Treatment Thresholds for Bollworm [Helicoverpa zea] in Second-Generation Bt Cotton

Dual-gene Bt cotton has reduced the need for supplemental insecticide treatments for bollworm, Helicoverpa zea, compared with original single-gene technology.  Bollgard II® (Monsanto) and WideStrike® (Dow Agrosciences) have the Cry1Ac gene in common but have a different combination of either Cry2Ab or Cry1F, respectively.  These second-generation technologies enhance control of lepidopteran pests but remain less than 100% effective against bollworm when population pressure is high.  Current threshold recommendations are based on those for single-Bt-gene technology, and there are no differences in the recommendations between Bollgard II and WideStrike. This research aims to explore the quantifiable differences between technologies and develop the best possible thresholds for each.  Test plots near Blackville, SC were planted in 2010 and 2011, containing non-Bt, WideStrike, and Bollgard II varieties. Each were scouted and treated weekly for one of the following: bollworm eggs, larvae in white blooms, or boll damage.  Thresholds in each of the three categories were selected, and tests plots were sprayed accordingly.  Based on inputs and yields, results from the 2010 season indicated that our currently recommended threshold of 5% boll damage might be adequate.  A more aggressive approach seemed to be appropriate in 2010, considering the heavy pressure experienced from bollworm.  Unusually good growing conditions late into the season during 2010 very likely allowed for significant yield compensation, masking some of the negative impact of bollworm.  The 2011 harvest has yet to take place. Bollworm pressure was much lower and seasonal conditions differed from 2010. Further conclusions await yield data.