Bollworm Management in Bt Cotton: Early July Update

We have been getting reports of bollworm moth activity and low levels of eggs for about 10 days now in cotton. This week seems to be the week when the activity has really picked up and there are a lot of sprays going out for bollworm right now. We recorded a podcast yesterday (July 11) talking about bollworm management in Bt cotton that you can listen to here:

Bollworm Management in Bt Cotton

Some of the highlights include:

• Know what type of cotton you are dealing with. The 3-gene cottons (Bollgard 3, Widestrike 3, and TwinLink Plus) are not immune to bollworm but should provide much better control than 2-gene cottons (Bollgard II, Widestrike, and TwinLink).
• The 2-gene cottons have Bt Cry proteins and bollworms in the Mid-South have shown some resistance to those. The 3-gene cottons have the same 2 Bt Cry proteins, but also have the Bt Vip protein. No resistance has been shown for the Vip protein.
• The threshold for 2-gene cotton is 20% eggs anywhere on plants. Plant terminals can be scouted for eggs, but the threshold for eggs in the terminal should be lowered to around 10% if you are only looking at terminals.
• The threshold for 3-gene cotton is 4% larvae (>1/8 inch) or 6% damaged fruit (squares and bolls)
• Acephate plus a pyrethroid is not recommended for control of bollworm anymore
• Prevathon, Besiege, or Intrepid Edge are the only three products available, but may buy a little time on subthreshold bollworms if a plant bug spray is needed
• In a perfect world where cost is not an issue, 20 oz of Prevathon or 10 oz of Besiege will provide the best control. Lower rates can provide acceptable control if they are applied in a timely manner which is why we moved to an egg threshold in 2-gene cotton.
• Finally, scouting and spraying on thresholds is critical and more than one spray may be needed in a lot of situations.