Field-evolved resistance of Helicoverpa zea (Boddie) to transgenic maize expressing pyramided Cry1A.105/Cry2Ab2 proteins in northeast Louisiana, the United States

Authors: KAUR, GAGANDEEP - Louisiana State University Agcenter; GUO, JIANGUO - Institute Of Plant Protection - China; BROWN, SEBE - Louisiana State University Agcenter; HEAD, GRAHAM - Monsanto Company; PRICE, PAULA - Monsanto Company; PAULA-MORAES, SILVANA - Southwest Florida Research And Education Center; Ni, Xinzhi; DIMASE, MARCELO - Louisiana State University Agcenter; HUANG, FANGNENG - Louisiana State University Agcenter

Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2019
Publication Date: 2/27/2019
Citation: Kaur, G., Guo, J., Brown, S., Head, G.P., Price, P.A., Paula-Moraes, S., Ni, X., Dimase, M., Huang, F. 2019. Field-evolved resistance of Helicoverpa zea (Boddie) to transgenic maize expressing pyramided Cry1A.105/Cry2Ab2 proteins in northeast Louisiana, the United States. Journal of Invertebrate Pathology. 163:1-20.

Interpretive Summary: The corn earworm, also known as cotton bollworm, is a major target pest of pyramided transgenic Bt maize and cotton in the U.S. In 2016 and 2017, notable maize ear damage and larval survival of the corn earworm were observed on the pyramided transgenic Bt maize in some fields in northeast Louisiana. 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 the distribution patterns of Bt resistance in natural corn earworm populations in the southern U.S. states, 12 populations of the corn earworm were collected from Bt transgenic and non-transgenic maize plants in multiple locations of three states (that is, Louisiana, Georgia, and Florida). Diet-overlay bioassays were performed to assess the susceptibility to two Bt toxins of the progeny produced from the field-collected corn earworm populations. Results of the bioassays showed that the lethal concentrations required to kill 50% of insects (also known as LC50s) of the two Bt toxins for the field-collected corn earworm populations in the region were 25 to 909 fold greater than that of a known susceptible strain of the corn earworm. This is the first documentation of field resistance to Bt maize in any target insect species in the southern U.S. states. However, susceptibility levels to the two Bt toxins varied greatly among the 12 corn earworm populations collected from the three states, indicating a mosaic distribution pattern throughout the region existed. The findings from this study have significant ramifications for developing effective Bt resistance management strategies for the sustainable utilization of transgenic crop technology in the southern states.

Technical Abstract: The corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), is a major target pest of pyramided Bt maize and Bt cotton in the U.S. In 2016 and 2017, notable maize ear damage and larval survival of H. zea were observed in some fields of maize hybrids with pyramided Bt toxins Cry1A.105/Cry2Ab2 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 H. zea were collected from Bt and non-Bt maize 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 LC50s of Cry1A.105 and Cry2Ab2 for the populations collected from the areas with control problem were as much as >909- and >25-fold greater than that of a known susceptible strain, respectively. The results documented that the increase of damage in the fields of Cry1A.105/Cry2Ab2 maize 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 maize in any target insect species in the U.S. mid-south region. However, susceptibility levels to Cry1A.105 and Cry2Ab2 varied greatly among the H. zea 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 maize plants in the insect. The documentation of the field resistance to Cry1A.105/Cry2Ab2 maize in H. zea should have important implication for development of effective resistance management strategies for the sustainable use of Bt transgenic crop technology in the region.