Whistler shares many parallels with its mountain-town cousin in Jasper, Alta.
Stunning mountain vistas, millions of annual tourists, forests as far as the eye can see.
And, in recent years, the colour of said forests.
In late 2017 and throughout 2018, the increasingly red-tinged forests surrounding Jasper started to make headlines.
In that case, the trees were killed by a mountain pine beetle infestation, sparking fire concerns among residents and researchers.
In 2024, those concerns turned into a nightmarish reality, when a lightning-caused wildfire burned through 33,000 hectares of forests, and the Jasper townsite itself, causing an estimated $1.23 billion in insurable damage.
Whistler watched with the rest of the world as Jasper burned, “heartbroken” for our alpine neighbours one province over—that other potential parallel placed squarely front of mind.
“My initial response is similar to most of us I expect… deep sadness,” said Mayor Jack Crompton in a statement to Pique at the time. “It does make me grateful for the comprehensive emergency planning we have done and motivates a desire to do even better.”
RED HERRING?
Luckily for Whistler, the parallels to Jasper don’t extend to the infestation itself.
Whereas Jasper’s trees were killed by mountain pine beetle, Whistler’s forests are currently dealing with an outbreak of Western spruce budworm—a native defoliator of interior Douglas-fir in British Columbia.
“Certain insects, like bark beetles—that’s what killed most of the trees in Jasper—they kind of outbreak and kill trees as a result of stand conditions,” explains Kate Mitchell, a provincial forest entomologist with the Ministry of Forests. “So if trees are unhealthy, they’ll go in and take advantage of that, and that leads to tree mortality. But defoliators are regulated by these predator/prey cycles, so we expect outbreaks to occur more or less around set periods of time.”
On the Coast or even in the Southern Interior, outbreaks of Western spruce budworm are expected about every 20 to 25 years, and can persist for five or six years at a time, “and then the populations kind of go down again,” Mitchell says. “That doesn’t mean that they’re not, like, around the forest chewing on things every once in a while, but just in terms of the really widespread, synchronous outbreaks, we kind of know when to expect those.”
While Whistler’s current outbreak began in 2023, Mitchell notes there weren’t many areas affected around the resort at that time.
According to Heather Beresford, executive director of Whistler’s Cheakamus Community Forest (CCF), the rapid spread came as a surprise.
“I’m not a forester, so for me, it kind of came out of the blue last summer, along with everybody else in the community, when we started seeing those trees turning red,” Beresford says.
Flash forward to 2025, and the impact is visible nearly everywhere.
“I think it should be something that we’re very aware of, and that it could have serious consequences on the forest health,” Beresford says—not just for the CCF, but throughout the Sea to Sky corridor and beyond.
“If those trees die, I don’t think you have to tell people that that’s not good from a variety of lenses, whether you look at it from forest health, from wildfire, First Nations cultural values and just pure aesthetics,” she says.
“So this is a big deal, and that’s why I’m pushing really hard to make sure the province can get through the public engagement piece on the pest management plan this year as best they can for my area of responsibility, because if 2026 rolls around and we see another big expansion of the infestation and trees starting to die, I want to be able to take action in our community to protect our forests.”
PAINTING THE TOWN RED
According to a CCF fact sheet, Western spruce budworm feeds primarily on Douglas-fir, though true firs, larch and spruce may also be affected.
“The caterpillars feed on the new needles, but will move onto the older needles if all new foliage is consumed. Trees appear reddish in colour once needles have been fed upon. Attack does slow down growth and can affect tree form,” it reads.
To hear it from Mitchell, the current outbreak is not altogether unexpected. The ministry has conducted regular aerial overview surveys of B.C.’s forests since 1999, compiling decades of historical data for insect infestation, disease, wildfire—”you name it, if it’s killing or damaging trees, we’ve been mapping it for a very long time,” Mitchell says.
Surveys would have shown a slight increase in red trees in 2023, followed by a big uptick last year, she says, noting more than a million hectares of forests were defoliated by Western spruce budworm this year, the vast majority—about 900,000 hectares—in the Southern Interior.
It’s not unprecedented. Mitchell says the last really large outbreak occurred in 1987, and impacted 800,000 hectares of forest.
“It’s not that the ministry is not concerned … like all things which affect and potentially damage large swaths of the province, we obviously want to understand, contextualize and manage it where we need to,” she says, noting the Interior has had a pest management plan for decades, and regularly sprays to manage native defoliators like spruce budworm.
It’s important to note Western spruce budworm infestations, unlike other insects, don’t necessarily lead to tree mortality, Mitchell adds.
“If you don’t know what you’re looking at, or you don’t think of bugs every day like we do, you might see them and go, ‘Oh my God, all these trees around Whistler are dead,’ right?’” she says. “That’s not true. They got munched on by caterpillars, which is what turned the leaves red and caused them to fall. Next year, if the defoliators don’t come back, those are going to flush green, and no one’s ever going to know anything happened.
“Really, the way that we deal with this is just by continually monitoring it.”
As they monitor the situation, officials are also working this year on a pest management plan, complete with public engagement, should spraying Whistler’s forests become a necessity.
WHAT IS Btk?
Said pest management plan is currently in draft form, and describes how and when the Ministry may take action to reduce the outbreak. It is currently out for referral to affected First Nations, and general public input will also be sought in the near future.
“At this time, there is no plan by the province to take action such as spraying in the corridor,” Beresford says. “The CCF will undertake egg mass sampling in about a month to gain insight into what to expect for 2026. Those results will be shared with the provincial team.”
If it comes down to it, Whistler’s forests will be sprayed with Bacillus thuringiensis var. kurstaki (Btk), “a biological agent that kills the caterpillar when it eats the treated tree needles and is harmless to other species,” according to the CCF.
But not everyone agrees with that assessment. In an email to Pique last summer, Bob Brett with the Whistler Naturalists disputed that last point.
“It’s of course BS that such a chemical can target only one species, the most obvious proof being that the same one is used to kill other insect ‘pests’ including [spongy] moths in Vancouver,” Brett said.
“I’m not keen on any non-specific chemical treatments since we already have a global crash in insect populations. Btk is designed to target lepidopterans, but the proponents don’t clarify that means it can potentially kill all moths (which include butterflies) and even other insects. A quick web search shows that even though it primarily affects insects, other invertebrates can be affected too ... That’s a big group!”
Btk is indeed also used to kill spongy moth, Mitchell says, though she clarifies it’s not about targeting species, but rather families or genuses of insects.
“The reason why it works on spongy month is because it’s related in a very similar way, when we think about taxonomy and the tree of life, to native defoliators. So it has that same alkaline gut content, which is why we use it to spray for eradication,” she says.
“And I think we even say that on our website, that it only affects the caterpillars of larvae and butterflies. So there are a group of insects which are affected by it, not just Western spruce budworm … we could use it for a number of defoliating species.
“So that that is true, it’s not just one species, but it is, I think, key to think that it’s this group of insects, and that there has been no evidence of toxicity to salmon and fish-bearing streams, or mammals or even humans.”
Btk is a naturally occurring bacterium found in the soil and water of forests across Western North America.
When caterpillars eat the Btk, it “goes into their gut system, and then their gut system activates the bacteria, because they have a highly alkaline gut, which is kind of cool,” Mitchell says. “And so it can only be activated in a very basic alkaline environment, which these very specific species of caterpillars have. And then basically it releases a bunch of toxins and they die.”
Entomologists culture the bacterium in a lab, which is then attached to booms on planes and sprayed in a diluted solution across targeted spray blocks, Mitchell says.
“And so what you’re doing is, you’re spraying the bacteria, it falls through the air column and then it bonds to all of the plant matter, like the foliage and leaves and everything else. And then it’s actually only active for like, a couple of days,” she says. “So you have to time it really tightly in a synchronized way with a certain life stage of the caterpillars, when you know they’re going to be feeding, and when they’re the most susceptible to the Btk, and it degrades very quickly in the presence of sunlight, and if it rains, it just kind of washes off.”
The bacterium has been used more than three decades in Canada and the U.S. to manage defoliator populations, Mitchell adds.
“Over that time period, there have been extensive studies in situ in these spray programs, in labs, and there is no evidence to date of it being toxic to humans, to other mammals, to fish, to birds or to honey bees or other insects of interest,” she says. “Basically, because it’s so specialized, with that alkaline gut activation, it can only release toxins and become harmful to caterpillar larvae of moths and butterflies.”
And for those with concerns about spraying the forests, Mitchell gets it.
“I totally understand why people are like, ‘you want to spray what?’ I get it … you should ask, right? But there’s a lot of misinformation out there. And so I think it’s important for people to know that it’s not a synthetic chemical, because it is naturally occurring, and it’s a bacteria,” she says.
“And so I think it’s kind of cool, because basically, as entomologists and forest managers, we’re using something from the environment just to our advantage at a very key life stage to try and do something which has very minimal impacts, or no impacts to other target species when we manage for this. Which I think is cool.”
NEXT STEPS
None of that is to say the outbreak is not cause for concern.
Beresford notes the current spread is of a magnitude not seen before in the Coastal region.
“Provincial staff shared [on July 2] that larger Douglas fir (roughly above 30-centimetre diameter) should be able to withstand the defoliation caused by the Western spruce budworm larvae for four to five years, while younger Douglas fir will experience more damage and mortality because the Western spruce budworm can feed on proportionally more of their needles making it harder for the tree to photosynthesize properly,” she says.
Officials are hopeful the outbreak will decrease before significant large tree mortality occurs. In the meantime, they will continue to monitor the outbreak and collaborate on a response if one is warranted in the next year or so.
“The main message for now is that the trees are not dying even though the tips where the new buds grow are turning red,” Beresford says. “The situation is being monitored. If the outbreak continues at this level into 2026, 2027, serious consideration will be given to undertaking a spraying program with Btk, the only viable option for knocking down the Western spruce budworm population.”
