Supplemental Control with Diamides for Heliothines1 in Bt Cotton

Supplemental control with diamides for bollworm, Helicoverpa zea (Boddie), in Bt cotton is becoming more frequent, but there is little information on net returns to growers. The value of spraying Bt cotton with commercially available diamides was evaluated at seven locations in the Mississippi Delta cropping region in 2014 and 2015. Plots of Bollgard II®, Widestrike 3®, and non-Bt cotton varieties planted at each location were divided into five subplots. Treatments randomly assigned to subplots of each cotton cultivar were: 1) lowest labeled rate of chlorantraniliprole applied at threshold, 2) lowest labeled rate of flubendiamide applied at threshold, 3) lowest labeled rate of λ-cyhalothrin + chlorantraniliprole applied at threshold, 4) sprayed check, which was the maximum labeled rate of chlorantraniliprole applied on a 3-week schedule and initiated before first bloom to minimize damage from heliothines, and 5) nontreated check. Each plot was monitored weekly for larval abundance and plant damage by examining 100 plants at random. Supplemental control applications were initiated when larval abundance (≥4 per 100 plants) and plant damage (>2% damaged bolls with larvae present) met economic thresholds outlined by the Mississippi State University Extension Service. Following first bloom, all non-target pests were controlled with broadcast sprays of insecticide with low or no known toxicity to heliothines. No differences were detected in yield among treatments for any given Bt cotton variety. In non-Bt cotton, all threshold treatments and the sprayed check yielded significantly more lint cotton per hectare than did nontreated plots. Mean larval numbers per 100 plants ranged from 0.75–5.95 in non-Bt cotton to 0.04–0.8 and 0.44–1.17 in Bollgard II and Widestrike 3 cottons, respectively. Non-Bt cotton required approximately one additional diamide application for heliothine control as compared to Bt cotton plots. Results indicated no economic benefit to supplemental spray of diamides for bollworms in Bt cotton at larval abundance observed during the study. Bt cotton alone seemed sufficient to control bollworms. This should provide confidence for producers to avoid overspray at near threshold densities of bollworms.