Time to scout for early season soybean pests

June 9, 2022 | Kriss Nelson

As soybeans emerge, it’s time to start scouting for pests.

Erin Hodgson, Iowa State University Extension entomologist specialist, says although there have been no reports of significant insect injury to soybeans this spring, farmers should still be on alert.

“It has been a cooler spring. Not only are plants growing slow, but insects are slow to get started, too,” says Hodgson. “There are some small plants out there right now and some that are bigger. Stages are asynchronous compared to normal.”

The Iowa Soybean Association (ISA) is working with Certified Crop Advisors to collect soybean pest incidence data from across the state.

“We will use this data to try and detect spatial trends where insect, weed and disease incidence is the greatest,” says Scott Nelson, ISA senior field services program manager. “We are also monitoring populations of corn rootworm adults in 120 fields across the state.”

There have been issues reported when farmers have been assessing stand counts and, after further investigation, note the presence of true armyworms – especially in those soybean acres planted into green cover crops.

Hodgson says the female true armyworm moth is attracted to the rye where she lays her eggs, and caterpillars will feed on soybeans.

“If there were rye cover crops that had a hard time with termination or were planted green with soybeans coming up, we have been hearing of true armyworm problems in central Iowa,” she says. “There has been some defoliation of soybeans this week and last week.”

Detecting these caterpillars can be challenging and treating them can be even more complicated.

“The issue is the dying rye creates a mat on the ground, and the true armyworms hide under the rye residue during the day and move over to the soybean and feed during the evening,” Hodgson says. “People have been noticing defoliation and not seeing what is causing it. You need to dig through the rye residue to find the suspect.”

Hodgson recommends spraying an insecticide if 8-10 true armyworms, a half-inch in size, are found per square foot. If the true armyworm is over an inch long, it may be too late to treat as they are wrapping up the end of their life cycle.

“Unfortunately, true armyworms are not noticed until they are bigger. It would be a timelier treatment when the caterpillars are small,” she says.

If treatment is warranted, Hodgson says to penetrate the residue with insecticide; a high-volume pressure spray is recommended. Timing the application as close to dawn or dusk will increase the odds of making contact.

True armyworms are migratory pests, and once they land in fields, they can produce a couple of generations of caterpillars. The good news is, Hodgson says, typically the first generation of caterpillars are problematic because soybeans are small.

High residue situations plus a cool spring could equal seed and seedling damage from soil pruning pests such as millipedes, pillbugs, slugs and snails.

“Farmers with cover crops or high residue fields have to be a little more thoughtful when planting,” says Hodgson. “There must be good soil conditions for seeds to germinate quickly and for vigorous growth. If the plant is growing quickly, they outpace pest activity most of the time. If they are sitting in the ground as a seed or emerging very slowly, that is when a pillbug or grub or similar pests can take them out.”

Regular scouting recommended

As the growing season progresses, Hodgson said farmers can expect another suite of insects potentially becoming active in June and July.

“Keep your eyes peeled. Regular time out in the field scouting will help you be more proactive instead of reactive,” she says.

The usual suspects to be on the lookout for include soybean aphids and the Japanese beetle. To detect the landing of migratory pests, regular scouting will be required.

“The mix of caterpillars and beetles could be different from field to field,” says Hodgson.

For farmers on the state’s western side, soybean gall midge is a pest to look for.

“The first detection of adult emergence happened in Nebraska this week,” says Hodgson. “That was similar to what happened last year. We expect adults to start popping up in the western part of Iowa next week.”

If soybean plants have surpassed the V2 stage, they are more susceptible to gall midge egg-laying.

“Those fields with older plants next week would be the ones I would scout first,” says Hodgson.

The photo above shows soybean gall midge larvae feeding on the stem of a soybean plant. (Photo credit: Erin Hodgson/Iowa State University).

To watch for and assist with soybean gall midge management strategies, the Soybean Gall Midge (SGM) Alert Network has been established. It’s a network of 36 sites across four states. Go to soybeangallmidge.org to learn more.

ISA’s Research Center for Farming Innovation field service program managers are also available to answer questions and can be reached by calling 515-251-8640 or visiting www.iasoybeans.com.

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