Tent caterpillars severely defoliating forests around Timmins, Ont.

Close up of forest tent caterpillars on a leaf with water droplets in Timmins Photo by Dan Rowlinson

Forest tent caterpillars may look cute and fuzzy, but don’t let their appearance fool you—these ravenous creatures can strip a tree bare in days. This year, they’re eyeing forests in Timmins, Ont.

Malacosoma disstria are two-inch-long hairy caterpillars native to Canadian forests populated by hardwood trees. They emerge in the night to feed on leaves, and if they’re populous enough, they can clear a forest up to 30 km wide in around two to three weeks, says Carleton University entomologist Jeff Skevington. Once they’re done with a tree, they march down to the ground as a group to search for more food, oftentimes walking across roads and scaling up homes.

“The trees are evolved to cope with this, so they aren’t going to die,” Skevington says. “But the caterpillars can be so abundant that cars will even slide on them as they’re moving.”

According to Health Canada, forest tent caterpillars have white dots on their backs, don’t form tents, and they prefer aspens and poplars. They may settle for many other types of trees, Skevington says, since they’re very hungry. 

The name tent caterpillar comes from a closely related species, the eastern tent caterpillar (Malacosoma americanum). Their brown back sports a thin white line down the middle, and they form silk shelters called tents to fend off predators. They’re also easily confused with the western tent caterpillars (Malacosoma californicum pluviale), who sport blue dots along their reddish-brown backs and don’t form tents. 

Both eastern and western species tend not to overpopulate and completely strip the cherry and apple trees they’re fond of, Skevington says, adding that western tent caterpillars may also like willows, poplars, and oaks. Forest tent caterpillar populations, however, tend to grow in regional cycles where they increase dramatically one year, hit their infestation peak the next, then decline until another eight to 10 years later.

Dan Rowlinson, the provincial program lead for Ontario’s forest health monitoring program, says forest tent caterpillar populations are hitting their second-year peak in Timmins, Ont., this summer. Based on his team’s current ground observations, Rowlinson says forests have lost 75 to 100 per cent of their leaves all the way north to Hearst and south to Lake Englehart along Highway 11, as well as west towards Foleyet along Highway 101.

“It certainly is intense,” he says. “Severe defoliation is the result of our surveys.”

Rowlinson says at least half a dozen Timmins residents, annoyed at caterpillars on their properties, have called his team this year. Some members of the Timmins Facebook group have also shared tricks to remove the insects, including wrapping aluminum foil around the base of trees, applying a layer of soap to the base of exterior home walls, and spraying insecticides.

Aluminum wraps or sticky materials such as honey, which will cause caterpillars to get stuck while climbing, only work on a small scale before becoming tedious, Rowlinson says; they’re best used to save individual decorative trees. He adds that soap is mostly effective at keeping caterpillars off homes instead of trees.

Skevington discourages using insecticides, saying tent caterpillars will become overwhelmed by their natural predators, “friendly flies,” in late June. Using an insecticide risks killing just as many friendly flies as tent caterpillars, which he says will delay the natural cycle and cause more caterpillars to return the next year.

Rowlinson says insecticides may not delay a cycle if used on tent caterpillars with a much larger population in the region, as is the case with Timmins. He recommends using BTK and carefully following the instructions on the label, otherwise it may harm plants or other animals.

Canadian Museum of Nature insect expert Bob Anderson also recommends picking off the caterpillars by hand. It can be difficult to find them all, and leaving even one caterpillar means leaves will continue being eaten, so he says it’s better to call pest control if they become a big issue.

“Some caterpillars have these stinging hairs, but tent caterpillars don’t,” Anderson says. “There’d be no health hazard at all. You can just pluck them off the tree.”

He adds that cottagers can prevent caterpillars from returning in the second-year peak by laying moth traps in early spring.

“When the moths come out in the spring, they lay eggs, and then those eggs will develop into caterpillars,” Anderson says. “Pest control will put a trap out that is scented with a chemical—a pheromone—that attracts one of the two sexes of moths.”

Health Canada recommends disposing of removed caterpillars, eggs, or cocoons in a sealed bag.

All three experts say tent caterpillars only threaten the appearance of a cottage environment; they make trees look unhealthy, but they don’t eat or enter homes or impact other animals. Rowlinson says they’re also a visual nuisance when they start to cocoon because they latch onto solid surfaces such as walls, porches, doors, and gardening tools.

This can drive down the value of a property in the summer, so Skevington recommends that people looking to rent or sell should wait until nature runs its course—most affected trees will regrow stronger leaves in mid-July.

“The cycle is self-regulatory,” Skevington says. “It’s a bit of an aesthetic nightmare, but people will manage.” 

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