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.