Impact of Various Bt Cotton Traits and the Application of an Insecticide on the Within Plant Distribution of Helicoverpa zea (Lepidoptera: Noctuidae) Larvae and Injured Floral Structures

Abstract

Previous studies have indicated that the expression of insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) in cotton can have a significant influence on the behavior of bollworm larvae (Helicoverpa zea, Lepidoptera:Noctuidae). This suggests that the particular Bt protein produced by a cotton variety may need to be considered when determining the most ideal scouting methods to utilize for bollworm. NonBt, WideStrike (producing Cry1Ac + Cry1F Bt insecticidal proteins), and Bollgard II (Cry1Ac + Cry2Ab) cotton varieties were planted and either treated with an insecticide or left untreated. The presence of H. zea larvae and their feeding injury were recorded according to their location in the canopy and type of floral structure where they were found. Results from comparison of larval and injury distributions indicated no significant differences between the different cotton varieties tested, and that insecticide treatment had minimal impact on this distribution. Larval size was generally associated with location in the canopy, suggesting that larvae tend to move towards the middle of the canopy as they age. The effect of different Bt cotton technologies appears to associate with how quickly larvae move to preferred feeding sites rather than their preference for particular feeding sites. These results suggest that scouting methods could be standardized independently of the presence of a Bt cotton trait or previous insecticide application. Focusing scouting efforts on the middle portion of the canopy (i.e., nodes 6-9) should increase the detection of small larvae and ‘fresh’ injury and be less influenced by previous insecticide applications.

A Modified F2 Screen for Estimating Cry1Ac and Cry2Ab Resistance Allele Frequencies in Helicoverpa zea (Lepidoptera: Noctuidae)

“…It is possible that using these high concentrations of Bt proteins resulted in the selection of major resistance alleles and the elimination of some minor resistance alleles, which may have underestimated the overall resistance allele frequency (Zhao et al 2002, Mahon et al 2010. Nevertheless, the results indicate that the resistance allele frequency for Cry1Ac and Cry2Ab in H. zea is high in the U.S. Midsouth, confirming previous results (Yang et al 2017(Yang et al , 2018b(Yang et al , 2022Kerns et al 2018Kerns et al , 2019Kerns et al , 2020 (Lin et al 2022, Yu et al 2022. The estimated resistance allele frequency of Cry1Ab in Lin et al (2022) andCry1A.105 in Yu et al (2022) is significantly lower than that of Cry1Ac documented in this study.…”

“…Therefore, the resistance allele frequency for Cry1Ac and Cry2Ab in H. zea in this study was estimated using the Bayesian statistics approach described in Andow et al (2000). Because the Cry resistance allele frequencies were expected to be high in the southern states (Yang et al 2017;Fleming et al 2018;Kerns et al 2018Kerns et al , 2019Kerns et al , 2020, we used formula #3 in Andow et al (2000) for calculating the corresponding resistance allele frequencies, which accounts for a high probability of detecting resistance alleles for either one or both parents carrying at least one resistance allele. Specifically, we calculated the expected resistance allele frequency in the parental male using the formula:…”