Cutworms invade Vermont cornfields

Farmers in several Vermont counties are waging battle against black cutworms, a destructive early-season pest that quickly can reduce plant populations in a field. Although not an uncommon occurrence in the state, what is unusual is the extent of the damage and large amount of acreage affected this year.

Heavier than usual damage has been reported by farmers in Franklin and Rutland counties thanks in part to the recent hot temperatures that have pushed the cutworms to the cutting stage. The widespread and heavy damage in cornfields also can be attributed to a combination of factors from a mild winter, recent storm fronts, actively growing weeds and crop residue.

“Storm fronts carry adult black cutworm moths from the Gulf states to Vermont,” Dr. Heather Darby, University of Vermont Extension agronomist, explains. “Moths ‘drop out’ of these storm fronts and seek attractive egg-laying sites, especially on common chickweed and other winter annuals.

“As planting is delayed, the larvae will hatch and begin feeding on the weeds,” she continues. “As the weeds are killed, the cutworms need additional food and will be large enough to cut corn plants as seedlings emerge from the soil. Fields that are most susceptible are no-till plantings, high-residue or weedy fields, areas previously in pasture or sod and low, wet areas.”

Scouting for this pest is essential, even if control measures such as seed treatment and planting Bt corn have been used. Look for cut, missing or wilted corn plants, all typical symptoms of larvae damage.

The larvae, which feed mainly at night, will move up the row as they feed. On average, one larva may cut three or four plants in its lifetime.

“To scout, check 20 plants in five locations for cutworm injury,” Darby says. “Post-emergence rescue treatment is suggested when five percent or more of the plants are cut and larvae are one inch or less in size. If they are larger than an inch, it is less likely that a control treatment will help.

“Apply rescue treatments late in the day since cutworms feed at night. Only the infested area and a 20 to 40 foot surrounding buffer need to be treated. The spray should be directed to the base of the plant.”

For information on approved treatments or other control strategies, contact Darby at (800) 639-2130 (toll-free in Vermont) or (802) 524-6501, ext.437.

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