Armyworm on the march in Manitoba

Cereals and grasses are taking a hit from armyworms, one of several pests that tend to blow in as adults from the south

Manitoba fields display the colour spectrum of armyworm larvae colouration.

Manitoba has an armyworm problem.

Eastern, central and southwestern Manitoba, as well as the Interlake have all reported spraying from farmers looking to keep the pests from chomping down on their cereal and forage grasses, according to Manitoba Agriculture and Resource Development.

“There’s been some control in pretty much every agricultural region except the northwest,” provincial entomologist John Gavloski said.

“It can be patchy,” he added. “It’s not every field. There are some fields you can find a lot and other fields, very little. A lot depends on what stage things were at when they were laying their eggs. In some areas, it seems they preferred to lay their eggs in some of the ditch vegetation, because it would have had a lot of lush, green, grassy material. Other places, it was the perennial grasses, your ryegrass and fall rye that seem to have the most egg laying.”

Why it matters: Many areas of the province have already fought with grasshoppers this year. Now, forage and cereal grasses have armyworm to contend with.

Some cereal crops have also noted armyworm problems, he said.

Not to be confused with its canola-munching cousin, bertha armyworm, the true armyworms currently causing issues in Manitoba fields prefer grassy plants over broadleaf crops, although Gavloski says they will consume broadleaf plants in some instances.

Other players

In some areas, armyworm is just the latest insect pest to threaten grasses and pastures.

Most regions of the province have also reported control efforts for grasshopper, although grasshoppers should be largely done their highest period for damage, Gavloski noted.

“Same story with the armyworms,” he said. “It’s not every field that would need it, but there are hot spots and people have been controlling them either full field or edge control for the grasshoppers.”

Both edge treatments and full-field spraying for grasshoppers have been noted in eastern Manitoba, joined now by treatments for armyworm.

Like diamondback moth, this year’s army worm problems can also be linked to winds blowing in adult moths. photo: John Gavloski

In the Interlake, producers have reported perennial ryegrass, fescue and timothy grass falling prey to armyworms.

At the same time, more areas in the Interlake have become grasshopper, “hot spots,” according to the July 7 weekly crop report, and the pests are now starting to move on from the region’s pastures to its annual crops.

The Interlake has, thus far, faced down another tough pasture year due to short precipitation, despite rains that have brought excessive moisture to other parts of the province.

As of the July 7 crop report, many pastures in the Interlake were still considered poor.

“It certainly can (compound things) if there’s other factors such as drought or other factors that are keeping the pastures from growing vigorously, then yeah, then the armyworms would compound that,” Gavloski said.

Ray Bittner, provincial livestock specialist in the Interlake, also noted cutworm damage in greenfeed and silage crops in his area in June.

Deciding to spray

The province puts the economic threshold for armyworm at four or more larvae per square foot if cereals have yet to head.

Those numbers drop, however, if farmers are actually losing seed heads. Once farmers start to see head clipping in the field, provincial guidelines suggest pulling out the spray at two worms per square foot.

For forages, the province suggests spraying if five or more armyworms beneath 2-1/2 centimetres long are found per foot. For seedlings, those numbers drop to two to three insects in the same space.

Levels of parasitism will also impact the decision to spray, provincial guidelines caution. Larvae that have fallen victim to a parasitoid insect, such as Manitoba’s various parasitic wasps and flies, have little time left to damage crops before the wasp or fly eggs hatch and make a meal of the armyworm.

Some of those flies lay their eggs right behind the head of the armyworm, the province says, urging producers to look for eggs along the backs and sides of the worms, and to factor in the number of parasitized larvae when deciding whether or not to spray.

“There is little benefit in applying an insecticide once the armyworm is nearly full grown, pupae are present, parasitism is extensive, or the crop is nearing maturity. By that time most of the damage will have been done,” provincial guidelines say.

Even then, the province urges patch control when possible.

If the producer does decide to spray, they may want to wait until evening, when the armyworms are feeding, the guidelines say.


Producers should shake a few plants as they go through their field to see what comes loose, the province said, as well as checking soil cracks and beneath soil cover where armyworms might be hiding.

“Pay special attention to patches of lodged plants,” the July 1 provincial pest update urged, pointing to the better environment for hiding within lodged crops.

Birds may also give a hint that there’s a problem, according to the province, since birds tend to pick armyworms off leaves and birds gathering in a specific part of the field should inspire the producer to go scout.

“Anyone who is growing either cereals or forage grasses, it’s good if they do go out and have a look, just to see what the armyworm level is like,” Gavloski said.

Looking at the pest’s life cycle, Gavloski noted that Manitoba could see another armyworm flush later in August, “but normally it’s happening late enough that it’s not a big concern,” he said.

The province estimates 16 to 19 days for armyworms to cycle out of their larval stages, assuming temperatures are around 25 to 30 C.

About the author


Alexis Stockford

Alexis Stockford is a journalist and photographer with the Manitoba Co-operator. She previously reported with the Morden Times and was news editor of  campus newspaper, The Omega, at Thompson Rivers University in Kamloops, BC. She grew up on a mixed farm near Miami, Man.



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