Winnipeg experiences 'year of the caterpillar' as 3 worm species wriggle across treetops

Already infamous for its insects, Winnipeg is experiencing an arboreal infestation entomologists say they haven't seen in decades: three different species of caterpillars crawling about the city's treetops at once.

Elm spanworms are infesting the inner-city tree canopy in neighbourhoods on either side of the Assiniboine River, from The Forks to Wolseley and Wellington Crescent.

The emergence of a large population of the purplish-black worms follows an even larger, city-wide infestation of forest-tent caterpillars and a small emergence of cankerworms.

Winnipeg insect-control branch superintendent Ken Nawolsky said he's never seen all three species chow down on the city's tree canopy at the same time.

"It's definitely the year of the caterpillar. We do have a lack of mosquitoes this year, so some other insect is replacing it and it's the caterpillars," Nawolsky joked in an interview on Thursday.

"This has been a very unusual year. Typically what happens is when you have the forest-tent caterpillars peaking, you don't really have other tree pests out there at the same time."

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Elm spanworms, which somewhat resemble cankerworms, feed on the leaves of elms and other hardwoods. They chew away at all soft parts, leaving behind only major veins and spines — the "spans" that give the moth larvae their name.

Elm spanworms are larger than cankerworms and can be identified by their orange heads and grippy, pincer-like feet. They also produce large volumes of black excrement that's bigger than cankerworm droppings but not as large as forest-tent caterpillar pellets.

"When they're sitting there digesting the leaf meal that they've eaten, they're pooping it out and that'll sit on your car, sit on your sidewalks, on the roads, and if it rains like it has over the last couple of days, it can be a slippery mess," said Taz Stuart, technical operations director at pest-control company Poulin's.

Nawolsky said Winnipeg's elm spanworm population is the largest he has seen in 20 years.

"We've noticed a big increase in their population this year," he said, adding Winnipeggers who don't venture downtown or into nearby neighbourhoods may not even notice this.

"Elm spanworms tend to be along the riverways, for whatever reason, and they don't go out into the suburban areas. That's kind of been puzzling to us as to why they seem to like those areas."

Weather permitting, Winnipeg's insect-control branch plans to spray trees in several inner-city neighbourhoods with the bacterial insecticide BTK this weekend in an effort to reduce the spanworm population. 

He said he expects elm spanworms to continue attacking elms for another three weeks. Cankerworms, which are green or dull brown and have more bulbous legs, are also expected to remain for several weeks, after hatching later than usual, Nawolsky said.

After first emerging in May, the fuzzier forest-tent caterpillars, which prefer to eat aspen, are starting to enter the coccoon phase, he said.

"It's been one of those weird springs," Stuart said. "We have them all at once."

The spanworms that don't get eaten by squirrels or birds will transform into pale white moths in August, Stuart said. Cankerworms will become dull grey moths at roughly the same time.

Forest-tent caterpillars turn into golden-brown moths, usually in July, Stuart said.

Worm species wriggle across treetops

7 years ago

Duration 1:49

Already infamous for its insects, Winnipeg is experiencing an arboreal infestation entomologists say they haven't seen in decades: three different species of caterpillars crawling about the city's treetops at once.