Spray Can Control Gypsy Moth, but Only in Spring

Last summer, three staffers at Zimmer Air, an aerial pesticide application company, barely kept up with the phone calls and emails from homeowners, cottagers and forest owners. Everyone asked the same question: how can I stop the gypsy moth from devouring my tree leaves and needles?

“I’ve never had so many calls,” said Paul Zimmer, president of Zimmer Air. His company, based in Blenheim in Chatham-Kent, owns a dozen helicopters and planes, and has been spraying for 45 years to combat gypsy moth and other pests. Along Lake Huron and in eastern Ontario “the population of gypsy moth has exploded this year,” Zimmer said.

Alas, most calls came too late. Aerial spraying from early May to mid-June is effective. When the caterpillars grow too large, says Zimmer, spraying is “a costly mistake.”

“A guy in Stirling called crying to come and spray his pine,” Zimmer said. “He said, ‘They are in there feeding on my pine. I’m probably going to lose them.’ But it was too late.”

The gypsy moth is an invasive insect. A French scientist imported gypsy moths from Europe to Boston in 1868 to spin silk. The moths escaped and spread. With few predators, the European gypsy moth today ranks among eastern North America’s worst defoliators. It prefers oak or maple but can eat up to 150 different species. In the Belleville Intelligencer newspaper, forester Jim McReady called this summer’s outbreak the worst since the mid-1980s.

Beginning in 2007, the City of Toronto and other municipalities have hired Zimmer Air to spray bacillus thuringiensis kurstaki (BTK), a natural bacterium found in soil. Zimmer compares producing the spay to “a yoghurt you make in a fermenter.” Health Canada confirms that other insects, mammals, birds, and fish are not affected by BTK.

After it feeds, the gypsy moth caterpillar pupates and becomes a moth, which lay eggs on tree trunks, buildings and even cars. (The moth got its name because it lays eggs on cars or campers, whose travels spreads the insect). An egg mass the size of a quarter can produce 500 larvae. (One can scrape the egg masses off tree trunks with a putty knife or stick, though given the number of eggs above your reach, it’s a Herculean task.)

Extreme winter cold can kill the eggs. “Now the best hope is we’ll get 30 or 40 degrees below zero for three or four days,” Zimmer said. A cool, wet spring could bring a fungus and/or virus that kills the moth. Cuckoos, blue jays and orioles eat the caterpillars.

In fall, towns hire firms to count egg masses and decide whether to spray in spring. Already, some cottage associations have booked Zimmer for next year. Business is booming. “The gypsy moth is an extremely prolific little creature,” Zimmer said.

Consult your municipality or cottage association for their plans to manage the pest.

Watch a 2020 webinar from Ontario’s Invasive Species Centre: “Forest Under Attack: The History, Dispersal and Management of Gypsy Moth.”