How to Tackle Insect Pests in Corn, Soybeans This Season

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Five tactics for identifying, and dealing with, these profit-robbing crop munchers


When you quiz an entomologist about what farmers should be scouting for in their corn and soybean crops, the first answer you might hear is "watch out for everything." And that's the first rule of scouting, see what's present and develop a plan of action.

Yet the 2021 crop year is presenting a range of challenges depending on where you farm. And that's probably the number one focus for an insect pest program. Know current conditions in the field.

1. Weather matters.

In any given season, weather varies widely across the country. Knowing conditions for your fields – beyond amount of rainfall – can be an early warning system for pest detection since some pests are more sensitive to these conditions. A good example of weather impact is with soybean aphid and spider mites in both corn and soybeans.

"One of the bigger things on soybeans coming up this season is soybean aphid," says Bryan Jensen, extension entomologist, University of Wisconsin. "We've seen that some states have already found them. It's early, but I like to do a little spot checking in fields to get a feel for what's happening."

Jensen notes that under the hot weather that hit the Midwest early in the growing season, he's not expecting to see a huge population increase in those aphids – this pest does not like hot weather. But if conditions change, "they can bounce back pretty quick."

If weather remains hot and dry, there's another pest that can raise a challenge. Spider mites can hit both corn and soybeans. "Warm, dry weather can set the stage for all of this, I'm not sounding an alarm but that's something to look for," he says.

2. Scouting pays.

When discussing scouting with an entomologist, they're talking about a thorough walk through a field taking a closer look at crops. While a pickup truck spot check may sound good, problems can develop in fields beyond your view. As more farmers deploy drones and aerial imagery, these tools can help identify areas to send scouts, which makes this tactic more efficient.

When scouting, it's about counting bugs or measuring damage. For example, if you're scouting using imagery, and it shows stress in some part of the field, targeting that area for added scouting can help spot trouble earlier. Then you can decide what treatment actions to take.

"The crop is more valuable [this year]," says Christian Krupke, extension entomologist, Purdue University. "Farmers' willingness to pay to keep risk low may be higher. Scouting is key to that."

"The crop is more valuable [this year]. Farmers' willingness to pay to keep risk low may be higher. Scouting is key to that."
-Christian Krupke

3. Managing treatment investment.

If you've identified a pest in the field, the next step is to determine if you need to treat. When combating insects there's a fine line to walk regarding when to treat and when to walk away. Rising commodity prices mean those per-bushel yield losses are dearer and may justify treatment.

Wisconsin's Jensen notes, however, that for each pest every state has its own guidelines for when to treat. These are available from state extension resources. For example, the timing of black cutworm injury may be different in one state versus another.

Jensen says Wisconsin farmers were seeing black cutworm damage in their crop early this season, as well as some true armyworm injury. The key is to assay the level of damage and the populations you're dealing with.

In Indiana, for example, black cutworm can have up to three generations, and your chances of having a problem depends on other risk factors including tillage and crop rotation. Yet treatment may be advised if primary leaf feeding tops 3 to 5%, and you find two or more cutworms per 100 plants.

For soybean aphids, Purdue's Krupke says the treatment threshold is pretty common across the Midwest – if you see 250 aphids per plant it's time to treat. "That threshold is used throughout the region and it's worked for years," he says.

For those spider mites, which can hit quickly in hot weather, the field edges will show damage early. The treatment threshold depends on timing. "Once we get into drought in corn, we will see spider mites start to move into the field from the edges," Krupke says. "But a good rain will cure spider mites for nothing."

4. Understanding treatment options.

Economic threshold is a phrase most farmers have heard more than once. It's that financial balancing act for determining when to pull the trigger on a treatment for an in-season pest. As noted earlier, this can vary by state for some pests. But another factor that enters the economic threshold conversation is efficacy.

Just spraying a problem could be money down the drain if timing, or the treatment approach, isn't tackled using best practices. There are treatments for black cutworm, aphids and spider mites.

Just spraying a problem could be money down the drain if timing, or the treatment approach, isn't tackled using best practices.

Yet timing for spider mites, if the problem arises due to continued drought, is that treatment options kill adult and immature mites, but do little to eggs already laid. "The insecticides we apply don't kill anything in the egg," says Wayne Ohnesorg, extension educator and entomologist, University of Nebraska. "It's not unusual to need a retreatment 10 to 14 days later depending on how severe the infestation was before you treated. This is a pest you have to watch closely."

5. Looking for new pests.

"Soybean gall midge is starting to emerge," says Ohnesorg. This is a pest that first appeared in Nebraska in 2011, and for now there are few treatment options. Entomologists are still learning about the pest, but for now it can't be stopped with seed treatments, and insecticide sprays have not been thoroughly tested for efficacy.

Scouting can help identify if fields are infested. Entomologists recommend scouting starting at V2 in soybeans. The pest has been identified in Nebraska, Iowa, Minnesota, Missouri, and South Dakota.

For the 2021 season, watching for everything makes sense, and given higher crop prices, spending time with a trusted adviser to reevaluate economic treatment options can help preserve income.

This content produced by Farm Progress for Corteva Agriscience.