Grass seed-Glassy cutworm

Crymodes devastator and an identical species east of the Cascades
Protagrotis obscura (no common name)

Pest description and crop damage The adults of glassy cutworm are heavy grayish moths with a wingspan of about 1.25 inches.

Larvae of both species look identical, have the same life histories, and cause similar damage. Their bodies are a translucent (glassy) dirty gray. Heads are red-brown. They can be 1.5 inches long when mature (late spring). They are true subterranean cutworms, spending nearly their entire larval life in the crown or in and among the roots.

West of the Cascades, glassy cutworm is one of the most common species infesting grasses. East of the Cascades, Protagrotis obscura is the primary pest.

Larvae bore into the crown and through roots, seldom feeding above ground or on leaves. They are pests primarily of established grasses, reflecting the egg-laying preferences of the moths. Damage is from fall through spring. In western Oregon, larvae feed through the winter. In eastern Oregon, Washington, and Idaho, larvae usually diapause (an inactive state, like hibernation). Heavy infestations seriously weaken and even kill mature plants. Extremely cold winters worsen injury; in fact, larval populations usually are detected in spring after damage erroneously labeled "winterkill."

Biology and life history Adults emerge in late June, July, and August, mate, and lay eggs on the soil surface near the crowns of grass host plants. Eggs hatch in about 10 days. Larvae begin to bore into and feed in grass crowns. Spring-seeded tall fescue in western Oregon should be scouted carefully, beginning in early October, for larvae that will feed just below and within crowns. Larvae feed through the winter west of the Cascades and diapause (hibernate) during the coldest winter months east of the Cascades. Larvae pupate in soil during spring.

Scouting and thresholds Scout for larvae in October. Dig plant crowns and roots to at least 3 inches and carefully inspect them for larvae.

No thresholds are established for this pest.

Note: Don't mistake sod webworm for glassy cutworm. Cutworm damage is from September through April. Sod webworm (cranberry girdler) larvae (western Oregon) complete feeding and prepare to overwinter by late October, they do not resume feeding in the spring, but pupate and emerge as moths in June.


Management-chemical control

Chemical control of this pest is seldom successful because most of the larvae occur too far below the soil surface to be reached by insecticides, even when irrigated.

  • 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.
  • cyfluthrin (Baythroid XL) at 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 0.025 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.
  • 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.