True Armyworm 5. June 2018

True armyworm (Mythimna unipuncta, formerly Pseudaletia unipuncta) is a North American agricultural pest in the insect family Noctuidae. 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. 


True armyworm moths may migrate to B.C. in April-May in unpredictable numbers, influenced by weather, temperature, winds, and conditions in their originating locations, which are more southerly locations in North America. Moths can live for weeks, and seek lush green grass for egg laying. Eggs are laid deep within folded grass blades and are nearly impossible to find in fields. Both egg masses and larvae can be abundant in localized areas. 

There are 6 larval instars, with the last two instars causing the majority of feeding damage (Figure 3). The larvae stage lasts approximately 1.5 months, depending on temperature, in which time the larvae grow from 4 mm up to 35 mm. Young larvae mostly feed during the day and are well hidden by grass blades. Older larvae are nocturnal feeders and move up the plants, and rest at the base of plants and under grass canopy during the heat of the day. Larvae are marked with stripes running the length of their body, with a yellow/brown head capsule with a netted appearance (Figure 5), while the body colour ranges from pale green while young, to nearly black when full sized. When larvae are ready to pupate, they burrow into the soil just below the surface where they remain for 1-2 weeks (Figure 6). 

Adult moths emerge from the soil and a second generation occurs in late summer (August-September). Adult moths are about 2cm long with brown, delta-shaped bodies and have a distinguishing white dot on each wing (Figure 4). Adult moths are nocturnal and may be seen flying around outdoor lights at night, while hiding at the base of plants or other dark protected areas during the day.