Transgenic cotton, Gossypium hirsutum L. (Malvales: Malvaceae), that produces two Bacillus thuringiensis Berliner (Bt) (Bacillales: Bacillaceae) toxins has reduced the need for insecticide treatments for bollworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), compared with single-gene Bt traits. Field trials were conducted in an area prone to high pressure from H. zea in South Carolina in 2010 and 2011 to develop action thresholds for this species in dual-gene Bt Bollgard II and WideStrike cotton. Plots containing non-Bt, WideStrike, and Bollgard II cotton varieties were examined weekly and treated according to threshold protocols for one of the following: bollworm eggs, larvae in white blooms, or boll damage. Although insecticide applications targeting H. zea increased yield in non-Bt cotton, differences in yield among the thresholds evaluated were not statistically evident when insecticides were applied within the sets of Bt traits. Insecticide applications exclusively targeting H. zea were not necessary in dual-gene Bt cotton. More H. zea larvae and damage occurred in WideStrike cotton compared with Bollgard II; however, lint yields for dual-gene Bt cotton did not differ among thresholds and did not support implementing protection strategies unique to each set of traits.
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