Western yellow striped armyworm, Spodoptera praefica (Grote), is known only from the western states, principally California, Washington, and Oregon. In California, S. praefica is much more important than the eastern yellow striped armyworm, S. ornithogalli.
Both species can be serious pests of alfalfa, especially in early fall following a hot dry summer which concentrates the larvae into “armies”.
There apparently are as many as three generations annually, with broods of adults present in March to May, May to June, July to August, and August to November. Some of the latter brood of yellow striped overwinters as pupae rather than emerging as adults.
Although eggs, larvae and adults of western yellow striped armyworm may be present in autumn or early winter they cannot withstand cold weather, and perish. Buildup over the past 11 dry warm winters in the PNW is enabling buildup of this pest. Development time, from egg to adult, is about 40 days.
Insecticide treatment of mass attacked fields is needed if the larvae are causing damage to the crowns of plants that will overwinter or to stands recovering from cutting which will be cut again.