The impact of Cry3Bb Bt-maize on two guilds of beneficial beetles

In a 3-year comparison of non-Bt untreated control maize, maize treated with the soil insecticide tefluthrin, and transgenic Cry3Bb Bt-maize (Bt) plots, average carabid catch per pitfall trap did not vary significantly except in 2001 when average catch in the insecticide treatment was lower than the average catches in the Bt and control treatment. Analyzing total number of carabids captured, in each individual year except 2003 and for all years summed, control plots exceeded Bt plots, and Bt plots exceeded insecticide plots in 2001 and all years summed. Fewer, Harmonia axyridis, a common coccinellid, were observed in Bt plots than in control maize plots. In laboratory assays, prey consumption for the most common carabid species, Agonum muelleri and Poecilus lucublandus, did not differ between treatments. The life span of H. axyridis fed on field-collected aphids from Bt was reduced by 38% versus those fed aphids from control maize. ELISA indicated Bt proteins were passed from the plant to the predator via the aphid. This is the first reported negative impact of Cry3Bb Bt-maize on carabid activity-densities in the field; and one of the first mechanistic examples of a negative indirect tritrophic level impact of a Cry3Bb Bt-maize on a coccinellid.

► Carabids were affected by insecticide and Bt maize used for corn rootworm control.
► Prey consumption by common carabids was not reduced by Bt-fed prey.
► Survival of the common carabid, Poecilus lucublandus, fed Bt-fed prey, was reduced.
► The coccinellid, Harmonia axyridis, had lower counts in Bt than in control maize.
► The H. axyridis lifespan was reduced by 38% by feeding on aphids from Bt maize plots.