Semester of Graduation

Fall 2021

Degree

Master of Science (MS)

Department

Department of Entomology

Document Type

Thesis

Abstract

Corn earworm, Helicoverpa zea (Boddie), is an important agricultural pest and a target species of Bt corn and Bt cotton. Recently, this pest in the U.S. has developed resistance to Cry1/Cry2 corn and cotton. The objectives of this study were to 1) estimate the frequencies of resistance allele to Cry1Ab and Vip3Aa20 proteins in Louisiana populations of H. zea; 2) determine if Cry1A.105/Cry2Ab2 single/dual-gene resistance in H. zea is associated with fitness costs; and 3) investigate the effect of three low concentrations of mixed Cry1A.105/Cry2Ab2 proteins on the growth, development, and reproduction of two known Bt-susceptible strains and six Cry1A.105/Cry2Ab2 single/dual-gene homozygous/heterozygous resistant strains of H. zea. Using a F2 screen of iso-female lines of H. zea derived from collections in non-Bt corn fields in Louisiana during 2019, a total of 64 resistance alleles to Cry1Ab were detected in the 55 iso-female lines screened. Thus, the expected resistance allele frequency to Cry1Ab in the Louisiana H. zea populations was estimated to be 0.293. In the screen against Vip3Aa20, no major resistance allele was found in the 57 lines screened, while 5 lines were considered to carry minor resistance alleles. The frequency of major resistance alleles to Vip3Aa20 was estimated to be ≤ 0.013, and the estimated minor resistance frequency to the Vip protein was 0.026. Results of the fitness study showed that the Cry1A.105/Cry2Ab2 single/dual gene resistance in H. zea was not associated with any observable fitness costs. On diet treated with the three low concentrations of mixed Cry1A.105 and Cry2Ab2, the two known susceptible H. zea strains could not complete their life cycle and produce no viable eggs, while all other insect strains containing resistance alleles completed the life cycle and produced viable offspring. With few exceptions, the effect of the mixed Bt proteins on the insect strains/genotypes carrying resistance alleles was significant only for the highest concentration assayed. Information generated from this study can be used to better understand the Bt resistance evolution in H. zea and to improve the current IRM programs to preserve the Vip3A susceptibility in H. zea.

Committee Chair

Fangneng Huang

DOI

10.31390/gradschool_theses.5431

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Entomology Commons

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