By Patricia Robertson
It’s hard not to notice the elaborate silvery curlicues on Yukon aspen leaves. They look like miniature mazes, or aerial views of winding rivers. Unfortunately, they’re not just pretty patterns, but the signature of insect infestation –
the handiwork of the aspen serpentine leaf miner.
The leaf miner, a native of the boreal forest, has always been part of the landscape. But in the last 20 years it’s become one of Yukon’s most successful tree pests. In the last 10 to 12 years, it has spread from northern Yukon
throughout the territory and even into BC.
“Ordinarily, an insect has all sorts of natural enemies that will eventually bring it back under control,” says Rod Garbutt, a retired forest health technician formerly with the Canadian Forest Service. “But this hasn’t happened with
the leaf miner and we don’t know why.”
“Cold doesn’t seem to be a factor in keeping the leaf miner population down,” adds Rob Legare, the forest health forester with the Yukon’s Forest Management Branch. “We’ve had some pretty cold winters and they’ve bounced
right back.”
The infestation in the Yukon is unprecedented in Garbutt’s almost 30 years of monitoring forest health. “The only place I’ve seen a tree that was free of leaf miner, and it was only a seedling, was down around Tagish. The damage
is less severe down there. Maybe 40 to 50 per cent of the leaves are mined, instead of 100 per cent, as in areas further north.”
The adult aspen leaf miner is a tiny moth that overwinters under the snow (actually in the organic earth layer called the duff). The females lay multiple eggs in May on the aspens’ budding leaves. The eggs then hatch into larvae,
which are so tiny that they actually live and feed inside the cells on the top and bottom surfaces of the leaf. As they eat their way along, they create the characteristic tunnels that turn the leaves from dark green to silver and give
the serpentine leaf miner its name.
The larvae don’t kill trees outright, but they do affect tree growth. Studies done in Alaska suggest that, because they feed only in the epidermal layer or “skin” of the leaf, they don’t interrupt the critical work of photosynthesis.
That’s because photosynthesis – the absorption of light by the leaves to produce the tree’s energy – takes place in the mesophyll, the layer beneath the epidermis.
“So if the leaf miner attacks only the epidermis, the machinery of the cell carries on almost as normal,” Garbutt says. “But what they’ve found in Alaska is that if the bottom of the leaf is also mined severely, then the stomata – the
‘mouths’ of the leaf – are damaged. And that interrupts the functioning of the cell machinery.”
Last year Garbutt and Legare made a list of the top 10 insect pests currently threatening Yukon forests. “Most of them are bark beetles – the spruce beetle (Dendroctonus rufipennis), the Ips perturbatus or northern spruce
engraver, and the balsam bark beetle (Dryocoetes confusus),” says Legare. “Then there are the defoliators – the aspen serpentine leaf miner (Phyllocnistis populiella) and the large aspen tortrix (Choristoneura conflictana). Finally,
there are four species of budworms, which mostly attack spruce.”
The aspen serpentine leaf miner is one of the insects that Garbutt and Legare are most concerned about. “We’re always concerned about the beetles,” says Garbutt. “They’ve caused a tremendous amount of damage and will
continue to do so. But the leaf miner is the only significant pest of aspen that’s active right now. There’s a lot of aspen in the Yukon, so anything that threatens a species to the degree that this one does is definitely a concern.”
A minor change in climate can allow an insect to be successful where it wasn’t before. One such insect is the willow blotch miner (Micrurapteryx salicifoliella). In the last three years, according to aerial surveys, it’s become
widespread in northern Yukon. It’s mostly found along the Yukon River and its tributaries, wherever there’s swampy willow habitat. Garbutt believes the river may be acting as a corridor for the insect to move in from Alaska.
Another example is the unusual spread of a bark beetle, the northern spruce engraver, between Dawson City and the Stewart Crossing area. “Two years ago we got a call from one of the natural resource officers in Dawson City,
saying that he’d seen some increased levels of red spruce trees,” says Legare. “So Rod and I did an aerial survey and saw patches of anywhere between a quarter hectare and a few hectares.”
When Legare and Garbutt examined the trees on the ground, they found that many of them had been attacked by the northern spruce engraver, plus some spruce beetle as well. “Usually it’s the other way round,” says Legare.
“Usually the spruce beetle comes in first, followed by the engraver beetle. And if you read about the northern spruce engraver, it always says that it doesn’t normally kill trees. In the Yukon we’re finding that it is killing trees.” Last
year, Legare and Garbutt mapped over 121 hectares of dead white spruce in the area.
Garbutt attributes the change to drought, which stresses the trees and makes them more vulnerable. “It definitely was drier for about four years in succession, and we were seeing a tremendous increase in the incidences of both
beetles, especially the northern spruce engraver.”
In the Shakwak Trench and within Kluane National Park, drought was the main cause of the largest outbreak of spruce beetle ever recorded in Canada. But there the dry conditions were due not so much to lessened rainfall but to
increases in temperature. These increases made the trees transpire more, in an area with relatively little stored moisture in the soil, “so in a roundabout way they induced their own drought,” Garbutt says.
Historically, spruce beetle infestations have often been caused by road construction, logging activity, or blowdown. “But what makes this one different,” says Legare, “is climate moderation. There’s a pattern of overmature spruce
in the Shakwak area, combined with drought conditions in the summer and mild winters.” It makes for what he calls “a perfect storm” of events. “It’s like setting a table for the spruce beetle,” adds Garbutt.
Are other areas of the boreal world having these insect problems, too? “I don’t know,” says Garbutt. “That’s a good question. I haven’t seen anything on the internet that suggests Russia is having the same problems, but I wouldn’t
be at all surprised. Actually I’d be surprised if they weren’t.”
For more information about drought stress on Yukon forests, see http://taiga.net/yourYukon/col464.html
For more information about the effect of climate change on treelines, see http://taiga.net/yourYukon/archives2005.html
This column is co-ordinated by the Northern Research Institute at Yukon College with financial support from Environment Yukon and Yukon College. The articles are archived at http://taiga.net/yourYukon.