In-field monitoring of beneficial insect populations in transgenic corn expressing a Bacillus thuringiensis toxin

In 1998 and 1999, field studies were conducted near Rosemount, MN to assess the potential impact of transgenic sweet corn, transformed to express the Cry1Ab toxin from Bacillus thuringiensis Berliner var kurstaki (i.e., Bt corn), on several beneficial insects, including predatory coccinellids, chrysopids and anthocorids. Beneficial insects in both Bt and in non-Bt sweet corn were also monitored in field cages in 1999. Plants were visually sampled for beneficial insects by arbitrarily selecting 3 consecutive plants from each plot or 6 plants/cage. Rank transformed data were analyzed using the Kruskal-Wallis test, which indicated no significant within-year differences in the overall density of beneficial insect populations between Bt and non-Bt sweet corn. Coleomegilla maculata (DeGeer) was the dominant predator species detected in 1998 and 1999. A significant trend (P < 0.05) was found for C. maculata larvae in open plots, with non-Bt treatments having higher C. maculata levels than Bt. Also, C. maculata larval and adult densities, for caged plots, showed a significant trend for higher counts in the in non-Bt corn. No additional differences in species diversity of beneficial insects were detected using Hills N1. Neither Hippodamia convergens Guérin-Ménville, Adalia bipunctata (L.), nor Coccinella septempunctata L. were observed during 1999. Although our test detected significant trends for higher densities of C. maculata in non-Bt corn, the results also suggest that longer-term in-field studies with higher sample sizes are needed to further characterize what may be relatively subtle population effects in the field.