Manitoba Agriculture entomologist John Gavloski is getting a lot of reports of armyworm damage.
The crop pests are now found at economic levels across the province.
“They will migrate north come springtime, and what we get is really a hit-and-miss situation,” he said. “Some years, we barely see them. Some years, it’s very localized. This year, not so lucky.
“It seems like a big population did migrate in, and not just to a particular part of the province; I’m getting calls from every agricultural region on them: northwest, southwest, central, Interlake, eastern — you name it.”
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In some years armyworm populations can get to levels that can cause economic damage to crops — this appears to be one of those years.
To some extent, the infestation came as a surprise to Gavloski. Manitoba Agriculture normally puts up pheromone beta traps early in the season. But this year, issues with their supplier meant they didn’t have enough of the proper traps.
Traps that were put out, however, picked up fairly heavy levels in eastern Manitoba in late May and early June.
“I was expecting high levels in the east. I was kind of caught off guard by the higher levels in the west,” Gavloski said.
What’s on the menu?
Fall rye is one of the crops where armyworms are chowing down.
“What they really like is a thick, dense grass stand to lay their eggs into,” Gavloski said. “So often, crops like fall rye and winter wheat are preferred egg-laying spots.”
But Gavloski said they can be found in spring cereals as well.
“I’m getting calls for pretty much all of the cereal crops, forage grasses and even pastures this year.”
With the numbers racking up, it’s probably wise to scout for armyworm, even in crops that are traditionally less bothered by the pests. Gavloski noted that occasional high populations are seen in corn. Armyworms are much less of a concern on broadleaf plants, but there are exceptions.
“Very rarely do we see them at high levels [in] dry beans or peas,” he said.
They’re even more rare in flax and canola, but can feed on these plants if they need to.
What to look for
Younger armyworms are a tricky to identify.
“They’re tiny little things, and they’re not hiding on the soil like they do when they’re big guys. The first couple of stages will be up on the leaves a bit more during the day,” Gavloski said.
At their early instars, larvae skeletonize the surface of the leaf blades or the inner surface of the sheath and later feed from the margins of the leaves, consuming all the tissues.
“You’ll see big, ragged edges of the leaves, and sometimes eventually the leaves might disappear,” Gavloski noted.
In most cereal crops and forage grasses, armyworms feed exclusively on the leaves until foliage is gone before they start clipping the heads. The exception is timothy grass.
“Timothy seems to be a crop they really like, and what we notice is that they tend to do more head feeding earlier than in other crops,” Gavloski said. “If you’re a timothy grower, that’s something you definitely want to watch for.”
Life cycle
Armyworms grow to about three and a half centimetres long when fully grown.
Perhaps predictably, the closer they get to being fully grown, the more they feed. That rapidly drops off, however, once they approach maturity. At that stage, Gavloski noted, they’ll feed for a few days, clean out their gut and then pupate.
“If most of the caterpillars in your field are fully mature and you notice that things are starting to diminish a little bit, that likely means they’re starting to turn to pupae. Unless you’ve got a very severe population, when you get to that point you can just ride it out.”
The length of time before the pupating stage depends on temperature. This year, Gavloski expects that will take roughly two and a half weeks, looking at Manitoba’s growing season so far. The pupa stage is also temperature-dependent, and Gavloski suggests it would be roughly 10 days before adults emerge and start laying eggs again.
Gavloski said Manitoba typically gets two armyworm lifecycles in a season, but only the first is usually a concern.
“Cycle No. 1 is the one that causes all the damage,” he said. “By the time we get to cycle No. 2, the crops that they prefer to feed on are gone.”
How many is too many?
In small grains, the general guideline for economic levels of armyworm is about four or more larvae per square foot.
“There are fields that are very legitimately above that, and spraying is going on in almost all our regions to some degree as we speak,” said Gavloski.
“If the crop is heading and if you are starting to see head clipping, drop that threshold to about two or more.”
In forage grasses, Gavloski says the threshold is five or more larvae per square foot.
Scouting techniques
“What you’re looking for is mostly the larger larvae,” said Gavloski.
He recommends measuring the area that is to be assessed, giving the plants a bit of a shake to knock off any larvae that might still be on the plants and then checking the soil for armyworms.
“Keep in mind that they are good at hiding and they like to hide,” he said.
He suggested pulling away stubble and trash on the soil before checking and carefully checking cracks in the soil.
“As long as you’re looking carefully at the ground, you don’t need to be out there at night with your flashlight.”
He also noted that infestations can be patchy.
“They like a nice, thick, dense grassy canopy to lay their eggs into. So if there are any lodged areas of your field, plants that were kind of flattened or knocked down from rain or for whatever reason, you will almost always find more of them in those lodged areas,” he said.
Gavloski said if farmers find a population that exceeds the threshold, they should keep in mind that the caterpillars are nocturnal. Spraying needs to be done late in the day or in the evening.
“You’ll get better contact, and they’ll be feeding on freshly applied insecticide.”
Predators and parasitoids
“If you’re on the borderline of the threshold, but you’re seeing a lot of signs of parasitism, that might be something you can factor into your decision-making,” Gavloski also noted.
Farmers with a highly parasitized armyworm population may see what looks like a web-cased egg cluster at the top of cereal and forage grasses.
“These are not eggs; these are pupal clusters of a parasitoid wasp called cortesia,” he said.
Another sign is the presence of a white egg on the side of some of the armyworms. This is likely evidence of tachinid flies.
Both tachinid flies and cortesia lay eggs on armyworms, and their larvae will feed on the host, eventually killing it.
Predators that like to eat armyworms include ground beetles, rove beetles (which will eat small armyworms), damsel bugs, wolf spiders and birds.
Gavloski said that, while all these predators and parasitoids can help keep populations of armyworm under control, they won’t help curb a really bad infestation.
“If you’re well above threshold, like some of the fields we’re seeing, then you have to spray regardless,” he said.
