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Metro Vancouver soon to be swarmed by moths, experts warn

Experts hoping for population to crash this fall

Provincial entomologists are prophesizing an awfully mothy few weeks, as the endemic western hemlock looper enters its final form.

In July, the province warned we were in our third year of a naturally occurring outbreak of the population, which defoliates hemlock, cedar and pine trees while in their caterpillar larval stage.

After spending the summer months turning much of the forest brown or orange, those larvae are now emerging as moths.

“A lot of people will see the moths flying. They tend to congregate. And lots of people hate the look of them. But the key thing to remember here is that this is not an invasive species. This is a native insect,” said Jeanne Robert, provincial forest entomologist.

They’re not loved during their short stays, but they are totally harmless in their moth form, Robert said. The one perk the moths bring for humans: compost.

“I do recognize they're kind of icky if they get in your house or stuck in your filters or air intakes,” she said. “There is sort of a nice little boost of nutrients for your garden plants.”

After a few weeks, when colder weather and rain arrives, the moths will start to die off, bringing an end to the third year of the outbreak. Looper moth outbreaks typically last three years before their population crashes.

 “I can't say for sure, but I'm hoping what has happened historically for this insect will happen this year,” Robert said.

While there may still be pockets of looper larvae munching away on trees in 2022, the next major population explosion on the South Coast isn’t expected until sometime between 2032 and 2036.

Provincial staff do trap moths to survey and estimate their population levels, along with aerial monitoring of the forests to see that things aren’t getting out of hand. While the province does have the option of using a non-toxic spray that targets just the looper larvae, Robert said the damage done hasn’t met that threshold.

In fact, Robert said she is optimistic many of the obviously stressed trees turning orange or brown today will survive and emerge green again when the outbreak ends.

“Yes, they look terrible. We all love our trees,” she said. “But the ecosystem is quite capable of bouncing back from this level of defoliation so this is definitely not a mothpocalypse.”

Although the species is found around the province, outbreaks in the Interior are less frequent – every 20 years. And North Vancouver and West Vancouver have been particularly hard hit this time around, especially on the west side of the Capilano Reservoir.

“I know it feels really big but a lot of what we’re seeing is fairly localized,” Robert said.

Metro Vancouver, which has jurisdiction over the watershed, is monitoring closely to see if tree mortality will impact water quality, slope stability or fire risk, particularly in the interface areas. But Jesse Montgomery, Metro’s division manager for the watershed, said he expects local forests will, in time, emerge stronger than they are now thanks to new holes in the canopy opening up space for younger trees, and nutrients from dead trees returning to the soil.

“The forest does quickly regenerate and from a natural forest ecology perspective, it's not unhealthy,” he said. “In second growth forest, it can result in a more resilient and biodiverse forest stand over time.”