Insect to Injury?

With plenty of doomsday hysteria circulating about the destruction of western forests from the mountain pine beetle epidemic, the U.S. Forest Service is attempting to allay fears about another beetle on the rise – a 2mm-long twig beetle, Pityogenes plagiatus knechteli, that’s killing younger trees in mountain pine beetle-affected areas throughout Colorado and Wyoming.

First observed in high numbers several years ago, twig beetles, and the tree mortalities they're causing, have generated more inquiries from the public of late. As a result, the U.S. Forest Service Rocky Mountain Region released an interagency memo recently updating officials about the status of the twig beetle, which is boring into the upper boles and branches of mountain pine beetle-infested lodge pole pines as well as un-infested smaller lodge poles, mostly less than seven inches in diameter. Homeowners should avoid transplanting lodge poles, it recommends, and keep trees well-watered, though no further actions such as preventative spraying are currently being taken.

“This is not an unexpected event,” says Bob Cain, entomologist for the U.S. Forest Service, who predicts this twig beetle infestation is unlikely to become an epidemic.

Really, it’s a textbook story in beetle population dynamics: Big beetle invades big forest, eats it down to a small forest; small beetle attacks small forest, eats it down to a smaller forest.

The process of small beetle following big, known as a secondary infestation, has been documented as far back as the 1930’s, when researchers tracked a mountain pine beetle epidemic in Montana’s Beaverhead National Forest between 1927 and 1938. The study, published in 1940 in The Journal of Forestry, noted twig beetles in high numbers only during the tail end of that period, from 1933 to 1936, when they killed 3.4 million trees – a small bite compared to the 20.2 million trees eaten over 11 years by the mountain pine beetles. Of those 3.4 million, nearly 80 percent stood less than seven inches in diameter.

Since most salable trees are greater than nine inches in diameter, the twig beetles pose little economic threat to remaining lodge pole forests in Colorado and Wyoming. Some trees will die, of course, but Cain doesn’t foresee any additional die-off having a significant impact on lodge pole forest ecology.

“Twig beetle-caused tree mortality should decline in the next few years,” he says. “And we do not anticipate that they will cause stocking concerns in stands of younger trees.”

Trees attacked by mountain pine beetles slowly fade to gray after about a year, whereas twig beetle-infested trees often quickly turn red, sometimes within weeks. Aerial surveys – perhaps the most effective method for measuring beetle damage – can hopefully delineate gray from red by as early as the new year.

Cain admits, however, that twig beetle activity tends to be scattered and less noticeable than that of the mountain pine beetle. "There's no way to distinguish the size of the tree that's fading," he says. "So it's impact is going to be difficult to detect from the air.”

Adam Petry is a biologist and HCN intern.