During April and May, true armyworm moths may migrate from southern USA and Mexico on wind currents to parts of Southern Canada including B.C., Manitoba, and Ontario. True armyworm is not known to overwinter in Canada. True armyworm prefers grass crops such as cereals, pasture, grass hay and corn. However, under high populations, true armyworm larvae may also feed on broad leaf plants. Adults feed on flower nectar or other sweet sources.
Young larvae will skeletonize grass foliage and eat small holes in leaves. From the third instar and older, larvae will devour entire leaves. Larvae will also sometimes feed on developing seed heads and corn tassels and ears. Once an area is sufficiently defoliated, larvae will move in a group to other grass stands to resume feeding. True armyworms can cause significant but often patchy crop damage in only a few days, and can cause rapid crop devastation when the larvae population is high. In southwest B.C., the first generation of larvae, feeding in June and July, can cause significant crop loss to grass hay and cereals. The second generation of larvae in August and September can be even more destructive, affecting both grass and corn crops. 2017 was the first year damage was recorded for this pest in B.C.