There is a lot of mining going on in the Yukon this summer, but these miners are not after gold. On aspen trees all over the Yukon, aspen leaf miners have been busily burrowing away, mining chlorophyll from the leaves and leaving winding silvery tracks behind them.

Any day now the larvae will finish with their feeding and begin the next stage of their life cycle, transforming into small whitish moths about five millimetres in length.

This is the largest outbreak of the leaf miners since they were first noticed in the Yukon in 1979, in the Dawson City area. Trained observers can spot infested aspen stands from a distance, and even from the air.

"If you drive west of Whitehorse towards the Takhini River, you can see large areas of it," says Don White, a forest technician with the Forest Management Branch of the Department of Energy, Mines and Resources.

But White says that foresters are not concerned that the insects are endangering the aspen. While leaf chewers like gypsy moths will eat the whole leaf, the leaf miners just tunnel through part of it.

"They just eat just a small portion of the photosynthetic tissue so the leaves still photosynthesize. This impacts the total amount of nutrients going into the trees and can make them more susceptible to disease and drought, but it does not kill off the trees."

White says that people should not worry about the insects damaging prized aspen in their yards either, particularly if the trees are getting enough water.

The outbreaks usually tend to be relatively short-lived, and last year's outbreak did not even affect the fall display of colour in the Takhini area.

If you had held one of the silvery leaves up to the light a few weeks ago, you would have been able to detect one of the small worm-like larvae inside of its tunnel.

Dark streaks in the tunnels are caused by the insects' excrement. Now they are in their pupal chambers, or cocoons, at the edge of the leaf, preparing for their emergence as moths.

In the fall the moths seek refuge in cracks in tree bark and under leaf litter. After spending their winters frozen solid, they emerge again in late spring, mating after the leaves emerge on the trees. Eggs are laid on the leaves, and once they hatch the larvae burrow into the leaves and start "mining" the chlorophyll.

Leaf miners are endemic, meaning that they always live here. White compares this year's infestation to a disease such as the flu; a few people always have it at any given time, but no one notices it until it reaches a certain level.

Leaf miners are just one of the many insects that can attack aspen. In addition to aspen blotch miners, leaf mining sawflies, and various leaf beetles, there are also aphids, which turned leaves on Yukon aspen shiny in the 1980s and 1990s. A few years ago leaf rollers also took their turn.

The leaf miner outbreak is affecting Alaska as well, and last summer aerial surveys detected more than 300,000 acres affected by the insects in the Fort Yukon area.

The Canadian Forest Service (CFS) is now surveying Yukon forests for insects and disease, looking for damage of all types both from the air and on the ground.

During these annual surveys, forest plots around the territory are checked both visually and by a "three tree beating." After placing a tarp underneath them, the technician gives three trees a good hard thrashing with a long stick.

The critters shaken out of the trees are used to help determine the severity of insect outbreaks. A sample of insects is also shipped off to a research laboratory in Victoria for identification.

When the CFS started its Yukon surveys in the late 1960s and early 1970s, these tree beatings produced barely any insects at all. But the increase in annual temperatures since then has also brought an increase in forest pests.

"The warmer winters in recent years could be contributing to the outbreak, enabling more adults to overwinter," says White. "The common wisdom is that if we had winters like we did back in the 1970s, we would not have the same problems with these insects."

For more information on forest insects, contact the Forest Management Branch at 456-3999.