Multi-year, large-scale comparison of arthropod communities in commercially managed Bt and non-Bt corn fields

Commercial scale propagation of Bt corn in the Philippines began in 2003. Since then, the country has become one of the leaders in the global adoption of transgenic agriculture. Bt corn expressing the transgenic insecticidal Cry1Ab protein successfully controls the Asian corn borer (ACB), Ostrinia furnacalis, the major insect pest of corn in the Philippines. One of the concerns in planting Bt corn is the adverse long term effect on non-target arthropods. The objective of the present study is to determine if Bt corn will significantly change species composition of arthropod communities. Principal response curve analysis, which is a multivariate method for the analysis of repeated measures, is designed to test and display treatment effects that change across time. Abundance data were log-transformed before analysis to reduce the effect of weights inflated because of highly abundant species. The deviation of principal response Cdt for Bt corn from control (i.e. non-Bt corn) on each sampling date was tested using the Monte-Carlo method (999 permutations) performed with Canoco 4.5. Species groups with species weights that contributed to the overall community response (from PRC) were further analyzed by comparing their distribution on Bt- and non-Bt corn throughout the season at all 3 sites. Visual counts from 100 plants per site per sampling date were pooled and analyzed by repeated measures ANOVA. PRC analysis did not detect a significant difference between Bt- and non-Bt corn arthropod communities. Only those species with weight 0.5 were included in the diagram. Species weights 0.5 are most likely to follow the abundance changes shown in the PRC plot whereas those less than -0.5 show a trend in the opposite direction. Species weights that are between -0.5 and 0.5 do not contribute strongly to the community response. Spiders and an unidentified hymenopteran wasp significantly contributed to predator and parasitoid abundance, respectively, throughout the 3-year study period. Other significant beneficial taxa observed in at least one cropping season were the red ant Solenopsis geminata, Chrysopidae, Trichogramma sp. and the coccinellid beetle, Microspis discolor. Repeated measure ANOVA detected significant increase in abundance in the spider population in Bt corn during the June-Sept 2008 cropping season. PRC analysis also predicted an increase in abundance of the corn planthopper Stenocranus pacificus when there is a negative arthropod community response to Bt corn. Taken together, the results of PRC analysis suggest that Bt corn does not alter species composition of arthropod communities.