Environ. Biosafety Res.
Volume 2, Number 3, July-September 2003
|Page(s)||181 - 206|
|Published online||15 November 2003|
Effects of Bacillus thuringiensis on non-target herbivore and natural enemy assemblages in tropical irrigated rice
Department of Biological Sciences, California State University,
Stanislaus, Turlock CA 95382, USA
2 Entomology and Plant Pathology Division, International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines
3 Current address: Department of Biological Sciences, CW405 Biological Sciences Building, University of Alberta, Edmonton AB T6G 2E9, Canada
4 Current address: Biosystematics Unit, International Centre of Insect Physiology and Ecology, PO Box 3072, Nairobi, Kenya
5 Research Institute of Entomology and State Key Lab for Biological Control, Zhongshan University, Guangzhou 510275, P.R. China
6 Washington State University Cooperative Extension, King County, 919 SW Grady Way, Suite 120, Renton WA 98055-2945, USA
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.
Key words: Bacillus thuringiensis / food web / Oryza sativa / tropical irrigated rice / arthropod community dynamics / ecological statistics / biological control / non-target impacts
© ISBR, EDP Sciences, 2003