City sees outbreak of hairy, multi-coloured caterpillars
Crawling and clustering together along tree trunks and branches, they have long threads of golden hair, keyhole-shaped white spots and radiant blue stripes. They’re forest tent caterpillars and, thanks to an outbreak, you might spot them around Montreal this spring.
Tent caterpillars can be an “extreme forest pest,” said Jim Fyles, a forest ecologist.
“People talk about highways or roads being just absolutely covered with them to the point that they ’re slippery to drive on,” Fyles said.
Here in Montreal, we’re not likely to see outbreaks at that level. Tent caterpillars are common to Quebec and always turn up around this time of year, Fyles said.
Yet forest tent caterpillars, part of the larger tent caterpillar family, are particularly intense right now. Entomologist Stéphanie Boucher thinks this will be the final year of a three-year outbreak of the fuzzy insects, which will then go into their cyclical decline for another seven to 11 years.
While they form a natural part of our ecological system, they are defoliators, which means they can eat all the leaves from a tree.
“Most of the trees will recover from this, so it normally doesn’t kill the tree. But if you have defoliation of the same tree multiple years in a row, it will weaken the tree,” Boucher said.
Home gardeners and other tree stewards can take natural measures to remove forest tent caterpillars, who rest in large groups to protect themselves from their predators, including skunks and raccoons.
Boucher said the most effective way to eliminate the caterpillars is by finding their eggs, laid in rings around tree branches in early spring. Perched high in trees, those eggs can be hard to find. The next best method is removing mature caterpillars from trees and drowning them in water and plain household soap. The caterpillars are not poisonous.
Eastern tent caterpillars, another variety, build characteristic silk tents where they rest at night and take refuge from rain. Clipping the branches where the caterpillars build these tents and throwing them away is an efficient control method.
“If I was a backyard person and had an apple tree that started to get these nests on it, I would just try to take them off. It’s the kind of thing that if you sort of let it go, they’ll defoliate the tree,” Fyles said.
Unlike eastern tent caterpillars, forest tent caterpillars secrete silk by creating a silk sheet around tree trunks. They are often found on aspen, oak and maple ash trees and are typically five centimetres long in their maturity.
They are born only once a year. The caterpillars will spend spring eating, then go inside cocoons in July. Small, brownish moths emerge in August, which go on to mate and lay their eggs.