Semester of Graduation

Summer

Degree

Master of Science (MS)

Department

Entomology

Document Type

Thesis

Abstract

The fall armyworm (Spodoptera frugiperda) and corn earworm (Helicoverpa zea) are two polyphagous pests that, in the Americas, are targeted by transgenic corn and cotton expressing Bacillus thuringiensis (Bt) proteins. A previous study during 2011-2013 demonstrated practical resistance to Cry1F corn in S. frugiperda in the southeastern seaboard of the U.S., while resistance of H. zea to Bt corn and cotton containing cry1A and cry2A genes has widely occurred in the U.S. The first objective of this study was to investigate the current susceptibility status to four Bt proteins commonly expressed in Bt corn plants, including Cry1F, Cry1A.105, Cry2Ab2, and Vip3A, in S. frugiperda populations from the southern region of the U.S. To accomplish this objective, a total of 23 S. frugiperda populations were collected during 2021 to 2022 from seven U.S. southern states, including 9 populations from the southeastern coastal region. Diet-overlay bioassays showed that one of the 23 populations exhibited a < 2-fold greater LC50 of Cry1F than a reference susceptible strain, while all other 22 populations were equally (8 populations) or more susceptible (14) to the Cry1F protein. Susceptibility ratios (LC50 of field population/LC50 of the susceptible strain) of the 23 field populations ranged from 50 of >10-fold greater than the susceptible strainto Cry2Ab2. Leaf tissue bioassays also showed that all S. frugiperda populations were highly susceptible to Bt corn hybrids/lines containing cry1F, cry1A.105, cry2Ab2, or cry1Ab/vip3A transgenes with an overall 7-day mortality of 95.8% for Cry1F corn, 98.7% for Cry1A.105, 99.8% for Cry2Ab2, and 100% on Cry1Ab/Vip3Aa plants. Overall, these findings do not provide evidence of practical resistance in S. frugiperda to any of the tested Bt proteins or Bt corn hybrids/lines. The detected Cry1F susceptibility in S. frugiperda could be the first case of documented practical resistance to a Bt protein becoming more susceptible and thus has important implications for IRM. The second objective of this study was to investigate larval survival and development of a Cry1A.105/Cry2Ab2 dual-protein resistant (VT2P-RR), a susceptible, and an F1 heterozygous (VT2P-RS) population of H. zea on nine Bt and three non-Bt corn hybrids. The Bt corn hybrids evaluated represent five common pyramided traits expressing two or three of the Cry1A.105, Cry1Ab, Cry1F, Cry2Ab2, and Vip3Aa20 proteins. In the laboratory, neonates of the three insect genotypes were placed on the silks of corn ears collected from fields at R1-R2 plant stages. The larvae of all three H. zea genotypes survived well on non-Bt corn hybrids. A varied number of VT2P-RR and VT2P-RS larvae also survived on ears of hybrids expressing Cry proteins without Vip3A. The dual-protein resistance on ears of Cry protein corn was incomplete, and the VT2P-RR was highly susceptible to Bt corn hybrids containing vip3A gene. Effective dominance levels of VT2P-RR on Cry protein corn varied from recessive to incompletely dominant. Data generated from the study should be useful in assessing the risk of resistance evolution to Vip3A and refining IRM modeling for the sustainable use of the Bt crop technology as a pest management tool.

Date

7-9-2023

Committee Chair

Huang, Fangneng

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Entomology Commons

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