Semester of Graduation



Master of Science (MS)



Document Type



The corn earworm (CEW), Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), is a major target pest of pyramided Bt corn and Bt cotton in the U.S. In 2016 and 2017, notable corn ear damage and larval survival of CEW were observed on pyramided Cry1A.105/Cry2Ab2 corn in some fields in northeast Louisiana. The objectives of this study were 1) to determine if the ear damage and larval survival observed in the area were due to resistance development to the Bt proteins in the plants, and 2) if resistance had occurred, to determine the approximate distributions of the resistance in the southern region of the U.S. To accomplish the proposed objectives, 12 populations of CEW were collected from Bt and non-Bt corn plants in multiple locations in Louisiana, Georgia, and Florida. Diet-overlay bioassays were conducted to examine the susceptibility of the progeny produced from the field-collected populations to Cry1A.105 and Cry2Ab2. Results of the bioassays showed that the median lethal concentrations (LC50s) of Cry1A.105 and Cry2Ab2 for the populations collected from the areas with control problem occurrence were as much as >909-fold and >25-fold greater than that of a known Bt-susceptible strain, respectively. The results documented that the observed field control problems of Cry1A.105/Cry2Ab2 corn in northeast Louisiana was due to resistance development of the insect to the Bt proteins in the plants. This is the first documentation of field resistance to Bt corn in any target insect species in the U.S. mid-south region. However, susceptibility levels to Cry1A.105 and Cry2Ab2 varied greatly among the CEW populations collected from the three states, suggesting a mosaic distribution of the resistance in the region. Several factors could have contributed to the rapid development of the resistance to Cry1A.105/Cry2Ab2 corn plants in the insect. The documentation of the field resistance to Cry1A.105/Cry2Ab2 corn in CEW should have important implication for development of effective resistance management strategies for the sustainable use of Bt crop technology in the region.



Committee Chair

Huang, Fangneng