Projected Cutting Dates for Black Cutworm in Corn

Farmers should be on the lookout for black cutworm in their corn fields.

The earliest projected cutting dates were late last week in Montgomery County. University of Illinois Extension Entomologist Nick Seiter says fields, especially at risk to having plants cut by the black cutworm, include those with later planted corn and those sown into grassy weeds or a late terminated cover crop. Seiter explains, “What you are going to want to do is to scout your field. Look for plants lying on the ground that appears to have been cut with scissors. This is different looking than damage from a bird digging up the plant looking for the seed. These corn plants will be cut off. When you start finding that, scrape around in the residue looking for the larvae. The black cutworm larva is dark colored, with a greasy appearance. It is not slimy, but it looks like it has been coated with Crisco. If you find the worms and about three percent of the plants have been cut throughout the field it is the time to initiate a treatment.”

Seiter says there are several pyrethroid insecticides that can be successfully used as a rescue treatment. He offers these black cutworm management pain on the University of Illinois the Bulletin website.

• Infestations are more likely in later planted corn, as delayed planting means larger cutworm larvae are present at earlier stages of corn development.
• Black cutworm moths prefer to lay their eggs on grasses, not bare ground. Therefore, fields with grassy weeds present at or shortly before planting are more likely to experience damaging populations. Similarly, monitor fields closely if a grass cover crop (e.g., cereal rye) is terminated while corn is susceptible to cutworm damage (emergence to ~V5).
• The economic threshold for black cutworm is 3% of plants cut with black cutworms still present in the field. Look for plants that look like they have been cut roughly with scissors close to the base; plants with intact roots were most likely dug up by birds and do not represent cutworm damage. Remember, larvae do their feeding at night and hide in residue or just below the soil surface during the day, so you will have to do a little bit of digging near the base of the plant to find them.
• Several Bt corn trait packages offer suppression of black cutworm, but these might be less effective under heavy infestations or against later stage larvae. Most pyrethroid insecticides labeled for use in corn will do an excellent job of controlling larvae as a rescue treatment; just remember that they only pay off when an economic threshold has been reached.

Kelly Estes at the Illinois Natural History Survey coordinates an insect trapping network throughout the state and those results, including the black cutworm cutting dates, are posted online at The Bulletin website - that’s bulletin.ipm.illinois.edu and on Twitter using the handles @ILPestBulletin or @ILPestSurvey.