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Food web dynamics of irrigated rice fields at five elevations in Luzon, Philippines
- Kenneth Schoenly, Joel E. Cohen, K.L. Heong, James A. Litsinger, G.B. Aquino, Alberto T. Barrion, Gertrude Arida
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- Journal:
- Bulletin of Entomological Research / Volume 86 / Issue 4 / August 1996
- Published online by Cambridge University Press:
- 10 July 2009, pp. 451-466
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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 Baños, 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.
Effects of Bacillus thuringiensis on non-target herbivore and natural enemy assemblages in tropical irrigated rice
- Kenneth G. Schoenly, Michael B. Cohen, Alberto T. Barrion, Wenjun Zhang, Bradley Gaolach, Vicente D. Viajante
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- Journal:
- Environmental Biosafety Research / Volume 2 / Issue 3 / July 2003
- Published online by Cambridge University Press:
- 15 November 2003, pp. 181-206
- Print publication:
- July 2003
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Endotoxins from Bacillus thuringiensis (Bt) produced in transgenic pest-resistant Bt crops are generally not toxic to predatory and parasitic arthropods. However, elimination of Bt-susceptible prey and hosts in Bt crops could reduce predator and parasitoid abundance and thereby disrupt biological control of other herbivorous pests. Here we report results of a field study evaluating the effects of Bt sprays on non-target terrestrial herbivore and natural enemy assemblages from three rice (Oryza sativa L.) fields on Luzon Island, Philippines. Because of restrictions on field-testing of transgenic rice, Bt sprays were used to remove foliage-feeding lepidopteran larvae that would be targeted by Bt rice. Data from a 546-taxa Philippines-wide food web, matched abundance plots, species accumulation curves, time-series analysis, and ecostatistical tests for species richness and ranked abundance were used to compare different subsets of non-target herbivores, predators, and parasitoids in Bt sprayed and water-sprayed (control) plots. For whole communities of terrestrial predators and parasitoids, Bt sprays altered parasitoid richness in 3 of 3 sites and predator richness in 1 of 3 sites, as measured by rarefaction (in half of these cases, richness was greater in Bt plots), while Spearman tests on ranked abundances showed that correlations, although significantly positive between all treatment pairs, were stronger for predators than for parasitoids, suggesting that parasitoid complexes may have been more sensitive than predators to the effects of Bt sprays. Species accumulation curves and time-series analyses of population trends revealed no evidence that Bt sprays altered the overall buildup of predator or parasitoid communities or population trajectories of non-target herbivores (planthoppers and leafhoppers) nor was evidence found for bottom-up effects in total abundances of non-target species identified in the food web from the addition of spores in the Bt spray formulation. When the same methods were applied to natural enemies (predators and parasitoids) of foliage-feeding lepidopteran and non-lepidopteran (homopteran, hemipteran and dipteran) herbivores, significant differences between treatments were detected in 7 of 12 cases. However, no treatment differences were found in mean abundances of these natural enemies, either in time-series plots or in total (seasonal) abundance. Analysis of guild-level trajectories revealed population behavior and treatment differences that could not be predicted in whole-community studies of predators and parasitoids. A more conclusive test of the impact of Bt rice will require field experiments with transgenic plants, conducted in a range of Asian environments, and over multiple cropping seasons.