Northern hay growers may want to watch for a winter cutworm that can damage alfalfa fields, grass pastures and small-grain crops, say Michigan State University experts. The cutworm is actually the larval stage of the greater yellow underwing moth, says Jerry Lindquist, Osceola County extension director. A somewhat unexpected outbreak caused damage in the northern half of Michigan’s Lower Peninsula in 2007.

“This is a new pest to the U.S., although it has been a problem in Europe and Canada for some time,” he says. “There hasn’t been documented economic damage in the U.S. from winter cutworms before. These cutworms are a lot like armyworms. The unusual thing about them is that they can survive cold temperatures. The larvae emerge and feed in September and then overwinter. There have been reports of people seeing them crawling over the snow. Hay producers should start looking for damage in April and May.”

The caterpillars are sandy brown to black and range from 1” to 3” long. “They feed in the evening or after dark and can be found on plant leaves,” Lindquist says. “On cooler days they may hide at the base of the plant. Signs of early feeding may appear when fields don’t green up as expected. You will often see large groups of the caterpillars at one time.” The caterpillar eventually forms a cocoon in May and June and then becomes a moth. Eggs are laid in August and the larvae hatch out in September and begin feeding again, so fall crop damage can also occur.

Harvesting the crop can remove the caterpillar’s food supply. Economic thresholds for treatment have not been established, but Michigan State entomologists recommend that action be taken if four or more larvae are seen per square foot. Growers who used pyrethroid insecticides last year reported good control, says Lindquist.

Learn more about the winter cutworm in the most recent issue of the Michigan State University Field Crop Advisory Team Alert newsletter at www.ipm.msu.edu/cat08field/fc04-03-08.htm#1. Contact Lindquist at 231-832-6139 or lindquis@msu.edu.