Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) Larval Distribution on Different Bt Technologies and Evaluating Cotton Plant Tissue Assays for Resistance Monitoring

Field trials were conducted in 2018 and 2019 to determine if an insecticide treatment and different Bt (Bacillus thuringiensis) cotton technologies had an effect on bollworm, Helicoverpa zea (Lepidoptera: Noctuidae), distribution in the cotton canopy. Non-Bt, Cry1Ac + Cry1F, and Cry1Ac + Cry2Ab cotton varieties were either treated with an insecticide or left untreated after a bollworm, infestation was detected. Cotton plants were mapped for signs of bollworm feeding on floral structures (i.e., bolls, squares, flowers) and the physical presence of larvae. 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 lyophilized 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 and a Cry1Ac, Cry1F, and Cry2Ab-resistant strain of H. zea. The resistant strain was over 95-fold and 54-fold less sensitive to Cry1Ac and Cry2Aa, respectively, compared with the susceptible strain. However, the resistant strain was at least 5-fold more susceptible to Vip3Aa39 than the susceptible strain. Lyophilized 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. Lyophilized plant tissue from white flowers was best able to detect the differences in susceptibility between the susceptible and resistant strain of H. zea.