True Armyworm 1. Information for BC Growers July 2017

True Armyworm (Mythimna unipuncta, formerly Pseudaletia unipuncta) is a North American insect in the family Noctuidae which is introduced annually, in April, to southern Canada on wind currents from the southern USA and Mexico. Southern areas of Manitoba and Ontario do experience outbreaks of this pest periodically.

Hosts: grass crops, including cereals, forage, and corn are the primary hosts, but true armyworm larvae will feed on broad leaved plants as well, included peas and canola.

Damage: Larvae feed on leaves of grass and less frequently on developing seed heads. Feeding occurs mostly at night, or during cool mornings. Once the area is defoliated the growing larvae move in groups to other grass stands to continue defoliation. Damage in a field can be spotty and variable, and quickly result in devastation if the population of larvae is high. The first generation (larval feeding in June and July) is the worst, and the second generation in August is less destructive.

Biology: Adult moths ‘blow in’ to BC in April, in unpredictable numbers, and then begin laying eggs in grass. Eggs are laid densely, and then larvae feed in close proximity to each other; distribution of eggs and consequently larvae is highly clumped or aggregated. Larvae have 6 instars, with the last two instars resulting in the most feeding damage. The larval stage lasts about 1.5 months, depending on temperature, and they grow to about 35 mm before pupating. The larvae burrow into the soil just below the surface to pupate, within an earthen cocoon, and remain there for 1-2 weeks. The adult moth emerges from the pupae and a second generation of eggs and larvae occur in late summer (August). Adults are brown delta-shaped heavy-bodied moths (3 cm long) that may be seen flying around outdoor lights at night. During the day they hide at the base of plants or other dark protected places.

Monitoring for True Armyworms: Adult moths can be watched for around lights at night, but also by using pheromone baited bucket-type traps (‘uni-traps’) from April-May and in late July and August. Trapping for adults is useful to show when the moths are present and if numbers could be high enough to be of concern. Additionally, trapping moths is useful for predicting when and where larvae will occur. Set traps in areas of concern, such as beside a field that had heavy larvae infestation earlier in the summer.

Field scouting for larvae should being in mid to late June by checking at least 5 areas of a field. During the day, larvae will be down low on the plant or in the thatch. Scout after sunset and larvae will be up on the plants, where they may be easier to count. Larvae can be dislodged by shaking plants over a drop-sheet or panel and then counted. Get an average number per square foot (30x30 cm) over the 5 sampling sites for an estimate of the field density.

Management: Insecticides are available for control of armyworms. Modifying harvest plans may be necessary to limit losses. Consider cutting, baling, or grazing earlier, as well as irrigation and fertilization to encourage regrowth will help limit losses in hay fields. Cereal crops or corn will face yield reductions if they are defoliated early, but will not be as impacted if feeding occurs later in plant development.

The working threshold for insecticide treatment in forage grass/hay is 5 larvae per 30x30 cm. The threshold is lower for annual cereals: 2-4 larvae per 30x30 cm. For the application to be most worthwhile, apply before most of the larvae reach 25 mm in length.

There are naturally occurring biological control agents that feed on or kill true armyworm larvae, including parasitic wasps and flies, ground beetles and rove beetles, and fungal, bacterial, and viral diseases. As well, several bird species will feast on the larvae. These agents do not prevent an outbreak but can curtail damage to a limited extent.

More information:

Insecticides registered for control of armyworm in forage, grass, corn, July 20, 2017
Labels searched on PMRA Label Search site:

PCP #Product nameActive IngredientCropsPre-harvest or grazing interval# applications/year
Group 28
corn (field, sweet), grass forage, fodder, and hay group, non-grass animal feed group, oilseeds and cereals forage grasses: 0 days PHI; forage corn: 14 day PHIup to 4 apps, 7 days apart; use high rate 
5821Malathion Malathion
Group 1 
cereals, grasses, legumes, alfalfa, clover for hay 7 days 1 app 
27876Sevin Carbaryl
Group 1A 
forage, pasture, cereals 1 day up to 2 apps, 8 days apart 
Group 3 
cereals, corn corn for silage: 14 days; cereals: 28 days; sweet corn: 1 day up to 3 apps, 4-7 days apart 
28778Delegate Spinatoram
Group 5 
cereals, field/forage corn cereals: 21 day PHI; forage corn 7 day PHI up to 3 apps, 5 days apart 

Compiled by Tracy Hueppelsheuser, BC Ministry of Agriculture, Abbotsford
phone: 604-556-3031