Keep an eye on cornfields for signs of black cutworm. This migratory insect cuts and feeds on the early vegetative stage of corn plants. BCW begins feeding on corn plants as soon as the plants emerge from the ground in the spring.

Black cutworm moths arrive in Iowa and other Northern states with spring storms each year. These moths lay eggs in and around crop fields. The eggs hatch, and the emerging BCW larvae can cut seedling corn.

This pest is sporadic, making it essential to scout fields to determine if management or treatment is needed, says Erin Hodgson, Iowa State University Extension entomologist. Scouting for BCW larvae helps to determine if an insecticide application will be cost-effective. This year, delayed corn planting can coincide with emergence of BCW larvae so scouting should be done early to protect seedlings.

Hodgson, along with ISU colleagues Ashley Dean and Adam Sisson, offer the following information and guidelines for scouting and controlling black cutworm on corn.

Scout fields to protect seedlings

When to scout for BCW caterpillars is based on the “peak flight” of moths and accumulating degree days after the peak flight. Degree days are a measure of temperature used to gauge insect development. A peak flight for BCW is defined as capturing eight or more moths over two nights in a wing style trap baited with a pheromone lure.

To find out when moths arrive in Iowa, cooperators around the state monitor pheromone traps and report moth captures. Cooperators started checking traps at the beginning of April and captures of BCW moths occurring the first week. Moth captures were picked up in mid-April and have continued through early May, with several peak flights recorded. Peak flights observed during this period in Iowa were in line with captures in surrounding states.

The map shows predicted BCW cutting dates for the nine Iowa crop reporting districts based on actual and historical degree day data and peak flights during late April and early May.

Predicted cutting dates for 2019

“We may continue to see peak flights occur in Iowa,” Hodgson says. “Adult moth trap captures don’t necessarily mean there will be economically significant BCW infestations in a particular location. Field scouting is essential to determine if an economically damaging infestation exists. As you are out in fields assessing stands, be on the lookout now for early-season insect injury in corn — BCW or otherwise.”

Several states bordering Iowa also track black cutworm moth flights and make estimates about cutting dates. Several of the predictions in these states are near the Iowa border, and some are for counties directly adjacent to Iowa. Data from these out-of-state locations may be informative for Iowans living nearby.

Extension resources on black cutworm can be found at these universities:

Tips for scouting pest

Poorly drained, low-lying or weedy fields, as well as those next to natural vegetation or with reduced tillage, may have a higher risk of BCW injury. Also, cornfields with green cover crops may be attractive to egg-laying females. Late-planted corn can be smaller and more vulnerable to larval feeding.

Some Bt corn hybrids provide suppression of BCW — Vip3A, Cry1A.105, Cry2Ab2 and Cry1F proteins — but larvae can still cut off young plants.

Crop scouts are encouraged to look for any activity during the early-season corn stand assessment. It’s best to start scouting at least a few days before estimated cutting dates. Early scouting is important because local larval development may be different due to weather variation within a climate division, and additional peak flights could prolong BCW larvae arrival to the field.

WARNING: Black cutworm larvae usually begin chewing on corn plants above the soil surface. Leaf feeding (left) may be seen. As larvae mature, they can severely damage or kill plants (right).

Fields should be scouted for larvae weekly until corn reaches V5 growth stage. Examine 50 corn plants in five areas in each field for wilting, leaf discoloration and damage, or those that are missing or cut. Flag the areas in the field where there is suspected feeding and return later to assess further injury. BCW larvae can be found by carefully digging the soil around a damaged plant.

Is this a black cutworm?

BCW larvae have grainy, light gray to black skin, and four pairs of fleshy prolegs on the end of the abdomen. There are pairs of dark tubercles, or bumps, along the side of the body. The pair of tubercles nearest the head is about a third to a half the size of the pair closest to the abdomen. BCW larvae can be confused with other cutworms and armyworms, so additional resources may need to be consulted to confirm identification.

DINGY CUTWORM: Black cutworm (left) can be distinguished from other larvae, such as dingy cutworm (right), by the size of the dark tubercles on the back.

Common thresholds for seedling, V2, V3 and V4 growth stage corn plants are two, three, five and seven plants cut out of 100, respectively. A dynamic threshold for BCW may be useful with corn price and input fluctuations. An Excel spreadsheet with calculations built in is available and can be used to help with black cutworm management decisions.

Preventive BCW insecticide treatments applied as a tank-mix with herbicides are a questionable practice. BCW is a sporadic pest, and every field should be scouted to determine insect presence before spraying insecticides.

If you see any fields with BCW larvae while scouting, contact ISU Extension. Send a message to [email protected]. This information could help refine future predictions.

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