Why an invasive moth caterpillar infestation breaks records in central Canada

A species of invasive moth caterpillars is stripping trees and raining canopy droppings across much of southern Ontario and Quebec, amid a record epidemic. Here’s why their population has exploded and what can be done about them.

What are these moths and what do adults and caterpillars look like?

Moths are known as the “LDD” butterfly due to their scientific name Lymantria dispar dispar. LDD moth is the name preferred by Ontario’s Invasive Species Program, which says its original common name of “gypsy moth” is derived from a culturally offensive insult.

The caterpillars are up to six centimeters long. They are furry and can be identified by the five pairs of blue dots and six pairs of red dots on their backs.

Adult male butterflies are brown and fly. Adult females are larger, white, and cannot fly.

Female LDD butterflies are white and cannot fly. Males can fly and are brown. (Emily Chung / CBC)

Where are they found?

At this point, they are found throughout much of southern Canada.

They are native to Europe and Asia, but were introduced to the United States near Boston in the 1860s by amateur astronomer and entomologist Étienne Trouvelot, who wanted to test their potential for making silk. They escaped and became an invasive species

They first reached southern Canada in 1969, according to David Dutkiewicz, entomological technician at the Invasive Species Center, a nonprofit based in Sault Ste. Marie, Ontario, a conservation organization focused on the prevention, detection, response and control of invasive species in Canada.

The current population explosion is mainly located 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, as well as smaller populations in the Mauricie region and near Quebec.

What kind of damage do these moths do?

These butterfly caterpillars have a voracious appetite and eat a wide range of foods including oak, birch, poplar, willow, and maple unlike many other caterpillars which are more picky eaters. They also feed for one time of year about twice as long as many native caterpillars, said Joel Harrison-Off, City of Toronto Forest Health Care Inspector. And although they are eaten by some birds and mammals, none of them consume enough of them to really reduce the population.

When there is an outbreak, they can completely strip the trees of their leaves, just as they did the oaks in High Park in Toronto.

By mid-June, the caterpillars had almost completely stripped the oaks of their leaves in Toronto’s High Park. (Oliver Walters / CBC)

Usually trees can recover. But some tree species, such as oak, have a harder time regenerating their foliage.

Harrison-Off said the stress on trees and the energy they need, especially if climatic conditions such as drought are also stressful, make them more difficult to defend against pathogens.

“These trees, some of them will start to die back in the next few years due to defoliation.”

How serious is this year’s epidemic?

These moth outbreaks are cyclical, leading to an outbreak or infestation every ten years or so and lasting one to three years. The numbers increase in years with good weather conditions and then decrease due to fungal or viral infections spreading through the population. Previous epidemics occurred in 1985, 1991 and 2002.

The current outbreak in Ontario began in 2019 and the following year, butterfly caterpillars defoliated over 580,000 hectares, a record. “You look at the size of Prince Edward Island, basically, which was defoliated last year,” Dutkiewicz said.

He said this year’s final tally would not be released until August, and at the moment we are on track to see a similar amount of defoliation and “another record year or close to a record year”.

WATCH | “To a lot of people the trees look like they’re dead”: Alistair MacKenzie of Ontario Parks says he’s not too worried about LDD moth caterpillars ravaging one of the rarest ecosystems of the province. 2:37

In comparison, 1985, the previous worst epidemic, peaked at 350,000 hectares of defoliation.

Harrison-Off said this year that LDD moths have been seen in parts of Toronto where they had never been seen before, including people’s properties: “We get a lot of feedback, and people are trying to manage and learn more about [LDD] moths. “

Why is it so bad?

This is partly due to the natural cycle. But this year’s weather conditions were also ideal, said Dutkiewicz – a warm winter, followed by a dry spring.

Butterflies cannot survive for long below -20 ° C, which is why they are confined to southern Canada. And a wet spring promotes the growth of a fungus that kills them.

“We did not have this type of natural event (…) in order to reduce the population,” he said. “It really was the perfect series of fair weather events.”

Will climate change make the situation worse?

David Featherstone, senior ecologist at the Nottawasaga Valley Conservation Authority, about 100 kilometers north of Toronto, predicts that moths will begin to spread north to central Ontario.

Extreme droughts and storms can also put stress on trees and forests, which can make trees less able to endure being stripped of their leaves by LDD moths and having to repel them again, a he added.

What can be done against LDD moth infestations?

Some municipalities in Ontario, such as London, Ontario, spray a bacterial insecticide called BTK to control disease outbreaks. Dutkiewicz said some conservation authorities were also carrying out targeted spraying to protect “high-value trees”. But Ontario Parks says it’s letting nature take its course.

Joel Harrison-Off, City of Toronto Forestry Health Care Inspector, said the city is tackling LDD moth infestations with targeted treatments that include vacuuming egg masses and spraying or spraying. injection of pesticides to protect specific individual trees. (Oliver Walters / CBC)

Harrison-Off said BTK has the potential to kill many beneficial and native caterpillars in environmentally sensitive areas like High Park.

Instead, the City of Toronto performs targeted spot treatments.

“We literally scraped millions of eggs using vacuum cleaners,” he recalls. Some trees, such as oaks, have been injected with pesticides or sprayed individually. The city also sent out 40,000 brochures about the pest as part of a public education campaign.

He noted that moths are mainly spread by humans, as they don’t fly well (and females don’t fly at all). For example, people can hang eggs from their cars when visiting city parks, and these can hatch in their cabin or elsewhere.

Can you do something about them on your property?

Dutkiewicz recommends that people who care for moths on a small number of trees on their own property could also use a technique that involves tying a strip of burlap around tree trunks at about breast height. . The caterpillars, which are there until mid-July, love the shade and hide there. Then they can be picked up and thrown into a bucket of soapy water.

WATCH | Volunteers work to thwart invasive moth caterpillars in Ottawa’s Hampton Park: Resident Sharon Boddy says many trees in Hampton Park are threatened by LDD moth caterpillars, leading volunteers to trap and kill them. remove as much as possible. 1:45

After Labor Day, when the adults have finished laying, you can also find their egg masses and use a butter knife or painter’s scraper to scrape them in 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 tree crevices.

But biologists who spoke to CBC News for the article noted that because these butterflies typically don’t kill trees, they’re actually less harmful than many other invasive species and are a species we’ll have to live with.

“They won’t be leaving anytime soon,” Featherstone said. “And we just have to adapt as people and as ecosystems.”

Female LDD butterflies are seen with egg masses, which can be scraped off trees to control the population the following year. A nymph is visible at the bottom left. (Emily Chung / CBC)

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