Dual-gene Bt cotton has reduced the need for insecticide treatments for bollworm, Helicoverpa zea (Boddie), compared with original single-gene Bt technology. Bollgard II¨ (Monsanto, St. Louis, MO) and WideStrike¨ (Dow AgroSciences, Indianapolis, IN), both produce the Cry1Ac protein and a second protein, Cry2Ab or Cry1F, respectively. These dual-gene Bt cottons provide enhanced control of lepidopteran pests, but remain less than 100% effective against bollworm, particularly when population pressure is high. Current recommended treatment thresholds for bollworm on cotton in South Carolina are as follows: treat with insecticides when three or more large larvae are found per 100 plants or when 5% boll damage is detected. Studies were conducted in an area prone to high bollworm pressure near Blackville, South Carolina, in 2010 and 2011 to develop appropriate thresholds in Bollgard II and WideStrike cotton. Plots containing non-Bt, WideStrike, and Bollgard II cotton varieties were examined weekly and treated according to treatment threshold protocols for one of the following: bollworm eggs, larvae in white blooms, or boll damage. Although yields increased with insecticide applications in non-Bt cotton, statistical differences in yield among thresholds were not evident within the Bt technologies. The conclusion drawn from this limited study was that insecticide applications exclusively targeting bollworm were not necessary in dual-gene Bt cotton. Higher levels of bollworm infestation and damage occurred in WideStrike cotton, however, WideStrike lint yields in this study did not differ among varying thresholds and so did not support the conclusion that protection strategies be amended for each technology
