Through the 1990s to the mid-2000s, the spruce bark beetle ravaged southwest Yukon.

All in all, about 400,000 hectares of white spruce were killed off, without a whole lot anybody could do about it, if anything.

Vast expanses of lush, green forest were turned red – the colour of dead coniferous needles – as far as the eye could see.

The incursion was fairly consistent from 1994 to its peak in 2004.

In 2005, the infestation cooled substantially. It continued to decline through to 2012, when the concentration of spruce beetles was back around normal levels, where it remains today.

It’s believed drought conditions had put the white spruce under stress, and the trees were unable to reject the beetles’ attack as they normally would when healthy, forester Rob Legare of the Yukon’s forest management branch explained in a recent interview.

Legare also co-ordinates the forest health program. Every year since 2009, he and his colleagues have conducted aerial surveys over one the five designated zones the Yukon is divided into.

They also conduct ongoing aerial monitoring in areas they want to keep an eye on, as they did during the collapse of the spruce beetle in the southwest.

“If something is out there, we’ll see it,” he says.

Legare says there’s nothing facing the Yukon now like the spruce beetle infestation, though curiosity and concern are brewing about what seems to be a change in the health of aspen stands that have been or are under attack from leaf-eating insects.

There was also a concern in recent years about the possibility the enormous mountain pine beetle infestation in B.C. would find its way this far north up through the Rocky Mountain Trench.

The trench runs along the west side of the rockies, and stretches from Montana to the Liard River, just south of the B.C.-Yukon border.

It ties into the bottom of the Tintina Trench, the hotbed of mineral resources which runs through the territory up into Alaska.

Just as the spruce beetle chewed through southwest Yukon, the mountain pine beetle has killed off millions of hectares of pine forest in B.C. since the late 1990s. It’s also hit Alberta on the east side of the Rocky Mountains.

As the main body of the infestation surged northward, it closed to 150 kilometres south of Watson Lake. But clumps of damage from the pine beetle – affecting 10 to 15 trees in one spot – were seen as close as 75 kilometres from Watson Lake.

The forest management branch began regular monitoring along the Yukon-B.C. border in 2009, and conducted its own survey in the northern areas of the trench inside B.C. during years of greatest concern.

A Yukon government interdepartmental committee was formed in 2011. A mountain pine beetle risk analysis was completed and a strategy has been developed.

Officials hosted a large public workshop in 2012 to explain the pine beetle’s encroachment and any options they might take to halt or hinder its progress.

Legare says there is evidence of success targeting the clumps of mountain pine beetles which leapfrog ahead of the main infestation by going after the clumps in the winter, cutting down the trees and burning them.

“You can control the beetle, but you have to hit it when it’s small.”

The infestation began retreating a number of years ago. The main body is now located some 500 kilometres south of Watson, though there remains continuous monitoring from the air and ground.

Every year, Legare and crew fly a 25-kilometre-wide strip inside B.C. along the Yukon-B.C. border looking for red patches of dead pine that might signal the resurgence of the threat by the mountain pine beetle. So far, so good.

Since 2009, they’ve also been setting out pheromone bait traps along the border that will not attract the beetles from long distances, but will draw them in if they’re in the area.

Not one mountain pine beetle has been found in any of the traps.

Also on the radar for the forest health program is what appears to be a changing trend in stands of aspen trees have been attacked by the leaf-eating large aspen tortrix and the aspen serpentine leafminer, particularly in the north – Dawson, Mayo, Pelly Crossing....

There’s some question whether a changing climate is adding to the stress of the aspen trees that have been or are under attack, as there is evidence of more trees dying after they’ve been hit by the tortrix or leafminer.

Legare says they can’t say for sure that it’s related to climate change, but they plan to do additional research on the ground beginning next summer.

Using ground surveys by road or foot, the former federal Forest Insect Disease Survey had been documenting the loss of aspen foliage related to the presence of the tortrix and leafminer going back to the 1950s.

It wasn’t until the late 1970s and early ’80s, however, that the federal survey started noting dead tree tops on aspen trees that had been attacked. Reports did not specify any cause for the dead tree tops, simply describing the new observations as “aspen decline.”

And through the 1990s, more aspen decline was seen.

“This is becoming more and more prevalent in our surveys,” Legare says of what they’re seeing from the air since the annual aerial surveys by the forest health program began in 2009.

“We are starting to see more and more aspen decline, especially in the North. It’s also consistent with what our colleagues in the Northwest Territories are seeing.”

Legare explains they’re now on their second round of flying one of the five zones each summer.

They’ve noticed in 82 per cent of the areas up north where they previously noted the presence of aspen tortrix or aspen serpentine, they’re now seeing “aspen decline.”

In 60 per cent of the areas in the Haines Junction region where they previously saw evidence of tortrix and the serpentine leafminer, they’re now seeing aspen decline, he says.

“You are seeing something that gives us an indication that this is environmental,” says Legare, suggesting there may be something working in combination with the impact from the tortrix and serpentine that is killing the aspen.

“That is why we want to get on the ground and do some more research.”