A species of invasive moth caterpillars is stripping trees bare and raining down feces from the canopy across much of southern Ontario and Quebec, amid a record outbreak. Here's why their population has exploded and what can be done about them.
What are these moths, and what do the adults and caterpillars look like?
The moths are known as the "LDD" moth because of their scientific name Lymantria dispar dispar. LDD moth is the name preferred by Ontario's invading species program, which says its original common name of "gypsy moth" is derived from a culturally offensive slur.
The caterpillars grow up to six centimetres long. They're hairy and can be identified by the five pairs of blue dots and six pairs of red dots on their backs.
The adult male moths are brown and fly around. The adult females are larger, white and can't fly.
Where are they found?
At this point, they're found throughout much of southern Canada.
They're native to Europe and Asia, but were introduced to the U.S. near Boston in the 1860s by astronomer and amateur entomologist Étienne Trouvelot, who wanted to test their potential for manufacturing silk. They escaped and became an invasive species
They first reached southern Canada in 1969, according to David Dutkiewicz, entomology technician at the Invasive Species Centre, a non-profit organization based in Sault Ste. Marie, Ont., a conservation organization focused on preventing, detecting, responding to and controlling invasive species in Canada.
The current population explosion is mostly in southern Ontario, although Quebec reports outbreaks in the Montérégie region and near Montreal (Boucherville, L'Île-Perrot, parc national du Mont-Saint-Bruno), as well as in the Outaouais region, along with smaller populations in the Mauricie region and near Quebec City.
What kind of damage do these moths do?
These moth caterpillars have voracious appetites and eat a wide range of food, including oak, birch, poplar, willow and maple, unlike many other caterpillars that are pickier eaters. They also feed for a period of the year about twice as long as many native caterpillars, said Joel Harrison-Off, a forest health-care inspector with the City of Toronto. And while they're eaten by some birds and mammals, none of them consume enough to really put a dent in the population.
When there's an outbreak, they can completely strip trees bare of leaves, as they've done to the oak trees in Toronto's High Park.
Generally, trees can recover. But some tree species, such as oak, have a harder time regenerating foliage.
Harrison-Off said the stress that puts on the trees and the energy that requires, especially if climate conditions such as drought are also stressful, makes it harder for them to defend against pathogens.
"These trees, some of them will start to die back in the next few years as a result of being defoliated."
How bad is this year's outbreak?
These moth outbreaks are cyclical, leading to an outbreak or infestation every decade or so and lasting one to three years. Numbers rise in years when weather conditions are good, then decline due to fungal or viral infections that spread through the population. Previous outbreaks have happened in 1985, 1991 and 2002.
The current outbreak in Ontario started in 2019, and the next year, the moth caterpillars defoliated more than 580,000 hectares — a record. "You're looking at the size of Prince Edward Island, basically, that was defoliated last year," said Dutkiewicz.
He said this year's final tally won't be out until August, and right now, we're on track to see a similar amount of defoliation and "another record-breaking year or close to a record-breaking year."
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In comparison, 1985, the worst previous outbreak, topped out at 350,000 hectares of defoliation.
Harrison-Off said this year, LDD moths have been seen in parts of Toronto where they haven't been seen before, including people's properties: "We're getting a lot of reaction, and people are trying to deal with and learn about [LDD] moths."
Why is it so bad?
Partly, it's due to the natural cycle. But this year's weather conditions were also ideal, Dutkiewicz said — a warm winter, followed by a dry spring.
The moths can't survive below –20 C for long, which is why they're restricted to southern Canada. And a wet spring helps grow a fungus that kills them.
"We haven't had those types of natural events ... in order to knock back the population," he said. "It really has been the perfect series of events of beautiful weather."
Will climate change make it worse?
David Featherstone, senior ecologist at the Nottawasaga Valley Conservation Authority about 100 kilometres north of Toronto, predicts that the moths will start spreading north into central Ontario.
Droughts and extreme storms can also put stress on trees and forests that can make trees less able to withstand being stripped of their leaves by the LDD moths and having to regrow them again, he added.
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What can be done about LDD moth infestations?
Some Ontario municipalities such as London, Ont., spray a bacterial insecticide call BTK to control outbreaks. Dutkiewicz said some conservation authorities also do targeted spraying to protect "high-value trees." But Ontario Parks says it is letting nature take its course.
Harrison-Off said BTK has the potential to kill a lot of beneficial and native caterpillars in environmentally sensitive areas like High Park.
Instead, the City of Toronto does targeted spot treatments.
"We scraped off literally millions of eggs using vacuum cleaners," he recalled. Some trees, such as oaks, were injected with pesticides or sprayed individually. The city also mailed out 40,000 pamphlets about the pest as part of a public education campaign.
He noted that the moths are mostly spread by people, as they don't fly well (and females don't fly at all). For example, people may get eggs attached to their car while visiting city parks, and those might hatch at their cottage or somewhere else.
Can you do anything about them on your property?
Dutkiewicz recommends that people dealing with the moths on a small number of trees on their own property can also use a technique that involves tying a band of burlap around the tree trunks at about chest height. The caterpillars, which are around until the middle of July, like the shade and hide under there. Afterward, they can be picked off and dumped into a bucket of soapy water.
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After Labour Day, when the adults are done laying eggs, you can also find their egg masses and use a butter knife or painter's scraper to scrape them into soapy water. Dutkiewicz said they are brown with a texture that resembles the skin of a tennis ball, and can be found under branches or in the crevices of trees.
But the biologists who spoke to CBC News for the article noted that because these moths don't typically kill trees, they're actually less harmful than many other invasive species, and are a species we're going to have to live with.
"They're not going away any time soon," Featherstone said. "And we just have to adapt as people and as ecosystems."