Occurrence, distribution, and ear damage of Helicoverpa zea (Lepidoptera: Noctuidae) in mixed plantings of non-Bt and Bt corn containing Genuity® SmartStax™ traits

Protein contamination on refuge kernels due to cross-pollination from Bt corn to non-Bt corn ears is a major concern in the use of a seed mixture refuge strategy (“RIB”) for resistance management of ear-feeding pests. In this study, occurrence, distribution, and ear damage of the corn earworm, Helicoverpa zea (Boddie), were evaluated in three planting patterns of non-Bt and Bt corn plants containing Genuity® SmartStax™ traits. The three planting patterns were 1) pure stands of 27 Bt plants; 2) pure stands of 27 non-Bt plants; and 3) one non-Bt plant in the center surrounded by 26 Bt plants. A total of six trials were conducted in open field conditions with natural infestations in 2011 and 2012. Egg populations of H. zea were distributed randomly or uniformly, and the number of eggs laid was similar between Bt and non-Bt corn ears regardless of the planting patterns, suggesting that females of H. zea have no egg-laying preference between Bt and non-Bt plants. Bt corn plants containing Genuity® SmartStax™ traits were equally effective in the control of H. zea in pure stands of Bt corn and “RIB” plantings. Occurrence of larvae and ear damage on Bt corn were significantly lower than on non-Bt plants and there were no significant differences between pure stands of Bt and “RIB” plantings across all trials. However, the limited numbers of live larvae in the pure stands of Bt plants were distributed non-randomly, suggesting a possibility of uneven expression of Bt proteins or elevated larval movement in the pure stands of Bt plants. Larval occurrence (3rd-5th instars) and ear damage on the refuge ears in “RIB” plantings were similar to or greater than found on ears of pure stands of non-Bt plants. However, more studies are needed to understand the effect of pollen movement on the full life cycle of H. zea before a final conclusion on the refuge function of RIB planting can be made.