Army Worm Q&A

  1. What do the worms look like?

The “worms” are caterpillars (larvae) up to 1.5 inches long (3.5 cm), with brown to black bodies with tan stripes on sides. Heads are tan and mottled with darker lines. Larvae have 6 instars (stages) with the last three stages being the most damaging to grass hay and corn crops.  Larvae feed for 4-6 weeks, depending on temperature.  There are two generations of larvae in B.C.: June-July and August-September. They molt their skin as they grow.

  1. What do the adults look like?

Moths:  heavy bodied, triangle/delta shaped, tan brown, night-flying.  They emerge from pupae in the soil in late July and August. They live within the plants low to the ground during the day and fly during the evening and are attracted to light.  Moths are present in spring (May-June) and from late July-August.

  1. When do moths appear and what is the cycle of True Armyworms?

The adult moths arrive in April-June on storms or wind currents from the south and lay eggs in lush grass. The first hatch of the larvae in 2017 was in late June. Once larvae are done feeding, they dig into the soil and pupate for 2 weeks.  Moths then emerge and a second generation of eggs and larvae occur in August and September.  This insect is NOT expected to overwinter in B.C.

  1. Can you spray for them?

Yes, if you scout (late June) grass and corn fields 1-2 times per week, and if seen, consider cutting the grass crop and then spray after while larvae are exposed and not protected by the canopy. Scout the same areas for the second generation of larvae starting the third week of August.   The action threshold for grass hay is 5 larvae per 1 square foot and size on average ¾ inch.  Larvae may only be present on field edges, in spots, or limited areas in a field. Corn can tolerate more larvae, however, if a few larvae are seen per plant and larvae are still small, spraying corn may be worthwhile. Scouting is key to most efficient and effective control—knowing the best timing and location of the pest.

  1. What deterrents and control methods are there?

Insecticide is used as a foliar spray when caterpillars/larvae are detected.  There are no seed or soil treatments that can prevent armyworm outbreaks or protect grass crops.  Traited corn seed (VT2P, G2, stacked, G8) options are available. Armyworm outbreaks are variable and difficult to predict: every 5-20 years.

  1. What do they eat?

Grass crops, including cereals, forage and corn but larvae will feed on broad leaf plants as well (peas, canola and other crops/vegetables) when grass hosts are not readily present.  Damage has been significant in grass hay and corn in B.C. in 2017.

  1. Do they inhabit all soil types?

Yes, Armyworm can be found across various soils.

10.Where did they come from?

Moths are introduced annually in the spring (April-May) to Southern Canada on wind currents from the southern USA and Mexico.

QA.Army Worm