What insect and disease pests should we scout for in May and early June in soybeans? Are there occasions where we would have to treat?
The Indiana certified crop adviser panel answering this question includes Gene Flaningam, Flaningam Ag Consulting LLC, Vincennes; Greg Kneubuhler, G&K Concepts, Harlan; and Tom Stein, manager of the Boswell and Templeton branches for Ceres Solutions Cooperative.
Flaningam: Early-season soybean pests will be mainly those insects that feed on the seed and soybean leaves. Soybeans planted into heavy residue should be scouted for insects such as slugs, seed corn maggots and bean leaf beetles. Look for small holes in the leaves and the cotyledon. Disease issues would be those primary seedling diseases such as pythium and rhizoctonia. Those early-season pests are primarily controlled with seed-applied insecticides and fungicides. Pests such as slugs would have to be controlled with baits or some type of tillage practice.
Kneubuhler: The only insects that tend to occur early in the season are bean leaf beetles and slugs. Soybeans that early in their vegetative stage can handle a lot of defoliation. So, they’re very forgiving before treatment is necessary. Generally, it takes greater than 30% defoliation before treatment is warranted.
Slugs are a rare occurrence but can occur if conditions are wet for a long time. They were prevalent in the central and southern part of the state in 2017. If stand is significantly reduced, then treatment is needed. The only real options for slugs are tillage or a few molluscicides on the market.
The most common diseases early in the season are phytophthora root rot and pythium. There is no in-crop rescue for these diseases. The best defense is planting varieties with good tolerance or resistance, and treating your seed before planting. Once identified in the field, it comes down to determining what kind of stand loss you’ll incur.
Stein: Early-season diseases you should be scouting for include seedling fungal diseases such as pythium, which is one of the greatest threats to soybean seedlings in the Midwest, along with phytophthora, rhizoctonia and fusarium. Unfortunately, if you have these early fungal disease problems, sometimes the best rescue treatment is to replant.
As far as early-season insects are concerned, scout for ones that cause stand loss, stunting and delayed emergence from feeding on seeds and seedlings. Look for foliage feeding on unifoliates, cotyledons, emerging trifoliates and stems caused by bean leaf beetles, cutworms and armyworms. Pay attention to damage done to the growing point. Check the underside of the top-most trifoliate leaves with a magnifying glass to inspect for soybean aphids.
Belowground pests can cause damage to soybean plants as well. Look for stunted, wilted plants resulting from wireworm and white grub feeding. Seed corn maggots can cause a reduction in stand. There are occasions where you might need to treat for these insects. There are several insecticide rescue options available to control aboveground pests once action thresholds are met. Since there are no rescue treatments for belowground pests such as wireworms, white grubs and seed corn maggots, replanting may be your only option.