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A very hungry caterpillar has invaded the Toronto region. Here’s what that means for the trees
July 2, 2021

Over the past few weeks, trees in many neighbourhoods around the GTA appear to have been ravaged, their branches stripped bare of leaves by an invasive species of moth caterpillar that is eating its way through the city’s green canopy.

Once known as the gypsy moth, a name considered ethnically insensitive, the preferred name for the insect is now LDD, derived from its scientific name, Lymantria dispar dispar.

Before it becomes a moth, it lives as a caterpillar and has been seen eating through a variety of trees in the city, particularly the north and east areas, such as Woodbine Beach, North York and Scarborough. Even on the sunniest of days in High Park, the sheer number of caterpillars munching make a sound like the pitter patter of raindrops.

“This is a very hungry caterpillar,” said Kristyn Ferguson, Ontario program director for large landscapes with Nature Conservancy of Canada.

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York Region has also identified LDD outbreak levels in all nine of its local municipalities, including Jefferson Forest area in Richmond Hill, Woodbridge in Vaughan, the Grandview Area in Markham and Case Woodlot in Aurora.

The reason for this year’s sudden increase in denuded trees is because the LDD is in the midst of a population explosion. According to Ferguson, these population booms happen every 10 to 12 years.

Though the caterpillar’s primary food source is oak trees, it will also eat about 400 other plant species, including maple and elm trees.

“They can have really widespread impacts when they’re having a population boom like they are right now,” Ferguson said, referring to the widespread defoliation.

The moth has been found in Canada since around the 1960s and is not only in Ontario, but also found in Quebec, New Brunswick and throughout most of the rest of eastern Canada.

Soil compaction, air pollution and other pests and diseases make urban areas most susceptible to LDD infestations, according to James Lane, York Region’s manager of natural heritage and forestry.

“While forested areas may be affected, trees in urban areas including street trees on local and regional roads and trees on private property in urban areas are most at risk,” Lane said.

Colder temperatures also help to control the population by killing exposed LDD eggs, but last winter was too warm to do so.  Paige Taylor White / Toronto Star

In Toronto, the city is managing the impacts of LDD by using “integrated pest management techniques,” which include more than 30,000 inspections of city-owned trees, egg mass removal for 4,387 trees, biological insecticide injections called TreeAzin for 493 trees and spraying 83 trees from the ground. There are about 600,000 city-owned trees in Toronto.

“The city of Toronto anticipated that the LDD moth population would be high in certain areas of the city, and an extensive management program was implemented to address this projection,” a city spokesperson said.

Even though this is a peak year for LDDs, the city said aerial sprays for the moths, which were last done in 2020, are not annual events and are only used when needed.

Torontonians can report LDD moth, caterpillar or egg mass sightings by using the city’s reporting tool. Those reports help the city plan egg mass surveys that will take place in the fall to help determine treatment plans for next year.

Last year’s egg mass survey provided evidence that populations were rebounding faster in natural areas, according to the city. Colder temperatures also help to control the population by killing exposed eggs, but last winter was too warm to do so.

LDD moth caterpillars have exploded in numbers this year, eating a large part of the city's tree canopy.  Paige Taylor White/ Toronto Star

Although some birds and larger insects do feed on the caterpillars, nature also has built in a self-regulating mechanism: a naturally occurring virus called NPV -- harmless to people -- that rages through a booming population, ultimately helping to limit it.

“When they have big populations, like now, a bacterium will knock down the population,” said Chris Darling, senior curator of entomology at the Royal Ontario Museum. “That’s why it’s cyclical. That’s why you have high years and low years.”

Meantime, if you’re worried about LDD on your trees, experts say you can wrap them in burlap so caterpillars can’t climb up. If you do find egg masses, they can be scraped off into a bucket of soapy water. If a caterpillar lands on you, you’re not supposed to touch it without gloves as its bristles can cause an allergic reaction.

These individual interventions could be helpful in preventing broader-scale outbreaks in years to come, according to Ferguson.

However, it could also come down to just waiting them out. The caterpillar stage is expected to conclude next month when most LDDs turn into moths. Then the insect will no longer have the same impact on foliage, and trees will begin to rebound.

The good thing is, experts say, many trees will recover from the damage, particularly deciduous trees -- the ones that lose their foliage at the end of growing season.

“They can withstand this kind of attack, though it’s certainly not pleasant,” Ferguson said.

One thing is for certain: these caterpillars will return next year, and the year after that. “They’re here to stay,” said Darling.