FAQ: Spongy moth caterpillars

A gypsy moth caterpillar crawling on young leaves. Photo by Oleksandrum/Shutterstock

If you happen to be in Ontario right now, chances are you’ve seen at least one or two (or maybe even 1,000) furry critters crawling up tree trunks and feasting on leaves. It’s been a boom year for the Spongy moth, formerly known as the LDD moth (Lymantria dispar dispar). The caterpillars of this invasive species have been ravaging trees across the province, especially in southern Ontario and the Sudbury area.

We spoke with Jolanta Kowalski, spokesperson for the Ministry of Natural Resources and Forestry, and Matt Logan, president of Logan Tree Experts, to learn more about spongy moth caterpillars and how to best handle them.

Why are there so many spongy moth caterpillars this year?

The spongy moth population is cyclical—and right now we’re in a peak year. Generally, a surge occurs every seven to ten years, resulting in a population increase that can last three to five years. Logan says that the population should (thankfully) level out, and by next year numbers will decline.

What do spongy moth caterpillars eat?

Although they can feast on a wide variety of trees, they prefer the leaves of hardwood trees—especially oaks. “They really, really like white oaks,” says Logan.

Can spongy moth caterpillars kill trees?

While the caterpillars are definitely a nuisance, they don’t cause whole tree mortality. “Once the feeding ceases, the trees can produce a second crop of leaves that will enable them to continue to grow throughout the summer,” says Kowalski. “Trees growing vigorously can withstand a few seasons of severe defoliation with little impact.” 

Over time, however, successive attacks will reduce a tree’s vigour and make it more susceptible to drought stress, attacks from other pests, and disease. “It takes away the tree’s ability to gather energy,” says Logan.

Are spongy moth caterpillars poisonous?

They’re not exactly poisonous, but some people may develop irritated skin or rashes if they come into contact with the caterpillar’s tiny hairs. Kowalski advises wearing gloves when handling the caterpillars and keeping them a safe distance from your face to avoid inhaling their hairs. The impact on pets, however, is less known. Play it safe by ensuring your dog or cat doesn’t interact with the furry critters.

How can I keep spongy moth caterpillars off my trees?

Different treatments exist depending on the life stage of the moth (see the graphic below). When they’re egg masses, you can scrape the eggs off the tree and then discard them. Once they’ve evolved into caterpillars, however, you’ll have to take a different approach. Wrapping trees in burlap bands is an effective way to reduce the number of larvae. “This will give the travelling larvae a place to congregate during the warm days, and they can then be physically removed and killed,” says Kowalski. Do this by attaching a 40–60 cm wide piece of burlap around the trunk with a piece of twine or rope. Drape the burlap over the twine, creating an overhang where the caterpillars can crawl underneath to seek shelter. Check the burlap every afternoon to remove the caterpillars, placing them in a bucket of soapy water to ensure they are destroyed.

A chart explaining the stages of LDD moths and how they can be destroyed.
Photo courtesy of the Ministry of Natural Resources and Forestry

Read more: How to control a gypsy moth outbreak

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