Several species
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 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 clover seed fields in western Oregon but significant damage has not been recorded.
Scouting and thresholds Look for armyworm and cutworm larvae in and around crowns, 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. Treatment in established clover seed 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 and cutworm adults are believed to migrate into Oregon from California in summer. Eggs are laid in grasses and other spaces with strong plant density. 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
Management- biological control
Populations of noctuid moths are generally kept under control by a number of natural enemies that include parasitic wasps (Trichogramma species and braconid wasps), parasitic flies in the family Tachinidae, nematodes and several bacterial and viral pathogens. Natural predators will feed on cutworms readily, including predaceous ground beetles, birds and rodents. However, the impact of these natural predators on cutworm populations in Pacific Northwest production systems is unclear.
Management-chemical control
- 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.
- chlorantraniliprole (Vantacor) at 0.047 to 0.098 lb ai/A. PHI 0 days. REI 4 hr. Do not make more than 4 applications per calendar year. Do not apply more than 0.2 lb ai/A per calendar year. Can be applied in-furrow at planting or as an overhead foliar spray. Is most effective through ingestion of treated plant material. Apply at egg lay, egg hatch or when larvae are newly hatched. Armyworms only.
- malathion (Malathion 5EC) at 1 to 1.25 lb ai/A. Minimum 14-day retreatment interval. Maximum 1.25 lb ai/A per cutting. PHI 0 days. Group 1B
Note: When bifenthrin is used to control other pests, armyworms and cutworms are effectively controlled as well.