Do’s and don’ts of treating soybean insects in September

• By Dominic Reisig •

corn earworm in soybeans
Corn earworm in soybeans — photo courtesy North Carolina State University

Do: Scout your crop. Identification is the first step so you know what you’re dealing with. The second step is counting what you have. Using these two things, you can apply Extension-recommended thresholds and save money. Use a sweep net for beans in 30 inch rows or less and a drop cloth for wide-row beans.

Do: Assess your growth stage and canopy. Thresholds are set for the average situation. Your beans are not average! We can relax thresholds, depending on the pest, as beans become more mature. A full canopy can tolerate much more defoliation than the thresholds that are set. It’s important to adjust thresholds for each unique situation.

Do: Choose the correct insecticide. Don’t be tempted to add something in the tank for pests that are below threshold. If you so, you might cause problems for yourself later on in the season (see the don’t section). For earworms, choose Blackhawk, Intrepid Edge or Steward (see previous article for reasons not to use Besiege or Prevathon for earworm).

If you have stink bugs above threshold, choose a pyrethroid. Note that pyrethroids sprayed now will open you up to armyworms, bean leaf beetle, and loopers. So only use them if you need to.

Don’t: Spray pests that are below threshold levels. ALL of our thresholds are conservative. That is to say, we tell growers to spray before they experience an economic loss to provide some buffer. Furthermore, many growers may be tempted to add something broad-spectrum in the tank (pyrethroids — bifenthin, for example — or Orthene) to clean up the field.

Doing so will clean up the field, including beneficial insects. See this article from last year, where we sprayed loopers with Prevathon and bifenthrin. Two weeks after treatment, looper numbers were actually higher in all plots where we used bifenthin! Remember the old adage…no such thing as a free lunch.

Don’t: Spray just because your neighbor did so or just because you found pests in a single field. Soybean pest pressure is extremely variable in each field due to differences in planting date and varieties. 2019 has been especially problematic since pest pressure has been so spotty. Treat each field as a unique situation.

Dr. Dominic Reisig is an associate professor and Extension specialist, Entomology & Plant Pathology, at North Carolina State University. He may be reached at dominic_reisig@ncsu.edu.

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