Black cutworm larvae feed on newly emerged vegetable crops. The worms are active feeders, clipping many seedlings at or below the soil line in a single night. They prefer crops sown as seed (rather than transplants); susceptible crops include beets, carrots, cucumber, leafy greens, melons, peas, potato, pumpkin, snap beans, squash, and sweet corn. If not controlled, these pests can destroy approximately a third of planting. Black cutworms are particularly problematic to fresh market growers and home gardeners. As an adult moth, the black cutworm may be referred to as the Dark sword-grass or the Ipsilon dart.

Appearance

The dark gray to black larvae have a greasy appearance and a grainy texture. Mature larvae grow to 1 ½ inches long. When disturbed, cutworms curl up into a tight C-shape. Adults are gray moths that have a series of distinctive dark markings on their forewings (including a dagger-shape on each outer forewing) and lighter colored hind wings.

Life cycle

The black cutworm doesn’t overwinter in Wisconsin. Instead, the adult moth migrates north on storm fronts in early spring, usually in May and early June. Females lay eggs on low-growing vegetation such as chickweed, curly dock, cruciferous weeds, and plant residue. Larvae hatch about 5-10 days later and over the course of a month will pass through six instars (growth stages) before pupating. Larvae migrate from mature vegetation to seedlings when they’re about ¾ inches long. About 2 weeks after entering the pupal stage, adult moths emerge, mate, and repeat the cycle. In Wisconsin, three generations emerge each year.

Symptoms and Effects

Newly hatched larvae feed on leaves but are unable to chew entirely through, creating a “window pane” effect. As they mature, they create small pinholes in the leaves; large infestations of cutworms can completely defoliate plants. During the fourth instar, larvae begin cutting plant stems. The ½ inch worms chew through the stem at or just below the soil surface. This type of injury is common during extended periods of dry weather. If plants are larger, larvae may not be able to cut through the stem but will burrow into it below ground level, causing plants to wilt and die.

Scouting

Timely detection is critical for insecticide treatments to be effective or economical. The best way to do this is to monitor for the arrival of the moths. Watch the Wisconsin Department of Agriculture, Trade & Consumer Protection (DATCP) pest bulletins (pestbulletin.wi.gov) for their reports.

Alternatively, monitor for the return of the adults using a black light trap. When 9 or more moths have been caught on each of two consecutive nights, begin calculating degree day accumulation.

Degree day modeling

Degree days offer a way to track insect development based on temperature and will help you determine when the damaging fourth instar will appear. Visit the Vegetable Disease and Insect Forecasting Network (VDIFN) website, which features a convenient map-based interface, to view the black cutworm model and many others. Expect leaf cutting to begin at 300 degree days and end around 1000, with a base temperature of 50°F and an upper threshold of 86°F.

Alternatively, you can calculate degree days from your own weather data using the formula below and keep a running total.

As a second check, begin scouting fields weekly when plants emerge, looking for evidence of spotty emergent. If seedlings aren’t coming up evenly, dig the soil to spot check for cutworms.

Management

Non-chemical control

A number of parasitic wasps, flies, and predaceous ground beetles help keep cutworm numbers down. Birds also help control the population by feeding on the larvae.

Cutworms are most problematic in low, wet, grassy areas. Since female moths prefer to lay eggs in weedy areas, keeping weeds controlled will lessen the possibility of damage. Avoid planting susceptible crops in low, wet areas or in rotations following sod. Cultivate as needed in fall and spring to disrupt the cutworms’ life cycle and to destroy host weeds.

Chemical control

If non-chemical control measures aren’t sufficient for adequate control, begin treating at the first sign of pests cutting and repeat every 7 days until 1000 degree days are reached.

For a complete listing of available insecticides, refer to the latest copy of Extension Publication Commercial Vegetable Production in Wisconsin (A3422).

Economic treatment thresholds for black cutworms have been developed for the following crops:

• Snap bean= 2 larvae/ row foot
• Potatoes= 4 larvae/row foot
• Sweet Corn= >5% of plants damaged
• Leafy greens= <3% of the stand affected