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 cor n), on several beneficial insects, including predatory coccinellids, chryso pids and anthocorids. Beneficial insects in both St and in non-Bt sweet cor n were also monitored in field cages in 1999. Plants were visually sampled for beneficial insects by arbitrarily selecting 3 consecutive plants from e ach plot or 6 plants/cage. Rank transformed data were analyzed using the Kr uskal-Wallis test. which indicated no significant within-year differences i n the overall density of beneficial insect populations between Bt and non-B t sweet corn. Coleomegilla maculata (DeGeer) was the dominant predator spec ies 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 Guerin-Menville , Adalia bipunctata (L.), nor Coccinella septempunctata L. were observed du ring 1999. Although our test detected significant trends for higher densiti es 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 characteri ze what may be relatively subtle population effects in the field.