Cutworms in Corn

We received a call about cutworms in some late planted corn in the Oran area. Lots of people had to plant corn later than normal. One of the ways to avoid insect injury is to plant early – the opposite is also true: planting late means you are more likely to have insect problems.


Black Cutworm curled up next to a damaged corn plant.

Cutworms are early season pests of corn. Plants are vulnerable from emergence to the V6 stage Cutworms are most common in years with wet springs. Young cutworms climb the plant and make small holes in the stalk; where, large worms can cut through the plant right above the soil. Most of the time you see plants cut near the base of the soil and the seedling plant lying on the ground near the base but you don’t find the worm. There are many species of cutworms but the most common cutworm in Southeast Missouri is the Black Cutworm. The best way to tell the black cutworm moth is by the black “dagger” shapes on its wings. The worms can be gray to nearly black, shiny, almost greasy, in appearance, with smooth skin and can be 2 inches long. Cutworms are mostly likely to be out at night; during the day they are usually hiding under leaf litter on under the soil. When disturbed, they curl into a “c” shape. In order to keep an eye on possible cutworm infestation, it is best to start scouting for cutworms within a week of emergence.

There has been a lot of black bird injury that resembles cutworm injury where the bird broke the top of the plants off trying to get to the seed. The picture below isn’t the best, but it gets the point across. Black birds snapped the young seedlings at the base of the plants. This injury can be easily confused with cutworm injury. What you can’t see in the picture is – where neighboring plants should be, there are characteristic holes where black birds had pulled out plants to get the seeds. These were pulled, but they couldn’t get the seed out and the plants broke off near the base of the plant. For characteristic black bird injury, I’ll refer you back to Kent Fothergill’s earlier post on black birds. So make sure that you have cutworm injury before spraying, or else it is a waste of money.

Black bird injury that mimics cutworm injury.

The University of Missouri Pest Management Guide says: Because the level of yield reduction is related to the site of injury on the seedling plants, different corn thresholds exist for damage above and below ground. Rescue insecticide treatments are recommended when 2 to 4 percent of the corn seedlings are cut below ground and 6 to 8 percent of the seedlings are fed upon or cut above ground. The smaller percent figure is for lower plant populations (15,000 to 20,000 per acre); whereas, the larger figure is for higher plant populations (22,000 or more per acre).

Categories: Corn, Insects, Monitoring, Scouting issues

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