Forest tent caterpillars feast in Calgary as outbreak spikes population across Alberta

In the se Calgary community of interests of mahogany tree, Samantha Tong was bemused when she thought she saw the skin on a tree moving — until she looked closer.

"It literally looks like a big patch of dark bark that's about to fall off, and as you get closer to it, you realize the whole thing is fuzzy and moving," she said.

Upon closer inspection, Tong realized the tree was covered in forest tent caterpillars.

"Then, when we were walking around the playground, we noticed that there were like probably seven other trees that had them," she said.

"They're there on a regular basis now, and I'm noticing more and more popping up on trees," she said.

She's not the only one who's noticing a caterpillar population spike: Red Deer, Olds and Devon are among the municipalities advising residents of an increase in the fuzzy invertebrate's numbers this spring.

Forest tent caterpillars are appearing in larger numbers in Calgary

Olds College entomologist Ken Fry says forest tent caterpillars are native to Alberta and relatively common, but their populations go through cycles in which they increase dramatically.

"Roughly every 10 years, every decade or so ... Populations increase enormously," he said.

Fry said the cyclical population explosion is called an outbreak.

He said the causes of these cyclical outbreaks are still being studied but are believed to be influenced by a variety of factors including weather, health status of trees, and other factors like predators, parasites and disease.

Forest tent caterpillars are perhaps best known for the damage they inflict on trees.

"When it comes to your trees, they're going to eat all the leaves," Fry said.

"The eggs usually hatch right in the spring, just as the buds are flushing on the poplars., but if they haven't flushed yet, they'll even get right into the buds and start chewing on the buds," he said.

Fry said the caterpillars will most likely defoliate their host tree before moving on to the next tree — but in most cases, a healthy tree can rebound and grow back its leaves.

"Early season defoliation, it may look bad, but the tree can usually recover," he said.

But if the tree's not entirely healthy, it might be a different story.

"If it is drought-stressed, if the roots have been compacted through construction or activities around the root zone, if it's somewhat diseased from other reasons, then the tree won't be able to rebound as easily, and that's a bit of a stress on the tree," Fry said.

That stress could put it at risk of other pests, like wood-boring beetles.

"Trees can usually withstand a one-season munching, but when it comes to prolonged persistent defoliation over two, three years, that can result in some twig death or branch death or die back, you know, vulnerability over the winter to winter kill, things like that," Fry said.

"You don't want to have this going on year after year after year. You might want to intervene in subsequent years."

If they're a problem, solutions other than pesticide include manually collecting the caterpillars and putting them in a freezer or drowning them, Fry said.

A biological pesticide called bacillus thuringiensis kurstaki, or BTK, is also a popular solution. The spray essentially destroys the caterpillar's digestive system when consumed, and doesn't affect non-target insects or other animals.

The larva stage makes up most of the forest tent caterpillar moth's lifespan. It spends about five to eight weeks as a caterpillar, metamorphoses after about two weeks in a cocoon, and then only lives up to ten days as an adult moth.

"That can be problematic in an outbreak season, where you have hundreds if not thousands of moths flying around, you know, clustering around lights over your door, something like that," Fry said.

The moths lay egg clusters in trees, where they are kept insulated to survive the winter before hatching the following spring.

Fry said if forest tent caterpillar eggs are found in a tree, it could be beneficial "to nip those little egg cases off so that there's nothing to hatch out come springtime."

Kath Smyth with the Calgary Horticultural Society stressed the importance of controlling caterpillar populations during the ongoing outbreak.

"They'll appear now for about four or five weeks," the horticulturist said. "What worries me is that if we're not vigilant ... To deal with them and eliminate them, we will end up with quite an infestation, because they'll overwinter, they'll overseason, and they'll come back twice as much."

The federal government says forest tent caterpillar outbreaks have been recorded since the 1930s, and that they're considered responsible for the defoliation of 4.8 million hectares of Canadian forest in 2015 alone.

But while the damage caused by forest tent caterpillar outbreaks can put trees at risk, they're considered a natural part of Canada's natural ecology.

The caterpillars are a valuable food source for numerous predators, and caterpillar feces - known as frass - is considered a highly effective fertilizer.

"Just as always with any kind of animal, if you can put it into context, yes, it may seem like a plague right now, but when you look step back and look at it from a larger ecosystem level, it is actually doing you a service," Fry said.

He said a tree that's not recovering from forest moth caterpillar defoliation might be a sign of other underlying problems with the plant.

"It's weeding out the the weaker members of that tree population, and, or, it's highlighting that your tree in your backyard or in the park is actually suffering," he said.

"If it's going to go into serious decline from an outbreak like this, then perhaps you have to think about, 'Well, how should I improve the care of my trees?'"

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