Ontario believed to be nearing end of gypsy moth cyclical boom: Nature Conservancy of Canada

Ottawa has been invaded by some hungry, hungry caterpillars this spring and summer — and these flying eating machines are devastating many plant and tree species across the city.

Also known by its scientific term, Lymantria dispar dispar (LDD), the European gypsy moth is eating its way across cities in Ontario, Quebec, New Brunswick, Nova Scotia and Prince Edward Island, and it’s stripping trees right down to their bare branches, and littering the ground around them chewed-up leaf confetti.

It’s a problematic species that is really wreaking havoc in the city — especially this year, and that’s because the moth goes through a population boom and crash cycle every 10 to 12 years.

Usually, it’s a viral disease or moth fungus that causes the population to crash after years of heavy outbreak.

Ontario, however, is nearing the end of its boom cycle and a crash is expected, hopefully for 2022.

“When mature, LDD caterpillars are approximately 50 mm long, hairy and dark-coloured,” Kristyn Ferguson, Nature Conservancy of Canada Ontario program director said. “Five pairs of blue dots and six pairs of red dots line their fuzzy backs. They feed on the leaves of over 400 plants and especially love deciduous trees, like sugar maples, oaks, elm and birch, but will feed on coniferous trees if their preferred leaves run out. 

One caterpillar, she says, can eat one square metre of leaves in one season.

While hardwood trees may survive up to three years of severe defoliation, coniferous trees would not survive one bad infestation, she adds.

It’s difficult to determine what 2022 will look like, however, as it is still somewhat of a crystal ball situation, Ferguson says — even if the NCC is armed with the best knowledge from egg mass surveys and assessment of larval mortality.

There are natural predators that eat the LDD caterpillars, like parasitic wasps, some mammals, birds and other insects.

Ferguson says it’s important to learn how to identify the LDD caterpillar from other, native lookalike species, like the eastern tent caterpillar, the forest tent caterpillar and fall webworm.

Identifying an LDD moth infestation

First of all, do not handle them with bare hands as some people develop an itchy rash from touching the tiny hairs on the caterpillars, which contain histamines.

These caterpillars can catch you by surprise when they fall from trees or if you stand under the canopy with caterpillars dangling from their silks, waiting to be carried by the wind to their next host plant.

Male LDD caterpillars are mainly brown and females are mainly white when they turn into moths. Females are also flightless and lay eggs on tree bark, branches and other sheltered places like rock piles, piles of logs lawn furniture, etc.

How to manage infested trees and plants

The adult moth's life stage is actually short-lived, so control measures are best done in the moth’s larval stage.

The NCC says to start by scraping off any egg masses you see from August to the following spring.

To manage infested trees and plants, homeowners can create caterpillar traps by wrapping trees with burlap bands from May to August. Secure the burlap to tree trunks at chest height with a piece of rope or twine. Then, fold the material above the rope or twine down over itself to create a trap. This should stop caterpillars from climbing the trees.

Then in the afternoon, check your burlap band traps in the afternoon, when caterpillars are likely to seek shelter from the mid-day sun inside the burlap.

Scrape the caterpillars into a bucket of soapy water and let sit for a day or two. Wear gloves to avoid direct contact with the caterpillars.

Continue to check the traps throughout the day and season.

And when scraping egg masses from trees, use a scraper tool to put them into a bucket of soapy water. Let sit for a day or two and dispose of in the garbage.

The NCC warns not to use adhesive tape around the tree to trap caterpillars as this can be a death trap for birds and other insects.