ABSTRACT

A delay in Bt resistance evolution in the cotton bollworm (Helicoverpa zea) could be enhanced if a fitness cost were associated with those individuals that have resistance alleles. Fitness comparisons were made over two generations measuring the responses between a bollworm strain found on Bollgard cotton and one found on non-Bt cotton plant, BG and NBT respectively. Measurements of larval growth and mortality were made for both strains after exposure to Cry1Ac and technical grade cypermethrin. Our expectation would be a decrease in survival after pyrethroid exposure and a decrease in growth on the non-toxin diet in the BG strain if a there were fitness cost due to Bt resistance. We did not find a statistically significant difference between strains in our pyrethroid evaluations, although there was a difference in LD₅₀ values for the F1 and F2 generation. Our Cry1Ac evaluations showed BG survival was higher than NBT larvae in the F1 generation, but not for the F2 generation. In contrast to our expectation, the BG strain larvae grew larger than the NBT for the F1 generation in the non-toxin control diet. Similar results have been reported in the past and are discussed within a context of environmental and maternal effects. Future experiments with an H.zea strain that has substantial Cry1Ac resistance should improve understanding of whether differences between strains are due to genetic or environmental causes.