A wet spring for some areas of the Corn Belt delayed tillage, planting and herbicide applications, setting the stage for heavy black cutworm pressure. Brian Weihmeir, an LG Seeds agronomist in south-central Illinois, started fielding calls about the tell-tale snipped plants around Memorial Day.
“It was something we were keeping an eye out for after a wet, cool spring delayed planting
Black cutworms snip corn plants
When cutworm moths return from overwintering to the south, they lay eggs in winter annuals and heavy vegetation. When the eggs hatch and the larvae start growing, cutworms, true to their name, cut the corn plant off “just like a pair of scissors right at the soil surface,” Weihmeir explains.
Black cutworms usually clip plants during the evenings or overcast days when it's a bit cooler. When it’s warm and sunny, they burrow into the soil. That’s why you’ll likely find a small hole about the size of a pencil a few inches away from a cut plant, Weihmeir says. “You can usually dig down with a trowel or a pocketknife, and a couple inches below the surface you’ll find the cutworms.”
The larvae are likely to be a gray or blackish color and “when you pick them up, they'll curl up into a ball,” Weihmeir says.
Black cutworms cause damage that’s not obvious from the road
Controlling black cutworm is challenging for two reasons: 1) the pest can cause a lot of damage quickly; and 2) cutworm damage occurs soon after planting.
“We usually start seeing cutworm damage mid-to-late May when farmers are just getting done planting,” Weihmeir explains.
Rather than doing a quick “windshield tour,” Weihmeir recommends growers get out of the vehicle and walk fields to check for black cutworms. Farmers should start scouting after planting and while corn plants are getting established, especially if they have heavy winter annual pressure and a cool spring.
Besides weedy fields, other high-risk areas for black cutworm include fields that had minimal tillage and poorly drained areas.
Depending on the growth stage of the crop, cutworms can cut corn plants above the growing point, which will keep the plant viable. “But these plants will be behind all season and yield less,” says Weihmeir. “Plants cut below the growing point may wilt and die, causing plant population to become compromised. Work with your agronomist to evaluate the need to replant.”
Farmers should make note of the larvae size. “If you start digging your fields and find a couple of plants cut but the larvae are near full-size – 1 ½ inches to 2 inches long – they’re probably done feeding. But if we start seeing larvae that are less than an inch long and we’re seeing quite a bit of damage, that’s probably going to warrant a rescue treatment,” Weihmeir says.
Management and rescue treatment plans
The first priority for managing black rootworm is to keep corn fields clean, Weihmeir says.
There are corn hybrids with above-ground traits like VT Double Pro®, Trecepta® and Viptera® that offer some control against black cutworm. But he warns producers against complacency. “Cutworms can still do damage if pressure is heavy enough,” he says. The story is similar for seed treatments.
When deciding if a rescue plan is warranted, there are many variables to consider. “We have the potential for a high-value crop this year, so the thresholds for an insecticide spray application may be lower than the 2% to 5% damage university extension offices often recommend for treatment,” Weihmeir explains. Consult with your crop advisor for guidance.
“When considering a black cutworm rescue treatment, keep in mind that a lot of insecticides that control cutworm can also wipe out some beneficial insects,” Weihmeir says.
If treatment is needed, farmers need to act fast. “If a farmer waits three or four days to make that rescue treatment, it could be too late,” Weihmeir warns. Therefore, he urges farmers to get their plan together now for protecting the crop.
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