1. Insect
  2. Legume, Grass, and Field Seed Crops
  3. Grass Seed Pests

Grass seed-Armyworm

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Army cutworm (Euxoa auxilaris)
True (common) armyworm (Mythimna unipuncta formerly Pseudoletia unipuncta)

Pest description and crop damage Moths in the family Noctuiidae have gray or brown forewings with slate or buff-color markings. Hind wings are silvery-gray or beige. Average wingspan is 1.4 to 1.6 inches.

Mature larvae are about 1.5 to 2 inches long, tan to brown, with a faint white or red midline stripe on the top of the body. These are bordered below by a white stripe on each side. A prominent black stripe is along each side bordered below by an orange-brown stripe. Larvae have an inverted white Y on the front of the head.

Pupae are brownish-red and found in the top soil layer, commonly near the crowns of host plants.

In eastern Oregon, army cutworm occasionally damages grasses grown for seed, pastures and cereals; look for larvae in fall or late winter if damage is seen. In western Oregon, the true armyworm infrequently infests grasses in the fall, usually tall fescue and orchardgrass. Late-summer flights of moths lay eggs in established grass seed fields after harvest. Larvae feed on fall regrowth.

Extensive but localized damage may result if the population is not treated. Large numbers of larvae feed so voraciously that mass migrations of larvae within and away from fields occur as regrowth fails.

Scouting and thresholds Look for armyworm larvae in and around crowns, where birds (swallows, crows) are feeding. Dig around in the thatch, grass residue, and at the base of plants. Armyworms typically feed at night. Also look for frass (excrement) that resembles tiny grass pellets. Treatment in established grass crops is suggested when 4 larvae per sq ft are observed. The threshold in seedling stands is 2 larvae per sq ft.

Biology and life history Armyworm adults migrate into Oregon from California in summer. Eggs are laid in grasses. They hatch in a week, and larvae feed through the fall. Larvae aggregate in small areas, where defoliation may be complete. Most armyworm damage is seen during late summer and fall in western Oregon. Outbreaks are infrequent and are associated with unusually high spring rainfall in California which favors survival of the first generation. Resultant moths migrate north following green plant material.

Management-biological control

Tachinid flies heavily parasitize armyworm larvae and often cause armyworm populations to collapse over large areas after a season or two.

Management-chemical control

Insecticides are most effective when larvae are young and small. It is advised to apply insecticides at night when armyworms are above ground and active.

  • bifenthrin (BrigadeÆ 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 (e.g., 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. recommended on 1st and 2nd instar larvae only. 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 6.0 to 10.0 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.
  • malathion at 1.25 lb ai/A. PHI 0 days. REI 12 hr. Maximum single application rate is 1.25 lb ai/A. Maximum one application of malathion allowed per year.
  • spinosad (Blackhawk) at 0.025 to 0.050 lb ai/A per season. PHI 0 days forage, 3 days hay or fodder. REI 4 hr. For resistance management, do not apply Success more than three times in any 21-day period. Do not exceed 0.186 lb ai/A per season. Do not make more than six applications per season.
  • spinosad (Success) at 0.031 to 0.062 lb ai/A. PHI 0 days forage, 3 days hay or fodder. REI 4 hr. For resistance management, do not apply Success more than three times in any 21-day period. Do not exceed 0.186 lb ai/A per season. Do not make more than six applications per season.
  • zeta-cypermethrin (Deadlock G) at 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 0.0175 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.