December 02, 2023

Scout fields for black cutworms

Time to consider fall weed control

CHATHAM, Ill. — Black cutworms have been observed in traps in Illinois, primarily in counties across the center of the state.

Black cutworm is an aggressive pest that arrives in the Corn Belt in late spring, feasting on many plants, including corn and turf grasses.

“It overwinters along the Gulf Coast and it migrates back up into the Midwest with spring storm fronts. Those moths that come back in with those spring storm fronts are going to look for a location that increases the chances of their young surviving,” said Matt Montgomery, Pioneer field agronomist based in west-central Illinois.

Upon arrival, the moths look for fields that have vegetation, including broadleaf and grass crops, weeds and other plants. Black cutworm adults feed on plant nectar.

“They’re going to look for fields that have green in them. The moths come in, lay those eggs, those hatch on the winter annuals and begin to feed,” Montgomery said.

Montgomery noted when the winter annuals are killed, the cutworm moves over to the cash crop.

“Take the idea of fall weed control seriously.”

—  Matt Montgomery, field agronomist, Pioneer

Black cutworms can be identified by their body segments. A gray body with four dots — two small and two large — distinguishes black cutworm from other common cutworms such as armyworm.

When smaller, black-cutworm larvae will chew holes in the leaves. Adult black cutworms begin cutting V1 to V5 plants and even drill into V6 to V8 plants, killing the growing point. In wet soils cutting will occur mostly above ground.

A majority of cutting will be below ground when the soil is dry.


Montgomery noted the following economic thresholds for black cutworm in corn:

• Two cut plants per 100 plants for seedlings.

• Three cut plants per 100 plants for V2 stage.

• Five cut plants per 100 plants at V3.

• Seven cut plants per 100 plants at V4.

“We’re going to look for cutoff plants. We’re going to look for leaves that have been fed upon and are cast aside. Sometimes, we’ll see the pest feeding on a leaf. Sometimes we’re going to see plants that are pulled into the ground,” Montgomery said.

“If we carefully dig around with our knife, we can actually dig up the cutworm itself. If we see 6% to 8% of the plants in the field showing injury, showing damage from black cutworm, that’s going to be a trigger for us keeping this field tamed down from black cutworm pressure with a rescue insecticide application.”


Montgomery said the fields with high black cutworm pressure was a because “they didn’t have a neonicotinoid, didn’t have a diamide insecticide, didn’t have a trait.”

“All of the winter annual pressure gave this pest plenty of stuff to feed on outside of the crop and get some extraordinary size to it, combined with some warm temperatures that helped it advance through its life cycle fairly quickly,” he said.

“By the time we got some really big sizes to this pest, we had killed off that winter annual pressure and then it moved over to the crop, and because it’s large now, that means it’s going to take a lot more to kill it. It’s going to get its licks in before it finally succumbs.

“That brings up an issue that I think we absolutely have to take seriously, the issue of winter annual pressure.”

Montgomery noted he saw numerous fields in early spring with butterweed, cressleaf groundsel and other weeds pressure that were ideal stops for black cutworm moths.

“We saw those fields everywhere. Why did we see that? We have no time to work with in the spring anymore, and because of that, I think it’s absolutely critical that we start thinking about fall weed control,” he said.

“If you haven’t been doing that, I think you have to from here on out. We just don’t have the time logistically to get the fields all sprayed and get them cleaned up while we’re trying to get a crop planted in a really tough season.

“It’s absolutely critical that we get those fields under control. Take the idea of fall weed control seriously.”

Tom Doran

Tom C. Doran

Field Editor