january-june, 1992 Economics of Nematode Control 31 Factors Affecting the Economic Threshold for Heterodera schachtii Control in Sugar Beef James G. Robb, Eric D. Kerr 2 and Daryl E. Ellis Panhandle Research and Extension Center University of Nebraska 4502 Avenue I, Scottsbluff, NE 69361 ABSTRACT The sugar beet nematode is a major pest for western Nebraska sugar beet (Beta vulgaris) farmers, and is costly to control. Three field studies were conducted to help establish the relationship between preplant nematode egg and larva concentration (eggs per cm 3 of soil) before planting, and final yield. Response functions were estimated and economic thresholds were calculated for each field study. Economic threshold levels from each field study were very different. For a selected study, the sensitivity of results to changes in important parameters (percent sugar, percent control, cost of control and sugar price) was evaluated. The threshold level was influenced by each of the parameters evaluated. Additional Key Words: Beta vulgaris, yield, nematode populations, ':~ost. IPublished as Paper No. 9444, Agricultural Research Division, Universify of Nebraska. The rcsearch was partly funded 1)1f The Western Sugar Company-Grower Joint Research Commi flee Inc. 2Comspondin,'? aufhor.
32 Journal of Sugar Beet Research Vol 29 No. 1&, 2 T he sugar beet cyst nematode (Heterodera schachtii Schm.) is a major problem in many sugar beet (Beta vulgaris L.) production areas of the world. It has been a serious pest on sugar beet in the North Platte River Valley of western Nebraska since the 1920s (Thorne, 1926) and is now distributed over more than half the sugar beet acreage in that region. The basic methods of control are crop rotation with non-host crops, and use of chemical nematicides. The primary crop rotation of sugar beet-corn-dry bean in this region is too short for satisfactory control of even moderate population levels of the nematode. Chemical nematicides provide effective control when applied as a soil fumigant in either fall or spring or as granular formulations band-incorporated into the soil over the row at planting time. Costs of chemicals alone are typically $75.00 to $120.00 /ac. Thus, it is important that chemicals are applied only in fields that are infested beyond an economic threshold population level. The concept of an economic threshold has become a principle of plant protection and pest management decisions (Carlson and Headley, 1987). Control is recommended when an economic threshold is reached, a point at which expected economic losses without treatment exceed treatment costs. Treatment generates positive net returns only when the pest population exceeds the threshold level. Carlson and Headley (1987) described four aspects of an economic threshold: 1) it is defined for a time before the expected damage (e.g., yield reduction) has occurred; 2) it will depend on the expected damage to the crop if the pest is not controlled at the time of decision; 3) it will depend on the value (price per unit of output) of the crop; 4) it will depend on the efficacy and cost of the control methods to reduce the population below the economic injury level (the relationship between economic injury level and economic threshold has been discussed and reviewed by Pedigo, et ai., 1986). Recent work focusing on the economic aspects of thresholds has included the incorporation of risk and long-term benefits. Osteen et al. (1989) described optimal combinations of dosage and action thresholds for corn nematode control incorporating measures of a farmer's risk-aversion level when there is uncertainty about nematode density. With the particular parameters used in that study, threshold and nematicide dosage increased as the measure of a farmer's risk-aversion increased. Torell et al. (1989) presented an approach to calculate economic injury level when the benefits from control continue for several years. Research on sugar beet nematode thresholds has been conducted in both greenhouse and field environments. Cooke and Thomason (1979) conducted two greenhouse experiments on the relationship among soil temperature, nematode concentration and root weight.
January-June, 1992 Economics of Nematode Control 33 They also conducted one field experiment in the Imperial Valley of California, and described some economic threshold relationships. In greenhouse studies in which soil temperature was 15, 19, 23, 27, or 31 C, H. schachtii populations increased most at 23 C, less at 27 C, and least at 19 C. At 15 and 31 C, populations decreased by 50 and 90%, respectively. The population tolerance limits, below which no damage is done, were estimated visually; limits were lowest at 23 27 C (65 eggs per 100 g soil), increased to 430 eggs per 100 g at 19 C, and plants were not harmed at 15 C. Similarly, in their field study, when soil temperature was near optimum for nematode development, the tolerance limit was 65 eggs per 100 g. Considering the market price of sugar beets, cost of using nematicides (at that time), and potential yield, Cooke and Thomason (1979) estimated the economic threshold to be 143 eggs per 100 g soil. Griffin (1974) showed that soil temperature at planting time was important in predicting the effect of H. schachtii populations on sugar beet yields. In fields with comparable nematode populations, soil fumigation with 1,3-dichloropropene resulted in a 19% increase in sugar beet yield when seed was planted at 3 C, and a 67% increase when planted at 11 C. Because soil temperature is an important factor in the magnitude of sugar beet yield losses caused by the sugar beet nematode, the economic threshold should differ among geographic areas with different average soil temperature. Thus, Griffin (1981b) estimated economic threshold levels of 3.5 and 2.0 eggs per g at Parma and Rupert, Idaho, where planting date soil temperatures were 6 and 12 C, respectively. He noted that in field studies where planting date soil temperatures ranged from 6 to 24 C, H. schachtii was most pathogenic at 24 C and least pathogenic at 6 C. Before making their decision to apply a nematicide, many sugar beet growers in western Nebraska depend upon commerciallaboratories to determine the nematode population level, reported as the number of eggs or cysts in a certain volume or weight of soil. It is only within the last two years that laboratories in Nebraska have reported the number of eggs; consequently, there is limited experience among commercial nematicide applicators and growers in evaluating egg counts for their potential yield effects. Griffin (1981a) compared preplant cyst and egg (including larvae) populations with sugar beet yield. The correlation between sugar beet yield and eggs per gram of soil was negative and high (r = -0.967), but the correlation between sugar beet yield and cysts per gram of soil was lower (r = -0.662). He suggested the use of number of eggs (and larvae) rather than number of cysts when determining economic threshold levels for H. schachfii in sugar beet production. Jones (1956) reported that populations of H. schachfii in land under fallow or non-host crops decreased by approximately 20, 40, and 50% per annum for cysts, cysts with contents, and eggs, respectively. Thus, egg populations decrease more rapidly than cyst
34 Journal of Sugar Beet Research Vol 29 No. 1 & 2 populations. He noted that in the cool soils of England, the 'economic zero' for growing a satisfactory sugar beet crop was about 10 eggs per gram of soil representing field size areas. H. schachtii populations from different geographic areas may differ in virulence. Griffin (1981c) reported that one Utah population differed from five other populations from Oregon, Idaho, and Utah in having greater seedling penetration, virulence, and rate of emergence; the Utah population also significantly diminished sugar beet growth as measured by root and top weight. He suggested that those differences in virulence would influence nematode threshold densities. This paper reports: 1) field experiments to determine the relationship of sugar beet yields to H. schachtii population densities for the Nebraska sugar beet production areas; and 2) effects of changes in percent sugar, percent nematode control, cost of control, and sugar price on a calculated economic threshold level. MATERIALS AND METHODS Field Studies. Experimental sites in Scotts Bluff County of the Nebraska Panhandle included a range of sugar beet nematode population levels; site selection was based on previous experience and preliminary soil sampling and analyses for egg populations. Two sites, both in commercial grower fields, were used in 1988 (1988A and 1988H). In 1989, the site was at the University of Nebraska Panhandle Research and Extension Center field laboratory, Mitchell, Nebraska. All three studies were on Tripp very fine sandy loam soil, with <11% slope at the 1988A site and 2 % slope on the other 2 sites. The fields were furrow irrigated with surface water, and conventional tillage systems were used for seedbed preparation and cultivation. Field plots consisted of sampling points on 15 meter grids along 2 field strips in 1988 and 4 strips in 1989. Thirty plots were used at the 1988A location, 22 at the 1988H location, and 64 plots in 1989. At planting, soil samples were collected at each study site on 15 and 22 April on the 1988H and 1988A site, respectively, and on 15 April in 1989. Twelve soil cores were collected in the row at each plot, bulked into one sample, and processed for cyst extraction with a Fenwick can. Cysts were broken in a test tube homogenizer, the contents were washed into a counting dish, and egg and larval populations were determined. Roots were hand harvested from 3.6 m of row at the 1988A site and 1988H sites, respectively, on 3 and 4 October, and machine harvested from 30.5 m of row on 4 October in 1989. Clean root weight and sugar percentage were determined by the Western Sugar Company tare laboratory. Response functions depicting the impact of eggs and larvae (eggs per cm 3 ) in the soil on root yield and sugar content (percent sugar) were estimated by ordinary least squares regression (equation 1); each plot provided a data point. Yield = f (eggs per cm 3 ) (1)
January-June, 1992 Economics of Nematode Control 3S Economic Methods. Economic thresholds were calculated as the eggs per cm 3 level at which the additional output (tons and/or percent sugar) obtained by control resulted in no added net return. Net return per acre (NR) consists of several components which can be simplified into three basic aspects: yield (Y), price (P), and summation of associated costs (C) (equation 2). NR = (Y x P) - C (2) Therefore, the economic threshold in eggs per cm 3 is the point at which the NR equals zero, where C includes control measures and Y is the yield increase due to control, based on equation 1. The rate of chemical application included in C was pre-specified based on typical requirements for nematode control in sugar beet fields. Baseline Economic Parameters. Sugar beets are grown under a contract between the farmer and processor. The price per ton received by the farmer depends on the percent sugar delivered, adjusted for pile loss, and the net price of sugar sold by the processor. Several other adjustments (e.g., trucking from receiving station or factory) are made before the grower's price (P) is established. Thus, several baseline economic parameters are required to evaluate thresholds for sugar beets. In this study, a microcomputer spreadsheet program (Ellis and Robb, 1990) was developed to incorporate the response function relationship and economic parameters to calculate economic threshold levels. Seven parameters are shown in Table 1, including the net price of sugar received by the processor ($23.50/cwt), shared pile loss (sugar loss in storage prior to processing), and shared freight between the grower and processor. The baseline cost of chemical, application, etc., was $90.40/ac. Trucking cost from the field to the receiving station was $2.20/ton. The baseline percent control achieved with a nematicide was set at 90 percent. That is, from the response function (equation 1), control results in a yield based on 10% of the initial eggs per cm 3 remaining. Table 1. Baseline parameters for evaluating economic thresholds. Item Baseline Value Company net price of sugar ($lcwt) $23.50 Storage pile loss (% sugar)............................ 0.6 Grower share of pile loss (%).............................. 60 % Growers share of freight to factory ($/ton)................ $0.08 Trucking cost from field or receiving station ($/ton)........... $2.20 Cost of chemical, application, etc. ($/ac).................... $90.40 Nematode control achieved with treatment (% )................ 90%
.36 Journal of Beet Re~arch Vol RESULTS AND DISCUSSION per Statistic 1988A 1989 Observations (N) 30 22 64 Coefficient of 0.67 Regression line (b) -2.888-1.068 Standard error of slope 0.643 0.198 0.050 statistic 4.49 5.40 Y axis intercept 29.66 30.00 25.25 Several factors to have caused the differences among tests in of eggs per cm 3 of soil Soil tenlpe-rature at and the season n..'... "...":.hlu ri,l-ho..""ri for 1988 and 1989. Another years was the date of sugar beet Due to conditions in the of 1989 and decreas COlmr:)Ollnc1ed and greater than
January-June, Economics of Nematode Control 37 V...U~H.""' nematode concen 35.0 30.0 25.0 20.0 15.0 10.0 o 2 4 6 8 10 12 14 16
38 Journal of Sugar Beet Research Vol 29 No.1 & 2 The range in yield response functions is probably representative of what a sugar beet grower would experience over a period of years. With its lack of compounding stress conditions, a typical crop emergence date, and uniform plant stands, we believe the 1988H study is most representative of average conditions in the North Platte River Valley of western Nebraska. Economic Thresholds for Field Studies. Given the different response functions, the resulting economic thresholds also differed. Threshold results for the three studies were approximately 6.1, 2.4 and 1.0 eggs per cm 3 for the 1989, 1988H and 1988A studies, respectively (Figure 2). Because no significant percent sugar response to eggs per cm 3 was found, thresholds also were calculated for the average percent sugar of 15.75% for the three field studies. Adjusting by average percent sugar resulted in thresholds (Figure 2) of approximately 5.2, 2.8 and 1.0 for the 1989, 1988H and 1988A studies, respectively. Sensitivity Analysis: 1988H Study. The three field studies depict some of the inherent variability in crop response due to year, cultural practices, etc., and depict a likely range of economic thresholds. In contrast to some other pests, nematodes must be controlled prior to planting, and follow-up treatments are not an option. Assuming the farmer makes his/her treatment decision expecting a normal crop, the results from the 1988H study were most typical. Based on the response function from the 1988H study, the sensitivity of economic threshold results was evaluated by changing the coefficient for one parameter while holding other conditions constant. Four parameters were varied over a range of likely situations. Percent sugar was evaluated over a range of 15% to 18%; percent control achieved by treatment (percent control) was varied from 80 % to 100%; cost of control ($/ac), including chemical and application costs (machinery, fuel, labor, etc. ), was varied between $60.00 and $160.00; and the net price of sugar ($lewt sugar price) was eva luated from $20.00 to $28.00/cwt. The percent sugar and price range depict possible conditions in western Nebraska. The percent control range reflects possible situations and is probably dependent on the type of chemical used, application method, and climatic conditions. Costs of chemicals have varied over the last five years. Chemical cost per acre is dependent on type of chemical used, row spacing when using granular nematicides placed in-furrow, manufacturer rebate incentives, etc. Also, there is a concern that the cost of chemicals to control nematodes will increase in the future. Each parameter (e.g., percent sugar) was evaluated individually over the range described previously for its sensitivity or influence on the economic threshold. All other coefficients were fixed at baseline levels: $23.50/cwt sugar price, $90.40 cost of control, 90% control achieved with treatment (Table 1), and 15.75% sugar (average of field studies). With these coefficients, the 1988H response function resulted
January-June, 1992 Economics of Nematode Control 39 Figure 2. Economic thresholds for three field studies at actual and average percent sugar. 7.0 - (t) E (,) -. en 6.0 5.0 C) 4.0 C) 1... ~ Actual Adjusted by Average Percent Sugar (15.75%) :..................... - > 3.0 -.J "'C o.r:. en 2.0. '.c 1.0 o 1989 1988H 1988A
40 Journal of Sugar Beet Research Vol 29 No. 1 & 2 in a baseline economic threshold of 2.B eggs per cm 3 Results of each sensitivity analysis are shown in Figure 3. The economic threshold dropped from about 3.0 at 15% sugar to about 2.3 at 1B% sugar (Fig. 3A). Treatment would be recommended at a lower level of eggs per cm 3 if a farmer expects to produce a higher value crop due to a higher percent sugar. At BO% and 100% control, the economic thresholds were approximately 3.1 and 2.5 eggs per cm 3, respectively (Fig. 3B). As percent control increases, the nematicide is more effective in saving yield and a lower threshold results. Cost of control is a major determinant of the economic threshold for nematodes in sugar beet (Fig. 3C). With a $60.00/ac cost of control, the threshold treatment level was about 1.B eggs per cm 3 Increasing the cost of control to $120.00/ac increased the threshold to about 3.7 eggs per cm 3 At a cost of control of $160.00/ac, the threshold increased to approximately 4.9 eggs per cm 3 As in the situation with higher value crop due to quality (percent sugar), increasing the net price of sugar resulted in a lower economic threshold (Fig. 3D). At a $20.00lcwt price for sugar the threshold was about 3.3 eggs per cm 3 ; at $2B.00lcwt the threshold declined to about 2.3 eggs per cm 3 Combined Influences on Thresholds. The sensitivity analysis described in the previous section showed how the economic threshold changed as one coefficient varied. Thresholds also were calculated for two sets of combined influences: 1) BO % control, 15% sugar and $21.00/cwt sugar; 2) 100% control, 17% sugar and $27.00/cwt sugar. These are reasonable situations a grower may face over a period of years. Three levels of control cost were evaluated: $60.00, $90.40 (baseline cost of control), and $120.00/ac. With baseline coefficients (90% control, 15.75% sugar and $23.50lcwt sugar) and cost of control, an economic threshold of 2.B eggs per cm 3 was calculated. Compared to this baseline resull, the sets of combined influences show a threshold range of approximately 1.3 eggs per cm 3 (100% control, 17% sugar and $27.00/cwt sugar) at $60.00/ac control cost to approximately 5.0 eggs per cm 3 (BO% control, 15% sugar and $21.00/cwt sugar) at $120.00/ac control cost. Figure 4 shows the intermediate results and the baseline for comparative purposes. Discussion. Prior to making recommendations on economic thresholds for nematode control in sugar beet for a particular year, the expected price of sugar should be estimated. With additional information on the control method used (e.g., granular nematicide applied and the rate of application per acre), insight into percent control expected with the chosen chemical, and percent sugar expected in the crop, better recommendations can be made from soil samples of eggs per cm 3
January-June, 1992 Economics of Nematode Control 41 Figure 3. Sensitivity of economic threshold to changes in parameters for selected field study. Fixed baseline coefficients were used for all parameters except as shown on the x-axis of each panel (e.g., percent sugar). Major baseline coefficients were $23.50/cwt sugar price, $90.40 cost of control, 90% control achieved by treatment, and 15.75% sugar. 6.0 A B r------ -------, 6.0 ;;-- 5.0 E ~ ~ 4.0 ~ g! 3.0 j ~ 2.0..r::. en..c 1.0!.................. 5.0 4.0 3.0 2.0 1.0 0.0 6.0 15% 16% 17% Percent Sugar C...-----------' 0.0 18% 80% 85% 90% 95% 100% Percent Control D r-------------,6.0 ~5.0..................... E ~ ~4.0 ~ ~3.0 -I ~2. 0..r::. en.c 1.0... -...... I 0.0 $60 $80 $100 $120 $140 $160 Cost of Control ($/ac)........... 5.0...... 4.0 ~'3.0... 2.0... 1.0 '----------... 0.0 $20 $22 $24 $26 $28 Sugar Price ($/cwt)
42 Journal of Sugar Beet Research Vol 29 No. 1 & 2 Figure 4. Influence of combined effects on economic threshold for selected field study (1988H). 6.0 5.0 Cost of Control m ($/ac) $60.00 $90.40 $120.00 --. C') 4.0 E 0 en 0> 0> 3.0 - >...J "'C 2.0 0.c. CJ) ~.c. t 1.0 0.0 Baseline 80% Control 100% Control 15% Sugar 170/0 Sugar $21.00/cwt Sugar $27.00/cwt Sugar
January-June, 1992 Economics of Nematode Control 43 This study supports the conclusion expressed by others (e.g., Carlson and Headley, 1987) that the development of an economic threshold does not result in a general rate for pest control actions, but rather provides a piece of information for decision making. Important parameters will change each year, and the field studies suggest that the yield loss due to a specific population of nematodes varies, due to weather conditions and probably to other factors (e.g., cultural practices). A microcomputer spreadsheet template approach can provide a mechanism to update threshold calculations on a periodic basis; with additional input from a sugar beet grower, this can provide a more individualized recommendation. LITERATURE CITED Carlson, G. A. and J. c. Headley. 1987. Economic aspects of integrated pest management threshold determination. Plant Dis. 71:459-462. Cooke, D. A. and I. J. Thomason. 1979. The relationship between popula tion density of Heferodera schachtii, soil temperature, and sugarbeet yields. J. Nematol. 11:124-128. Ellis, D. E. and J. G. Robb. 1990. Sugarbeet contract analysis: two spreadsheet programs. University of Nebraska-Lincoln, Panhandle Research and Extension Center, Panhandle Farm Management #90-4, 6 p. Griffin, G. D. 1974. Determination of Heferodera schachtii populations and their relation to economic losses of sugarbeets. J. Nematol. 6:141. (Abs). Griffin, G. D. 1981a. The relationship of Heferodera schachtii population densities to sugarbeet yields. J. Nematol. 13:180-184. Griffin, G. D. 1981b. The relationship of plant age, soil temperature, and population density of Heferodera schachtii on the growth of sugarbeet. J. Nematol. 13:184-190. Griffin, G. D. 1981c. Pathological differences in Heferodera 5chachfii populations. J. Nematol. 13:191-195. Jones, F. G. W. 1956. Soil populations of beet eelworm (Heferodera schachtii schm.) in relation to cropping II. Microplot and field plot results. Ann. Appl. BioI. 44:25-56. Osteen, C. D., L. J. Moffitt and A. W. Johnson. 1988. Risk efficient action thresholds for nematode management. J. Prod. Agric. 1:332-338. Pedigo, L. P., S. H. Hutchins and L. G. Higley. 1986. Economic injury levels in theory and practice. Ann. Rev. Entomol. 31:341-368. Thorne, Gerald. 1926. Control of sugar-beet nematode by crop rotation. U.S.D.A. Farmers Bulletin No. 1514. 85 pp. Torell, L. A., J. H. Davis, E. W. Huddleston and D. C. Thompson. 1989. Economic injury levels for interseasonal control of rangeland pests. J. Econ. Entomol. 82:1289-1294.