Twig beetle: The next nemesis for Colorado trees?
July 10, 2010
SUMMIT COUNTY, Colorado – As if Colorado’s forests didn’t already have enough enemies.
In the wake of the mountain pine beetle epidemic, another tree-killing beetle is on the rise. The twig beetle, Pityogenes knechteli, is attacking smaller trees the pine beetle has overlooked.
Officials from the Colorado State Forest Service first saw high numbers of the twig beetle last summer.
“We’ve been doing some periodic field monitoring to see what this bark beetle is going to do,” said district forester Ron Cousineau. “Right now, we just don’t know to what extent it’s going spread or how much impact it’s going to have.”
Like the pine beetle, the twig beetle burrows through the bark of lodgepole pines into the trees’ cambium, which facilitates the flow of nutrients. The twig beetles feed on the trees’ living tissue, essentially shutting down its circulatory system.
Pine beetles, which are projected to kill about 90 percent of mature lodgepole pines in Summit County, target the main stems of trees greater than 6 inches in diameter. Twig beetles prefer branches and young trees between 1 and 5 inches in diameter. The twig beetles do not attack seedlings.
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“You have trees that have survived the mountain pine beetle epidemic that are now being attacked by the twig beetle,” said Bob Cain, a U.S. Forest Service entomologist.
At about 2 millimeters long, twig beetles are smaller than pine beetles, and the holes they burrow into trees are about as wide across as a pencil lead.
A surge in twig beetles following a pine beetle epidemic has been reported in scientific literature dating back as far as 1939. So it’s no surprise to forest health experts that the smaller insect’s population has risen recently.
Cain doesn’t expect the outbreak to be as severe or long-lasting as the pine beetle epidemic has been, partly because there aren’t many living lodgepoles left. Also, mortality among younger trees won’t have nearly the visual impact that the death of mature lodgepoles has had.
“I think it’s going to be a matter of beetles using up the resources that are available. Maybe for the next year or two, we’ll have continued evidence of high populations,” Cain said.
Stressed trees are the most vulnerable to the twig beetle. Recently transplanted trees are at higher risk of dying from an attack. The bugs are also particularly common in dense stands of young lodgepole, where individual trees are competing against one another for nutrients.
But Cousineau advised against thinning stands now, since the exposed resin of cut trees will attract the beetles. Instead, thinning should take place in late September, he said.
When twig beetles do infest a tree, mortality takes place much more quickly than in trees hit by mountain pine beetles. A tree attacked by twig beetles will likely turn red and die in one summer, whereas those hit by pine beetles take a year to die.
“When they become active in a pocket of trees, you’ll start to see a whole portion of it turning red very quickly. It might turn in just a few weeks,” Cousineau said.
SDN reporter Julie Sutor can be reached at (970) 668-4630 or firstname.lastname@example.org.