Moth traps and field reports indicate the beginnings of the bollworm moth flight that could affect both cotton and soybean. Below are my suggestions on deciding if and when to make an insecticide application, and with which insecticides.
- Don’t jump the gun! Scouting is step one. This is even more important because the bollworm has developed some level of resistance to some Bt toxins and to pyrethroid insecticides. It will also be more important to scout for eggs than in previous years.
- Know your Bt technologies. It is far more likely that treatment for bollworm infestations will be needed in WideStrike, TwinLink, and Bollgard 2 (and of course non-Bt) varieties … compared with WideStrike 3, TwinLink Plus, and Bollgard 3 where treatment is relatively unlikely.
- After first bloom, treat when 4 or more ‘surviving’ larvae are found per 100 plants. An alternative threshold is when 6% or more of ‘fruit” are damaged; for example, if you found 4 damaged squares and 2 damaged bolls in a sample of 50 squares and 50 bolls. This damage threshold has worked well for making treatment decisions in non-Bt and Bt technologies. However, I’d treat anytime boll damage exceeded 3% and larvae are still present.
- Treatment should be considered if 20-30% or more of plants are infested with eggs but only for WideStrike, TwinLink, and Bollgard 2 varieties. I do not recommend treating based on eggs for WideStrike 3, TwinLink Plus, or Bollgard 3 varieties.
- As I’ve mentioned previously, I strongly suggest using diamide insecticides, specifically Prevathon or Besiege. Minimum rates are 14 oz/acre and 7 oz/ace, respectively. However, you’ll get better control if you bump the rates. My personal minimum is 16 oz or Prevathon or 8 oz of Besiege, but consider even higher rates in later maturing fields to increase residual control.
- Scouting is important but easier than in cotton. Use your sweep net! Serious infestations in soybean often occur in flowering but late maturing fields (especially those with an open canopy).
- Make treatments when larval counts exceed the economic threshold (see the table below).
- There are a number of good insecticide options in soybean, and residual control is typically not as important as in cotton. You can see UT’s recommended insecticides for corn earworm (bollworm) in soybean at http://guide.utcrops.com/soybean/soybean-insect-guide/corn-earworm/. Some “go to” products on my list included Besiege, Blackhawk, Intrepid Edge, Prevathon, and Steward.
- Heligen, a virus, has worked well to control corn earworms in soybean when used correctly, and it is an economical option. However, this product should be used more proactively than the other insecticides, primarily when larvae are small. I suggest using this product in high risk soybean fields when small larvae are being found. This is not a good option for fields with corn earworm populations well over the treatment threshold, and especially if there is a mix of small and large larvae.