Prevent Cutworm Damage to Cut Losses

Cutworms are cool-weather, early-season pests of many cultivated plants. Often before crops are even planted, the small instar cutworms may be feeding on winter-annual weeds in fields. When winter-annual weeds are burned down, and spring crops are planted, half-grown cutworm larvae may be ready to attack emerging seedlings. Cool conditions and excessive moisture that can delay planting and/or slow growth of crops can favor cutworms by extending the period of vulnerability. Cutworms will attack plants from the seedling stage until they are about 6 to 12 inches or so in height, depending on the crop. But plants up to about 4 inches in height are most vulnerable.

Figure 1. Black cutworm at base of corn seedling (Photo: Ric Bessin, UK).

There are several species of cutworms that can be serious pests, including black cutworm (Figure 1), bronze cutworm, and dingy cutworm. Black cutworms moth captures have been slowly increasing this spring. If an intense moth flight occurs, a degree-day model base 50°F is used to predict when larvae are large enough to start cutting seedlings.  Research has shown that about 300 Degree Days after an intense moth flight (this date is the biofix), larvae will begin to cut plants.

Scouting fields at this time will determine whether or not a cutworm spray is needed and helps reduce unnecessary at-planting soil insecticide applications. When scouting, watch for cutworms hiding beneath residue or in the soil on clear sunny days; at night or on overcast days, cutworms leave their burrows to feed on various plants. Cutworm burrows are often within 6 inches of the plant they are feeding on. To sample for cutworms in the soil, dig around damaged plants at a 6-inch radius and about 2 to 3 inches down.

There are several insecticides that provide excellent cutworm control, but the challenge is knowing when to use them. I recommend beginning to scout fields for cutworms when degree-day totals hit 300 after an intense moth flight (biofix), then continuing to scout twice a week until plants reach 6 inches in height, then weekly thereafter.  Some of the traited field corn hybrids provide cutworm control.

Another tactic to reduce cutworms is early field preparation. Killing weeds with a burn-down herbicide or soil tillage 2 or more weeks before planting will help to starve small cutworms before seedling emerge.


By Ric Bessin, Entomology Extension Specialist

Posted in General Pests