Gypsy moth caterpillars 'virtually everywhere' in SW Ontario, expert says

The City of Sarnia completed its second annual gypsy moth spray on Friday, due to another severe outbreak of the gypsy moth caterpillars — but the infestation has spread to regions across Southern Ontario. 

'This is the worst infestation I have seen since the eighties,' said Paul Zimmer

Gypsy moth caterpillars can eat a square meter of foliage. Defoliated trees are vulnerable to disease. (The Canadian Press )

The City of Sarnia completed its second annual gypsy moth spray on Friday, due to another severe outbreak of the gypsy moth caterpillars — but the infestation has spread to regions across Southern Ontario. 

"This is the worst infestation I have seen since the eighties," said Paul Zimmer, president and operations manager of Zimmer Air Services, the company that conducts aerial biological spray programs. 

Zimmer's company performed sprays for the city of Sarnia on May 14, and then again on May 21.  Four regions in the city were sprayed after being identified as having severe or moderate to severe infestations.

"This year it is virtually everywhere from Sarnia to Peterborough up to Midland," said Zimmer. "It encompasses a very large area and it is moving north."

Zimmer Air conducted its first spray over Sarnia on May 14, 2021 (Patti Ross, Manager of Parks, Forestry & Horticulture City of Sarnia )

According to Zimmer, the company has committed to do aerial sprays for over 6000 individual contracts, eight municipalities and six reservations this year.

He expects to cover about 50,000 acres of land throughout Ontario, ranging from municipal park lands, maple bushes, cottage areas and forestry woodlots.  

Paul Zimmer, President and Operations Manager of Zimmer Air Services, says this year is the worst gypsy moth outbreak since the 1980s. (Submitted by Paul Zimmer)

What are gypsy moths?

Gypsy moths are an invasive species brought to Canada from Europe in the 1860s. They have been found in North Africa, Asia and Japan, but according to Natural Resources Canada, it's the European moths that are seen throughout Quebec and Ontario. 

The eggs of a gypsy moth caterpillar hatch in early spring to mid May. Once the larvae emerge they eat large amounts of foliage. One caterpillar can eat up to a square meter of foliage, according to Chris MacQuarrie, a research scientist with Natural Resource Canada's Great Lakes Forestry Centre.

Chris MacQuarrie is a research scientist at the Great Lakes Forestry Centre in Sault Ste Marie. (Amanda Roe)

Over time, this defoliation can weaken or kill trees. Oak trees are particularly vulnerable, but according to MacQuarrie, gypsy moths do not discriminate. 

"They really like oak and birch and certainly in the north they like aspen. They also eat maple and beech and they'll even go after softwood trees," said MacQuarrie.

Gypsy moth infestations come in cycles that tend to peak about every 10 years or so and each outbreak can last a few years.

In 2019 the Ontario Ministry of Natural Resources and Forestry surveyed approximately 47,000 hectares of land infected by gypsy moths. In 2020, more than 580,000 hectares were mapped out. 

"Their survey predicted a fair amount of damage to some parts of Ontario this year," said MacQuarrie. 

How to deal with them

There are a few ways to deal with gypsy moth infestations, such as purchasing individual bottles of biological spray from hardware stores to be applied on an infested tree but MacQuarrie suggests wrapping a bag of burlap around the trunk of the tree. 

"The caterpillars will come down during the day to hide underneath the burlap to get away from predators," he said.

"You can come along in the evening to pick the caterpillars out from underneath the burlap and drop them in a bucket of soapy water and that will kill them. That will keep the population from going on your trees in your yard." 

Gypsy moth action group

Last year's outbreak of gypsy moths in Lambton Shores sparked the development of an action group. The Gypsy Moth Citizens Action group was created by a number of Port Franks residents who were badly affected by the caterpillars. 

"It was horrendous," said Romayne Smith Fullerton, spokesperson for the group. 

"These moths created a landscape devastation that is shocking." 

Fullerton says her backyard was covered in frass, gypsy moth feces. 

A tree located in Port Franks, covered in gypsy moth caterpillars in spring 2020. (Submitted by Romayne Smith Fullerton)

Due to the legs from the caterpillars that were released into the air and the frass the caterpillars left, many people with allergies had bad reactions.

"Many people could not go outside last year," said Fullerton. 

The group says it's still early in the season, so they haven't seen the full impact of this year just yet.

The group also advocated for a municipal aerial spray in 2020, but the city council objected to a blanket aerial spray. 

"Our local council voted to not object to private spraying, but they would let us individually organize ourselves and they would not object to that," said Fullerton.

Fullerton said the municipality agreed to spray a small buffer area of their municipal property with BTK, a biological spray, if adjacent private property owners were spraying their properties as well as pay for a buffer spray for a private property if the owner paid for the spray.

This year, Paul Zimmer is contracted to do approximately 1,800 individually contracted sprays in Lambton-Shores. but the municipality completed its first aerial spray on May 17. 

"We've been doing this for many many years, it's all in the planning," said Zimmer. 


Lauren Toffan


Lauren Toffan is a Digital Reporter with CBC Windsor. Prior to joining Windsor, she worked as an Associate Producer with CBC P.E.I. and a Chase Producer with CBC Toronto. Her background is in health and the arts but her passions are journalism and Archie Comics trivia.