Pest description and crop damage True (common) armyworm (Mythimna unipuncta, formerly Pseudoletia unipuncta) and variegated cutworm (Peridroma saucia) are the most common species in western Oregon. They are present from early bloom through seed set. 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. Winter cutworms (Noctua pronuba) have been recently detected in vetch seed fields in western Oregon but significant damage has not been recorded.
Sampling and thresholds Look for armyworm and cutworm larvae around the base of vetch plants, where birds (swallows, crows) are feeding. Dig around in crop residue and at the base of plants. Armyworms and cutworms typically feed at night. Also look for frass (excrement) that resembles tiny grass pellets. Treat when larvae are less than 0.5 inch long and there are >1 per sq ft on seedling stands.
- Bacillus thuringiensis (Bt)-Use according to label directions. For armyworms, effective only on first and second instar larvae Some formulations are OMRI-listed for organic use.
- malathion at 1 to 1.25 lb ai/a. PHI 7 days. REI 12 hr. Avoid applying spray on bloom. Malathion is toxic to all bees. If necessary to control pests when bloom is present, apply only in evening or early morning, when bees are not actively foraging in vetch. Always contact beekeeper before applying. Armyworms only.
- zeta-cypermethrin (Mustang MAX) at 0.014 to 0.025 lb ai/a. PHI 3 days for cutting or grazing; 7 days hay, seed. REI 12 hr. Retreatment interval 7 days. Do not exceed 0.025 lb ai/a per cutting nor 0.75 lb ai/a per season.