J. A. Litsinger

A food web approach to evaluating the effect of insecticide spraying on insect pest population dynamics in a Philippine irrigated rice ecosystem

Summary 1. Data from a 645-taxa Philippines-wide food web and multiple regression models were used to predict population fluctuations of insect pests in a rice field. Independent variables of pest models included the biomass of rice plants in the field, the abundance of each pest, and the abundances of five highly correlated enemies of the pest, all as functions of time. 2. To test the ability of the models to reveal effects of insecticide spraying, a rice field at the International Rice Research Institute (IRRI) in the Philippines was divided into deltamethrin-sprayed and unsprayed plots. Data on the abundance of seven pest species (Nephotettix virescens, Recilia dorsalis, Sogatella furcifera, Nilaparvata lugens, Hydrellia philippina, Ne. nigropictus and Cofana spectra) and their natural enemies (predators and parasitoids) were collected during the dry season of 1990. 3. Spraying insecticide disorganized the population dynamics of insect species feeding in the IRRI field. Multiple regression models were less able in the sprayed plot than in the unsprayed plot to forecast the population fluctuations of pest species on the basis of various numbers and combinations of independent variables. For example, current pest abundance, by itself, was a significant predictor of future pest abundance for four of the seven pests (Ne. virescens, R. dorsalis, S. furcifera, H. philippina) in the unsprayed plot, but significant fits were found for only two pests (R. dorsalis, H. philippina) in the insecticide-sprayed plot. 4. In the unsprayed plot, independent variables were significant predictors of future pest abundance in four of seven initial models compared to one of seven models in the sprayed plot. Step-wise removal of independent variables in the models enhanced their forecasting power in both the sprayed and unsprayed plots, but significant models in the unsprayed plot nearly always outnumbered those in the sprayed plot. 5. In the unsprayed plot, Ne. virescens retained five of seven independent variables as significant predictors, compared to four for S. furcifera, three for C. spectra, and one for the remaining four pests. Classical models that contain one or two species as independent variables may not be sufficient to forecast future abundances of some Philippine rice pests in unsprayed and sprayed plots. 6. In general, models that included interaction terms and either the presence or absence of sprays among the independent variables did not improve the forecasting power of models in either the sprayed or unsprayed plot. 7. The methods developed here for studying the impact of spraying on the organization of arthropod communities in rice fields could be applied to other interventions

Management of the golden apple snail Pomacea canaliculata (Lamarck) in rice.

Abstract The golden apple snail Pomacea canaliculata (Lamarck) is a recently introduced rice pest in Asia. Its management is based on preventative and corrective methods emphasizing cultural and mechanical practices, with molluscicide as a measure of last resort. The planting method greatly influences the period of susceptibility to snail damage: wetbed-transplanted 20-day-old seedlings were less damaged than dapog-transplanted 13-day-old seedlings or direct-seeded rice. Seedlings ⩾ 30 days old were more tolerant of snail damage than younger seedlings. Drainage immobilizes snails. More preventative measures should be undertaken in high-risk (⩾ 2 snails m −2 ) than in low-risk ( −2 ) fields. Snails can be controlled in low-risk fields by employing one or two cultural methods. Dapog or direct-seeded methods would benefit from drainage and mechanical control (hand-picking). If cultural methods cannot be carried out, snails can be controlled with molluscicide at low dosage. High-risk fields, on the other hand, should be protected with a combination of cultural and mechanical methods. These methods can be substituted with low or high dosages of molluscicide. Molluscicide alone, however, requires a dosage of 0.5 kg a.i. ha −1 , which most farmers cannot afford.

Food web dynamics of irrigated rice fields at five elevations in Luzon, Philippines

The above-water food webs of arthropod communities in irrigated rice fields on Luzon Island, Philippines, were studied over the growing season at five sites (Los Banos, Cabanatuan, Bayombong, Kiangan, Banaue) ranging in elevation from 22 m to 1524 m. Arthropod populations were vacuum-sampled at roughly weekly intervals from the date after seedlings were transplanted to flowering at each site. Site- and time-specific webs were constructed from a 687-taxa cumulative Philippines web and time-series of species present. Taxonomic composition, food web structure, and arthropod phenology were broadly similar across different sites. Arthropod abundance was inversely associated with altitude across the five sites, but numbers of taxa and links and six food web statistics showed no obvious increasing or decreasing trend with altitude. The rise of taxa, links and mean food chain length over the growing season at each site reflected an increase in plant size with age and, at some sites, an orderly accumulation of newly arriving herbivore, predator, parasitoid and omnivore species. At each site, herbivores built up faster than predators and parasitoids, and predators arrived faster than parasitoids; the difference between the latest and earliest sampling dates of first arrivals, averaged over the five sites, was 38, 63 and 73 days for herbivores, predators and parasitoids, respectively. Site-to-site consistencies in food web properties and first arrivals suggest that such patterns may be influenced more by crop age than by geography or altitude. Sampled predator, parasitoid and omnivore taxa potentially encountered only a subset of their lifetime prey and predator species at any particular time in the rice field. Prey lists cumulated over time may underestimate the temporal specificity of predation by potential biological control agents. Research opportunities linking rice food webs and integrated pest management with East Indies biogeography are proposed.

Bacillus thuringiensis isolates from the Philippines: habitat distribution, δ-endotoxin diversity, and toxicity to rice stem borers (Lepidoptera: Pyralidae)

Bacillus thuringiensis Berliner isolates were detected in 57% of 801 samples of rice grain dust, soil, rice field arthropods, and miscellaneous habitats (rice straw compost and mammal faeces) collected at 100 sites in the Philippines. The collection yielded 3950 isolates of B. thuringiensis (8.7 isolates/positive sample). Grain dust from rice mills was the richest source (63% of the samples were positive, with 10.2 isolates/positive sample), followed by rice field arthropods, soil, and miscellaneous habitats. Polyclonal antibodies to six δ-endotoxin groups (Cry1A, Cry1B, Cry1C, Cry1D, Cry1E, and Cry3A) were used in enzyme-linked immunosorbent assays (ELISA) to characterize the toxins produced by each isolate. Subsamples of isolates representing the diversity of isolate sources and δ-endotoxin profiles were bioassayed against the yellow stem borer, Scirpophaga incertulas (Walker) and striped stem borer, Chilo suppressalis (Walker). Eighteen isolates highly toxic to both species were selected for characterization of δ-endotoxin genes by polymerase chain reaction (PCR) with primers specific to 14 genes or gene subfamilies, and Western blotting with Cry2A antibodies. At least two novel δ-endotoxin genes, related to cry1B and cry1F , were detected by DNA sequencing of PCR products.

Rice crop loss from insect pests in wetland and dryland environments of Asia with emphasis on the Philippines

Yield losses from insect pests were determined by the insecticide check method in 60 crops in the Philippines from 1976–1986 in three rice environments with traditional and modern cultivars in 10 sites. Plot size of 100 m2 minimized interplot interference from insecticide protection on untreated plots. Low and high crop losses were recorded across sites in drylands (2-39%, -X = 25%), rainfed wetlands (13-24%, -X = 21%), and irrigated wetlands (5-40%, -X = 18%). Weighted crop loss for the Philippines based on production by environment and cultivar type averaged 18.3% or 0.57 t/ha per crop. Consistently greater yield loss occurred in the vegetative (50%) than reproductive (30%) or ripening (20%) stages across environments and cultivars. No epidemics affected the trials therefore crop losses were those from chronic pests. The major chronic pests in the wetlands were stem borers, leaffolders, whorl maggot, and rice bug; and in the drylands were ants, seedling maggots, white grubs, leaffolders, stem borers and rice bug. Yield loss variability was greater between fields than crops or sites, therefore a control strategy based on corrective actions when pest numbers reach threshold levels is more efficient than prophylactic actions. Yield loss was more in early maturing cultivars stressing the importance of compensation in tolerating insect damage. Insect stresses perhaps can be compensated by total crop management to increase the plant’s ability to tolerate insect damage.RésuméLes pertes de rendement de cultivars modernes et traditionnels, dues aux ravageurs, ont été évaluées de 1976 à 1986 aux Philippines, par comparaison avec des traitements insecticides, lors de 60 cycles culturaux dans dix sites correspondant à trois situations agroécologiques rizicoles. L’utilisation de parcelles de 100 m2 a permis de minimiser l’interférence entre parcelles traitées et non traitées. Les pertes de rendement observées ont varié de 2 à 39% (moyenne: 25%) en riziculture sèche, de 13 à 24% (moyenne: 21%) en riziculture aquatique et de 5 à 40% (moyenne: 18%) en riziculture aquatique irriguée. Les pertes de rendement observées pour les différentes situations agroécologiques et cultivars se sont réparties autour d’une moyenne de 18% correspondant à 0.57 t/ha. Les pertes au cours de la phase végétative (50%) ont été régulièrement plus élevées que lors de la phase reproductive (30%) ou de maturation (20%). Etant donné qu’aucune épidémie de ravageurs n’a été observée au cours de l’étude, les pertes de rendements sont attribuables aux ravageurs endémiques. Les plus importants sont: (1) la chenille foreuse des tiges, la chenille tordeuse des feuilles, la mouche des feuilles, et la punaise coreide en riziculture aquatique; (2) les fourmis, la mouche des plantules, les larves blanches (white grubs), la chenille tordeuse des feuilles et la punaise coreide en riziculture sèche. La variabilité des pertes de rendement observée entre les différents champs d’essai a été supérieure à celle observée entre les types de cultures et les situations agroécologiques évaluées. Une stratégie de contrôle des ravageurs fondée sur la détermination des seuils critiques des populations est plus efficace que des actions prophylactiques. Les pertes de rendement plus élevés observées avec les cultivars précoces mettent en évidence l’importance de l’aptitude de la plante à compenser les dommages des ravageurs. Les pratiques culturales qui augmentent la tolérance de la plante aux ravageurs pourraient compenser la pression de ces derniers.

A Bibliography of Rice Leaffolders (Lepidoptera: Pyralidae)

This bibliography on rice leaffolders contains 886 published and unpublished references for the period 1854 to June 1987. References are arranged alphabetically by the name of author and also classified by subjects. A distribution map, tables of alternate host plants, outbreaks, resistant rice varieties, resistant wild rices, natural enemies and developmental stages, and a brief description of current status of the leaffolders are also provided.RésuméCette bibliographique sur la chenille tordeuse des feuille du riz (rice leaffolders) contient 886 références, publiées et non publiées, couvrant la période de 1854 à Juin 1987. Les références sont classées par ordre alphabétique suivant le nom des auteurs et également par sujet. Une carte de la distribution du ravageur, des tableaux sur l’apparition des hautes densités de population du ravageur, sur les varietés résistantes de riz cultivées et sauvages, sur les ennemis naturels et les tables de vie plantes hôtes ainsi qu’une description à jour du statut de la chenille tordeuse des feuilles sont également presenteés.

Second generation insect pest problems on high yielding rices

Abstract Traditional lowland rices of Asia are photoperiod sensitive and ripen at the end of the monsoon rainy season, producing stable but low yields even under environmental extremes. The dry season fallow of these single rice crop systems breaks insect and plant virus disease cycles limiting pest buildup. Modern rices developed in the sixties to feed a growing human population attain their high yielding potential with good water control but are intolerant of drought or floods. Production of modern rices in Asia has kept pace with human population growth. Modern photoperiod insensitive varieties encouraged irrigation system development to realize their high yielding potential through dry season cropping and increased yield per crop. But multiple rice cropping over large areas raised the carrying capacity of the environment by allowing yearround pest development. Numerous outbreaks subsequently occurred over the past two decades through resurgence caused by the increased use of government‐subsidized, bro...

Rice crop stage susceptibility to the rice yellow stemborer Scirpophaga incertulas (Walker) (Lepidoptera: Pyralidae)

From field trials with artificial infestation of rice yellow stemborer Scirpophaga incertulas (Walker) (YSB) egg masses over the post-transplanted crop cycle and reports in the literature, it was concluded that rice passes through three stages of general morphological resistance interceded by two stages of susceptibility. Resistance occurs on very young rice, at mid-growth, and after panicle exsertion. The resistant period at mid-growth varied by plant maturity. On an early maturing rice variety (IR72), resistance occurred from panicle initiation (PI) to pre-booting, while on the medium maturing IR70, resistance was extended from the late vegetative stage to booting. High deadheart (severed tiller) densities occurred post transplanting because, being small, up to three tillers are damaged by a single larva. Whiteheads (severed panicles) were formed from infestations from booting onwards and were relatively higher than deadhearts on the early maturing variety but relatively less on the medium maturing variety. Zinc deficiency delayed maturity of IR72, and the period of naturally high resistance of seedlings was extended throughout most of the vegetative stage as well as after PI through pre-booting. Resistance at mid-growth and after panicle exsertion could be from: (1) lignin and cellulose deposits on the cell walls, (2) tight wrapping of the leaf sheath, and (3) deposition of silica. All three trials produced similar quadratic models of the yield response to crop age when infested. Deadheart damage was compensated to a large degree, but lowest yields occurred from whitehead damage when infested at or just after the pre-booting stage.

Quantifying the Impact of Insecticides on Food Web Structure of Rice-Arthropod Populations in a Philippine Farmer’s Irrigated Field: A Case Study

Rice is grown on 10% of the arable land worldwide and provides one-third of the total calorie supply for two-thirds of the human population (International Rice Research Institute, 1991; Food and Agriculture Organization, 1992). In the middle to late 1960s, the widespread adoption of new high-yield, pest-resistant rice cultivars, nitrogenous fertilizers, and pesticides brought significant increases in rice production (popularly called The Green Revolution (Barker et al., 1985)) in south and southeast Asia. Extensive irrigation systems were built in Asia to capitalize on the insensitivity to the photoperiod of the new cultivars. Eliminating the former fallow period during the dry season removed one form of pest protection and brought new pest problems (Litsinger, 1993). Major outbreaks in the 1970s of the rice brown plant hopper, Nilaparvata lugens (Stal), a secondary rice pest before 1964, were traced to overuse of insecticides (Kenmore, 1980; Kenmore et al., 1984; Heinrichs and Mochida, 1984). The pest problems led to integrated pest management (IPM) strategies for tropical rice in the late 1970s that emphasized host plant resistance, cultural practices, and biological control, minimizing the use of pesticides.

Insecticide decision protocols: a case study of untrained Filipino rice farmers

Abstract Surveys in four irrigated rice sites in the Philippines over a span of eight years (1984–91) focused on farmers’ insecticide decision making protocol before formalized farmer field school training programmes. Despite past extension campaigns to the contrary, farmers based their decisions more on crop monitoring than prophylaxis. Farmers were deficient in pest identification skills using terms such as worms, moths, and hoppers while some farmers targeted beneficials. Farmers mostly based decisions on planthoppers, leafhoppers, and moths by seeing the insect pest, on whorl maggot, defoliators, and leaffolders by noting damage, and on rice bug and seedbed pests by prophylaxis. Decision protocols were largely site specific based on different pest complexes and outbreak histories as well as being highly farmer specific. A large proportion of prophylactic applications during the early crop stages was timed with fertilizer application. Farmers’ frequency of field visitation was as recommended for pests, but reasons to visit the field at the time of a spray decision were predominantly water management. A generalized pattern emerged with monitoring starting on the way to the farmers field. Some decisions were prompted upon seeing infestation in an earlier planted field or a neighbour spraying. A number of farmers sought lower lying more flooded parcels or downwind sides of parcels to visit first as these microhabitats favour greater pest densities. Most decisions were made while walking along the field border observing patches of damage or flushed moths with the unit of measure being the parcel. The field was entered only as a last resort. Farmers’ action thresholds were lower than those of researchers and sampling was less rigorous. A minority of farmers, however, expressed insect pest abundance in quantitative terms using sampling units of several rice hills, distance of rice rows, and per panicle. Some of the more innovative farmers’ pest assessment methods were tested later by researchers.