Virus Can Cause Collapse of Gypsy Moth Population

By Peter Kuitenbrouwer

The Gypsy Moth, which defoliated trees in many parts of Ontario in 2020, may return this spring to cause more damage, warns a scientist with the Ontario Ministry of Natural Resources and Forestry (MNRF).

“The Gypsy Moth is seemingly naturalized in southern Ontario,” Dan Rowlinson, who works in the biodiversity and monitoring section at MNRF, said during the Forest Health Review section at Forests Ontario’s annual conference. “It took a really big spike in 2020.”

The moth, in its caterpillar stage, defoliated about 45,000 hectares in 2019, Rowlinson said. In 2020, the moths, an invasive insect from Asia, ate the leaves off trees in a whopping 600,000 ha across the province. “It likes to get its foothold on the hilltops,” he added, noting that the caterpillars prefer oak, birch and aspen leaves in the north, and oak, Sugar Maple and beech leaves in the south.

Rowlinson, who is based in Sault Ste Marie, said that based on egg mass surveys his team has carried out with the Canadian Food Inspection Agency, he believes Ontario could get hit with another Gypsy Moth feeding frenzy this spring. But he predicted that the population will collapse through transmission of a fungus or a virus, as has happened before.

Canadians know all about viruses given the Covid-19 pandemic, Rowlinson noted. Indeed, Forests Ontario held its conference virtually for the first time, to avoid physical contact between participants that could spread the coronavirus.

“If you look at us humans, we want people social distancing because of Covid,” he said. “We know the impact a virus can have on a population. In the case of the Gypsy Moths, we want them climbing all over each other, spending Christmas together, because that’s what’s going to cause a collapse in the population.” Nuclear polyhedrosis is the virus that kills the moths, he said.

In Ontario, previous Gypsy Moth outbreaks peaked in 1985, 1991, and 2002, though the highest peak, 1991, at 350,000 ha, harmed far fewer trees than in 2020.

“There are going to be a large number of private control programs for Gypsy Moth this season,” Rowlinson said.

“Gypsy Moth egg masses on Colorado Spruce doesn’t mean it’s going to be defoliated,” he added. “Their preferred species is oak. Once they have eaten themselves out of house and home on the oak, they will move on to ornamental spruce. They’ll eat flagging tape, they’ll eat anything. The population is so intense it is a perfect storm for a virus to spread over the landscape.”

Talk to your municipality or cottagers’ association about Gypsy Moth control. The Ministry of Natural Resources and Forestry’s Forest Health Report for 2020 is available on its website: www.ontario.ca.