| Abstract: | Helicoverpa zea is a target pest of Bt transgenics with resistance to the Cry proteins produced in these crops. Evolved resistance to Cry proteins may accelerate selection for resistance against Vip3Aa, considered the only effective toxin remaining in Bt pyramids to control H. zea. An experiment was conducted to collect information regarding factors influencing the rate of Bt resistance evolution. Those were the Bt resistance allele frequency, the dominance of Cry resistance, and redundant killing of pyramided Bt crops against H. zea. F2 screens were conducted from 2018-2020 in populations of H. zea from Texas and the U.S. Midsouth. A total of 128 neonates per F2 family were exposed to discriminating concentrations of Cry1Ac (10 µg/cm^2), Cry2Ab2 (10 µg/cm^2) and Vip3Aa39 (3 µg/cm^2) using diet over-lay bioassays. In Texas, 93.4, 35.0 and 1.75% of the F2 families survived the discriminating dose of Cry1Ac, Cry2Ab2, and Vip3Aa39 respectively. The resistance allele frequency ranged from 0.4150 to 0.4975 for Cry1Ac, from 0.1097 and 0.1228 for Cry2Ab2 and 0.0065 for Vip3Aa39. The percentage of surviving families in the Midsouth was 92.7% for Cry1Ac, 38.5% for Cry2Ab2, and 2.60% for Vip3Aa39. The resistance allele frequency was 0.722 for Cry1Ac, 0.217 for Cry2Ab2 and 0.0155 for Vip3Aa39. Two H. zea resistant strains were derived from the F2 screens. The first H. zea strain was resistant to Cry1 and resulted incompletely dominant on Cry1Ac+Cry1F cotton. Pyramided crops producing Cry2 and/or Vip3Aa proteins showed complete redundant killing of Cry1 resistant H. zea. The second strain was resistant to Cry1+Cry2 and displayed non-recessive resistance on Bt crops producing Cry1 and/or Cry2 proteins. The redundant killing of the Cry1+Cry2 resistant H. zea was complete on Bt crops producing Vip3Aa protein. Our results indicate that Cry1Ac and Cry2Ab2 resistance allele frequency is high. However, the Vip3A39 resistance allele frequency in H. zea is low within the sampled region. Non-recessive inheritance of multiple Cry resistance and high Cry resistance allele frequency may accelerate Vip3Aa resistance evolution in H. zea. The findings of this study may support the implementation of adaptive resistance management to preserve the susceptibility of H. zea to Vip3Aa technologies. The electronic version of this dissertation is accessible from https://hdl.handle.net/1969.1/198610 |
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