1 Assessments of crop losses in rice ecosystems due to stem borer damage (Lepidoptera: Pyralidae) K. Muralidharan, I.C. Pasalu Department of Crop Protection, Directorate of Rice Research (ICAR), Hyderabad , India Received 31 March 2005; received in revised form 21 June 2005; accepted 23 June 2005 Abstract A database created from insecticide control experiments conducted under the All-India Coordinated Rice Improvement Project from 1965 to 1992 was used to derive empirical estimates of yield losses caused by stem borers. Each unit percent damage due to white earhead damage had a much greater impact on rice yield in the irrigated ecosystem than did damage due to dead heart. White earhead damage occurs later in the season and results in direct loss of a yielding panicle, and thus, no compensation (or very little) is possible. The grain yield loss from the two phases, dead heart and white earhead damage to rice, is more than additive. Based on 770 experimental units from 28 years data, our projections for damage over rice ecosystems due to 1% dead heart or white earhead, or to both phases of stem borer damage are 2.5%, 4.0%, and 6.4% yield loss, respectively. In terms of grain production loss over ecosystems, 1% dead heart, or white earhead, or both phases of stem borer damage would be 108, 174 and 278 kg/ha, respectively. In irrigated ecosystem, 1% dead heart resulted in 0.3% or 12 kg/ha loss whereas, 1% white earhead caused 4.2% or 183 kg/ha loss in grain yields; the loss due to 1% infestation in both phases of stem borer damage was 4.6% or 201 kg/ha. In rainfed lowlands, for 1% dead heart or dead heart and white earhead caused 2.3% or 76 kg/ha yield loss. Even at levels below the currently used economic threshold considerable losses can occur. This perception on losses assumes more importance because of the inadequate host-plant resistance to stem borer in rice. Although no insecticide gave total control of stem borer damage, many increased grain yields significantly. Emulsifiable concentrates of monocrotophos and chlorpyriphos appeared more economical for adoption by farmers as their application caused maximum mortality of larvae and unhatched eggs. Insecticide granules such as diazinon and carbofuran were equally efficient in preventing stem borer damage. r 2005 Published by Elsevier Ltd. Keywords: Rice; Oryza sativa; Stem borer; Dead heart; White earhead; Economic threshold; Yield loss; Insecticide control 1. Introduction Over 100 species of insects attack and damage rice (Pathak, 1968, 1977; Grist and Lever, 1969). Many of them often appear sporadically but do not cause economic loss. A few species, however, do cause significant damage and are extremely important. The stem borers, gall midge (Orseolia oryzae Wood-Mason), brown planthopper (Nilaparvata lugens Stal), leaf folder Corresponding author. Tel.: ; fax: address: (K. Muralidharan). (Cnaphalocrocis medinalis Gunee), and green leafhopper (Nephotettix virescens Dist.) are the major pests of economic importance in India and are among the production constraints consistently encountered in various rice growing environments (DRR, ; DRR, ). Stem borers in the order Lepidoptera are widely prevalent and serious insect pests of rice. In India, 18 stem borer species in the family Pyralidae and three species in the family Noctuidae have been recorded (Banerjee, 1964; Kapur, 1967). Usually one to four species are important in any given area. The predominant species in India include yellow stem borer, Scirpophaga (Tryporyza) incertulas (Walker), striped /$ - see front matter r 2005 Published by Elsevier Ltd. doi: /j.cropro
2 stem borer, Chilo suppressalis (Walker), and pink stem borer, Sesamia inferens (Walker). Of these species, S.inferens is restricted primarily to hill regions in northern India and Bengal in eastern India. Occasionally, other species like the white rice borer, S.innotata (Walker), may be encountered. The yellow stem borer, S.incertulas is the most dominant species in India (DRR, ; Kulshreshta et al., 1970). Varietal resistance to yellow stem borer has been investigated (Israel and Abraham, 1967). Land races of rice such as TKM 6, CB1 and CB2 have been used as resistant donors since 1964 in India (Roy et al., 1971). The resistance of TKM 6 was reported to result partially from non-preference and antibiosis (DRR, 1969). Research elsewhere on varietal resistance to rice stem borer has received a low priority and none of the varieties developed so far have more than a moderate degree of stem borer resistance (DRR, ; Chaudhary et al., 1984; Paroda and Siddiq, 1993). Stem borer adults are moths and three or more generations occur in a season. Most borer species are capable of flying only a short distance; however, they can travel 8 16 km if carried by wind (Pathak, 1968). A single female can lay eggs. An egg mass contains eggs and is covered with pale brown hairs from the anal tufts of female moths. The larvae live and feed inside the stem or rice culm. Both traditional cultivars and the modern semi-dwarf indica varieties produce numerous tillers (15 20), and thus provide conditions conducive for stem borer infestation. The newly hatched larvae may feed externally for some time, bore into the stems, usually throughout the upper nodes, and eat their way down to the base of the plants (Pathak, 1968). Crop production practices such as clipping seedlings to remove eggs laid at the leaf tips, density of plant populations, and N fertilizer application are known to alter the pest incidence (Pathak, 1977; Singh et al., 1990). Rice plants are most prone to stem borer infestation at the tillering and flowering stages (Viajante and Heinrichs, 1987). In a transplanted crop, stem borer larvae cut off the growing points of tillers causing them to die, a condition commonly known as dead heart. When the plants are attacked later, during the flowering stage, larvae feed on the meristem and empty, whitish-looking panicles called white earheads appear. In the infested fields these white earheads stand erect and contain empty and unfilled glumes. In addition to reducing rice yields, stem borer damage may also make plants more prone to invasion by pathogens. The estimated loss from stem borer damage varies from 3 95% (Ghose et al., 1960). In areas where 2 to 3 crops of rice are grown every year, the first crop is damaged severely (Israel and Abraham, 1967). The annual loss from pests and diseases on paddy rice in India was projected to be around $ 100 million; the share of paddy stem borer alone was considered as $ 10 million (Mehta and Varma, 1968). The most commonly cited crop loss figures from rice are those of Cramer (1967), who estimated worldwide losses in rice production due to insect damage to be 34.4%. An appraisal team charged with assessing the nature and scope of pest problems affecting the food supply in Southeast Asia concluded that stem borers were among the insect pests deserving the highest research priority (Glass, 1971). Yet, all these reports were only subjective estimates at best with extremely limited data. The magnitude of the loss caused by stem borers becomes apparent only when the grain yield harvested in an insecticide-protected plot is compared with that from an unprotected plot. Researchers at the Central Rice Research Institute in India estimated that for every 1% increase in white earheads, yields were reduced by 2.2% (Israel and Abraham, 1967). Stem borer attack was not completely prevented despite a regular treatment with insecticide (Israel and Abraham, 1967). However, a 48% yield increase in insecticide treated plots over untreated controls was found during the first two crop seasons in Tamil Nadu state in India (Ramakrishnan, 1972). Although stem borers are known to seriously affect crop yields in rice (Pathak and Dhaliwal, 1981; Litsinger et al., 1987; IRRI, 1990), few data are available on the actual yield losses derived for the different rice ecosystems. Savary et al. (1997) studied the relationship between rice cropping pattern, biotic constraints and yield levels in 251 farms. With principal component analysis of 14 injury variables, factors were identified for developing a multiple regression model in which the largest individual mean yield reduction (0.46 t/ha) was attributed to deadhearts. However, this model explained only 17.8% of yield variations. There are numerous reports on the ability of young rice plants to compensate for the loss from deadhearts to some extent by producing new tillers. The estimates available are from comparison of yields in unprotected and insecticide protected plots. The data included for such analyses were mostly from only one site in a single type of ecosystem. Injury yield relationships have been developed for stem borers (Israel and Abraham, 1967; Pathak, 1967; Pathak and Dyck, 1973; Gomez and Bernardo, 1974; Barr et al., 1981; Waibel, 1996). The relation between injury and yield is considered to be generally non-linear and to exhibit a high degree of tolerance to initial injury. Plants with as high as 30% dead hearts from stem borer attack may have no significant yield losses and as much as 10% white earheads can be tolerated (Teng et al., 1993). A higher degree of tolerance has been recorded under higher doses of fertilizer applications. Insect damage functions are thus speculated to depend on the crop age and nutritional status when the crop was infected, in addition to factors such as insect densities and feeding durations.
3 The use of inputs for plant protection was unimportant for rice prior to the mass introduction of modern varieties. Farmers had traditionally relied on host-plant tolerance, natural enemies, cultural practices and mechanical methods to contain the stem borer damage. In India, which has the world s largest area under rice production (44.6 m ha annually), losses increased in farmers fields as area planted to high yielding varieties increased. Most farmers protect their fields from the principal insect, stem borer, at least to a certain degree with insecticide application. Research and development in India is achieved mainly through the All-India Coordinated Rice Improvement Project (AI- CRIP). This project tests breeding material, and promising production and protection practices in various experiments by involving the available scientific force at different institutions to find solutions to problems through joint efforts (Muralidharan and Siddiq, 1997). Insecticide control experiments on rice stem borers study the impact of pest on yield and identify products for commercial use. Thus, the objective of this study was to use the database created from AICRIP insecticide control experiments in rice from 1965 to 1992 to derive empirical estimates of stem borerinduced yield losses. 2. Materials and methods We used data from 86 AICRIP insecticide control experiments on stem borer control performed from 1965 to 1992 at 16 sites in different rice growing states in India (DRR, ). A total of 770 observations on the final percent incidence of dead heart and white earhead, and yield were available. In these experiments, the use of various insecticides resulted in the occurrence of a range of levels of both stem borer infestation and yield. Therefore, these relationships were used to provide estimates of the yield loss from stem borer infestation and damage. Efforts were made at each experiment site to ensure uniform stem borer infestation by using a highly susceptible local cultivar and adjusting the time of planting and fertilizer application to favour maximum pest infestation. However, locational influences on both stem borer incidence and yield were expected. Based on the assumption that losses from stem borer infestation would be more uniform within a climatic region, the data sets were regrouped into two major ecosystems as irrigated lands, and rainfed lowlands. The irrigated environment is more favourable for both crop growth and pest population build up and is more homogenous than the rainfed lowlands, where rain water starts accumulating in the fields from mid-july and starts receding with the cessation of the southwest monsoon in late October. In 69 of the 86 experiments, Jaya, a high yielding, fertilizer-responsive variety with a maturity period of 130 days was used; in the remaining experiments, a local variety with a similar duration to maturity was used (DRR, ). A randomized complete block design with four replications was used for each experiment. Treatments represented a range of insecticides such as acephate, agronule, ambithion, anthio, azinphos ethyl, bendiocarb, BPMC, carbaryl, carbofuran, cartap, chlorpyriphos, cytrolane, diazinon, endosulfan, ethofenprox, ethoprop, fenthion, fenitrothion, isazophos, leptophos, methyl parathion, MIPC, monocrotophos, phosphamidon, quinalphos and triazophos. In each experiment, water sprayed control plots were included. Thirty-day-old seedlings were planted at two seedlings per hill with a spacing of cm between rows and hills, respectively. All plots measured 6 4m in size. Prior to planting, 40 kg N, 60 kg P 2 O 5 and 60 kg K 2 O/ha were broadcast applied and incorporated in the final puddling. Top dressings were made twice (at tillering and panicle initiation stages) with 40 kg N/ha per application. All insecticides were applied at 30 and 50 days after planting, to control dead heart and white earhead damage, respectively, from stem borer. Percentage dead heart damage was calculated by counting healthy and infested tillers on all 10 plants selected at random from each plot; border rows in each plot were excluded from selection. Similarly, percentage white earhead damage was counted a week prior to harvest. In all treated and untreated plots, one border row was excluded and grain yield from the remaining plants was harvested and expressed in kg/ha at 14% moisture (DRR, ). In each ecosystem, irrigated or rainfed lowlands, the same variety was grown for all plots in a single year. In 17 experiments, rice cultivars other than Jaya were grown, and therefore, potential yield among varieties would differ. Each insecticide, depending on its efficacy, reduced the damage from stem borer and increased yields in treated plots in comparison to untreated plots. The influence of climate and environment preclude the direct use of the two independent variables, i.e. dead heart and white earhead. Hence, to circumvent this problem in our study, data from the gradation in both stem borer infestation and grain yields created by application of various insecticide treatments in each experiment were used. However, the overall percentage decrease in stem borer damage or the percentage increase in yields were not chosen as variables for this analysis because of the potential differences in actual yield among locations, ecosystems, years and varieties, which would cause large variations in yield values for control plots. Therefore, the reduction in dead heart (%) and white earhead (%) and the yield increase (kg/ha) were calculated in comparison with values obtained for unprotected plots for each location-year combination.
4 The initial regression model evaluated was lnðyþ ¼loc_yr b 1 ð%dhþ b 2 ð%weþ b 3 ð%dh þ %WEÞ. After a further evaluation of the graph of the original data, regression analysis was performed such that damage due to white earhead could be proportionately greater or less than 10% incidence a value commonly cited as an economic threshold (ET) value for stem borer damage (Teng et al., 1993). Yield values were transformed on ln (Y) prior to analysis to stabilize variance. All regression analyses were performed using Statistical Analysis Software System (SAS Institute, 1988). Residual plots were evaluated for occurrence of visual pattern and coefficients of determination were calculated. Percent grain yield loss at various projected levels of dead heart and white earhead incidence were calculated for both ecosystems combined and each ecosystem individually. 3. Results The degree of stem borer infestation varied between the two rice ecosystems (Table 1). The maximum levels of dead heart damage were higher than levels of white earhead damage in both ecosystems. Mean damage due to dead heart was slightly greater than due to white earhead damage in the rainfed lowlands; both types of damage occurred at similar mean levels in the irrigated ecosystem plots. Variability about the mean was fairly large which indicated the relatively large degree of variability of damage levels among the 28 years of data. Among the 770 experimental units (year location - treatment combinations), 71 in the irrigated ecosystem had only white earhead damage. A total of 281 (195 in irrigated and 86 in the rainfed lowlands) units had only dead heart damage. In both ecosystems, for both phases of stem borer damage, the majority of experimental units (426 for dead heart and 288 for white earhead) were in the range of 0 5% incidence of damage (Figs. 1 and 2). Overall, only in 22 units for dead heart and 10 for white earhead, were the damage levels greater than 25% in these insecticide control experiments on stem borer in rice. Such high damage levels were found in both insecticide treated and untreated plots. Of the 770 experimental units, a total of 489 could be used for regression analysis to produce a yield loss model. Similarly, for the irrigated ecosystems and rainfed lowlands a total of 295 and 194 units, respectively, could be used for the analysis. Our regression analysis indicated that the loss in yield due to damage by dead heart could be estimated with single regression coefficient throughout the range of damage encountered for all experimental units (Table 2). For white earhead, however, the relative yield loss at damage levels below ET (o10%) was greater than that which occurred at levels above ET (410%) for the model describing data from the irrigated ecosystem and the overall data set. The computation of grain yield loss was confined for illustrative purpose to three levels of stem borer damage, i.e. 5% (one-half of ET), 10% (ET) and 12% (overall means) (Table 1). Each unit percent damage due to white earhead damage had a much greater impact on rice yield in the irrigated ecosystem than did damage due to dead heart (Table 3). For example, calculated mean yield loss (using the regression equation given in Table 2) for 5% white earhead damage with no dead heart damage would be 19.35% or 844 kg/ha, whereas calculated mean yield loss for 5% dead heart damage with no white earhead damage would be only 1.32% or 58 kg/ha. The damage at 5% is slightly more than additive for a calculated mean yield loss of 23.44% or 1023 kg/ha. If both types of damage were present at levels of 12%, a loss of nearly 50% of the yield would be expected to occur. In the rainfed lowlands, only dead heart damage had a statistically significant effect on yield. Although a significant regression was found, the R 2 value (0.07 or 7%) was quite low and the predictive power of the model is doubtful. The indication is, however, that dead heart damage had an 8 10 fold greater effect on yield in the rainfed lowlands than in the irrigated ecosystem. The seven insecticides viz., carbofuran G, chlorpyriphos EC, chlorpyriphos G, cytrolane G, diazinon EC, Table 1 Incidence and severity of stem borer damage in AICRIP insecticide control experiments on rice, a Ecosystem Overall Irrigated Rainfed lowlands DH (%) WE (%) DH (%) WE (%) DH (%) WE (%) Maximum Minimum Mean Standard deviation a AICRIP All-India Coordinated Rice Improvement Project, Directorate of Rice Research, Hyderabad; DH Dead heart damage; and WE White earhead damage.
5 No. of observations Overall Irrigated Rainfed lowland >25 Dead hearts (%) Fig. 1. Frequency distribution of observations (total n ¼ 770) from the all India coordinated insecticide control experiments for for percent dead hearts damage in rice caused by stem borers overall ecosystems, and in the irrigated and rainfed lowland ecosystems. No. of observations Overall Irrigated Rainfed lowland >25 White earheads (%) Fig. 2. Frequency distribution of observations (total n ¼ 770) from the all India coordinated insecticide control experiments for for percent white earheads damage in rice caused by stem borers overall ecosystems, and in the irrigated and rainfed lowland ecosystems. monocrotophos and quinalphos listed in Table 4 did not eliminate white earhead or dead heart damage in rice. Levels of damages for both phases of the pest damage were, however, reduced relative to the untreated controls. Yield increases with two applications using one of the seven insecticides listed ranged from 28% with quinalphos to 42% with diazinon (Table 4). Mean yield increase was 35.3% among the seven insecticides. 4. Discussion In earlier studies to analyze crop yield-stem borer relationships, the percent incidence of dead heart and white earhead damage has been used as a predictive set of independent variables for estimating losses (Israel and Abraham, 1967). The potential yield of cultivars has also often been derived from a plot with maximum protection against damage (Pathak, 1969; Waibel, 1996). Ours, however, is the most extensive study conducted in terms of numbers of years and is one of the first to take into account the potential differences in crop loss due to stem borer that result from ecosystem differences. The all India coordinated insecticide control experiments in rice provided a database of 28 years from which to calculate yield losses due to the two distinct phases of stem borer damage dead heart and white earhead. Location year differences in potential yield were taken into account through the use of a location - year term as the intercept in our regression analysis. This use of a variable intercept for the zero percent damage level allowed us to make valid comparison among years within and across rice ecosystems. The estimation of the regression coefficients could then proceed without the possible introduction of undesirable covariance s into the data set which could have occurred if data from the various plots were normalized in relation to either locations- or year-specific control plot yields. The coefficients of determination for our regression models were low, but acceptable for the combined data (R 2 ¼ 0:25) and for the irrigated ecosystem (R 2 ¼ 0:32) (Table 2). For the rainfed lowlands, the coefficient of determination value was very low (R 2 ¼ 0:07) and the model is of questionable value. Our results show that white earhead damage in irrigated ecosystems was of much more importance relative to yield loss than was dead heart damage. There are many reports on the ability of rice plants to compensate for the early loss of tillers to dead heart infestation by stem borers (Wyatt, 1957; Israel and Abraham, 1967; Rao et al., 1987). Plants have time to compensate for damage due to dead heart, because it occurs earlier in the season. White earhead occurs later in the season and results in direct loss of a yielding panicle, and thus, no compensation (or very little) is possible. Unlike the rainfed lowlands where the water level depends on the actual rainfall, in irrigated ecosystems, there is always an assured availability of water for the pest to survive and multiply. This may be the reason for the greater importance of stem borer in the irrigated ecosystem than in the rainfed lowland system. The grain yield loss from the two phases, dead heart and white earhead damage to rice is more than additive (Table 3). At 5% level of stem borer damage in irrigated
6 Table 2 Regression model for yield projection in percent for stem borer damage in rice ecosystems a Regression Degree of correlation b Ecosystem combined R 2 ¼ 0:25; n ¼ 489 If white earhead is o10%: lnðyþ ¼8:3739 0:0253ð%DHÞ 0:0410ð%WEÞ If white earhead is 410%: lnðyþ ¼8:3739 0:0253ð%DHÞ 0:0410ð10%WEÞ 0:0109ð%WE 10Þ Irrigated ecosystem R 2 ¼ 0:32; n ¼ 295 If white earhead is o10%: lnðyþ ¼8:3801 0:0027ð%DHÞ 0:0430ð%WEÞ 0:0016ð%DH þ WEÞ If white earhead is 410%: lnðyþ ¼8:3810 0:0027ð%DHÞ 0:0430ð10%WEÞþ0:0067ð%WE-10Þ 0:0016ð%DH þ WEÞ Rainfed lowlands ecosystem R 2 ¼ 0:07; n ¼ 194 lnðyþ ¼8:0898 0:0236ð%DHÞ a DH Dead heart damage and WE White earhead damage b R 2 ¼ Coefficient of determination all significant at Pp0:05; R 2 is the proportion of within year and location sum of squares that is accounted by the significant variables, DH, WE or DH*WE in the model. The parameter estimates (the relative rate of loss) for WE and DH*WE in rainfed lowlands ecosystem were non-significant, and hence were omitted; for all other variables, the estimates were significant at Pp0:05. n ¼ number of observations. Table 3 Computation of grain yield loss in relation to the level of stem borer damage in rice ecosystems a, based upon equations presented in Table 2 DH (%) b WE (%) Grain yield loss c Overall Irrigated Rainfed lowlands % kg/ha % kg/ha % kg/ha a Computation was limited for illustrative purpose to 5% (one-half ET economic threshold), 10% (ET) and 12% (overall mean DH and WE). Mean yields (over all years and appropriate locations) predicted without stem borer damage. b DH is dead heart damage and WE is white earhead damage. c Overall ¼ 4332 kg=ha; irrigated ¼ 4363; rainfed lowlands ¼ 3261 kg=ha. ecosystem for example, the derived yield loss would be: 1% from dead heart, 19% from white earhead, and over 23% for both dead heart and white earhead. This greater than additive effect is reported for the first time and is more pronounced as the degree of damage increases. Initial infestation and the wide occurrence of dead hearts promote the pest population (DRR, ). Further, if there were early dead heart damage, mother tillers or primary tillers would be the most affected. Insect feeding is also known to induce excessive tillering in rice. However, these secondary tillers have a lesser ability to produce grain yield compared to mother tillers or primary tillers. As a result, a greater than additive effect on yields was observed with the later phase, white earhead infestation.
7 Table 4 Overall performance of insecticides in controlling stem borer in rice Treatment n Mean damage xy (%) Grain yield increase Dead heart White earhead kg/ha Percent a Carbofuran G b Chlorpyriphos EC Chlorpyriphos G Cytrolane G Diazinon EC Monocrotophos Quinalphos Control (untreated with insecticide) n ¼ Number of observations. a Percent increase in yields estimated over the respective control plots at experimental sites where a particular insecticide was used. b Granules (G) at 1.0 kg a.i./ha (in 2 00 deep standing water by manual broadcast), and emulsified concentrates (EC) at 0.75 kg a. i./ha (as foliar sprays with hand compression knapsack sprayer) were applied respectively, at 30 and 50 days after planting. IRRI (1965, 1966) reported that plants can compensate for a low percentage of early dead hearts, but for every 1% white earheads a 1 3% loss occurred. Israel and Abraham (1967) estimated that for every 1% increase in dead hearts, the yield decreased by 1.6%, and for every 1% increase in white earhead, the loss was 2.2%. Based on a regression analysis, 2% dead heart and 2% white earhead were estimated to cause 4.4% yield loss at a level of a 3 t/ha and 6.4% yield loss at a level of 4 t/ha (Gomez and Bernardo, 1974). Based on 770 experimental units from 28 years data, our projections for damage due to 1% dead heart or white earhead, or to both phases of stem borer damage at the 1% level are 2.5%, 4.0%, and 6.4% yield loss, respectively. In our studies the most dominant variety was Jaya; other varieties were very similar to Jaya in yielding ability; and most of the high-yielding, semidwarf rice varieties released in India have yields comparable to Jaya (Muralidharan et al., 1997). Therefore, in terms of grain production loss over ecosystems, 1% dead heart or white earhead, or a 1% occurrence of both phases of stem borer damage would be 108, 174 and 278 kg/ha, respectively. In irrigated ecosystem, 1% dead heart resulted in 0.3% or 12 kg/ha loss whereas, 1% white earhead caused 4.2% or 183 kg/ha loss in grain yields; the loss due to 1% infestation in both phases of stem borer damage was 4.6% or 201 kg/ha. In rainfed lowlands, for 1% dead heart, or dead heart and white earhead caused 2.3% or 76 kg/ha yield loss. There are many reports on the use of 10% stem borer incidence as a threshold level for taking plant protection action (Israel and Abraham, 1967; Teng et al., 1993). In deep water rice in Bangladesh and Thailand, 1% yield loss was associated with 2% damaged stems at harvest. Yield loss was mainly due to a loss of bearing stems and lighter panicles borne by compensatory nodal tillers. Still a tentative damage threshold of 10% damaged stems at the booting-flowering stage and 20% damaged stems at plant maturity was proposed (Catling et al., 1987). Our results indicate clearly that even at levels below the threshold limit considerable losses can occur. This perspective on losses assumes more importance because of the non-availability of host-plant resistance to stem borer in rice. Variable results were obtained in the evaluation of commercial insecticides for controlling yellow stem borer in the Philippines (Heinrichs et al., 1986). At white earhead stage, no single compound effectively reduced damage (Das and Ray, 1984). In the all India coordinated trials during the 28 years studied, although no insecticide gave total control of stem borer damage, many increased grain yields significantly. Among from the insecticides studied, emulsified concentrates of monocrotophos and chlorpyriphos appeared more useful for adoption by farmers as their application caused maximum mortality of larvae (Pandya et al., 1987a) and unhatched eggs (Pandya et al., 1987b). Compared to other equally efficient insecticide granules such as diazinon and carbofuran, monocrotophos is known to be more economically viable (Sasmal et al., 1983). In fields of hybrid rice, stem borer damages were 28 36% higher than in other rice fields and there were % more egg masses. There were 7 17 times as many egg masses of S.incertulas in hybrid rice than in other rice. Survival rates of both species were also higher in hybrid rice, and larvae and pupae weighed more (Chieng, 1985). In order to increase the productivity to meet the food requirement, more areas are producing hybrid rice. The fine-grained aromatic and Basmati rice cultivars that receive premium price at markets are known to attract stem borers, and therefore, suffer serious damage from dead hearts and white earheads (Siddiq et al., 1994). The optimum time and
8 concentrations of insecticide application to control stem borer in rice depend on many factors including crop susceptibility to damage, population dynamics, crop yield in unit area, unit price of production, insecticide availability, degradation rate of insecticide and its efficiency, and weather. Therefore, there is a need to focus research attention on all these aspects and to reconsider the economic threshold level for taking plant protection action. Acknowledgements We dedicate this article to Late Prof. C. L. Campbell, North Carolina State University, USA for motivating us to venture into yield loss studies. The authors express their gratitude to all the scientists responsible for the conduct of all India, coordinated insect control experiments at different centers in various years. The support from World Bank (National Agricultural Research Project on IPM in Rice) and Food and Agricultural Organization is gratefully acknowledged. 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