The mountain pine beetle is a small, dark, cylindrical insect that usually completes its life cycle over a 12-month period, sometimes taking longer at higher elevations. Being a beetle, it has two pairs of wings, the outer set horny and the inner set more membranous, and a pair of antennae on its head. It also passes through a complete metamorphosis, featuring egg, larval, pupal and adult stages.
During the summer, the female converges on a new tree and bores through the bark to reach the phloem, a layer under the bark that feeds the tree by conducting the sugary products of photosynthesis throughout it. From there she sends out pheromones, chemical messages that attract males. After they arrive and mate with her, she continues digging through the phloem, building vertical galleries that are essentially tunnels. The male dies soon after mating and is buried in a gallery, covered by boring dust from other beetles.
A female mountain pine beetle lays an average of 60 eggs in the galleries. The eggs hatch in 7 to 10 days into brown-headed grub-like larvae. These larvae create tunnels similar to those dug by their mother except horizontal. To ensure a sustained food supply, the female injects the tree with a blue-stain fungus that she carries in her mouth. This shuts down the tree’s ability to defend itself, allowing the larvae to feed until winter. At that point the larvae will produce an antifreeze agent enabling them to survive, while their mother dies.
The larvae eventually carve out oval-shaped chambers where they transform into pupae in the spring. From mid-July to mid-August the newly formed adults drill holes in the bark in order to leave the tree and emerge to begin the cycle all over again in a new tree.
Habitat and behaviour
The mountain pine beetle lives most of its life under the bark of pine trees, primarily the lodgepole. However, it can live in virtually all pine varieties including ponderosa and western white. The beetle prefers mature timber, which is how lodgepole pine is classified after 80 years. Over the last century, advances in fire fighting methods and equipment have led to a three-fold rise in the occurrence of mature lodgepole pine in British Columbia. This increase in older trees has likewise increased suitable habitat for this small beetle.
The trees, for their part, do mount defences against the insects. But these are often defeated by the combination of sheer beetle numbers, their blue-stain fungus, and the aggressive tunnelling that chokes off the tree’s circulation.
Nor are affected trees even obvious, until newly emerged adults set off for unaffected ones. Sawdust at the base of a tree might indicate an attack in progress, but this will not always be visible. Trees known as “faders” sometimes lighten the colour of their foliage, giving researchers a valuable clue to the extent of an infestation.
Fader or not, a tree’s foliage turns bright red once the adult insects leave. This colour will then fade in the following year, and in the next couple of years most of its foliage will disappear. All that remains is a grey skeleton of a once vibrant tree.
Historically, the beetle and the tree enjoyed a more balanced co-existence. The insects culled aging trees, leaving most of a forest stand healthy and intact. The trees in turn provided the insects with both a food source and a reproductive site. Harsh winters also served to keep the beetle in check, since the larvae cannot survive temperatures below -35ºC. Nevertheless, as milder winters have become more common, and tree populations have swelled, so too have the numbers of mountain pine beetles. With this boom in beetle numbers, the trees are being killed in unprecedented numbers.
The mountain pine beetle can be found from central British Columbia as far south to Mexico, where temperate pine forests exist. Typically found in southwestern Alberta, by 2002, it was being seen north of Banff.