They’re small, they’re furry, and they can strip a forest of leaves in a matter of weeks. Thousands of forest tent caterpillars are invading Sudbury and other northeastern Ontario areas this summer.
The two-inch insect is known for its population outbreaks. Every 10 or so years, forest tent caterpillar populations boom with the critters coating boreal forest trees like wallpaper. They’re North America’s most widespread defoliator of hardwood, with a particular taste for trembling aspen and other poplars.
They emerge at night, feeding on leaves. Once they’ve stripped a tree bare, they’ll search out their next food source en masse. They’ve been known to cross roads and even scale people’s houses.
But preparing in advance for these outbreaks is trickier than you’d think. “Once you do a deep dive, you realize that predicting the timing and intensity of these semi-periodic eruptions is as challenging as forecasting the weather,” says Barry Cooke, a research scientist with the Great Lakes Forestry Centre in Sault Ste. Marie.
Cooke has published several papers on the population dynamics of forest tent caterpillars and even he admits that researchers haven’t quite figured out why the outbreaks happen.
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Forest tent caterpillar outbreaks have been recorded in Canada since 1791. What researchers have discovered is that population explosions happen when there’s a breakdown in natural controls. Predators such as birds, ants, beetles, parasitic wasps, and friendly flies fail to suppress the population.
The species’ larvae thrive during warm, dry springs, collecting in northern boreal forests where there is an abundant source of hardwood trees. The insects have even been found north of the 60 degree mark in the Northwest Territories. And climate change is pushing them further north.
But this doesn’t exempt Ontario’s cottage country. While outbreaks are less common in the Muskoka, Haliburton, Kawartha, and Frontenac areas than they are around Sudbury, Cooke says the forest tent caterpillar “has been cycling there throughout the entire Holocene, the last 10,000 years.”
The caterpillars hatch in the spring and feast on leaves for six weeks before spinning yellowish cocoons. Later in the summer, they’ll emerge as forest tent caterpillar moths.
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Despite their wriggling appearance and populous numbers, researchers stress that forest tent caterpillars are harmless. In fact, their cyclical population is an important part of the forest ecosystem. The trees grow back sturdier leaves later in the summer. Plus, these outbreaks generally only last one season, says Cooke.
But on rare occasions, the caterpillar outbreak can last three to four years. That’s when you can get some tree death, says Cooke. And this is exacerbated if the forest tent caterpillar’s habitat overlaps with other outbreak-inclined species, such as the budworm or the forest tent caterpillar’s cousin, the eastern tent caterpillar. (Although, these species tend to prefer other types of foliage.)
If forest tent caterpillars have wriggled their way onto your property, experts suggest removing them with water from a hose or sweeping them off with a broom. You can also apply a layer of soap around the base of your home to prevent the caterpillars from climbing your walls. And wrap a layer of aluminum foil around the base of any trees you’re hoping to protect.
“These outbreaks are ‘slow-fast,’” says Cooke. “They can take a long time to reach an eruption threshold, but when they reach it, they rise quickly.”
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