Doctor of Philosophy (PhD)
The corn earworm, Helicoverpa zea (Boddie), is a target pest of Bacillus thuringiensis (Bt) maize and cotton in the U.S. Previous studies have reported that this pest has developed field resistance to pyramided Cry1A/Cry2A maize and cotton in certain areas of the southern United States. The objectives of this study were to 1) determine the current status and distribution of the resistance to Cry1A.105 and Cry2Ab2 in H. zea in the southeastern United States; 2) estimate the resistance allele frequencies to the two Bt proteins in H. zea field populations in the region, and 3) assess the inheritance of the Cry1A.105 and Cry2Ab2 resistance in the insect. Diet over-lay bioassays with 31 field populations collected from seven U.S. southeastern states during 2018 and 2019 showed that Cry1A.105 and Cry2Ab2 resistance in H. zea is widely distributed in the regions. The resistance to Cry1A.105 appeared to have plateaued, while selection for Cry2Ab2 resistance is likely still occurring. F2 screens with 103 isolines established using mass-mating of H. zea populations collected from the region in 2018 and 2019 exhibited that the resistance allele frequencies in H. zea populations were high for both Bt proteins, averaging 0.405 to Cry1A.105 and 0.330 to Cry2Ab2. Various genetic crosses and backcrosses revealed that the Cry1A.105 resistance in H. zea was inherited as a single, autosomal, and non-recessive gene. Meanwhile, the Cry2Ab2 resistance in the insect was more likely to be autosomal, non-recessive, and polygenic. The information generated from this study is helpful in resistance risk assessment, refining resistance modeling, and improving IRM programs to mitigate the great challenge of the Cry protein resistance in H. zea for the sustainable use of the Bt crop technology in the region.
Yu, Wenbo, "Geographical Distribution, Allele Frequency, and Inheritance of CRY1A.105 and CRY2AB2 Resistance in Helicoverpa zea in the Southeastern Region of the United States" (2022). LSU Doctoral Dissertations. 5889.
Available for download on Tuesday, June 27, 2023