Extended evaluation of Bt protein cross-pollination in seed blend plantings on survival, growth, and development of Helicoverpa zea feeding on refuge ears

BACKGROUND: A seed blend, also called refuge in a bag (RIB), is used as a strategy to provide refuge for planting pyramided Bt corn in the U.S. Corn Belt. A major concern for the RIB strategy is cross-pollination between Bt and refuge plants, which may reduce susceptible populations of ear-feeding insects like Helicoverpa zea and affect the evolution of resistance. Previous studies showed a 5:95% (Non-Bt: Bt) RIB would be unlikely to provide sufficient refuge populations of H. zea for resistance management. In this study, we extended our research and conducted multiple trials to determine if three RIB plantings with greater refuge sizes (10, 20, and 30% refuge) could provide sufficient H. zea susceptible populations to delay resistance development. RESULTS: Experimental results showed that cross-pollination in 10:90%, 20:80%, and 30:70% RIB plantings still significantly reduced larval survival, delayed larval development and decreased the pupal size of H. zea on refuge plants. Regression analysis showed that the percent reduction of neonate-to-adult survivorship, relative to the survivorship on pure non-Bt ears, was significantly negatively correlated with the refuge percentage. CONCLUSION: These findings suggest that, approximately a 15% non-Bt refuge plants in RIB plantings could produce a similar number of susceptible adult moths as a 5% structured refuge planting, while an approximately 30% non-Bt refuge plant in RIB plantings could provide a similar number of susceptible moths as a 20% structured refuge planting. Information generated from this study should be useful in refining resistance management strategies for Bt crop technologies.