.62-ha experimental site at each location during each year preceding the experiment. Late-planted maize attracts ovipositing corn rootworm beetles and is used commonly as a trap crop by rootworm researchers throughout the Corn Belt. Standard weed control and fertility programs were under the supervision of university farm superintendents. Conventional tillage systems (chisel plow/spring harrow) were used at each location. The experimental design was a randomized complete block with split plots. The whole-plot treatments (4 rows, 0.76 m) were the 12 commercial maize hybrids, and the subplots consisted of 2 rows treated with terbufos (Counter 15 G [granular]; American Cyanamid, Princeton, NJ) and 2 rows not treated. Terbufos was applied as a 17.8-cm surface band ahead of the Þrming wheels of the planter (John Deere, 7000 series, 4-row MaxEmerge; Moline, IL) at each location and year except for the DeKalb 1993 experiment. Terbufos was added to the experiment after 1993 so we could evaluate the hybrids responses in the absence of signiþcant rootworm larval injury. The following 12 maize hybrids were selected because they were among the most popular hybrids grown by Illinois producers in the early 1990s (Gray et al. 1993): ÔAsgrow RX707Õ, ÔBurrus BX58Õ, ÔCargill 6337Õ, ÔCrows 401Õ, ÔDeKalb 591Õ, ÔFS 6774Õ, ÔGarst 8501Õ, ÔHughes 5500Õ, ÔNorthrup King N6560Õ, ÔPioneer 3394Õ, ÔRenk RK839Õ, and ÔWyffels W707Õ. Except for 1996, all 12 hybrids were available commercially and were included in our experiments. In 1996, we did not receive a shipment of Cargill Measurements. Plant populations were estimated each spring by counting the number of plants in 5.3-m sections of 2 treated and 2 control rows (no insecticide used) for each plot. Root injury (Hills and Peters 1971) and root volume measurement techniques were identical for each location and year of the study. Precipitation data were recorded daily and monthly totals are provided in Tables 1 and 2 for DeKalb and Urbana, respectively. Plots were machine harvested at the end of each growing season; yields were adjusted to a 15.5% moisture level. DeKalb, Each hybrid was planted in plots that were 4 rows wide and 24 m long. During 1993, De- Kalb was the only location for our study, and no soil insecticide was applied; therefore, the whole-plot

3 June 1998 GRAY AND STEFFEY: CORN ROOTWORM INJURY AND HYBRID COMPENSATION 725 Table 1. Precipitation totals (centimeters) for DeKalb experimental plots for the months of April September, Month April May June July August September Total DeKalb precipitation data obtained from onsite records at the Northern Illinois Agronomy Research Center. treatments (hybrids) were not split. Ten replicates of each hybrid were planted on 17 May. On 17 June, populations of each maize hybrid were estimated by counting the number of plants in 5.3-m sections in the center 2 rows of each plot. On 20 July, 10 plants were removed (roots dug from a volume of soil 17.8 cm 3 ) from the 2 center rows (5 plants per row) of each plot in each replication. The soil was washed from the roots with pressurized water, and roots were evaluated for larval injury on a 1Ð6 scale (Hills and Peters 1971): 1, no damage or only a few minor feeding scars; 2, feeding scars evident but no roots eaten off to within 3.8 cm of plant; 3, several roots eaten off to within 3.8 cm of plant; 4, 1 node of roots destroyed; 5, 2 nodes of roots destroyed; and 6, 3 or more nodes of roots destroyed. After each root system was evaluated for larval injury, it was submerged into a 10-cm-diameter, 4,000-mlcapacity graduated cylinder to estimate root volume. Measuring root volumes by water displacement has been reported previously (Musick et al. 1965). Approximately 1 mo later on 16 August, 10 additional plants were dug from the 2 center rows (5 roots per row) of each plot in each replication. The soil was washed from the roots and volumes were measured as described. Plots were machine harvested on 28 and 29 October. Because of the wet spring and standing water in 1 replication, we analyzed plant samples and harvest data from 9 replicates. DeKalb and Urbana, Because of land space limitations in 1994, only 9 replicates of the hybrids could be planted (4 May) at the DeKalb location. Plots were 4 rows wide and 43 m long; 2 rows were treated with terbufos at planting and the other 2 rows were not treated. Plant populations were estimated on 2 June by counting plants in a 5.3-m section of a treated and Table 2. Precipitation totals (centimeters) for Urbana experimental plots for the months of April September, Month April May June July August September Total Urbana precipitation data obtained from the Illinois Climate Network, Illinois State Water Survey, Champaign, IL. untreated row in each plot. On 14 July, 5 roots from 1 treated and 1 untreated row per replicate by hybrid combination were evaluated for larval injury, and root volumes were measured. On 16 August, the same number of plants were dug again, and root volumes were measured. Plots were machine harvested on 7Ð8 and 10Ð11 November. In Urbana, 10 replicates of the hybrid and insecticide treatment combinations were planted on 13 May Plots were 4 rows wide and 40 m long; 2 rows were treated with terbufos at planting and the other 2 rows were left untreated. Stand counts were taken on 26 May by counting plants in a 5.3-m section of a treated and untreated row in each plot. Root injury and volume were measured on 19 July; 5 roots were extracted from 1 treated and 1 control row per replicate by hybrid combination. Nearly 1 mo later on 17Ð18 August, the same number of roots were dug and volumes again were determined. Weather conditions forced us to machine harvest intermittently (by replicate) during November. DeKalb and Urbana, Ten replicates of the hybrid and insecticide treatment combinations were planted on 22 May. Each plot was 4 rows wide but varied in length because of constraints with land availability; 1 replicate was 25 m long, 2 replicates were 43 m long, and 7 replicates were 32 m long. Stand counts were taken on 15 June as described previously. On 26 July, 5 roots for each hybrid and insecticide combination were evaluated for injury, and volumes were measured. Root volumes were measured a 2nd time on 21 August. All plots were machine harvested on 5 November. Unusually wet (May precipitation, 25.7 cm) and cool conditions delayed planting until 3 June at Urbana in Ten replicates of the hybrid and insecticide treatment combinations were planted in plots with the same dimensions as described previously for Urbana. Stand counts were made on 19 June. Initial root volumes and injury ratings were made on 18 July using the same number of plants and procedures as before. On 23 August a 2nd set of roots (5 roots per insecticide by hybrid combination per replicate) were dug and root volumes were measured. The plots were harvested during November. DeKalb and Urbana, During the Þnal year of the experiment in DeKalb, only 8 replicates of the treatment combinations could be planted (3 May) because of constraints with land availability. Four replicates were 24 m long and the other 4 replicates were 43 m long. Cargill 6337 was not planted because it was not commercially available. Plant populations were estimated on 17 June. Root injury ratings and initial volume measurements were delayed until 2 August, 2 wk later than in previous years, because the cool spring and summer temperatures delayed rootworm egg hatch, resulting in late-season injury. The 2nd set of root volumes was measured on 20 August, slightly 3 wk after the 1st set of volumes was recorded. The number of roots evaluated and procedures followed were similar to those described for other years and locations. Plots were harvested on 4Ð5 and 10Ð11 No-

4 726 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 91, no. 3 vember. Because of human error, only 7 replicates were harvested. Planting was delayed in Urbana until 20 May because of abundant rainfall during the month (21.0 cm). Ten replicates of the insecticide and hybrid combinations were planted, with plot dimensions the same as in previous years at the Urbana site. Stand counts were taken on 17 June. On 16 July, root injury was assessed and root volumes were measured following previously described procedures. The 2nd set of root volumes were measured on 21 August. All plots were machine-harvested on 4Ð7 November. Statistical Analysis. Root rating, root volume, and yield data were analyzed with SAS (SAS Institute 1989). The general linear models procedure was used to perform an analysis of variance (ANOVA) on root rating, root volume, and yield data for each year and location. Actual means, variances, standard errors for means, least signiþcant difference (LSD) values (P 0.05), and the results of t -tests (Snedecor and Cochran 1967) are provided. Linear regression analyses were conducted and coefþcients of determination (R 2 ) calculated for relationships between yield and root volume parameters. Curvilinear regressions were performed and coefþcients of determination (R 2 ) presented for relationships between yield and root rating data. Table 3. Root ratings, root volumes, and yields of 12 maize hybrids from plots at the Northern Illinois Agronomy Research Center near DeKalb, IL, 1993 Root rating a Root vol. I b Root vol. II c Yield ql/ha d Asgrow RX Burrus BX Cargill Crows DeKalb FS Garst Hughes NK N Pioneer Renk RK Wyffels W S , SEM LSD SEM, standard error of the mean for means in a column; LSD, P 0.05 for means in a column. a Roots were rated on 20 July 1993 using the Iowa State 1Ð6 scale (Hills and Peters 1971); n, 90 per mean. b Root volumes I (milliliters) were measured on 20 July 1993; n, 90 per mean. c Root volumes II (milliliters) were measured on 16 August 1993; n, 90 per mean. d Harvest dates were 28 and 29 October 1993; n, 9 per mean; planting date was 17 May Results and Discussion DeKalb, A signiþcant hybrid effect on stand count was observed at the DeKalb location in 1993 (F 2.48; df 11, 99; P 0.009). Plant populations ranged from 23.1 to 26.1 plants per 5.3 m of row for FS 6774 and Renk RK839, respectively. Differences in plant densities for hybrids between these extremes were not statistically distinct (LSD 1.3, P 0.05). No explanation was evident for the difference in stand counts between FS 6774 and Renk RK839. Root injury evaluations indicated slight to moderate pruning of all hybrids during the experiment (Table 3). The overall effect of hybrid on root injury was not signiþcant (F 1.54; df 11, 88; P 0.13). Injury ratings ranged from 3.16 to 4.16 for Garst 8501 and Hughes 5500, respectively. Differences in root injury among the hybrids within this range were negligible, suggesting that the maize hybrids were equally susceptible to root injury by corn rootworm larvae. A signiþcant hybrid effect was evident for root volumes (Table 3) measured on 20 July (F 2.02; df 11, 88; P 0.04). Volumes ranged from 14.5 to 23.5 ml for Renk RK839 and Wyffels W707, respectively. Root volumes for the other hybrids were similar to one another. Slightly less than 1 mo later on 16 August, differences in root volumes among the hybrids became more pronounced (F 3.31; df 11, 88, P ). Renk RK839 again had the smallest average root volume (69.4 ml), approximately one-half the volume of DeKalb 591 (128.5 ml). In 1993, precipitation at DeKalb was plentiful throughout the growing season (Table 1). In July and August alone, the period in which root regrowth was measured, nearly 23 cm of rain fell. This amount of precipitation allowed all of the hybrids to compensate for root injury by regrowing abundant amounts of new root tissue. DeKalb 591 increased its average root volume by ml from July to August (82.5% increase). Renk RK839 increased its root volume by only 54.9 ml during this period, the least increase in root volume of all hybrids tested. However, even this increase represented a 79.1% gain in root volume. A signiþcant hybrid effect on yield (Table 3) was observed (F 4.64; df 11, 88; P ). Yields in general were far below the 1993 statewide average yield of ql/ha (Illinois Agricultural Statistics Service 1996), perhaps because of the larval root pruning. Yields ranged from to ql/ha for DeKalb 591 and Burrus BX58, respectively. Differences in yield among hybrids within this range generally were not signiþcant. Because DeKalb 591 had the largest root system among the hybrids on 16 August, we did not expect this hybrid to have the lowest yield. This suggests that hybrids that compensate for rootworm larval injury by regrowing copious amounts of root tissue may not yield optimally in a growing season characterized by average to above average levels of precipitation. DeKalb, The analysis of stand count data for DeKalb did not reveal a hybrid insecticide interaction. Therefore, data were pooled across these treatments and analyzed. The combined analysis revealed that the hybrid effect was not signiþcant. Plant densities were within a narrow range of 24.7 to 25.7 plants per 5.3 m of row for Asgrow RX707 and Hughes 5500, respectively.

5 June 1998 GRAY AND STEFFEY: CORN ROOTWORM INJURY AND HYBRID COMPENSATION 727 Table 4. Root ratings, root volumes, and yields of 12 maize hybrids from plots at the Northern Illinois Agronomy Research Center near DeKalb, IL, 1994 Hybrid Root rating a Root volume I b Root volume II c Yield ql/ha d Insecticide e Control Insecticide Control Insecticide Control Insecticide Control Asgrow RX ** * Burrus BX ** Cargill ** * * Crows ** DeKalb ** * FS ** * Garst ** * Hughes ** * NK N ** ** ** Pioneer ** Renk RK ** Wyffels W ** S , , , , SEM LSD SEM, standard error of the mean for means in a column; LSD, P 0.05 for means in a column. *, P 0.05; **, P 0.01; t -alpha values (88 df used in S 2 determination) for root rating, root volume, and yield differences between insecticide and control columns are for P, 0.05 and for P, a Roots were rated on 14 July 1994 using the Iowa State 1Ð6 scale (Hills and Peters 1971); n, 45 per mean. b Root volumes I (milliliters) were measured on 14 July 1994; n, 45 per mean. c Root volumes II (milliliters) were measured on 16 August 1994; n, 45 per mean. d Harvest dates were 7, 8, 10, and 11 November 1994; n, 9 per mean. e Terbufos 15 G was applied in a 17.8-cm band at planting on 4 May The analysis of root rating data indicated a signiþcant hybrid insecticide interaction (F 2.54; df 11, 955; P.0037). Because of this signiþcant interaction, root rating data were analyzed separately (treated versus untreated). A clear separation in the severity of root injury was evident between plants in treated plots and those in control plots (Table 4). Root ratings for hybrids in the treated plots ranged from 2.07 to 2.44 for Crows 401 and Cargill 6337, respectively. This level of root injury was well below the commonly accepted economic injury index of 3.0 (Turpin et al. 1972, Stamm et al. 1985, Mayo 1986, Sutter et al. 1990). Root injury was not signiþcantly different (F 1.63; df 11, 88; P 0.105) among treated hybrids. Root injury ratings among the hybrids in the control plots ranged from 3.51 to 4.53 for Pioneer 3394 and Northrup King N6560, respectively. This level of injury was sufþcient to cause economic loss. The effect of hybrid on root injury was signiþcant (F 3.30; df 11, 88; P ) for the control plots. The level of root injury of all hybrids was signiþcantly greater (t -tests; Snedecor and Cochran 1967; Table 4) in control plots. The hybrid insecticide interaction was not signiþcant (F 1.42; df 11, 951; P 0.156) for root volume measurements taken on 14 July (Table 4). Because the interaction was not signiþcant, root volume data were pooled across hybrid and insecticide treatments and analyzed, indicating a signiþcant hybrid effect on July root volumes (F 5.15; df 11, 88; P ). Root volumes ranged from 69.5 to ml for Burrus BX58 and Hughes 5500, respectively, in treated plots. Root volumes for plants in control plots ranged from 57.6 to 92.6 ml for Northrup King N6560 and Asgrow RX707, respectively. Half of the 12 hybrids, Cargill 6337, DeKalb 591, FS 6774, Garst 8501, Hughes 5500, and Northrup King N6560, had plants with signiþcantly (t -tests, Table 4) smaller root systems in the control plots. As with July root volumes, the hybrid insecticide interaction was not signiþcant (F 0.39; df 11, 959; P 0.962) for root volumes measured on 16 August. Consequently, data were pooled, resulting in a significant hybrid effect (F 7.43; df 11, 88; P ) for root volumes measured in August. Root volumes for plants in the treated plots ranged from 88.3 to ml for Cargill 6337 and DeKalb 591, respectively. These 2 hybrids also represented the extremes in the control plots with root volumes of 82.3 and ml, respectively. Unlike the July root volumes, no signiþcant differences (t -tests) in August root volumes were detected between treated and control plots among any of the hybrids. The increase in root volume in the treated plots from July to August was least for Asgrow RX707 (8.8%) and greatest for Wyffels W707 (35.3%). This relationship also was evident in the check plots where Asgrow RX707 and Wyffels W707 represented the extremes in root volume increases, 17.6 and 46.7%, respectively. DeKalb 591 again had the greatest numeric root volume in August. Yields at the DeKalb location were much greater in 1994 than in No signiþcant hybrid insecticide interaction (F 1.09; df 11, 96; P 0.376) was detected, so yield data were combined for the analysis. Hybrids signiþcantly (F 5.18; df 11, 88; P ) affected yield. Yields in the treated plots ranged from to ql/ha for Pioneer 3394 and Crows 401, respectively. In the control plots, yields ranged from to ql/ha for Wyffels W707 and Hughes 5500, respectively. SigniÞcant yield differences between treated and control plots oc-

6 728 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 91, no. 3 Table 5. Root ratings, root volumes, and yields of 12 maize hybrids from plots at the Shaw Entomology Research Farm located near Urbana, IL, 1994 Hybrid Root rating a Root volume I b Root volume II c Yield ql/ha d Insecticide e Control Insecticide Control Insecticide Control Insecticide Control Asgrow RX ** ** ** ** Burrus BX ** ** ** ** Cargill ** ** ** ** Crows ** ** ** DeKalb ** ** ** ** FS ** ** ** ** Garst ** ** ** Hughes ** * ** ** NK N ** ** ** ** Pioneer ** ** ** ** Renk RK ** ** ** ** Wyffels W ** ** ** ** S , , , , SEM LSD SEM, standard error of the mean for means in a column; LSD, P 0.05 for means in a column. *, P 0.05; **, P 0.01; t -alpha values (99 df used in S 2 determination) for root rating, root volume, and yield differences between insecticide and control columns are for P, 0.05 and for P, a Roots were rated on 19 July 1994 using the Iowa State 1Ð6 scale (Hills and Peters 1971); n, 50 per mean. b Root volumes I (milliliters) were measured on 19 July 1994; n, 50 per mean. c Root volumes II (milliliters) were measured on 17Ð18 August 1994; n, 50 per mean. d Harvest was conducted in November of 1994; n, 10 per mean. e Terbufos 15 G was applied in a 17.8-cm band at planting on 13 May curred for only 3 hybrids: Asgrow RX707, Cargill 6337, and Northrup King N6560 (t -tests, Table 4). Yields were greater in the treated plots of the other 9 hybrids, but these differences were not signiþcant. Even though the increase in root volume of Wyffels W707 from July to August was the greatest among all hybrids, it had the lowest yield in the control plots and nearly the lowest yield in the treated plots. These data suggest that in a favorable growing season like 1994, in which timely precipitation occurred (Table 1), many hybrids compensated for moderate levels of root pruning and produced acceptable yields, even in control plots. The 1994 statewide average yield was ql/ha (Illinois Agricultural Statistics Service 1996). Urbana, An analysis of stand count data revealed no signiþcant hybrid insecticide interaction (F 0.69; df 11, 106; P 0.747) at the Urbana location in Therefore, stand count data for treated and control plots were pooled and analyzed. A signiþcant hybrid effect on plant population (F 2.32; df 11, 97; P 0.014) was detected. Densities ranged from 23.5 to 26.0 plants per 5.3 m of row for DeKalb 591 and Garst 8501, respectively (LSD 1.4, P 0.05). Root injury ratings for all hybrids in the treated plots were below the economic injury index of 3.0 (Table 5). Root injury in the control plots was severe, with root ratings near or exceeding 5.0 (2 nodes of roots destroyed) for all hybrids. Injury of this magnitude would be expected to cause signiþcant yield losses in a producerõs Þeld. An analysis of root rating data revealed the hybrid insecticide interaction was not signiþcant (F 0.97; df 11, 1,042; P 0.475). A pooled analysis of root rating data indicated a nonsigniþcant hybrid effect (F 1.39; df 11, 97; P 0.191) onroot injury. Root ratings in the treated plots ranged from 2.12 to 2.68 for Crows 401 and Cargill 6337, respectively. Injury in the control plots varied from a rating of 4.98 for Asgrow RX707 and Crows 401 to 5.46 for Renk RK839. Root injury for all hybrids was signiþcantly (t -tests, Table 5) more severe in control plots than in treated plots. Root volume measurements taken in July (Table 5) revealed no signiþcant hybrid insecticide interaction (F 1.64; df 11, 1,041; P 0.082). Ananalysis of pooled data disclosed a signiþcant hybrid effect (F 6.06; df 11, 97; P ) on July root volumes. Root volumes in the treated plots ranged from 76.9 to ml for Burrus BX58 and Wyffels W707, respectively. In the control plots, volumes were considerably less, ranging from 32.4 to 75.1 ml for Burrus BX58 and DeKalb 591, respectively. Root volumes of all hybrids were signiþcantly less (t -tests, Table 5) in control plots than in treated plots. Similar to results in July, measurements of root volumes in August (Table 5) indicated no signiþcant hybrid insecticide interaction (F 1.25; df 11, 1,041; P 0.248). A signiþcant hybrid effect (F 7.95; df 11, 97; P ) onaugust root volumes was detected when the pooled data were analyzed. Root volumes in treated plots ranged from to ml for Garst 8501 and Asgrow RX707, respectively. In the control plots, root volumes ranged from 71.0 to ml for Cargill 6337 and Wyffels W707, respectively. Root volumes in the control plots were signiþcantly less (t -tests, Table 5) than those in the treated plots for all hybrids except Crows 401 and Garst In the treated plots, the percentage increase in root volume from July to August was greatest for Burrus BX58 (38.5%) and least for Northrup King N6560 (15.2%). Increases in root volumes in the con-

7 June 1998 GRAY AND STEFFEY: CORN ROOTWORM INJURY AND HYBRID COMPENSATION 729 Table 6. Root ratings, root volumes, and yields of 12 maize hybrids from plots at the Northern Illinois Agronomy Research Center near DeKalb, IL, 1995 Hybrid Root rating a Root volume I b Root volume II c Yield ql/ha d Insecticide e Control Insecticide Control Insecticide Control Insecticide Control Asgrow RX ** ** Burrus BX ** ** Cargill ** * Crows ** * ** DeKalb ** FS ** ** Garst ** ** Hughes ** ** NK N ** ** Pioneer ** ** Renk RK ** * Wyffels W ** ** S , , , , SEM LSD SEM, standard error of the mean for means in a column; LSD, P 0.05 for means in a column. *, P 0.05; **, P 0.01; t -alpha values (99 df used in S 2 determination) for root rating, root volume, and yield differences between insecticide and control columns are for P, 0.05 and for P, a Roots were rated on 26 July 1995 using the Iowa State 1Ð6 scale (Hills and Peters 1971); n, 50 per mean. b Root volumes I (milliliters) were measured on 26 July 1995; n, 50 per mean. c Root volumes II (milliliters) were measured on 21 August 1995; n, 50 per mean. d Harvest date was 5 November 1995; n, 10 per mean. e Terbufos 15 G was applied in a 17.8-cm band at planting on 22 May trol plots ranged from 20.1 to 58.5% for Hughes 5500 and Asgrow RX707, respectively. An analysis of yield data (Table 5) revealed a signiþcant hybrid insecticide interaction (F 4.42; df 11, 104; P ); therefore, the effect of hybrid on yield was analyzed separately for treated and control plots. The effect of hybrid on yield was signiþcant in both the treated (F 5.67; df 11, 96; P ) and control plots (F 8.43; df 11, 95; P ). Yields of all hybrids were signiþcantly less (t -tests, Table 5) in the control plots than in the treated plots. In the treated plots, yields ranged from to ql/ha for Asgrow RX707 and FS 6774, respectively. In the control plots, the range was from to ql/ha for Garst 8501 and Wyffels W707, respectively. Asgrow RX707 had the largest August root volume (169.4 ml) in the treated plots; however, it also had the lowest yield (68.05 ql/ha) among hybrids in these plots. Conversely, Wyffels W707 had the greatest August root volume (121.7 ml) and greatest yield (73.31 ql/ha) among the hybrids in the control plots. Wyffels W707 also had a competitive yield in the treated plot (89.02 ql/ha). The growing season at this location was very dry in 1994, with a total of only 12.6 cm of rainfall in July and August. Despite the dry soil conditions, large root volumes in August did not necessarily translate into large yields for certain hybrids (e.g., Asgrow RX707) in the treated plots. DeKalb, No signiþcant hybrid insecticide interaction (F 1.53; df 11, 108; P 0.131) was detected for stand counts. Data were pooled and analyzed, revealing a signiþcant (F 3.13; df 11, 99; P ) inßuence of hybrid on stand density. Stand counts ranged from 20.2 to 23.8 plants per 5.3-m section of row for DeKalb 591 and Cargill 6337, respectively. Plant densities between these extremes typically were not different among hybrids (LSD 1.6, P 0.05). In DeKalb, root injury (Table 6) was below the economic injury index for most of the hybrids in both treated and control plots. No signiþcant hybrid insecticide interaction (F 1.52; df 11, 1,063; P 0.120) was detected with respect to root injury. The effect of hybrid on root injury also was not signiþcant (F 1.41; df 11, 99; P 0.178). Injury in the treated plots ranged from 1.82 to 2.12 for Crows 401 and Renk RK839, respectively. Root ratings in the control plots ranged from 2.64 to 3.18 for Hughes 5500 and Pioneer 3394, respectively. Even though root injury was not severe in any of the plots, root ratings for all hybrids were signiþcantly (t -tests, Table 6) greater when the soil insecticide was not used. No signiþcant hybrid insecticide interaction (F 1.28; df 11, 1,065; P 0.230) was detected for July root volumes (Table 6). A signiþcant hybrid effect (F 3.66; df 11, 99; P ) was observed for pooled July root volume data. In the treated plots, root volumes ranged from 64.6 to 97.8 ml for Burrus BX58 and DeKalb 591, respectively. July root volumes in the control plots were similar to those in the treated plots because of the minimal larval injury, ranging from 62.0 to ml for Burrus BX58 and Northrup King N6560, respectively. No statistical differences in July root volumes of all hybrids were observed between treated and control plots. The root volumes measured in August (Table 6) revealed a trend similar to that observed in July. No signiþcant hybrid insecticide interaction (F 1.36; df 11, 1,062; P 0.187) was evident. As with July root volumes, a signiþcant hybrid inßuence (F 4.28; df 11, 99; P ) was detected when August root volume data were pooled across treat-

8 730 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 91, no. 3 Table 7. Root ratings, root volumes, and yields of 12 maize hybrids from plots at the Shaw Entomology Research Farm located near Urbana, IL, 1995 Hybrid Root rating a Root volume I b Root volume II c Yield ql/ha d Insecticide e Control Insecticide Control Insecticide Control Insecticide Control Asgrow RX ** ** ** Burrus BX ** ** ** ** Cargill ** ** * ** Crows ** ** ** DeKalb ** ** ** ** FS ** ** ** ** Garst ** ** ** ** Hughes ** ** ** ** NK N ** ** ** Pioneer ** ** ** ** Renk RK ** ** ** Wyffels W ** ** ** ** S , , SEM LSD SEM, standard error of the mean for means in a column; LSD, P 0.05 for means in a column. *, P 0.05; **, P 0.01; t -alpha values (99 df used in S 2 determination) for root rating, root volume, and yield differences between insecticide and control columns are for P, 0.05 and for P, a Roots were rated on 18 July 1995 using the Iowa State 1Ð6 scale (Hills and Peters 1971); n, 50 per mean. b Root volumes I (milliliters) were measured on 18 July 1995; n, 50 per mean. c Root volumes II (milliliters) were measured on 23 August 1995; n, 50 per mean. d Harvest was conducted in November of 1995; n, 10 per mean. e Terbufos 15 G was applied in a 17.8-cm band at planting on 3 June ment effects. Root volumes in the treated plots ranged from 87.5 to ml for Burrus BX58 and Wyffels W707, respectively. In the control plots, root volumes ranged from 84.8 to ml for Garst 8501 and DeKalb 591, respectively. With one exception (Crows 401), root volumes of all hybrids did not differ signiþcantly (t -tests, Table 6) between treated and control plots. Increases in root volumes (July to August) in the treated plots ranged from 17.9 to 35.6% for Garst 8501 and Pioneer 3394, respectively. Increases in root volumes were similar in both treated and control plots, ranging from 16.5 to 38.9% for Northrup King N6560 and Burrus BX58, respectively. Yields at the DeKalb location during the 1995 growing season were below the statewide average of ql/ha (Illinois Agricultural Statistics Service 1996). Because of wet conditions during the spring, planting was delayed until 22 May, 2 wk beyond the optimum planting date. Although 23.5 cm of precipitation occurred during JuneÐAugust, summer temperatures were well above normal during much of the critical pollination process. No signiþcant hybrid insecticide interaction (F 1.10; df 11, 105; P 0.370) occurred regarding yield. Analysis of pooled yield data indicated a signiþcant hybrid effect (F 2.82; df 11, 96; P 0.003) on yield. Yields in the treated plots ranged from to ql/ha for DeKalb 591 and Garst 8501, respectively. Interestingly, root volumes in the treated plots during August indicated that Garst 8501 had a very small root system, whereas DeKalb 591 had a very large root system. The lower yield of De- Kalb 591 may have been linked to lower stand counts for that hybrid. Yields in the control plots ranged from to ql/ha for Hughes 5500 and Cargill 6337, respectively. Yields of all hybrids in the control plots except, DeKalb 591, were signiþcantly less (t -test, Table 6) than in the treated plots. The yield responses of the hybrids to the insecticide were greater than we anticipated, especially when considering the low rootworm larval pressure (low root ratings). However, plants were under severe heat stress during the summer of 1995, particularly during pollination. Urbana, Planting was delayed 3 wk beyond the optimum planting date (10 May) at the Urbana site during 1995 until 3 June because of the extreme amount of precipitation that fell during May (25.7 cm, Table 2). After the unusually wet May, precipitation in June (4.9 cm) and July (5.1 cm) was scarce, during which time extremely high temperatures were common. Because of these extreme environmental conditions, plants were under stress during critical phases of development like pollination. No signiþcant hybrid insecticide interaction (F 0.96; df 11, 108; P 0.488) was detected for stand counts. An analysis of pooled data revealed a signiþcant hybrid inßuence (F 6.22; df 11, 99; P ) onplant population. Stand densities per 5.3 m of row ranged from 23.6 to 26.3 plants for FS 6774 and Cargill 6337, respectively. Differences in stand counts between these extremes were not signiþcant statistically for most hybrids (LSD 0.9, P 0.05). Root ratings (Table 7) revealed extreme larval injury in the check plots; all hybrids had 1.5Ð2 nodes of roots destroyed. The hybrid insecticide interaction for root injury was signiþcant (F 2.85; df 11, 1,058; P 0.001). Separate analyses of the potential inßuence of hybrid on root injury were conducted for root rating data from treated and control plots. The inßuence of hybrid on root injury in the treated plots was not signiþcant (F 1.76; df 11, 99; P 0.071). Root ratings in these plots ranged from 2.04 to 2.54 for Crows 401 and Northrup King N6560, respectively.

9 June 1998 GRAY AND STEFFEY: CORN ROOTWORM INJURY AND HYBRID COMPENSATION 731 The effect of hybrid on root injury in the control plots also was not signiþcant (F 0.73; df 11, 99; P 0.707). Injury in the control plots ranged from 4.56 to 5.04 for Northrup King N6560 and Wyffels W707, respectively. Northrup King N6560 had the greatest root injury in the treated plots and the least amount of injury in the control plots. This may explain, in part, the signiþcant interaction between the hybrid and insecticide effects. Root injury for all hybrids was signiþcantly worse (t -tests, Table 7) in the control plots than in the treated plots. Because of the late planting date and low amounts of rainfall in June and July, root volumes measured on 18 July were very low (Table 7), especially for plants in the control plots. The hybrid insecticide interaction for July root volume measurements was significant (F 4.49; df 11, 1,053; P ); therefore, separate analyses were performed on root volume data from treated and control plots. The hybrid effect in the treated plots was signiþcant (F 3.45; df 11, 99; P ), with root volumes ranging from 30.8 to 45.7 ml for Renk RK839 and Wyffels W707, respectively. The inßuence of hybrid in the control plots was not signiþcant (F 1.62; df 11, 99; P 0.104), with volumes ranging from 11.4 to 19.0 ml for Renk RK839 and Northrup King N6560, respectively. Root volumes of all hybrids in the control plots were signiþcantly (t -tests, Table 7) less than those in the treated plots. A signiþcant hybrid insecticide interaction also was detected for August root volume measurements (F 3.79; df 11, 1,057; P ). Because of this signiþcant interaction, data from treated and control plots were analyzed separately. The root volumes of hybrids in August differed signiþcantly (F 5.67; df 11, 99; P ) in the treated plots, ranging from 67.0 to ml for Renk RK839 and Wyffels W707, respectively. Similarly, root volumes differed signiþcantly (F 5.88; df 11, 99; P ) in the control plots, ranging from 51.8 to 97.8 ml for Burrus BX58 and Northrup King N6560, respectively. With the exceptions of Asgrow RX707, Crows 401, Northrup King N6560, and Renk RK839, root volumes were signiþcantly less (t -tests, Table 7) in control plots than in treated plots. Percentage increases in root volumes in the treated plots ranged from 50.1 to 66.1% for Asgrow RX707 and Pioneer 3394, respectively. Percentage increases in root volumes in the control plots ranged from 72.4 to 82.4% for Burrus BX58 and Asgrow RX707, respectively. In August of 1995, 13.6 cm of rain fell at Urbana (Table 2), probably enabling all hybrids to grow new root tissue after the severe larval root pruning. In 1995, yields at Urbana (Table 7) were the lowest that occurred during this 4-yr investigation. The hybrid insecticide interaction on yield was signiþcant (F 3.09; df 11, 101; P ). An analysis of yield data for treated plots revealed a signiþcant hybrid effect (F 14.01; df 11, 95; P ), with yields ranging from to ql/ha for Hughes 5500 and Wyffels W707, respectively. The hybrid effect on yield also was signiþcant in the control plots (F 8.30; df 11, 92; P ), with yields ranging from to ql/ha for Crows 401 and Wyffels W707, respectively. Wyffels W707 had a very large root system in both treated and control plots during July and August, which may have aided its yield production during this dry, stressful growing season. Yields of all hybrids were signiþcantly less (t -tests, Table 7) in the control plots than in the treated plots. DeKalb, Precipitation was abundant at the DeKalb experimental site during 1996; 61.2 cm of rainfall was recorded from May through August (Table 1). The month of July was especially wet with nearly 22 cm of precipitation. The hybrid treatment interaction was not significant (F 0.77; df 10, 253; P 0.657) for stand counts. An analysis of pooled stand count data revealed a signiþcant hybrid effect (F 8.53; df 10, 70; P ) on plant density. Plant densities varied from 16.2 to 24.6 plants per 5.3-m section of row for Northrup King N6560 and Crows 401, respectively (LSD 2.2; P 0.05). Northrup King N6560 had signiþcantly fewer established plants than any of the hybrids. The explanation for this is unclear; however, the very wet and cool conditions in May might have affected this hybrid more adversely. An analysis of root rating data (Table 8) revealed a signiþcant hybrid treatment interaction (F 2.00; df 10, 769; P 0.031). Therefore, root injury data from treated and control plots were analyzed separately. The effect of hybrid on root injury in the treated plots was not signiþcant (F 1.49; df 10, 70; P 0.160), with ratings ranging from 1.78 to 2.15 for Pioneer 3394 and FS 6774, respectively. Similarly, the inßuence of hybrids on root injury was not signiþcant (F 1.31; df 10, 70; P 0.240) in the control plots, with ratings ranging from 2.87 to 3.54 for DeKalb 591 and both FS 6774 and Wyffels W707, respectively. Injury to all hybrids in the control plots was signiþcantly greater (t -tests, Table 8) than in the treated plots; however, even in the control plots, root pruning of all hybrids was slight to moderate. Because of the very wet and cool spring, corn rootworm egg hatch was delayed 2 wk at DeKalb in Consequently, root ratings and volumes were measured roughly 2 wk later (2 August) than in previous years because of the late-season larval injury; therefore, initial root volumes (Table 8) were very large. The hybrid treatment interaction for volumes measured on 2 August was not signiþcant (F 0.89; df 10, 767; P 0.543). An analysis of pooled data indicated a signiþcant (F 8.37; df 10, 70; P ) hybrid inßuence on root volumes measured in early August. In the treated plots, volumes ranged from 77.5 to ml for Burrus BX58 and Northrup King N6560, respectively. This pattern also occurred in the control plots where volumes ranged from 58.7 to ml for these same hybrids, respectively. Statistical differences (t -tests, Table 8) in root volumes between treated and control plots occurred for only 4 hybrids: DeKalb 591, FS 6774, Northrup King N6560, and Pioneer For each of these hybrids, root volumes were smaller in the control plots.

10 732 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 91, no. 3 Table 8. Root ratings, root volumes, and yields of 11 maize hybrids from plots at the Northern Illinois Agronomy Research Center near DeKalb, IL, 1996 Hybrid Root rating a Root volume I b Root volume II c Yield ql/ha d Insecticide e Control Insecticide Control Insecticide Control Insecticide Control Asgrow RX ** ** Burrus BX ** ** * Crows ** * DeKalb ** ** * FS ** * ** Garst ** ** Hughes ** NK N ** * ** Pioneer ** ** ** ** Renk RK ** ** Wyffels W ** * S , , , , SEM LSD SEM, standard error of the mean for means in a column; LSD, P 0.05 for means in a column. *, P 0.05; **, P 0.01; t -alpha values (70 df used in S 2 determination) for root rating, root volume, and yield differences between insecticide and control columns are for P, 0.05 and for P, a Roots were rated on 2 August 1996 using the Iowa State 1Ð6 scale (Hills and Peters 1971); n, 40 per mean. b Root volumes I (milliliters) were measured on 2 August 1996; n, 40 per mean. c Root volumes II (milliliters) were measured on 20 August 1996; n, 40 per mean. d Harvest dates were 4Ð5 and 10Ð11 November 1996; n, 7 per mean. e Terbufos 15 G was applied in a 17.8-cm band at planting on 3 May Root volumes were measured again 18 d later (Table 8), the shortest span of time between volume measurements throughout this investigation. Again, no signiþcant hybrid treatment interaction was evident (F 0.57; df 10, 761; P 0.840) for the 2nd set of root volume measurements. A signiþcant hybrid effect (F 7.85; df 10, 70; P ) on root volumes (20 August) was detected when data were pooled and analyzed. Root volumes in the treated plots ranged from 77.8 to ml for Garst 8501 and Northrup King N6560, respectively. In the control plots, volumes ranged from 52.8 to 99.3 ml for Garst 8501 and DeKalb 591, respectively. With 2 exceptions, Garst 8501 and Hughes 5500, root volumes were signiþcantly less (t -tests, Table 8) in the control plots than in the treated plots. Unlike in previous years and locations, the percentage change in root volume (2Ð20 August) was minimal for plants in the treated plots, ranging from 10.4 to 13.4% for Garst 8501 and Burrus BX58, respectively. Root volumes measured for all hybrids in the control plots indicated that a consistent decline occurred between the 2 August sampling dates. The percentage root decrease in volumes ranged from 26.0 to 2.3% for Renk RK839 and DeKalb 591, respectively. These declines in root volumes were not expected because they had never occurred in any of our previous trials. We speculate that the cool and frequently saturated soils during the spring and summer months may have contributed to premature root senescence. Despite the declines in root volumes that occurred in August, yields for many hybrids (Table 8) were above the 1996 statewide average of ql/ha (Illinois Agricultural Statistics Service 1997). Only 1994 yields from DeKalb were greater during this investigation. No signiþcant hybrid treatment interaction (F 0.73; df 10, 55; P 0.694) was detected for yield data. An analysis of pooled yield data indicated a signiþcant (F 11.52; df 10, 60; P ) hybrid inßuence on yield. In the treated plots, yields ranged from to ql/ha for Northrup King N6560 and Pioneer 3394, respectively. Yields in the control plots ranged from to ql/ha for Northrup King N6560 and Wyffels W707, respectively. The low yields of Northrup King N6560 were likely related to its poor plant establishment. Although yields of all hybrids were less in control plots than in treated plots, signiþcant differences (t -tests, Table 8) occurred for only Burrus BX58, Garst 8501, and Pioneer Urbana, Precipitation at Urbana was abundant throughout May 1996 (21.0 cm, Table 2) and planting was delayed until the 20 May. Rainfall also was plentiful during June (14.4 cm) and July (8.5 cm), more than doubling the combined monthly totals (June and July) that had occurred at Urbana during the previous 2 yr. However, the precipitation total in August (3.6 cm) was the least for that month for either location in any year of this investigation. This low monthly total may help explain why some hybrids, particularly in the control plots, regrew less root tissue than they had in previous years. An analysis of stand counts indicated no signiþcant hybrid insecticide interaction (F 0.89; df 10, 319; P 0.544); therefore, stand count data were pooled and analyzed. The inßuence of hybrid was signiþcant (F 3.64; df 10, 90; P ), with stand counts ranging from 25.7 to 27.8 plants per 5.3 m of row for FS 6774 and both DeKalb 591 and Renk 839, respectively. Stand counts of most hybrids within this range did not differ signiþcantly (LSD 0.9; P 0.05). Moderate levels of root pruning (root rating 4.0) of all hybrids except Pioneer 3394 were observed

11 June 1998 GRAY AND STEFFEY: CORN ROOTWORM INJURY AND HYBRID COMPENSATION 733 Table 9. Root ratings, root volumes, and yields of 11 maize hybrids from plots at the Shaw Entomology Research Farm located near Urbana, IL, 1996 Hybrid Root rating a Root volume I b Root volume II c Yield ql/ha d Insecticide e Control Insecticide Control Insecticide Control Insecticide Control Asgrow RX ** ** ** * Burrus BX ** ** ** ** Crows ** ** ** ** DeKalb ** ** ** ** FS ** ** ** ** Garst ** ** ** ** Hughes ** ** ** ** NK N ** ** ** ** Pioneer ** ** ** ** Renk RK ** ** ** ** Wyffels W ** ** ** ** S , SEM LSD SEM, standard error of the mean for means in a column; LSD, P 0.05 for means in a column. *, P 0.05; **, P 0.01; t -alpha values (90 df used in S 2 determination) for root rating and root volume differences are for P, 0.05 and for P, t -alpha values (83 df used in S 2 determination) for yield differences between insecticide and control columns are for P, 0.05 and for P, a Roots were rated on 16 July 1996 using the Iowa State 1Ð6 scale (Hills and Peters 1971); n, 50 per mean. b Root volumes I (milliliters) were measured on 16 July 1996; n, 50 per mean. c Root volumes II (milliliters) were measured on 21 August 1996; n, 50 per mean. d Harvest dates were 4Ð7 November 1996; n, 10 per mean. e Terbufos 15 G was applied in a 17.8-cm band at planting on 20 May within the control plots (Table 9), and even Pioneer 3394, on average, had nearly a full node of roots destroyed. A signiþcant hybrid insecticide interaction (F 2.77; df 10, 976; P 0.002) was detected for root rating data. In the treated plots, the inßuence of hybrid on root injury was signiþcant (F 2.33; df 10, 90; P 0.017), with ratings ranging from 2.0 to 2.38 for Garst 8501 and both DeKalb 591 and FS 6774, respectively. The hybrid effect also was signiþcant (F 2.18; df 10, 90; P 0.026) inthecontrol plots, with ratings ranging from 3.78 to 4.46 for Pioneer 3394 and Wyffels W707, respectively. Injury to all hybrids in the check plots was signiþcantly worse (t -test, Table 9) than in the treated plots. An analysis of root volume measurements in July (Table 9) indicated a signiþcant hybrid insecticide interaction (F 2.19; df 10, 974; P 0.016). For the treated plots, the effect of hybrid on July root volumes was signiþcant (F 2.82; df 10, 90; P 0.004), with volumes ranging from 38.6 to 56.5 ml for Hughes 5500 and DeKalb 591, respectively. The inßuence of hybrid on root volume also was signiþcant (F 2.11; df 10, 90; P 0.032) inthecontrol plots during July, with volumes ranging from 20.5 to 29.5 ml for Northrup King NK6560 and DeKalb 591, respectively. Root volumes of all hybrids in the check plots were signiþcantly less (t -tests, Table 9) than in the treated plots. A signiþcant hybrid insecticide interaction (F 11.33; df 10, 976; P ) was detected for August root volume measurements (Table 9). An analysis of root volumes in the treated plots revealed a signiþcant hybrid effect (F 11.89; df 10, 90; P ), with volumes ranging from 76.4 to ml for Renk RK839 and Wyffels W707, respectively. Similarly, August root volumes were affected signiþcantly by hybrid in the control plots (F 2.64; df 10, 90; P 0.007), with volumes ranging from 25.4 to 47.5 ml for Garst 8501 and Pioneer 3394, respectively. Root volumes (August) of all hybrids in the control plots were signiþcantly less (t -tests, Table 9) than in the treated plots. The percentage increase in root volume for plants in the treated plots ranged from 44.7 to 70.9% for Crows 401 and Wyffels W707, respectively. Percentage increases in root volumes were considerably less for plants in the control plots, ranging from 4.7 to 44.6% for Garst 8501 and Asgrow RX707, respectively. The lack of precipitation in August 1996 limited the ability of many hybrids to regrow root tissue as effectively as they had in previous years (e.g., 1995 in Urbana [increase in root volume of hybrids in control plots ranged from 72.4 to 82.4%]). An analysis of yield data (Table 9) indicated a signiþcant hybrid insecticide interaction (F 2.99; df 10, 91; P 0.003). The inßuence of hybrid on yields in the treated plots was signiþcant (F 3.20; df 10, 83; P 0.002), with yields ranging from to ql/ha for Asgrow RX707 and Wyffels W707, respectively. Wyffels W707 had the largest August root volume (treated plots) and greatest percentage increase (treated plots) in root volume among the hybrids. A signiþcant hybrid effect (F 6.68; df 10, 85; P ) on yield also was detected in the control plots, with yields ranging from to ql/ha for Renk RK839 and Wyffels W707, respectively. Yields of all hybrids were signiþcantly less (t -tests, Table 9) in control plots than in treated plots. Regressions of Yield on Root Injury, Root Volumes in July and August, and Changes (Milliliters and Percentage) in Root Volumes. Regression equations and coefþcients of determination (R 2 ) for relationships between yield and root injury, root volumes in July and August, and changes in root volumes (1993Ð1996) are reported in Tables 10Ð14. Analyses were per-

12 734 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 91, no. 3 Table 10. Regression equations and coefficients of determination (R 2 ) for relationships between yield and root ratings for corn rootworm injury of 12 maize hybrids, Regressions of yield on root ratings a n R 2 P SE a b 1 b 2 Root ratings across years and location Yield x 4.12x Root ratings by year and across locations Yield (1993) x x 2 Yield (1994) x x 2 Yield (1995) x 0.88x Yield (1996) x x 2 Root ratings by location and across years Yield (DeKalb) x x 2 Yield (Urbana) x x 2 Root ratings by year and locationðdekalb Yield (1993) x x 2 Yield (1994) x x 2 Yield (1995) x x 2 Yield (1996) x x 2 Root ratings by year and locationñurbana Yield (1994) x 2.10x Yield (1995) x x 2 Yield (1996) x 9.41x SE, standard error; a, intercept; b 1, coefþcient 1; b 2, coefþcient 2. a Root ratings were derived from the Iowa State 1Ð6 scale (Hills and Peters 1971). formed across years and locations, by year and across locations, by location and across years, and by year and location. Regarding root injury data and its potential usefulness in predicting yield, curvilinear analyses improved coefþcient of determination values as compared with linear regressions and are thus presented. Root Injury. When yield was regressed against root ratings (Table 10) across years and locations, only 12% Table 11. Regression equations and coefficients of determination (R 2 ) for relationships between yield and root volumes I of 12 maize hybrids, Regressions of yield on root volumes I a n R 2 P SE a b Root volumes I across years and locations Yield x Root volumes I by year and across locations Yield (1993) x Yield (1994) x Yield (1995) x Yield (1996) x Root volumes I by location and across years Yield (DeKalb) x Yield (Urbana) x Root volumes I by year and locationñdekalb Yield (1993) x Yield (1994) x Yield (1995) x Yield (1996) x Root volumes I by year and locationñurbana Yield (1994) x Yield (1995) x Yield (1996) x SE, standard error; a, intercept; b, slope. a Root volumes in mid-july were measured by water displacement in a graduated cylinder.

13 June 1998 GRAY AND STEFFEY: CORN ROOTWORM INJURY AND HYBRID COMPENSATION 735 Table 12. Regression equations and coefficients of determination (R 2 ) for relationships between yield and root volumes II of 12 maize hybrids, Regressions of yield on root volumes II a n R 2 P SE a b Root volumes II across years and locations Yield x Root volumes II by year and across locations Yield (1993) x Yield (1994) x Yield (1995) x Yield (1996) x Root volumes II by location and across years Yield (DeKalb) x Yield (Urbana) x Root volumes II by year and locationñdekalb Yield (1993) x Yield (1994) x Yield (1995) x Yield (1996) x Root volumes II by year and locationñurbana Yield (1994) x Yield (1995) x Yield (1996) x SE, standard error; a, intercept; b, slope. a Root volumes in mid-august were measured by water displacement in a graduated cylinder. (R , n 152, P 0.001) of the variation in yield could be explained by root injury. The coef- Þcients of determination improved when regression analyses were conducted by year and across locations during 1994Ð1996 (0.58, 0.64, and 0.52, respectively). In 1993, the very low coefþcient of determination (0.08) probably was the result of the narrow range (3.16Ð4.16) in root injury because no soil insecticide was used in the 1st yr of the investigation. Consequently, all roots sustained some light to moderate pruning. Regressing yield against root ratings by location and across years resulted in lower coefþcients of determination compared with regression analyses conducted by year; however, 17% (R , n Table 13. Regression equations and coefficients of determination (R 2 ) for relationships between yield and change in root volume (milliliters) from volume I to volume II measurements of 12 maize hybrids, Regressions of yield on change SE (ml) a n R in root volume P a b Volume change (ml) across years and locations Yield x Volume change (ml) by year and across locations Yield (1993) x Yield (1994) x Yield (1995) x Yield (1996) x Volume change (ml) by location and across years Yield (DeKalb) x Yield (Urbana) x Volume change (ml) by year and locationñdekalb Yield (1993) x Yield (1994) x Yield (1995) x Yield (1996) x Volume change (ml) by year and locationñurbana Yield (1994) x Yield (1995) x Yield (1996) x SE, standard error; a, intercept; b, slope. a Change (ml) in root volume from mid-july to mid-august was measured by water displacement.

14 736 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 91, no. 3 Table 14. Regression equations and coefficients of determination (R 2 ) for relationships between yield and percentage change in root volume from volume I to volume II measurements of 12 maize hybrids, Regressions of yield on percentage change in root volume a n R 2 P Percentage change across years and locations Yield x Percentage change by year and across locations Yield (1993) x Yield (1994) x Yield (1995) x Yield (1996) x Percentage change by location and across years Yield (DeKalb) x Yield (Urbana) x Percentage change by year and locationñdekalb Yield (1993) x Yield (1994) x Yield (1995) x Yield (1996) x Percentage change by year and locationñurbana Yield (1994) x Yield (1995) x Yield (1996) x SE, standard error; a, intercept; b, slope. a Change (ml) in root volume from mid-july to mid-august was measured by water displacement. a SE b 70, P 0.002) of the variation in yield at the Urbana site across years was caused by root injury. Throughout the study, root injury was consistently more severe and yields were affected more signiþcantly at the Urbana site. Regression analyses conducted for De- Kalb and Urbana by year clearly indicated the greater importance of root injury to yield at the Urbana site. At DeKalb from 1994 to 1996, coefþcients of determination were 0.24, 0.69, and 0.29, respectively. During this same period at Urbana, coefþcients of determination were 0.70, 0.65, and 0.71, respectively. Even though minor root injury was observed at DeKalb in 1995, a high percentage (69%) of the variation in yield was still associated with root injury. Several investigators have attempted to establish a relationship between root ratings and yield. Spike and Tollefson (1989b, p. 1760), who relied on artiþcial infestations of western corn rootworm eggs, concluded that root ratings were not consistent predictors of yield during their 2-yr study. Earlier work by Branson et al. (1980), who also relied upon artiþcial infestations of western corn rootworm eggs, revealed a signiþcant R 2 value of 0.29 when yield was regressed against root ratings (1Ð6 scale). Turpin et al. (1972, p. 1618), reporting on the results of a large Þeld study, concluded, The yield data showed little relationship with damage ratings 2.5 and observed yield. Damage ratings 2.5 were linearly related to decreasing yields where a damage rating increase of 1.0 was associated with 10 bushels/acre yield reduction. Foster et al. (1986, p. 306) reported that root damage (1Ð9 scale) was not signiþcantly correlated with yield loss (P r 0.12). Based on these Þndings in the literature, the large and signiþcant R 2 values we obtained when yields were regressed against root ratings at Urbana during each of the 3 yr were somewhat unexpected. Despite these large coefþcients of determination at Urbana, 30% of the variation in yield in each year of the experiment could not be explained by rootworm larval injury. Root Volumes, July. Our results have demonstrated that a large root volume in July (Table 11) is not always an indication that a given hybrid will yield more than another hybrid with a smaller root system. However, when yield was regressed against July root volumes across years and locations, 29% (R , n 152, P 0.01) of the variation in yield could be explained by this measurement. Overall, a large root volume in July was a positive inßuence on yield during our experiments. This conclusion was especially reinforced by the 1995 regression analysis by year and across locations (R , n 48, P 0.01), which indicated that a large July root volume was a very important and positive inßuence on yield. As mentioned previously, the 1995 growing season was very hot and dry at both locations. Regression analyses across years indicated that July root volumes at DeKalb (R , n 82, P 0.01) were less important predictors of yield than July root volumes at Urbana (R , n 70, P 0.01). In fact, regressions conducted for DeKalb by year failed to produce any signiþcant coefþcients of determination. For Urbana, July root volume proved to be a very good indicator of potential yield throughout the study; coefþcients of determination for 1994Ð1996 were 0.60, 0.64, and 0.67%, respectively.

15 June 1998 GRAY AND STEFFEY: CORN ROOTWORM INJURY AND HYBRID COMPENSATION 737 Root Volumes, August. The analyses of yield regressed against August root volumes (Table 12) resulted in many similarities with the regressions conducted for July root volumes. The overall regression analysis across years and locations resulted in a low but signiþcant coefþcient of determination (R , n 152, P 0.05), which indicated that a large root volume in August generally is a positive inßuence on yield. However, as our data revealed, a large August root volume for certain hybrids does not always result in the greatest yield. Regression analyses conducted by year and across locations resulted in highly significant coefþcients of determination for 1995 (R , n 48, P 0.01) and 1996 (R , n 44, P 0.01). These results suggest that a large August root volume for both years was an important factor in explaining the variation in yield. This was especially true for 1995 when the results were similar for the July root volumes. As stated previously for July root volumes, August root volumes were more important predictors of yield at the Urbana location, as indicated by the regressions conducted across years (R , n 70, P 0.01). Regression analyses conducted by year at DeKalb did not result in any signiþcant relationship between August root volume and yield, similar to the results for July root volumes at DeKalb, and most likely related to less larval injury at DeKalb than at Urbana. CoefÞcients of determination were highly signiþcant for Urbana when yield was regressed against August root volumes from 1994 through 1996 (0.41, 0.57, and 0.66, respectively). These results were very similar to those for July root volumes at Urbana and indicated that, in general, large root systems in July and August were positive yield factors when rootworm larval feeding (moderate to severe pruning of roots) was intense. Spike and Tollefson (1989b) reported that root biomass of the maize hybrid they used was a better predictor of yield than root ratings. They also suggested (p. 1763) that regrowth after injury plays an important part in protecting the plant from yield losses. Our results, analyzed 12 maize hybrids, agree with their Þndings that root biomass is a signiþcant predictor of yield. However, we also found that root ratings could be useful predictors of potential yield. Root Regrowth (Milliliters). Although large July and August root volumes in our experiments generally were positively linked to yield, regression analyses of yield against root regrowth (milliliters) suggested that those hybrids that invested too heavily in root compensation sacriþced potential yield. When yield was regressed (R , n 152, P 0.01) against the change (milliliters) in root volume from July to August across years and locations, a general decline in yield resulted as root volume increased (Table 13). This differs from the Þndings of Spike and Tollefson (1989b) who found a positive relationship between root regrowth and yield for the hybrid in their study. Godfrey et al. (1993b, p. 1563) found that root biomass generally increased with increasing amounts of damage for the uninfested, 200, and 500 eggs per row-cm treatments. Despite good evidence of root compensation, rootworm larval injury reduced yields by 15.0 and 40.7% for their hybrid during the 2 yr of their study. In our experiments, when yield was regressed against the change (milliliters) in root volume by year across locations, increases in root volumes generally were associated with lower yields in 1994 (R , n 48, P 0.01) and 1995 (R , n 48, P 0.01). The regression analysis for 1996 data, by year and across locations, indicated a general increase in yield for those hybrids that compensated by regrowing more root tissue (R , n 44, P 0.05). This signiþcant relationship most likely was due to the contribution of 1996 data from Urbana that showed a strong positive relationship between root regrowth and yield (R , n 22, P 0.01). The least amount of precipitation in August during this study occurred at Urbana in These data suggest that in a dry period after moderate larval injury (root ratings 4.0Ð4.5), hybrids that compensate by growing greater amounts of root tissue may produce greater yields. When yield was regressed against the change (milliliters) in root volume (milliliters) by year for DeKalb, no signiþcant coefþcients of determination were found. However, when data from this location were combined across years, a signiþcant relationship was detected (R , n 82, P 0.01). This relationship indicated that hybrids generally were penalized in yield potential at the DeKalb site for regrowing root tissue after root injury had occurred. Compared with Urbana, precipitation at DeKalb typically was greater for the months of June, July, and August (Table 1), suggesting that when soil moisture is plentiful, an investment in regrowing root tissue generally does not result in a positive yield gain. Root Regrowth (Percent Change). Regression analyses across years and locations of yield against the percentage change in root volume from July to August (Table 14) produced more evidence that hybrids that invest too many resources to grow new root tissue during this period sacriþce yield (R , n 152, P 0.01). When regression analyses were conducted by year and across locations, signiþcant coefþcients of determination for 1994 (R , n 48, P 0.01) and 1995 (R , n 48, P 0.01) again indicated a negative relationship between root regrowth and yield. Further regression analyses performed by location and across years conþrmed this negative relationship at both DeKalb (R , n 82, P 0.01) and Urbana (R , n 70, P 0.01). The negative association between yield and root compensation after larval injury also was substantiated for DeKalb in 1994 (R , n 24, P 0.05) and Urbana in 1994 (R , n 24, P 0.01) and 1995 (R , n 24, P 0.01). In 1996 at Urbana, a signiþcant (R , n 22, P 0.01) positive relationship between percentage regrowth and yield was detected, supporting the previously described (Table 13) analysis of yield regressed against the change in volume (milliliters) (July to August) for this year and site. We believe this evidence indicates that hybrids that regrow root tissue after larval injury in a dry season (e.g., Urbana in 1996

16 738 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 91, no. 3 Table 15. Profit margins for soil insecticide use compared with control plots across years ( ), locations, and hybrids Range of root injury a in control plots No. of positive yield beneþts with soil insecticide Avg yield beneþt ql/ha b Avg proþt marginsc per ha at different grain prices/ql $7.87 $9.84 $11.81 $ Ð2.99 (n 11) $ $ $ $ Ð3.99 (n 21) $ $ $ $ Ð4.99 (n 26) $ $ $ $ Ð6.0 (n 12) $ $ $ $ a Root rating scale, 1Ð6 (Hills and Peters 1971). b Average yield difference between treated and control plots. c Calculated on the basis of an insecticide cost of $3.80/kg of formulated product applied at 9.75 kg/ha. [Table 2]), are more likely to optimize yield potential). A signiþcant positive relationship (R , n 22, P 0.05) between the percentage root regrowth and yield also was detected for DeKalb in However, the regression analysis (Table 13) for the change in volume (milliliters) against yield was not signiþcant for this site during Riedell and Evenson (1993) suggested that large root systems provide tolerance to rootworm injury to maize hybrids grown in the northern United States. The newest hybrids they evaluated were from the 1980s. Our results with 12 maize hybrids grown commonly in Illinois during the mid-1990s support their general conclusion. Root compensation, in the form of regrowth after larval injury, was not examined by Riedell and Evenson (1993). Our results concerning the positive relationship of large roots to yield agree with those of Riedell and Evenson (1993); however, the degree of root compensation following larval injury and the availability of soil moisture also appear to be equally important predictors of yield potential. Riedell (1994) evaluated differences in root pull resistance among 1960, 1970, and 1980-era hybrids after maximum larval injury and again 2 wk later. Root injury was created by artiþcially infesting plots with corn rootworm eggs. Riedell (1994) indicated that the difference in root-pull resistance of 1970 and 1980-era hybrids was greater than that for 1960-era hybrids and concluded that hybrids of the 1970s and 1980s had greater tolerance to rootworm larval injury, especially in dry seasons. Our results suggest that hybrids in wetter growing seasons are penalized in yield potential for excessive root regrowth after rootworm larval injury has occurred. This relationship apparently is reversed in a very dry year, particularly a dry August. Chiang et al. (1980, p. 665) cautioned researchers on this very point by concluding... rainfall needs to be considered in assessing the economic threshold of this insect. In an investigation that involved a single hybrid and artiþcial infestation techniques, Godfrey et al. (1993b, p. 1563) further emphasized the importance of soil moisture when they suggested that root regrowth apparently was stimulated more by dry soil conditions than by moist soil conditions. Similar to our Þndings, their research also suggested that vegetative compensation for larval injury occurred at the expense of yield. Producers have been encouraged to monitor their Þelds for rootworm beetles (Gray and Luckmann 1994) during the late-summer months and, if the beetle density exceeds a given threshold (0.75 beetle per plant in continuous corn, 0.5 beetle per plant in rotated corn), to consider the application of an insecticide. Insecticides may be directed at egg-laying beetles or applied at planting the following spring to target the larval population. In spite of the Foster et al. (1986) recommendation always to treat continuous corn with an insecticide during planting and not bother with sampling for adults, extension entomologists continue to support scouting and use of thresholds for corn rootworms (Gray and Steffey 1997). By accepting these recommendations, growers are encouraged to believe that Þelds with non-economic levels of corn rootworms can be predicted accurately (Stamm et al. 1985). What index of economic root injury are thresholds of rootworm adults attempting to predict? Turpin et al. (1972) suggested that larval injury above a root rating of 2.5 (Hills and Peters 1971) could contribute to economic yield losses. Others have indicated that a rating of 3.0 (Mayo 1986) or 2.75 (Stamm et al. 1985) is a root injury index of potential yield loss. Sutter et al. (1990, p. 2418) suggested that... under good growing conditions (i.e., adequate soil moisture as we experienced), root damage ratings of 4.0Ð5.0 (for untreated maize) must be reached before yield losses would be minimized by insecticides. The economic injury index for corn rootworm larval injury has remained simple and static in the management literature offered to growers. Crop value, management costs, and crop susceptibility to injury have not been factored into recommendations for producers, partially because of a lack of critical information regarding the reaction of multiple maize hybrids to corn rootworm larval injury under a variety of environmental conditions. Our research efforts, as well as the contributions of others (Spike and Tollefson 1989b; Gibb and Higgins 1991; Godfrey et al. 1993a, b; Riedell and Evenson 1993) are beginning to show more precisely how maize reacts to larval injury. The interactions of root injury, insecticide cost, and 4 market prices of maize on proþt margins is depicted in Table 15. Root rating and yield data from our experiments (1994Ð1996) were used to calculate proþt margins. The use of a soil insecticide at a cost of $3.80/kg applied at 9.75 kg/ha was proþtable at each combination of root injury and grain price. Even when root injury occurred in the 2.0Ð2.99 range in control plots, the use of a soil insecticide was proþtable. Most

17 June 1998 GRAY AND STEFFEY: CORN ROOTWORM INJURY AND HYBRID COMPENSATION 739 root injury data in the 2.0Ð2.99 range were obtained from the 1995 DeKalb experiment which was planted on 22 May, nearly 2 wk beyond the optimum planting date. These data verify that even very low levels of root injury can cause economic losses in certain years. Perhaps not surprisingly, proþt margins for the use of a soil insecticide were greatest when root injury in check plots was in the 5.0Ð6.0 range. However, a soil insecticide application also was proþtable for intermediate ranges of root injury in control plots. Average yield beneþts of 6.08 and ql/ha were obtained in treated plots when injury in the 3.0Ð3.99 and 4.0Ð4.99 ranges occurred for control plots, respectively. Our data indicate that root ratings well below 4.0 contributed to economic losses. This clearly differs from the conclusion reached by Sutter et al. (1990, p. 2418) who suggested that root ratings from 4.0 to 5.0 ( under good growing conditions ) were necessary before soil insecticide beneþts began to accrue. In summary, the results of this investigation have shown that large root systems in July and August generally are positive factors in contributing to yield; however, compensatory root regrowth, particularly when soil moisture is adequate, can affect yield negatively. Root regrowth after larval injury had a positive effect on yield when soil moisture was inadequate. Root ratings were as useful in predicting yield as root volumes in July and August and root regrowth measurements during this period. Finally, proþt margins associated with different economic parameters demonstrated the very dynamic nature of root injury and its impact on yield under different environmental conditions. Acknowledgments The generous donation of seed from all seed companies is greatly appreciated. Our sincere thanks also go out to the following individuals for their numerous hours devoted to this study: Richard Bonham, Doyle Dazey, Jim Finger, Kyle Krapf, Hassan Oloumi-Sadeghi, Lyle Paul, and John Shaw. In addition, we express our gratitude to Eli Levine, Illinois Natural History Survey, and Walter Riedell, USDAÐARS Northern Grain Insects Research Laboratory, for their critical reviews of this manuscript. Finally, this research is dedicated to Karl Kinney, whose memory burns brightly. References Cited Branson, T. F Larval feeding behavior and host plant resistance in maize, pp. 159Ð182. In J. L. Krysan and T. A. Miller [eds.], Methods for the study of pest Diabrotica. Springer, New York. Branson, T. F., G. R. Sutter, and J. R. Fisher Plant response to stress induced by artiþcial infestations of western corn rootworm. Environ. Entomol. 9: 253Ð257. Chiang, H. C Bionomics of the northern and western corn rootworms. Annu. Rev. Entomol. 18: 47Ð72. Chiang, H. C., L. K. French, and D. E. Rasmussen Quantitative relationship between western corn rootworm population and corn yield. J. Econ. Entomol. 73: 665Ð666. Foster, R. E., J. J. Tollefson, J. P. Nyrop, and G. L. Hein Value of adult corn rootworm (Coleoptera: Chrysomelidae) population estimates in pest management decision making. J. Econ. Entomol. 79: 303Ð310. Gibb, T. J., and R. A. Higgins Aboveground dry weight and yield responses of irrigated Þeld corn to defoliation and root pruning stresses. J. Econ. Entomol. 84: 1562Ð1576. Godfrey, L. D., L. J. Meinke, and R. J. Wright. 1993a. Effects of larval injury by western corn rootworm (Coleoptera: Chrysomelidae) on gas exchange parameters of Þeld corn. J. Econ. Entomol. 86: 1546Ð b. Vegetative and reproductive biomass accumulation in Þeld corn: response to root injury by western corn rootworm (Coleoptera: Chrysomelidae). J. Econ. Entomol. 86: 1557Ð1573. Gray, M. E., and W. H. Luckmann Integrating the cropping system for corn insect pest management, pp. 507Ð541. In R. L. Metcalf and W. H. Luckmann [eds.], Introduction to insect pest management. Wiley, New York. Gray, M. E., and K. L. Steffey Insect pest management for Þeld and forage crops, pp. 1Ð36. In 1997 Illinois agricultural pest management handbook. Cooperative Extension Services, University of Illinois, Urbana-Champaign. Gray, M. E., K. L. Steffey, and H. Oloumi-Sadeghi Participatory on-farm research in Illinois cornþelds: an evaluation of established soil insecticide rates and prevalence of corn rootworm (Coleoptera: Chrysomelidae) injury. J. Econ. Entomol. 86: 1473Ð1482. Gray, M. E., E. Levine, and K. L. Steffey Western corn rootworms and crop rotation: have we selected a new strain? pp. 653Ð660. In Proceedings of Brighton Crop Protection Conference, British Crop Protection Council, Brighton, UK. Hills, T. M., and D. C. Peters A method of evaluating postplanting insecticide treatments for control of western corn rootworm larvae. J. Econ. Entomol. 64: 764Ð765. Illinois Agricultural Statistics Service Annual summary bulletin Illinois Agricultural Statistics Service, SpringÞeld, IL Annual summary bulletin Illinois Agricultural Statistics Service, SpringÞeld, IL. Kahler, A. L., A. E. Olness, G. R. Sutter, C. D. Dybing, and O. J. Devine Root damage by western corn rootworm and nutrient content in maize. Agron. J. 77: 769Ð 774. Krysan, J. L., and T. A. Miller [eds.] Methods for the study of pest Diabrotica. Springer, New York. Levine, E., and H. Oloumi-Sadeghi Management of diabroticite rootworms in corn. Annu. Rev. Entomol. 36: 229Ð255. Mayo, Z B, Jr Field evaluation of insecticides for control of larvae of corn rootworms, pp. 183Ð203. In J. L. Krysan and T. A. Miller [eds.], Methods for the study of pest Diabrotica. Springer, New York. Musick, G. J., M. L. Fairchild, V. L. Fergason, and M. S. Zuber A method of measuring root volume in corn (Zea mays L.). Crop Sci. 5: 601Ð602. O Neal, M., M. Gray, J. Spencer, K. Steffey, and E. Levine Western corn rootworms in corn after soybeans: efforts towards an economic threshold, pp. 66Ð72. In Proceedings Illinois Agricultural Pesticides Conference, 3Ð4 January Cooperative Extension Service, University of Illinois, Urbana-Champaign. Pike, D. R., K. D. Glover, E. L. Knake, and D. E. Kuhlman Pesticide use in Illinois: results of a 1990 survey of major crops. Circ. DP-91-1, Coop. Ext. Serv. Univ. Illinois, Urbana-Champaign.

18 740 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 91, no. 3 Pike, D. R., K. L. Steffey, M. E. Gray, H. W. Kirby, D. I. Edwards, and R. H. Hornbaker Biologic and economic assessment of pesticide use on corn and soybeans. Report 1-CA-95. USDA-National Agricultural Pesticide Impact Assessment Program, Washington, DC. Riedell, W. E Rootworm and mechanical damage effects on root morphology and water relations in maize. Crop Sci. 30: 628Ð Root responses of maize hybrids following corn rootworm larval feeding damage. Cereal Res. Commun. 22: 327Ð335. Riedell, W. E., and P. D. Evenson Rootworm feeding tolerance in single-cross maize hybrids from different eras. Crop Sci. 33: 951Ð955. Riedell, W. E., T. E. Schumacher, and P. D. Evenson Nitrogen fertilizer management to improve crop tolerance to corn rootworm larval feeding damage. Agron. J. 88: 27Ð32. Rogers, R. R., J. C. Owens, J. J. Tollefson, and J. F. Witkowski Evaluation of commercial corn hybrids for tolerance to corn rootworms. Environ. Entomol. 4: 920Ð922. SAS Institute SAS/STAT userõs guide, version 6, 4th ed. SAS Institute, Cary, NC. Snedecor, G. W., and W. G. Cochran Statistical methods. Iowa State University Press, Ames, IA. Spencer, J., E. Levine, and S. Isard Corn rootworm injury to Þrst-year corn: new research Þndings, pp. 73Ð81. In Proceedings Illinois Agricultural Pesticides Conference, 3Ð4 January, Cooperative Extension Service, University of Illinois, Urbana-Champaign. Spike, B. P., and J. J. Tollefson. 1989a. Relationship of plant phenology to corn yield loss resulting from western corn rootworm (Coleoptera: Chrysomelidae) larval injury, nitrogen deþciency, and high plant density. J. Econ. Entomol. 82: 226Ð b. Relationship of root ratings, root size, and root regrowth to yield of corn injured by western corn rootworm (Coleoptera: Chrysomelidae). J. Econ. Entomol. 82: 1760Ð Response of western corn rootworm-infested corn to nitrogen fertilization and plant density. Crop Sci. 31: 776Ð785. Stamm, D. E., Z B Mayo, J. B. Campbell, J. F. Witkowski, L. W. Andersen, and R. Kozub Western corn rootworm (Coleoptera: Chrysomelidae) beetle counts as a means of making larval control recommendations in Nebraska. J. Econ. Entomol. 78: 794Ð798. Sutter, G. R., J. R. Fisher, N. C. Elliott, and T. F. Branson Effect of insecticide treatments on root lodging and yields of maize in controlled infestations of western corn rootworms (Coleoptera: Chrysomelidae). J. Econ. Entomol. 83: 2414Ð2420. Turpin, F. T., and J. M. Thieme Impact of soil insecticide usage on corn production in Indiana: 1972Ð1974. J. Econ. Entomol. 71: 83Ð86. Turpin, F. T., L. C. Dumenil, and D. C. Peters Edaphic and agronomic characters that affect potential for rootworm damage to corn in Iowa. J. Econ. Entomol. 65: 1615Ð1619. Received for publication 17 June 1997; accepted 2 February 1998.