Are we overreacting to the tent caterpillar problem?

By Taylor MacPherson
Saskatchewan residents are currently waging war against tent caterpillars.
Although the wriggly pests tend to show up every year, the province is experiencing higher than average numbers of the caterpillars this spring, causing no small amount of distress to some.
Yet despite their shocking numbers, tent caterpillars pose almost no real risk to humans, crops or urban trees.
“Most of the time they’re pretty much just a nuisance,” says Tyler Wist, field crop entomologist with Agriculture and Agrifood Canada, and president of the Saskatchewan Entomological Society.
“They go in ‘boom and bust’ cycles,” Wist told the News-Times, “and right now they’re booming. Typically it’s two to three years, sometimes four, then they’ll disappear for seven to 10 years.”
Currently the caterpillars are in the third year of their “boom” cycle, although the south has largely been unaffected until this season.
“It’s slowly gotten worse, province-wide,” says Wist. “Last year was bad in the north and central regions. Now they’ve come down to the south.”
Following a “boom” cycle, the caterpillar population is typically reduced down to “bust” levels by parasites and disease.
Interestingly, entomologists are able to track tent caterpillar cycles back hundreds, and even thousands, of years through the science of dendrochronology – the study of fossilized tree-rings. “You can see it in the rings,” said Wist, noting stunted growth is often visible in years with higher caterpillar populations.
Although the recent spike in caterpillar population may seem alarming, the only real risk from tent caterpillars is damage to trees caused by repeated defoliation.
“After three heavy years of defoliation they can start to effect the growth of trees,” says Wist. “Other than that, the plants will leaf back out and recover from it.”
The tent caterpillars most often found in Saskatchewan are Malacosoma disstria, commonly known as forest tent caterpillars. This species feeds almost exclusively on the leaves of tress such as aspen, oak and maple, causing almost no damage to crops or other plants (although they may travel through crops in search of food or a place to pupate).
In addition, tent caterpillars are harmless to humans and animals. According to Wist, the caterpillars provide a source of food for both birds and bats, and the only risk posed to humans is “making the roads slick when there are thousands of them squished.”

In terms of prevention, it may already be too late to control the caterpillar population. Most pesticides which target the species need to be consumed, and the caterpillars are feeding less now that their growth cycle is nearly complete.
Wist argues that control methods may not be practical in the first place, when the minimal damage caused by the caterpillars is taken into consideration.
“Is it really economical to spray a bunch of aspen bluffs for tent caterpillars? Probably not.”
If a tree in your yard is home to a large number of the pests, Wist recommends spraying them off with water and then applying a sticky band to the tree.
“All of these caterpillars that we’ve got here,” says Wist, “they’re going to be pupating, and that can be a messy process.”
Wist describes cabin doors being sealed shut by a layer of abandoned cocoons, but notes such damage is almost unknown in more urban areas.
Although the caterpillars are harmless, many residents will be glad to hear that their life-cycle is coming to an end. “Probably they’ll be starting to pupate any day now, if they haven’t already,” says Wist.
After the caterpillars pupate, they end their lives as moths, with the females laying eggs in trees which will remain dormant until next spring.
“They’re just nondescript brown moths,” says Wist. “There will be a lot of them.”

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