An onslaught of tiny western pine beetles can bring down a mighty ponderosa pine. But the forest fights back by waging a sticky attack of its own. Who will win the battle in the bark?
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Bark beetles are specialized, with each species attacking only one or a few species of trees. Ponderosa pines are attacked by dark brown beetles the size of a grain of rice called western pine beetles (Dendroctonus brevicomis).
In the spring and summer, female western pine beetles fly around ponderosa pine stands looking for trees to lay their eggs in. As they start boring into a ponderosa, the tree oozes a sticky, viscous clear liquid called resin. If the tree is healthy, it can produce so much resin that the beetle gets exhausted and trapped as the resin hardens, which can kill it.
“The western pine beetle is an aggressive beetle that in order to successfully reproduce has to kill the tree,” said U.S. Forest Service ecologist Sharon Hood, based in Montana. “So the tree has very evolved responses. With pines, they have a whole resin duct system. You can imagine these vertical and horizontal pipes.”
But during California’s five-year drought, which ended earlier this year, ponderosa pines weren’t getting much water and couldn’t make enough resin to put up a strong defense. Beetles bored through the bark of millions of trees and sent out an aggregating pheromone to call more beetles and stage a mass attack. An estimated 102 million trees – most of them ponderosa – died in California between 2010 and 2016.
-- What is resin?
Resin – sometimes also called pitch – is a different substance from sap, though trees produce both. Resin is a sticky, viscous liquid that trees exude to heal over wounds and flush out bark beetles, said Sharon Hood, of the Forest Service. Sap, on the other hand, is the continuous water column that the leaves pull up to the top of the tree from its roots.
--- Are dead trees a fire hazard?
Standing dead trees that have lost their needles don’t increase fire risk, said forest health scientist Jodi Axelson, a University of California extension specialist based at UC Berkeley. But “once they fall to the ground you end up with these very heavy fuel loads,” she said, “and that undoubtedly is going to make fire behavior more intense.”
And dead – or living – trees can fall on electric lines and ignite a fire, which is why agencies in California are prioritizing the removal of dead trees near power lines, said Axelson.
---+ Read the entire article about who’s winning the battle between ponderosa pines and western pine beetles in California, on KQED Science:
---+ For more information:
Check out the USDA’s “Bark Beetles in California Conifers – Are Your Trees Susceptible?”
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