Date of Award
Master of Science
Entomology and Plant Pathology
Scott D. Stewart
Tyson Raper, William Moar, Juan Luis Jurat-Fuentes
Field trials were conducted in 2018 at the West Tennessee Research and Education Center (WTREC) in Jackson, TN and in 2019 at locations in College Station, TX, Tillar, AR, and Jackson, TN. Non-Bt, Cry1Ac + Cry1F, and Cry1Ac + Cry2Ab cotton varieties were either treated with an insecticide or left untreated. After five days, cotton plants were mapped for signs of bollworm, Helicoverpa zea (Lepidoptera: Noctuidae), feeding on floral structures (i.e., bolls, squares, flowers) and the physical presence of larvae. Bt technologies reduced the number of H. zea larvae and the amount of feeding injury, but no major differences in the pattern of feeding injury and distribution of larvae were found among the different cotton varieties. Most larvae and damage were found in the middle portion of the canopy. H. zea feeding appeared to occur slightly lower in the canopy of cotton treated with a pyrethroid when compared with untreated cotton. Results suggest that a standardized scouting methodology for H. zea infestations in cotton could be developed, regardless of if or what Bt technologies were used. Floral structures from the middle portion of the canopy appeared most indicative of H. zea infestation levels.
Laboratory experiments were done to evaluate Bt resistance monitoring techniques using purified proteins or various cotton plant tissues. Leaves, bolls, squares, white flowers, and pink flowers were collected from non-Bt cotton or cotton varieties expressing Cry1Ac + Cry2Ab, or Cry1Ac + Cry1F + Vip3A. Collected plant structures were lyophilized and ground into fine powders. Diet-overlay assays using purified proteins (Cry1Ac, Cry2Aa, and Vip3Aa39) and cotton plant tissues were conducted on a Bt-susceptible strain of H. zea and a strain known to be resistant to Cry1Ac, Cry1F, and Cry2Ab. The resistant strain was over 95.2% and 54.5% less sensitive to Cry1Ac and Cry2Aa compared with the susceptible strain, respectively. However, the resistant strain was slightly more susceptible to Vip3Aa39 than the susceptible strain. Boll and leaf tissue from non-Bt cotton severely stunted larval growth, suggesting that these tissues may not be ideal for assessing bollworm Bt resistance. Plant tissue from white flower was best able to detect the differences in susceptibility between the susceptible and resistant strain of H. zea. Assays using plant tissues in conjunction with Bt protein diet-overlay assays would likely provide a better indication of how a bollworm population with resistance to multiple Bt proteins may perform under field conditions.
Kerns, Dawson David, "Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) Larval Distribution on Different Bt Technologies and Evaluating Cotton Plant Tissue Assays for Resistance Monitoring. " Master's Thesis, University of Tennessee, 2020.