Natural Resources Canada
www.nrcan-rncan.gc.ca
Mountain Pine Beetle Biology
The
mountain pine beetle is a small, dark-coloured, cylindrical beetle, about
the size of a grain of rice.
It attacks and kills standing, living, lodgepole pine trees. It generally
completes its life cycle in one
year. In mid-summer, large numbers of adult female beetles
attack new trees by boring
through the bark to the sapwood. They construct vertical galleries
in the phloem between the bark
and the sapwood where, after the males join them, they mate and the
females deposit their eggs.
These eggs hatch into legless larvae that feed outwards from the vertical galleries on the phloem tissue of the host tree.
The beetles introduce a bluestain fungus into the sapwood of the tree that prevents the tree from repelling and killing the attacking beetles with pitch flow. It also blocks water and nutrient translocation within the tree. The joint action of larval feeding and fungal colonization kills the host tree within a few weeks of successful attack (the fungus and feeding by the larvae girdles the tree cutting off the flow of water and nutrients).
The larvae continue their development under the bark over winter, turn into a transformation stage called pupae next spring, and finally emerge to fly and attack new host trees in the summer following the initial attack.
Foliage symptoms of attacked lodgepole pine trees generally are not obvious until shortly before the mature adult beetles fly from the tree in the summer following attack. However, in some conditions, a slight lightening of the foliage can be seen in the spring preceding adult flight. These trees are commonly known as ‘faders’ and can yield visual clues regarding the extent of the infestation. In the summer of adult flight, the attacked trees will hold bright red foliage – it is these trees that are mapped in aerial overview surveys. The foliage will fade to a dull red in the second year following attack. Generally very little foliage will remain on the tree three or four years after the attack. The trees, called ‘grey attack’ will resemble grey skeletons.
At low (endemic) populations the mountain pine beetle survives in weakened or stressed trees. As populations increase or more trees become stressed because of drought or other causes, the population may quickly increase and spread. Healthy trees are then attacked and huge areas of mature pine stands may be threatened or killed. Warm summers and mild winters play a role in both insect survival and the continuation and intensification of an outbreak. Adverse weather conditions (winter low of -40°C or high winds during dispersal period) can reduce the beetle populations and slow the spread, but the insects can recover quickly and resume their attack on otherwise healthy forests.
The current outbreak is progressing as one might expect, considering the relatively mild winters that have been experienced since the mid 1980s and the generally favourable (to mountain pine beetle development) summer weather patterns. The outbreak is likely to continue until an early cold winter kills overwintering larvae. In fact, it was two back-to-back unseasonably cold fall periods in 1984 and 1985 that caused the collapse of the Cariboo-Chilcotin outbreak. In both of those years, early sustained temperatures in the -30 to -40°C were experienced.