Expanded use of pyramided transgenic maize hybrids expressing novel bacillus thuringiensis toxins in the southern U.S. Potential for areawide suppression of Helicoverpa zea (Lepidoptera: Noctuidae) in the Mississippi Delta

The corn earworm or cotton bollworm, Helicoverpa zea (Boddie), is an economically important pest throughout much of North America, but particularly in the southern U.S., where many crops are vulnerable to damage. Although transgenic cotton, Gossypium hirsutum L., expressing selected insecticidal toxins of Bacillus thuringiensis Berliner (Bt) has been available since 1996 in the southern states, the Cry1Ac toxin in cotton and Cry1Ab in maize, Zea meys L., are not known to cause consistent mortality in H. zea. Consequently, long-term areawide suppression would not be expected. However, with the advent of pyramided Bt events for cotton and field corn, and in the absence of the evolution of field-level resistance to Bt toxins, the potential exists for these crops to suppress H. zea. Simulation modeling predicted the initial decrease in abundance to be rapid, illustrating the impact of an immediate adoption of the two Bt crops, with high efficacy. Abundance of H. zea remained low and approximated an 85% reduction from baseline. This decrease or areawide suppression effect was proportional to the 80 and 90% Bt use rates for maize and cotton, respectively. The residual population that remains is largely the result of the net influx of moths via annual migration from outside Mississippi. These results are discussed within the context of resistance management and alternative migration and crop composition scenarios.