NC State Extension has two thresholds for bollworm, depending on the type of cotton planted:
Bollgard II, TwinLink, WideStrike:
25 total eggs on 100 leaves or fruiting structures (search throughout the canopy on multiple plants)
Bollgard 3, TwinLink Plus, WideStrike 3:
3 live second-stage bollworms (1/8 inch or longer) per 100 fruit (pay particular attention to bollworms in or under yellow, pink, or dried blooms stuck to young bolls),
2 second-stage bollworms on 2 consecutive scouting trips,
1 second-stage bollworm on 3 consecutive scouting trips
We are adding an additional threshold for Bollgard 3, TwinLink Plus, WideStrike 3 of 4% damaged bolls.
Why are we adding this threshold? Recent research in North Carolina found that yield losses were more closely aligned with damaged squares and bolls than number of live larvae. This difference in feeding behavior is an important consideration for Bollgard 3, TwinLink Plus, and WideStrike 3 cotton in NC. In addition, a similar damaged boll threshold for bollworm is used throughout much of the Cotton Belt, however, at a higher level (6% damaged bolls).
One difference for this damage threshold difference could result from our shorter season compared to the Midsouth (less time to compensate due to shorter growing season). In our research, yield response was variable, with results ranging from 2 to 6% damaged bolls as the economic injury level (economic thresholds are set below this point). An economic threshold of 4% captures the variability in this research and is a good point for growers to determine if a spray is needed in Bollgard 3, TwinLink Plus, WideStrike 3 varieties.
We don’t want to jettison the live second-stage live bollworm threshold at this point. Our research also verified the utility of these thresholds, but found that percent damaged bolls was a more consistent threshold connected to yield. At this time, we can use these two thresholds in tandem.
One word about the egg threshold on Bollgard II, TwinLink, WideStrike varieties. Our research found that the egg threshold was not well-correlated with yield. Specifically, we observed situations where egg numbers were below threshold, but yield was impacted at the end of the season. In these situations, we could have justified a spray.
Please use this threshold as a guide to time when sprays should be initiated. It is much better to be early with a spray than late. If a grower has 10 eggs per plant on week one of scouting and 10 eggs per plant on week two of scouting, it might pay to spray.
Because bollworm resistance is variable across the state and bollworm survival is highly dependent on environmental conditions, a one size-fits-all egg threshold isn’t applicable. As with many threshold decisions, context is very important. For example, growth stage, potential for compensation during the fall, resistance levels in the bollworm, etc. can influence the impact of bollworms on yield.