Leaf Miner in Aspen leaf
South Okanagan Nature Leaf Miner in Aspen leaf leaf miner 3
Another Threat to BC Forests

By now everyone is aware of the major attack on BC forests being made by the pine beetle. Virtually all of interior BC has suffered at least some damage to the forests, especially in those areas where pine trees are abundant. Many areas of northern and central BC have suffered extensive damage to the forests.

However another, much less well known, forest epidemic is taking place in northern BC, Yukon Territory and Alaska. This is an infestation of the aspen leaf miner which, like most pests, has been around for millennia, but every once in awhile, occurs in epidemic proportions.

The aspen leaf miner is the larva of a small white moth that occurs over most of North America. The adult, unlike most moths, survives throughout the winter even in northern climes. It does this by basically hibernating under the bark scales of various trees – not necessarily aspens – or in the organic litter on the forest floor. In the spring, after emerging and mating, the female lays an egg (generally only one per leaf) on the edge of newly emerging aspen leaves. After the egg is ladi, the adult folds the leaf over to form a protective cover for the egg. When the egg hatches, the worm-like larva burrows into the leaf and then proceeds to eat or “mine” the layer of the leaf between the top and bottom surfaces leaving a serpentine trail of destruction through the leaf. Towards the end of summer the larva pupates into an adult moth and the cycle begins again. As the larva is eating its way through the leaf it destroys much of the chlorophyll and thus interferes with the leaf’s ability to produce food for the tree. However, unlike the pine beetle which invariably kills every tree that is infected, leaf miners don’t generally kill a tree.

In “normal” times less than 5% of the leaves of an aspen would generally be infected by leaf miner. This is the case right now in the Okanagan where many trees have no leaf miners at all and others have a few to some. This week I found several trees along White Lake Road with leaves infected by leaf miner but certainly less than 5% of the leaves were affected. In contrast, earlier this summer I spent two months driving through northern BC, Yukon and Alaska where many of the trees had 80-100% of their leaves affected. Heavily infected trees appear silver from a distance instead of green as much of the chlorophyll has been destroyed.

Major infestations, such as current in the Yukon, can have a detrimental effect on the trees and if it continues for several years has the potential to kill or seriously damage the trees. The Okanagan has relatively few aspens whereas parts of northern BC, Yukon and Alaska are predominantly aspen so a major kill-off by the leaf miner would be a significant event.

The current outbreak began in Alaska in 2000 when it is estimated about 1,400 acres in Alaska were heavily infested. By 2007 this number had grown to nearly 700,000 acres in Alaska and had spread throughout the Yukon and northern BC. The cause of this particular outbreak of leaf miner is not known nor is it known whether it will spread to more southern areas. Aspen leaf miner epidemics occurred in northern BC along the Alaska Highway in the 1950’s and in Alaska in the 1970’s as well as elsewhere in western North America. For some reason, even though Aspen leaf miners occur all across North America, outbreaks seldom occur in the east. As mentioned, there are now (and always have been) leaf miners here in the Okanagan but certainly not in epidemic proportions. There is no known way to control an infestation of these interesting creatures.

The South Okanagan Naturalists’ Club will resume regular monthly meetings in September. If you are interested in any aspect of nature you might like to attend one of our meetings held on the fourth Thursday of each month. Do not be put off by the word “Naturalists” in our Club name. Most of the members are not professional biologists or scientists but simply average people interested in the nature of the Okanagan, whether it be wild flowers, geology, birds or simply being outdoors.
Nature Wise