Evaluation of action thresholds for chronic rice insect pests in the Philippines. I. Less frequently occurring pests and overall assessment

Transcription

1 International Journal of Pest Management, January March 2005; 51(1): Evaluation of action thresholds for chronic rice insect pests in the Philippines. I. Less frequently occurring pests and overall assessment J. A. LITSINGER 1, J. P. BANDONG 2, B. L. CANAPI 3, C. G. DELA CRUZ 2, P. C. PANTUA 2, A. L. ALVIOLA 2, & E. H. BATAY-AN III Jacobs Place, Dixon, CA, USA, 2 International Rice Research Institute, Metro Manila, Philippines, 3 Monsanto Philippines, Makati, Metro Manila, Philippines, and 4 Philippine Department of Agriculture Abstract Action thresholds as decision tools for insecticide application were developed and tested against the major insect pests of rice at four sites in the Philippines over a 13-year period. Action threshold treatments were compared to the farmers practice, prophylactic insecticide usage, and an untreated check. Yield loss data using the insecticide check method partitioned yield losses over three crop growth stages in the same test fields. Chronic pests that exceeded action thresholds in 79% of fields were whorl maggot Hydrellia philippina Ferino (Diptera: Ephydridae), defoliators Naranga aenescens Moore and Rivula atimeta (Swinhoe) (Lepidoptera: Noctuidae), leaffolders Cnaphalocrocis medinalis (Guenée) and Marasmia patnalis Bradley (Lepidoptera: Pyralidae), and stemborers Scirpophaga incertulas (Walker) and S. innotata (Walker) (Lepidoptera: Pyralidae). Minor chronic pests reached threshold levels in only one site each: rice bug Leptocorisa oratorius (F.) (Koronadal), whitebacked planthopper Sogatella furcifera (Horvath) (Zaragoza) and green leafhopper Nephotettix virescens (Distant) (Guimba); brown planthopper Nilaparvata lugens (Stål) did not exceed a threshold in any field. Stemborers were the most important pest group in terms of yield loss. Despite the insecticide check method underestimating losses, a mean crop loss of 0.62 t/ha was measured which showed ample scope for corrective action. But loss was evenly distributed across crop growth stages ( t/ha) reducing the impact of insecticides. Action threshold treatments overall outyielded the untreated check, more so in the two sites with highest pest density. The benefit of thresholds was to reduce insecticide usage, as a cost saving. However all the practices showed poor economic returns including the farmers practice. Farmers practice employed low insecticide dosages and timing was not consistent with pest damage, but yields were often similar to threshold treatments. Farmers appear to use insecticide more for risk aversion than for profit. The best threshold characters when evaluated against resulting pest density and yield loss criteria showed accuracies 4 90% correct decisions. Future work is needed to improve the insecticide response rather than monitoring tools. Thresholds need to be incorporated into improved crop management, which was often found suboptimal by farmers, to take advantage of the high levels of tolerance in modern high tillering cultivars. Crop husbandry practices which improve yield potential such as selection of longer maturing varieties and nitrogen fertilizer may be a more effective pest management strategy than insecticides. Keywords: Pest control, irrigated rice, insecticides, decision making, yield loss, plant tolerance, planting date, colonisation pattern, nitrogen fertilisation 1. Introduction Modern rice varieties possessing genetic resistance against brown planthopper Nilaparvata lugens (Stål) and green leafhopper Nephotettix virescens (Distant), the major epidemic insect pests, have been widely grown by farmers since the mid 1960s (IRRI 1985). Genetic resistance against chronic insect pests has been less successful (Heinrichs 1986). Whorl maggot Hydrellia philippina Ferino, defoliators Naranga aenescens Moore and Rivula atimeta (Swinhoe), leaffolders Cnaphalocrocis medinalis Guenée and Marasmia patnalis (Bradley), stemborers Scirpophaga incertulas (Walker) and S. innotata (Walker), rice bug Leptocorisa oratorius (F.), and whitebacked planthopper Sogatella furcifera (Horvath) are recurring pests causing significant yield losses in the Philippines as determined by the insecticide check method (Litsinger et al. 1987). Shortages in rice production in the 1970s and 1980s due to epidemic pests spurred the development of integrated pest management (IPM) strategies with the focus on minimising insecticide usage, which had been found to upset natural enemy populations leading to pest resurgence and secondary pest outbreaks. It is ironical that farmers now often overuse insecticide from fear of losses caused by a past history of crop failures from epidemic pests (Kenmore et al. 1987; Litsinger 1989). Farmers also overestimate losses from chronic pests based on their experience from epidemic pests prompting further overuse (Heong and Escalada 1997). Chronic insect pests are the main targets for IPM training programmes now widespread in Asia where Correspondence: J. A. Litsinger, 1365 Jacobs Place, Dixon, CA 95620, USA. Tel: Fax: jlitsinger@thegrid.net ISSN print/issn online # 2005 Taylor & Francis Group Ltd DOI: /

2 46 J. A. Litsinger et al. farmers learn crop monitoring and management decision skills (Matteson 2000). One of the basic tenets of IPM is to minimize insecticide usage, thus pest populations are tolerated until pest density has reached an economic threshold level or when other control methods are impractical or uneconomical (Norton and Mumford 1993). The main tools for insecticide decisions are economic thresholds which are pest densities that trigger a corrective action before the damage reaches the critical economic injury level (Morse and Buhler 1997). Economic thresholds are based on damage relationships between abundance and yield which along with economic parameters can predict the most optimal new thresholds as costs change. Damage functions have not been worked out for chronic rice insect pests, thus empirically derived action thresholds (ATs) have been utilised along with surveillance methods (Dyck et al. 1981; Reissig et al. 1986; Way et al. 1991). In the developed world, farmers often contract surveillance activities to professional scouts to make pest control decisions. In the developing world, small-scale farmers cannot afford to hire scouts, thus the use of ATs must be able to be mastered by farmers. Thresholds have often been said to be too difficult for farmers to master (Goodell 1984; Matteson et al. 1994; Morse and Buhler 1997), as those developed by researchers usually are not in a form that farmers understand. Goodell et al. (1982) advocated that for IPM technology to be easily assimilated, research should start with the farmers and not from research stations. Studies have shown that most farmers develop their own thresholds and monitoring techniques (Bandong et al. 2002). A number of the threshold characters being evaluated in this study were ideas that came from farmers (Bandong et al. 2002). A reciprocal relationship between the farmers and researchers perception of ATs needs to be fostered as both stand much to gain from each other s perspectives and individual skills (Goodell 1984; Heong and Escalada 1997). This study represents the field testing of ATs against chronic insect pests of rice following on from the early work reported in Heinrichs et al. (1978), Waibel (1986), and Smith et al. (1988). ATs are composed of a character to measure, a level for that character, a monitoring protocol, and a corrective response, normally an insecticide that would entail a recommended dosage and timing. ATs were compared to the farmers practice, a prophylactic best insecticide practice, and an untreated check. The resilience of rice crops to pest damage was evaluated by site and season using yield loss data. There are reports of exceptional examples of modern rice s ability to compensate from abnormally high insect infestation: crops suffering 50 and 82% rice whorl maggot damaged leaves (Viajante and Heinrichs 1986; Shepard et al. 1990), 67% damaged leaves from leaffolder (Miyashita 1985), 30% deadhearts and 10% whiteheads (Rubia et al. 1996), and three whiteheads/hill (Litsinger 1993). Nitrogen (N) contributes to this resilience and was tested as a contrasting strategy to insecticide in response to ATs. This paper is the first of a four-part series. The succeeding papers focus on the development for ATs for whorl maggot and defoliators (Litsinger et al. 2005) while those on leaffolders and stemborers will follow. This first part reports on the ATs for the less common pests and gives an overview. 2. Materials and methods 2.1. Study sites ATs were tested on-farm over a 13-year period in four irrigated, double-crop rice areas typical of the Philippines major rice bowls (Pingali et al. 1997) 23 crops in Zaragoza (141 fields), 15 crops in Koronadal (109 fields), 13 crops in Guimba (88 fields), and 17 crops in Calauan (81 fields). Farm sizes ranged from 5 1 to 4 ha with land preparation done by rotary tillers. Two sites Zaragoza (mean 2.1-ha farms) and Guimba (mean 0.9-ha farms) were in Nueva Ecija province, Central Luzon, while Calauan (mean 2.1-ha farms) lies in Southern Luzon, Laguna province. A fourth site, Koronadal (mean 1.0-ha farms), South Cotabato province is in Mindanao southern Philippines. Zaragoza comprised the villages of Marawa, Malabon Kaingin, Imbunia, Rajal Norte, and Batitang in portions of Zaragoza, Jaen, and Santa Rosa towns. Zaragoza farmers fields lie at the lower end of the Upper Pampanga River Irrigation System and consequently were planted late in the wet season due to delayed water delivery. Crops often reach maturity during the time of the strongest typhoons (between October 15 and November 15) and as a result suffer severe damage from flooding or lodging, or indirectly by diseases spreading from wind-whipped foliage or due to harvested grain that cannot be dried. Further description of farmers and the area is given by Goodell et al. (1982). The Guimba site was located in the village of Bantug next to the International Rice Research Institute (IRRI) and Philippine Regional Department of Agriculture Cropping Systems Program research site in nearby San Roque and Macatcatwit. Deep well pumps, one per village, each irrigated ha. Due to the small irrigation systems farmers plant their wet season crop earlier than in Zaragoza, avoiding the severe typhoons. Tungro disease is endemic, occurring sporadically. The electric pumps are expensive to operate and break down often leading to drought stress. Further site description is given in Entomology Department (1985). Calauan is located 10 km south of the IRRI research centre in the villages of Pulong and Dayap. The rice area is irrigated by river diversion from small streams feeding Laguna de Bay lake from the

3 Action thresholds for chronic rice insect pests 47 watershed of Mt. Makiling volcano. The villages are located along the lake edge but away from the perennial flood zone. Calauan farmers experienced large scale tungro outbreaks in the early 1970s. In Koronadal the villages of Namnama, Magsaysay, Barrio 1, Avancenia, and Morales were selected in the Marbel River Irrigation System. Koronadal lies outside of the typhoon belt but has been the site of outbreaks of tungro, grassy stunt, and brown planthopper hopperburn in the early 1980s. With its longer rainy season, the double crops are referred to as first and second rather than wet season and dry season. Its volcanic soil is still fertile and high yields are obtained with lower amounts of inorganic fertilizer. Further site description is given in Entomology Department (1985, 1988) Research teams ATs were developed and tested in farm communities by resident research teams for each site. Field workers were recruited from the surrounding villages representing major ethnic groups. A field office was rented and staffed by a laboratory assistant trained in ricefield arthropod identification and equipped with a dissecting microscope. The resident entomologist and team were encouraged to improve on current thresholds with farmers as co-partners. Thus there was constant revision of threshold characters, levels, insecticides, and methods of application Experimental design Thresholds were tested under farmers agronomic conditions as much as possible including their currently used cultivars and seedbed methods. The only stipulation made was not to include direct seeded crops as this has been found to affect pests (e.g., whorl maggot Litsinger 1994). The purpose was to conduct the trials under farmers field conditions and to only vary the insect control variable. The many cultivars used, for example, possessed genetic resistance against a similar complex of insect pests and diseases. The dapog seedbed method adds 2 weeks to the crop in the field. Farmers vary their management practice as they see fit throughout the season more commonly than following a standard set of practices. We did not attempt to dictate crop management practices for them. The results of the research will yield technology that is more robust and adaptive. Each season four to nine new farmers (replications) were identified from each site to be cooperators. These farmers were dispersed geographically within each community and grew the most popular variety at the time. Planting dates were staggered over the planting season to allow expression of the full range of pest densities. The most popular varieties changed over time but included IR36, IR42, IR62, IR64, IR74 (maturities ranging from 90 to 110 days) in most sites. Locally farmers had other selections: Koronadal with IR60 (released only in Mindanao) and an unofficial line #90; Calauan with C1 and Malagkit (both 120 days) and IR70; Guimba with IR58; Zaragoza with IR52 and IR56. To be released, Philippine varieties must be resistant to green leafhopper and brown planthopper which caused severe epidemics in the 1970s before improved multiple pest-resistant varieties were developed. As resistance is not durable, new varieties must be developed continually. Crops were transplanted from wet seedbeds or dapog. The latter was popular in Calauan and Koronadal where seeds are sown on banana leaves thus roots do not enter soil to minimize transplanting shock (from roots torn off while pulling seedlings), field establishment occurs 2 weeks earlier than a wetbed. Each season farmers, including the current co-operators, were interviewed every few weeks to record their cultural practices (e.g., Department of Entomology 1985, 1988). Within each co-operator s field, a 0.2-ha research area was demarcated with plastic string tied to bamboo stakes (Litsinger et al. 1980a). Typically eight to 10 treatments were included in the experimental design each season combining yield loss assessment along with two thresholds compared to the farmers practice, a prophylactic insecticide regime, and an untreated check. With the exception of the farmers practice (1000 m 2 ), plot sizes were m 2. This design and plot size arrangement have been determined to have unbiased effects on pest abundance (Litsinger et al. 1987). Five of the treatments were designed to measure yield loss using the insecticide check method which had been tested earlier in other sites (Litsinger 1991). Three crop stages were recognised following Yoshida (1981) vegetative (transplanting to panicle initiation), reproductive (panicle initiation to flowering), and ripening (flowering to maturity 10 days before harvest). In a typical 110-day variety, the reproductive stage would begin about 40 d.a.t. and end about 30 days later. The seedbed had been included in experiments prior to the current study, but as no significant yield loss was ever measured, the seedbed portion of the vegetative growth stage was eliminated. The first treatment termed full protection attempted to show the yield potential with the least possible insect damage. Weekly insecticide sprays at the manufacturers recommended dosages were applied to each plot with interplot spray drift minimised by a mosquito cloth on a m wood frame held downwind by two assistants. Insecticides were selected both for their efficacy as well as proven neutrality regarding phytotoxic or phytotonic effects on rice (Venugopal and Litsinger 1984). Insecticides were applied as foliar sprays with 19-l, lever-operated, knapsack sprayers fitted with hollow cone nozzles using a l/ha spray volume (increased with crop growth). From the second to fourth treatments,

4 48 J. A. Litsinger et al. insecticide was withheld from each successive growth stage, with the fifth treatment being the untreated check. The prophylactic treatment involved soil incorporation of 0.5 kg a.i. carbofuran granules/ha before transplanting followed by two foliar sprays of 0.4 kg a.i. chlorpyrifos/ha 10 days apart during the late reproductive stage to prevent stemborer whiteheads. All treatments were in randomised complete block design with replications as fields. Yield was taken from five 5-m 2 crop cuts in a stratified sample per treatment and dried to 14% moisture. Total yield loss was calculated both in terms of grain weight and percentage. Total yield loss was the difference between the full protection and untreated. To calculate percentage, total loss was divided by the untreated yield and multiplied by 100. Loss in each of the three growth stages was calculated in separate treatments where protection was omitted from each stage sequentially. Yield was subtracted from that of the full protection treatment. The loss in each of the growth stages was summed and adjusted upwards or downwards proportionally so that the total of the three stages equalled the total yield loss. Crop compensation was measured by regression analysis using the yield loss dataset for each of the two seasons per location using crop averages that varied over a range of yield potentials. Compensation would occur if the rate of yield loss did not rise proportionally with increasing yield (i.e., slope of the linear regression equation was insignificant) Sampling methods Pest incidence was sampled weekly in the threshold treatments and untreated check, but only once per growth stage in the yield loss treatments. Pest monitoring for AT decision making was carried out in the respective threshold plots. Pest or damage densities were usually measured on a per-hill basis with the sample size of 20 hills selected individually in a stratified pattern exclusive of a 1-m border zone. Mechanical hand counters were used to record the number of tillers and leaves per hill with pest damage recorded on those plant parts as appropriate. One staff scored the hill while another recorded Action thresholds Thresholds are multifaceted involving a number of variables, any one of which can affect efficacy. The first variable is a character such as an insect stage (adult, nymph) or its damage symptom (damaged leaves, deadhearts). Second is the sampling unit and number of samples that express the character (planthoppers = 20 hills, green leafhopper = 25 net sweeps, rice bug = per hill). Third is the density of the character per sampling unit (e.g., one planthopper or rice bug per hill, one leafhopper per sweep). Normally a single character with two threshold levels was tested each season per site, termed low level (e.g., one planthopper per hill) and high level (e.g., two planthoppers per hill), in separate treatments. New characters were continually being developed in an effort to improve performance. Another set of variables is associated with the corrective response triggered by a threshold, usually an insecticide or N. As development of ATs was iterative there was no balanced design to test the many characters and response variables in a given field. Most characters were tested in multiple sites. Data analysis after each season entailed comparing yield in the threshold treatments to that in the untreated check, farmers practice, and prophylactic treatment. An economic analysis was performed on each practice where marginal returns and benefit:cost ratios were calculated. Included in the analyses were crop monitoring and insecticide application labour, cost of labour, and interest. The farmers practice was what each individual farmer collaborator carried out in the trial field. Results were scrutinised to determine if yield loss occurred in each growth stage where thresholds were reached. If no yield loss were recorded but thresholds were reached, levels were raised the following season and vice versa. Figures were drawn to illustrate the weekly pest abundance and degree of control obtained field by field (e.g., Department of Entomology 1985, 1988) Planthopper thresholds. The whitebacked and brown planthoppers were monitored weekly in the vegetative and reproductive stages by recording the number of adults and nymphs as well as their predators from 20 hills along a zigzag transect in each plot (Matteson et al. 1994). Each hill was bent over the water with one hand and slapped with the other open hand to dislodge arthropods including predators onto the water surface for easier detection. The average number of tillers per hill was calculated. Only the brownish mature nymphs (stadia 4 5) and adults were counted. Densities of immature white nymphs are particularly prone to predation. Predator density was incorporated into the ATs, as for each found, five planthoppers were subtracted from the total. The AT levels ranged from 0.5 to 1 planthoppers (adults and nymphs) per tiller. Once half of the AT level were reached, monitoring was increased to twice a week. Fenobucarb (BPMC) insecticide was applied at 0.4 kg a.i./ha timed to when the planthopper population was in the late nymphal stage. Timing with mature nymphs was to minimize the egg load inside the tillers, as the insecticide has only minimal activity against eggs. If the crop were to be sprayed when adults were dominant, adults would be killed but eggs would survive inside the tillers to emerge under very low predator densities, often leading to resurgence Green leafhopper threshold. Green leafhopper was only monitored if tungro virus was observed in the community. The orange yellow discolouration, diagnostic of tungro, is masked in mature plants but

5 Action thresholds for chronic rice insect pests 49 becomes particularly noticeable in regrowth after harvest. A sweepnet was used in the seedbed through the vegetative stage, taking 25 strokes while walking. The AT level was set at 0.5 adults + nymphs/sweep with 0.4 kg a.i. fenobucarb/ha as the response Ricebug threshold. Twenty hills were inspected during the milk stage of grain development for rice bug adults and nymphs. The range of AT levels tested was four to 10 bugs/20 hills with a response of 0.4 kg a.i. endosulfan/ha as a single spray Insecticide response When a threshold were reached and insecticide was the given response, it was applied within a day of monitoring. Insecticide technology was developed in an iterative process as well, thus if efficacy was low, adjustments were made, normally changing the chemical but in certain instances involved research on other application methods. Tests to determine the minimum effective dosage were undertaken with a view to cost savings. Foliar sprays were applied as described in the yield loss trials. Percentage control of each threshold character was based on the untreated check Nitrogen substitution Application of N has been shown to increase the rice crop s tolerance of pest damage (Litsinger 1993, 1994; Rubia et al. 1996) and was tested in one of the threshold treatments in all four sites during the final 2 years as a substitute for insecticide. The motive was derived from the often discouraging results with insecticides. Urea was broadcast at 25 kg N/ha, equivalent in cost to an average insecticide application, in response to surpassing an AT for whorl maggot (eggs per hill from a neighbouring field), defoliators (larvae per hill), leaffolders (larvae per hill), and stemborers (egg masses per m 2 ) Statistical analysis All statistical analyses were performed by SAS and, unless otherwise stated, we use P as the criterion for significance. Results were subjected to ANOVA and regression/ correlation analysis where appropriate. Treatment means were separated using the paired t-test for two variables or least significant difference (LSD) test for more than two variables. Means are shown with standard errors of the mean (SEM) using a pooled estimate of error variance. 3. Results 3.1. Insect pests and densities The chronic pests that regularly exceeded thresholds were whorl maggot, defoliators, leaffolders, and stemborers (Scirpophaga incertulas was prevalent in all sites except Koronadal where S. innotata was dominant). Less common pests that reached threshold levels only in single sites were rice bug in three crops in Koronadal (5.8% of all test fields in the site), whitebacked planthopper in two crops in Zaragoza causing patches of hopperburn (9.1% of fields), and green leafhopper in one crop in Guimba with tungro symptoms occurring sporadically (0.8% of fields). The latter three pests occurred too infrequently for different threshold characters to be tested. Brown planthopper affected 5 10 fields nearby test fields causing isolated patches of hopperburn on susceptible cultivars in all sites except Calauan, but never in the test fields themselves Yield loss Measurement of yield loss is an essential element in evaluating thresholds. With use of the insecticide check method it was expected that the full protection treatment would provide 4 80% control for each pest based on damage. This goal was only achieved with leaffolders averaging % damaged leaves. Control of damage by defoliators ( % damaged leaves) and stemborers ( % control based on deadhearts and whiteheads) nearly reached the goal but the greatest disappointment came with whorl maggot. Despite weekly applications of high dosage foliar sprays, only % control was achieved based on damaged leaves over the four sites. Stemborers were the only pest to show significant differences by season ( vs % in the wet and dry seasons, respectively, by paired t-test with P = 0.04, df = 126). Yield loss, despite the suboptimal insecticide protection, showed considerable scope for IPM when viewed as a total (0.62 t/ha or 12.7%) (Table I). Yields across sites and seasons in the full protection treatment averaged 4.99 t/ha. Highest seasonal yields were recorded in Zaragoza dry season (6.23 t/ha) and lowest in the Guimba wet season (4.39 t/ha), the two closest sites. Untreated crops averaged 4.37 t/ha overall with the highest and lowest site yields occurring in the Zaragoza dry season (5.50 t/ha) and in the Guimba wet season (3.67 t/ha). Lowest yield per field was in Guimba 0.77 t/ha in the 1984 wet season. Highest yield loss per crop was also in Guimba dry season (0.77 t/ha); lowest was in Calauan wet season (0.30 t/ha). Losses during the vegetative stage (0.23 t/ha) were significant in all sites and seasons except the dry seasons in Guimba and Calauan. No one site or crop had significantly higher losses than another during this stage. The reproductive stage loss (0.24 t/ha loss) was significant in all site season combinations except Koronadal second crop and Calauan wet season. Least loss occurred in Calauan wet season

6 Table I. Measurement of yield loss by season in four irrigated rice sites by the insecticide check method, Philippines a. 50 J. A. Litsinger et al. Yield (t/ha) Yield loss a / Total Vegetative stage Reproductive stage Ripening stage Crops Fields Full Site Season 3 (no.) (no.) protection Untreated t/ha % P t/ha % P t/ha % P t/ha % P Zaragoza WS b b ab a ns DS a a bc ab Koronadal 1st b b bc ab ns ns 2nd b b ab a ns ns Guimba WS b b ab a DS b b a ns a Calauan WS b b c b ns ns DS b b bc ns ab total avg P ns 0.02 ns F df In a column, means+sem followed by a common letter are not significantly different (P40.05) by LSD test. 2 Probabilities show significant differences of losses measured by treatment which lacked insecticide protection in that stage compared to the insecticide protected treatment. Total yield loss was measured as the difference between the untreated and insecticide protected. Only the losses are presented for the vegetative, reproductive, and ripening stages. 3 WS = wet season, DS = dry season.

7 Action thresholds for chronic rice insect pests t/ha. The ripening stage loss (0.15 t/ha) was significant in only half the site-crop combinations. No one site or crop combination was significantly different. A wide range of yield losses and yield potentials occurred within each site and season. With yield loss as the dependent variable, regressions were made for each site and season (Figure 1). In Zaragoza a distinct seasonal difference in compensation ability was seen (Figure 1a). The wet season data showed an increasing rate of yield loss with rising yield levels leading to a significant regression showing a lack of compensation. But in the dry season a high degree of compensation was evident as crops with rising yield potentials from 1.5 to 7 t/ha showed an insignificant rise in yield loss. Data from Koronadal (Figure 1b), however, showed a lack of compensation in either the first or second crops as the regressions were both significant. Whereas in both wet and dry season crops in the low pest density sites of Guimba and Calauan (Figure 1c d) high rates of crop compensation were evident Nitrogen substitution The results of substituting insecticide with fertiliser as a response to thresholds showed significant yield gains over the untreated plots in three of the eight pest season combinations (Table II). Due to the few seasons of testing, the results were pooled over locations. Only responses to stemborers showed a yield gain in the wet season, while both whorl maggot and defoliators did in the dry season. There were no significant yield gains with leaffolder thresholds in any season Threshold treatments compared to other practices Thresholds, both high and low levels, were compared to three other pest control strategies: (1) doing nothing (the untreated check), (2) farmers practice (among the farmer co-operators), and (3) prophylactic insecticide scheduling. The full protection treatment served as a check showing yield potential with maximum insecticide protection. The variables analysed for each treatment were percentage of fields Figure 1a d. Relationship between yield and insect pest-caused yield loss in four sites showing compensatory capacity in those crops with significant slopes in the regression equation: (a) Zaragoza, (b) Koronadal, (c) Guimba, and (d) Calauan, Philippines. Table II. Yield gain from pooled fields across sites where nitrogen fertiliser replaced the insecticide response to action thresholds 1. Wet season Dry season Pest Yield gain (kg/ha) 2 P df Yield gain (kg/ha) 2 P df Whorl maggot ns Defoliators ns Leaffolders ns ns 24 Stemborers ns 19 1 Nitrogen was applied as urea at 25 kg N/ha when a threshold was reached. 2 Difference between threshold treatment and untreated by paired t-test at P40.05, mean+sem.

8 52 J. A. Litsinger et al. treated with insecticide, application frequency of treated fields, dosage load per crop, efficacy against target pest groups, yield gain, total yield, marginal returns from insect control, and benefit cost ratio. Over all sites, ca. twice the percentage of fields were treated with the low value than the high value thresholds (68 vs. 36%) (Table III). This was much less than the average for farmers (90%) and the standardised prophylactic regime (100%). As determined from interviews, farmers in three of the sites treated 4 92% of fields for an average crop, with Guimba farmers treating the fewest (76%). Guimba farmers may have tailored applications to the relatively low pest densities recorded there, but, more likely, they were more strapped for cash to purchase insecticide than farmers in other sites due to the high cost of running the electrical pump. Highest treatment frequency occurred with Calauan farmers ( 4 98%), also a low pest density site. Significant differences occurred within and between each threshold treatment with regard to pest density. The frequency of fields treated in the high pest density sites with the low threshold level was equivalent to farmers (90% or more) but was 5 57% from the high threshold treatment. But in the low pest density sites, the low threshold resulted in significantly fewer (39 53%) fields being treated, and even fewer ( 5 25%) with the high threshold level. The same general trends were evident in terms of the mean number of applications per field, but the differences were less distinct. This statistic was, however, based on treated fields and not all fields. Farmers averaged one more application (2.3 times) than the high threshold treatment (1.3 times), with the low threshold treatment closer to the high threshold (1.6 times). This indicated that the threshold levels may have been set too close together, and thus were more likely to be exceeded in both treatments in each field. Zaragoza was unusual as there was no difference between the farmers practice and the threshold treatments, all ranging between 1.9 and 2.0 times. This was due both to the greater whorl maggot pressure and the response of double sprayings, as well as more than one pest exceeding thresholds per field. Greatest frequency occurred with Koronadal farmers (3.1 applications per crop), probably due to farmers responding to recent history of pest outbreaks (Waibel 1986). In Guimba where only 76% of farmers had treated, the number of applications was equal to all sites but Koronadal. This indicated that of the Guimba farmers who sprayed, they applied more frequently than farmers in other sites. The threshold treatments resulted in lower application frequencies than the farmers practice in three sites for the high thresholds and two sites in the low thresholds. There was no statistical difference between threshold treatments in any site. The most popular insecticides of farmers, accounting for 74% of total applications, were mono- Table III. Comparison of corrective actions taken between thresholds and farmers practice insecticide application frequency 1. Fields treated (%) Insecticide applications (no./field) 3 Pest Crops Fields Low High Farmers Low High Farmers Site density (no.) (no.) threshold 2 threshold 2 practice P F df threshold threshold practice P F df Zaragoza High A a A b A a A a A a B a ns Koronadal High A a A b A a AB b AB b A a Guimba Low B b B c B a B ab B b B a Calauan Low B b B c A a B b B b A a avg P F df Applications not counted in the seedbed. In a column, means+sem followed by a common upper case letter are not significantly different (P40.05) by LSD test. In a row, means+sem followed by a common lower case letter are not significantly different (P40.05) by LSD test. 2 Fertiliser as substitute for insecticide was included. 3 Average of insecticide users only. Double sprays in response to a threshold was counted as two applications.

9 Action thresholds for chronic rice insect pests 53 crotophos (33%), chlorpyrifos + fenobucarb (15%), endosulfan (9%), methyl-parathion (8%), and cypermethrin (7%). All are broad spectrum materials and, with the exception of methyl-parathion, did not differ from those used in the threshold treatments. The mean dosage per application of farmers ranged from 0.20 to 0.23 kg a.i./ha for organophosphate, organochlorine, and carbamate insecticides (Table IV). This is half the recommended dosage which was set at the minimum effective dosage level. Insecticide timing, ranged among farmers from 20 to 26, 36 to 42, and 43 to 51 d.a.t. for the first through third applications per crop. The late timing of the first application showed the farmers practice likely would have minimal effect against whorl maggot. The calculation of mean dosage load is based on multiplying the mean frequency of treated fields, mean number of applications per treated field, and mean dosage per application. Highest to lowest loads per crop over all sites were ranked from the low value threshold 0.46 kg a.i./ha, farmers practice 0.45 kg a.i./ha, with the high value threshold at half the load (0.21 kg a.i./ha) of the other two treatments (Table IV). These data can be compared to the prophylactic which was standardised at 1.3 kg a.i./ ha. Highest loads ( kg a.i./ha) occurred in the low threshold treatments of the high pest density sites and in the farmers practice in Koronadal. In order for the dosage loads of the threshold treatments to be equal to those of farmers, Koronadal farmers had to have sprayed twice as frequently per crop. This did not happen in Zaragoza where spray frequency was similar, thus the low threshold load was twice the farmers practice (0.35 kg a.i./ha). Among the threshold treatments, significantly higher loads occurred in the high pest density than low density sites reflecting the higher pest pressure. This trend did not occur for the farmers practice. In Guimba, because of the low frequency of fields treated by farmers, there was no difference between the farmers practice and the low value threshold. In all sites, there were higher loads in the low threshold level than high level (0.46 vs kg a.i.). Only in Calauan was the farmers practice significantly higher than the low threshold. The pest control strategies were compared for overall efficacy in controlling each pest group with the full protection serving as a reference (Table V). Prophylactic scored the highest overall control (41%) among the practices which was significantly lower than the full protection (60%) for all pests except defoliators. Low and high thresholds were statistically similar (31 34%) to each other and the farmers practice (24%) with only slight numerical superiority in regard to whorl maggot and stemborers. Low threshold was similar to prophylactic for all pests but leaffolders, while high threshold was similar but for leaffolders and whorl maggot. Farmers practice was only similar to prophylactic regarding defoliator control. In terms of yield gain from control of each pest group (comparing only those fields where thresholds were reached), all practices were statistically lower ( kg/ha) than the full protection (729 kg/ ha) and, with the exception of stemborers, were statistically undifferentiated. Yield gain from stemborer control by prophylactic was superior to that of the farmers practice. Further comparisons were made regarding total yield over all fields. Comparing all sites and seasons, the prophylactic was the only treatment equal to the full protection (4.99 vs t/ha) (Table VI). Total yield loss was 0.62 t/ha as the difference between the full protection and the untreated. The prophylactic treatment closed 66% of that yield gap. There was no significant difference in yield between the prophylactic, both threshold treatments, and farmers practice ( t/ha), but all were higher than the untreated (4.37 t/ha). The results from Calauan and Guimba mirrored those of the overall analysis, but more treatment separation occurred in the high pest density sites. Table IV. Comparison of mean insecticide load per field between thresholds and farmers practice in four test sites, Philippines. Dosage load per field (kg a.i./ha) 1 Farmers insecticide spray dosage 2 Pest Low High Farmers (kg a.i./ha) Site density Threshold threshold practice P F df (A) Zaragoza High A a A b B b Koronadal High A a A b A a Guimba Low B a B b B a Calauan Low B b B c AB a avg P F df Data averaged over all fields, derived from Table III by multiplying percentage fields treated by insecticide application frequency by the dosage (0.4 kg a.i./ha used in thresholds and data in column (A) used for farmers. In a column, means+sem followed by a common upper case letter are not significantly different (P ) by LSD test. In a row, means+sem followed by a common lower case letter are not significantly different (P ) by LSD test. 2 Dosage per application, synthetic pyrethroid insecticides not included.

10 54 J. A. Litsinger et al. Table V. Comparison of corrective actions taken between thresholds and other practices degree of insect control and resulting yield gain 1. Control (%) 1 4 WAT 2 Damaged leaves Stemborers Yield gain (kg/ha) 3 Treatment Whorl maggot Defoliators Leaffolders (deadhearts) Average Whorl maggot Defoliators Leaffolders Stemborers Average Full protection a a a a a a a a 729 Prophylactic b ab b b b b b b 342 Low thresholds bc b c bc b b b bc 226 High thresholds c b c bc b b b bc 222 Farmers practice c b c c b b b c 206 P F df In a column, means+sem followed by a common letter are not significantly different (P40.05) by LSD test. WAT = weeks after treatment. 2 Based on damage. 3 Data only include fields where thresholds for each respective pest was exceeded. Averaging Zaragoza wet season yield data, all treatments were significantly lower than the full protection. Yields in both the threshold treatments were equivalent to the prophylactic (all closing the yield gap 47 50%), but the farmers practice did not raise yield above the untreated. Dry season averages had higher yield potential and in this case both the prophylactic and low threshold were equal to the full protection, closing the yield gap 90 and 69%, respectively. The high threshold and farmers practice closed the yield gap 43 and 57%, respectively, and were significantly higher yielding than the untreated. With the first crop in Koronadal, only the prophylactic equalled the full protection closing the yield gap 81%. All the other treatments closed the yield gap 20 29% but were statistically similar to the untreated. The lower yielding second crop registered a higher yield gap, and all treatments were superior to the untreated but indistinguishable among themselves closing the yield gap 23 49%. Among sites Calauan recorded the most positive marginal returns across treatments followed by Zaragoza (Table VII). There was no trend based on pest density. In Calauan the $48.10 marginal return in the high threshold treatment was worth 376 kg of unhusked rice. The two other sites had mostly negative results. Among treatments the results were highly varied by site with no clear best treatment between the threshold treatments and farmers practice (Table VII). The farmers practice had the greatest mean benefit ($5.80) but only slightly higher than the two threshold treatments. Low threshold had the highest returns in Zaragoza but the results were not significantly different than the high threshold or farmers practice. Benefit cost ratios should be 2.0 or above to be attractive to farmers (Smith et al. 1989) which was only achieved in Calauan for both threshold treatments. In no other treatment-site combination did the benefit cost ratio exceed Discussion 4.1. Insect pests and densities In this 13-year study in four prominent rice growing areas in the Philippines, pest census and yield loss measurement determined that whorl maggot, defoliators, leaffolders, and stemborers were the key chronic pests. Thresholds were surpassed for at least one of the four pests in over three-quarters (79%) of fields for each crop on average. These figures include all the threshold characters tested but would have been less if only the best characters had been employed, lowering the should not have treated error rate. With the exception of defoliators in Calauan, all pests surpassed thresholds in each study site at least once. Other rice pests rice bug, whitebacked planthopper, and green leafhopper occurred minimally; each surpassed thresholds in only one site. The brown planthopper never reached

11 Action thresholds for chronic rice insect pests 55 Table VI. Comparison of threshold treatments to other practices in terms of yield. Yield (t/ha) 1 Zaragoza 3 Koronadal 3 Over all sites and Treatment Seasons 2 Wet season Dry season First crop Second crop Full protection a a a a a Prophylactic ab b ab ab b Low thresholds b b abc bc b High thresholds b b c bc b Farmers practice b c bc bc b Untreated c c d c c P F df In a column, means+sem are not significantly different (P40.05) by LSD test. 2 Data from 68 crops in Zaragoza, Guimba, Koronadal, and Calauan sites. 3 Data by fields, not crop averages. 4 Low level characters, thus lower threshold values for a given character. 5 High level characters, thus higher threshold values for a given character. threshold levels in a single study field, mainly due to resistant varieties adopted by most farmers. The most ubiquitous pests were stemborers, whose densities (2 3% damaged tillers over the entire crop) and mode of damage, contributed more significantly to yield loss than any of the other pest groups. The same conclusion was reached by Savary et al. (1994) but particularly in association with weeds. Stemborer damage was measured in units of the more important tillers rather than leaves. Each mature tiller bears leaves (Yoshida 1981), thus for the same percentage, its damage is more profound. Also a deadheart or whitehead is a severed non-bearing tiller, while damaged leaves were rarely entirely defoliated. In addition stemborer larvae infest more tillers than are manifested as withered tillers (unpublished data) further raising the percentage of injury. All four pest groups have been favoured by inorganic fertilizer and irrigation, cultural practices associated with modern varieties (Litsinger 1989). Stemborers reputably have been the most important insect pest of long-maturing, low-tillering, traditional rices (Cendaña and Calora 1967) but their impact has been lessened by the earlier maturing, hightillering and narrow-stemmed modern rices. It was surprising, therefore, that in Calauan, where many farmers grow longer maturing varieties, that higher stemborer damage did not occur. In Calauan the 120-day varieties would allow an extra (third) stemborer generation to occur than is the case for most modern rices. Natural enemies may have been able to cope with this increase, whereas van der Goot (1925) reported that with 6 9-month traditional rices they usually could not. Whorl maggot and defoliators, not mentioned in the pre-green Revolution literature, emerged as new pests. Whorl maggot, along with stemborers, were the only pests that averaged damage levels above the benchmark standards on a per-crop basis. Defoliators and leaffolders are probably not normally a yield threat in their own right due to the nature of the damage (Heong 1990), but there is evidence that defoliator injury can be significant when associated with whorl maggot (Litsinger 1993). Both Zaragoza and Koronadal had the highest combination of whorl maggot and defoliators, and highest pest abundance in general. Such densities may be related to extensive irrigated rice areas (Loevinsohn et al. 1988), as both sites lie in large irrigation systems Insecticide check method Accurate yield loss estimates formed the basis of threshold character evaluation. Unfortunately the full protection treatment, which is the core of the insecticide check method, only achieved the desired 4 80% control with leaffolders. Control was especially low in wet seasons due to monsoonal rainfall which reduced insecticide residual activity. The difference in control by season was most noticeable in stemborers (least control in the wet season). Typhoons were common in Luzon and when they struck near harvest, the most vigorous growing treatments (usually the full protection ) were most prone to lodge, biasing yield loss estimates downwards. Significant yield gain from thresholds, despite high frequencies of correct decisions not to treat scores, indicated that the insecticide check method is not a good tool to match yield loss with a particular pest in multipest crops such as rice. A further confounding effect in trying to associate losses with a single pest comes from synergistic losses from multiple pests, each at subeconomic densities, that attack jointly causing significantly higher losses than those caused by each acting singly (IRRI 1983, 1984; Wu et al. 1995). Furthermore the yield loss contribution of each pest is significantly influenced