Wheat School: Fending off an armyworm invasion

True armyworms hungry for grassy plants, including wheat, have arrived in large numbers in parts of the Prairies this summer.

The pest, which migrates north as a light brown moth, arrived in Manitoba during the last week of May, explains John Gavloski, entomologist with Manitoba Agriculture, in this armyworm-focused Wheat School episode filmed at the 2023 Manitoba Crop Diagnostic School.

“This year there was a really big migration that came in,” he says, noting armyworms have been found in all agricultural regions of Manitoba, as far north as The Pas.

As he demonstrates in the video, scouting for armyworms includes shaking plants to knock any climbing larvae to the ground, and then counting the number of larvae or caterpillars in a square foot. Since they’re nocturnal feeders, they may hide under debris or in cracks in the soil, he notes.

The economic threshold for armyworms in wheat and other small grains is four larvae per square foot. If they’re clipping wheat heads, the threshold drops to two per square foot, explains Gavloski.

Depending on temperature, the larvae will start pupating and likely go through a second cycle during the growing season, but the second cycle typically isn’t as damaging due to a possible combination of parasitoid activity and cereal grains approaching maturity, he says. In research studies where the larvae were kept at a constant temperature, the pest remained at the larval stage for 16 days at 29 degrees Celsius, 26 days at 21 degrees Celsius, and 40 days at 17 degrees Celsius.

Armyworm moths

As crops mature or are harvested, he suggests keeping an eye on neighbouring fields, including canola, soybeans, and other crops.

“It’s not where they want to be, but it’s where they might end up once they’ve eaten what they prefer,” he says. “If you do see a big movement of them — they call them armyworms because they do defoliate a stand pretty severely and they can move across the road, say into a neighbouring crop — just do an edge pass along the secondary host, and that is usually enough.”

White masses that look like an egg sack near the top of wheat plants are a sign a parasitoid — the cotesia wasp — is infecting armyworms, explains Gavloski. The wasp lays its eggs inside the caterpillar, and causes it to climb to the top of a plant, at which point the cotesia larvae climb out of the armyworm and spin the highly visible pupa cases.

Watch the video above for more on armyworm scouting, spray thresholds, natural predators, and more from John Gavloski at the 2023 Crop Diagnostic School in Carman, Man.

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