Gilreath, Ryan Tyler (2019-11). Determining the Susceptibility and Cross-Resistance in a VIP3A Resistant Strain of Fall Armyworm Spodoptera frugiperda, to Purified Bt Proteins, Bt Corn, and Bt Cotton. Master's Thesis.
Thesis
The fall armyworm, Spodoptera frugiperda is one of the major target pests of Bt corn and cotton in the U.S. Current control strategies for FAW rely heavily on transgenic plants. Negative side effects of extensive use have resulted in field-evolved resistance. Gene pyramiding has been used to delay these resistance issues; however, the durability of this technique can be greatly reduced by cross-resistance. In this study, we investigated the susceptibility and cross-resistance of different genotypes of fall armyworm carrying Vip3A resistant alleles to purified Bt proteins, Bt corn and Bt cotton. Purified Bt protein assays, utilized to determine cross-resistance to other proteins, indicate that the resistant (RR) larvae tested 39.5 fold more resistant to the Vip3Aa51 toxin when compared to the susceptible (SS) strain, and tested highly susceptible to all other Bt proteins. SS and the heterozygote (RS) larvae were highly susceptible to all proteins. To confirm the susceptibility found in the protein bioassay, and to determine how the genotypes behave on Bt plants, corn leaves and whole plant corn bioassays were used. This bioassay found that RR survive well on non-Bt and Vip3111 (Vip3a, Cry1Ab) corn. RR had moderate survivorship on Herculex (Cry1F) corn, however survivorship diminished on all other technologies. These data suggest that RR may have some low level resistance to Cry1F. SS showed high survivorship on non-Bt but no survivorship on any other technology. Cotton leaf and square bioassays were utilized to determine the cross-crop resistance of Vip3A resistant FAW. During the leaf bioassay RR, RS, and SS genotypes had low survivorship across all varieties. However, in the square bioassay RR showed high survivorship across Bollgard III (Cry1Ac, Cry2Ab, Vip3A), Bollgard II (Cry1Ac, Cry2Ab), Widestrike (Cry1F, Cry1Ac), Widestrike 3 (Cry1F, Cry1Ac, Vip3A), regardless of technology. RS and SS showed similar results with high survivorship on non-Bt, Bollgard II (Cry1Ac, Cry2Ab), and Widestrike (Cry1F, Cry1Ac), however very little or no survivorship on Bollgard III (Cry1Ac, Cry2Ab, Vip3A), and Widestrike 3 (Cry1F, Cry1Ac, Vip3A). Results generated from these studies provided important information for insect pest management and aid in developing effective resistance management strategies for the sustainable use of Vip3A technology.
