Wheat School: An abundance of armyworms


A certain caterpillar that blew in on the wind may be wreaking havoc in your cereal crops. John Gavloski, provincial entomologist for Manitoba Agriculture and Resource Development, joined Kara Oosterhuis for this episode of Wheat School to talk all things armyworm.

Armyworm is different from bertha armyworm in a few ways: their preferred food is grassy plants and they don’t overwinter on the Prairies. Bertha armyworm will not eat grassy plants and do overwinter on the Prairies, he says.

“Armyworms blow in, they’re looking for lush dense grass stands to lay their eggs in,” says Gavloski. This particular year, in Manitoba, Gavloski thinks they blew in on the wind in early to mid June. A lot of the armyworm eggs ended up in taller grass stands at the time — rye, perennial forages, barley, oats, and wheat.

Rye and perennial forages seem to be taking the worst of it this year, but if those fully grown larvae — or caterpillars — get into a crop, they can eat a lot says Gavloski, so it’s really important to scout your cereals and forage grasses.

Armyworm have a lot of variation in their colouring — black, dark green, lighter green, or a beige-like colour. One thing in common amongst them is that they’ll have an orange stripe down the side. They also have two prominent dark lines on their head, which can be difficult to see in the lighter coloured caterpillars.

Armyworm hide under debris or in soil during the day and come out at night to feed, so the method of scouting is different than just using a sweep net. Gavloski suggests using a foot square, shaking the plants in case there are larvae there, picking through debris in that foot square, and checking cracks in soil. Do this in several areas to get a representative sample size.

Any more than four armyworm per foot square in a cereal crop, and you’ll probably have a population that will do damage, says Gavloski. If you notice any clipped heads, then that threshold goes down to two per foot square. For forage grasses it’s five per foot square as the threshold.

If you’ve scouted and have seen what look like egg masses on the heads, they’re actually not armyworm, they’re pupal clusters of a parasitic wasp that attacks armyworm. Even with parasitic wasps, if the population of armyworm is really high, Gavloski says an insecticide is the best control option.

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