Irresistible aroma responsible for mountain pine beetle attacks, suggests Calgary research

Research suggests pine trees with strong natural defences are their own worst enemy when it comes to the destructive mountain pine beetle.

Although the sap produced by the lodgepole pine is toxic to the beetles, the smell of the sap can attract large numbers of the tiny bugs that can collectively end up killing a tree.

Prof. Mary Reid, a University of Calgary researcher, says toxic chemicals in the resin within a healthy pine can ward off a few beetles, but not a swarm.

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"They are effective, but it could be that because they're also attractive that there can be enough beetles that can pour into that tree and kill the tree even though the individual beetles don't really do that well," Reid said.

"They really rely on the mass attack to overwhelm the tree's defences."

The beetles, which are about the size of a grain of rice, affect pine trees by laying eggs under the bark. The hungry offspring eat beneath the bark, eventually cutting off the tree's supply of nutrients and preventing it from repelling and killing the attacking beetles.

Reid said the research involved examining trees killed by the beetles in Banff National Park where larger pine trees were dead but smaller ones, with less defences, were fine.

The aroma of a strong tree can act like a beacon to the beetles.

"We know a lot of the chemicals in those resin ducts are attractive to mountain pine beetles and to other bark beetles, so it may be the trees that have more defences in them are actually more detectable to the beetles and are more likely to be attacked," Reid said.

"Those beautiful smells you have as you walk through a pine forest are ... volatile chemicals that the tree produces in the resin and beetles are attracted to those smells and presumably that helps them identify which their host tree is."

The current pine beetle outbreak in Western Canada began in 1996.

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The beetles have already destroyed about 163,000 square kilometres of timber in British Columbiaan area more than five times the size of Vancouver Island.

The bugs also have a firm foothold in parts of Alberta and some have been found in Saskatchewan.

Reid said controlling the pine beetle outbreak is nearly impossible. You can't spray for them and the only solution is a sustained freezing temperatures in the winter.

That cold snap didn't happen last winter, said Caroline Whitehouse, a forest health specialist with Alberta Agriculture and Forestry.

She said population surveys done in May and June indicate pine beetle numbers are in the moderate to high range in the northern part of the province.

Whitehouse said the numbers are higher closer to the B.C. boundary and stable further east.

"We had such moderate temperatures this winter and also we had a relatively warm spring, so we didn't really see much mortality in these populations," she said.

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