Grass seed-Black cutworm

Agrotis ipsilon

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Pest description and crop damage Black cutworm larvae are 1.2 to 1.6 inches long when mature. They can be gray, brown, or black and have lighter brown stripes running the length of the body. The head capsule is dark brown or black. By day, they hide in the soil or under plant debris on the soil surface. The adults of this moth are brownish gray with a spot and a light silvery band on the front wings. The wingspan is about 1.4 inches.

This cutworm, one of the most common species in western Oregon, is a pest of new seedlings. Larval populations are often in soil when grass is seeded in summer or early fall and follows fallow ground with weeds, legumes, crucifers, sugar beets for seed, pasture, or other grass seed crops. Generally, one can expect black cutworm larvae in fields where no-till and conservation tillage follow these cropping regimes.

Larvae feed at night, hiding in soil or resting under clods or plant residue by day. Even a few larvae in an area can cause substantial clipping and defoliation. Stand loss can occur if seedlings are clipped off repeatedly or if the growing points are destroyed.

Biology and life history Apparently, either moth or pupa may overwinter in the Pacific Northwest or moths migrate into the region during late spring; females deposit eggs during this time. Eggs hatch soon after deposition.

Young larvae feed and survive on many grasses and broadleaf plants. In the first week or two, the small larvae tend to stay above ground on a host plant. Later, they move to the soil and feed at night above ground at the soil surface. They also feed by day on some crops like table beet seedlings. They clip roots underground and pull foliage into the soil.

Larvae feed for 1 to 3 months, depending on soil and ambient temperature. When mature, they pupate in the soil. Adults emerge 2 to 3 weeks later. There appears to be a second generation of larvae occurring in late summer and fall that are present in soil and cause stand reduction when fall grasses are sown.

Scouting and thresholds Seedling grasses emerge well but later begin disappearing overnight. Suspect black cutworm when you notice that many of the injured grasses are clipped at ground level (as a rodent might do). Often, individual blades are left wilting by the seed furrow. Occasionally, seedlings will be severed underground.

Sift through soil in the seed furrow, dig and screen soil from cracks in the soil near the seed line, and inspect under clods, organic matter, and hollow stems of the previous crop's residue to find black cutworm larvae. Damage usually is in irregular patches of less than an acre to many contiguous acres.

There are no established thresholds. Treat when the potential for stand reduction is great.

Management-cultural control

Plowing and disking prior to seeding may not always reduce larval populations below damaging levels.

Management-chemical control

  • bifenthrin (Brigade 2EC and WSB) at 0.1 lb ai/A. Apply in spring and fall when insects are present at their economic threshold level. Maximum amount allowed is 0.2 lb ai/A per season. Applications made no less than 14 days apart. PHI 30 days prior to harvest for forage, hay and seed.
  • carbaryl (eg. Sevin XLR Plus, 4F) at 1.0 to 1.5 lb ai/A product. PHI 14 days. REI 12 hr. Up to two applications per year, but not more than once every 14 days. Do not exceed 3 lb ai/A per season.
  • cyfluthrin (Baythroid XL) 0.013 to 0.015 lb ai/A. PHI 0 days. REI 12 hr. Maximum amount allowed per 5 day interval is 0.022 lb ai/A. Maximum amount allowed per crop season is 0.089 lb ai/A.
  • lambda-cyhalothrin (Warrior) at 0.015 to 0.03 lb ai/A. PHI 0 days for grazing and cut for forage, 7 days for straw and seed crop. REI 24 hr.
  • lambda-cyhalothrin/chlorantraniliprole (Besiege) at 5.0 to 8.0 fl oz/A. PHI 0 days for grazing and cut for forage, 7 days for straw and seed crop. REI 24 hr. Do not exceed a total of 27.0 fl oz of Besiege or 0.09 lb ai of lambda-cyhalothrin or 0.2 lb ai of chlorantraniliprole per acre per year.
  • spinosad (Blackhawk) at 0.025 to 0.050 lb ai/A per season. PHI 0 days graze, 3 days hay. REI 4 hr. No more than 6 applications per season or a total of 0.186 lb ai/A.
  • zeta-cypermethrin (Deadlock G) 10 lb/A Distribute granules evenly in the furrow at planting. Only one application is allowed per season. Maximum amount that can be applied is 10 lb/A per season. No rotation crops can be planted within 30 days of the last application. REI 12 hr.
  • zeta-cypermethrin (Mustang MAX) at 0.014 to 0.025 lb ai/A. PHI 0 days forage, hay; 7 days straw and seed screenings. REI 12 hr. For forage and hay use no more than 0.10 lb ai/A per season; make subsequent applications no closer than 7 days. For straw and seed screenings use no more than 0.125 lb ai/A per season; make subsequent applications no closer than 17 days.

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