Will the spongy moth again wipe out Quebec tree canopies? Scientists are trying to find out

Tree leaves were eaten on Montreal's Mount Royal in July 2021, leaving many trees bare. The spongy moths particularly target oak and apple trees. (Paul Chiasson/The Canadian Press - image credit)
Tree leaves were eaten on Montreal's Mount Royal in July 2021, leaving many trees bare. The spongy moths particularly target oak and apple trees. (Paul Chiasson/The Canadian Press - image credit)

For the first time in more than four decades, swaths of tree canopies across southern Quebec were wiped out when spongy moth populations exploded to near biblical proportions in June of 2021.

But the leaves grew back.

The gooey masses of abandoned cocoons, glued to tree bark in growth-like clumps, eventually faded into the forest's embrace and the very hungry caterpillars, which were responsible for eating all those leaves, have yet to come back for seconds. In fact, they're all gone.

Now scientists like Emma Despland want to better understand what happened.

She's a professor of biology at Concordia University who researches plant-insect interactions, including outbreaks like that of the spongy month which affected southern Quebec as well as New Brunswick, Nova Scotia and Prince Edward Island.

She is overseeing research projects at places like McGill University's Gault Nature Reserve in Mont-Saint-Hilaire, Que., about 40 kilometres east of Montreal. The research is exploring if parasitoids and the moth's inability to survive low winter temperatures played roles in the insect's abrupt disappearance.

Leaf-munching stops in adult stage

The spongy moth, or Lymantria dispar, is an invasive insect brought to North America from Europe in the late 1800s.

As temperatures warm up in the spring and early summer, the insect, in its caterpillar stage, consumes large quantities of leaves with a particular appetite for oak and apple trees.

Paul Chiasson/The Canadian Press
Paul Chiasson/The Canadian Press

Once they form a cocoon and fly off as moths, the leaf munching stops. And while trees were able to recover from the outbreak this time, Despland said repeated outbreaks like this, year after year, could have disastrous consequences. Trees wouldn't be able to survive the repeated devastation.

She said she first noticed an uptick in spongy moths back in 2019 and began studying them, but her research was derailed by the pandemic in 2020.

"2021 is when it really exploded. I think that's when people really started to notice, and saw Mount Royal, the Morgan Arboretum, Oka park, Mont-Saint-Hilaire — many forest areas that people usually enjoy, there were no leaves on the trees," said Despland.

"Then in 2022, they were pretty much completely gone."

Survival rate down, mortality rate up

Studies so far have shown that as the outbreak progressed, survival rate went down as mortality rates went up, with the moths dying of fungal and viral infections as well as parasitoids, which are insects that live inside and feed on spongy moth eggs.

The population had remained stable for decades because, even if females were laying up to 800 eggs, the mortality rate remained high.

Celeste Decaire/CBC
Celeste Decaire/CBC

"The outbreak starts when, for some reason, the survival rate is higher," Despland said, but when the population then explodes, that creates more resources for the parasitoids.

"So we know that these natural enemies, these diseases and parasitoids, were part of the reason the population collapsed. Another potential part of the reason is over-winter mortality."

Now Noa Davidai, a PhD student at Concordia University who is co-supervised by Despland and Dr. Carly Ziter, is looking at mortality over the winter.

Despland said adult spongy moths lay eggs in July and those eggs can survive down to around –28 degrees. So the thinking is, cold snaps may increase mortality, she said.

There's also the possibility that eggs laid below the snow may be insulated enough from the extreme cold. And that's what researchers are trying to figure out.

Along with winter's impact, Marie-Ève Jarry, an undergraduate student at Concordia, is also looking at the effects of spongy moth outbreaks on native moth species.

No more spongy moths to study

"What's tricky now is there are no spongy moths," said Despland.

So rather than monitoring eggs directly, for example, researchers are studying the temperature below and above the snow around trees to understand their environment, she explained.

"There are a ton of other things I would like to do, but one of the things that's hard about working with an outbreaking species is, when an outbreak happens, you have to scramble and start now, and by the time you're ready, it's over."

The hope is that they won't come back, but if and when they will is still anybody's guess, Despland said.

Among those who don't want to see another surge in spongy moths is Frédérique Truchon, a biologist in charge of communications for the Gault Nature Reserve.

"The great thing about nature is that it is pretty resilient," she said, recalling the devastation at the reserve back in 2021. "The trees were able to grow their leaves back. By August, we had a normal and healthy forest."

This time, there was no lasting damage and by the summer of 2022, after a cold winter, the moths didn't come back, she said. Were they to have surged anew, trees would likely have been lost, she said, because they would have been unable to longer produce enough energy through photosynthesis without their leaves.

Spongy moths will likely come back, Truchon said, and with temperatures warming up, she's concerned the outbreaks will become more frequent.

"We ourselves don't have any way to predict that," she said. "But scientists like Dr. Emma Despland are trying to better predict these outbreaks."