Impact of Plant Phenology and Spray Timing on Thresholds for Helicoverpa Zea (Lepidoptera: Noctuidae) in Bt Cotton

Since its introduction in 1996, cotton expressing toxins from Bacillus thuringiensis (Bt) has effectively controlled a major target pest Heliothis virescens (Lepidoptera: Noctuidae). As a result, there was a significant reduction of foliar insecticide used for controlling this devastating pest in cotton. However, Bt cotton is only moderately toxic to Helicoverpa zea (Lepidoptera: Noctuidae), resulting in the need for occasional foliar oversprays to effectively control this pest. This experiment, replicated in GA, NC, and SC during 2017, was designed to look at the effect of H. zea economic foliar spray thresholds across Bt pyramids and plant phenology. Three different Bt cotton pyramids: 1) Cry1Ac + Cry2Ab, 2) Cry1Ac + Cry1F, and 3) Cry1Ab + Cry2Ae, and each of the three expressing Vip3A (pyramids 4-6), were planted in small experimental plots. Foliar insecticide treatments were sequentially triggered after cotton blooming at each week of bloom to both expose and protect different plant growth stages from H. zea injury. Foliar spray thresholds for different Bt cotton pyramids may be re-evaluated from these results.