Beetle puts trees at risk

The Department of Natural Resources announced Wednesday that mortality of eastern larch has been reported in many areas of the Upper Peninsula and in some areas of the northern Lower Peninsula. Eastern larch, also commonly known as tamarack, is succumbing to continued attacks by the eastern larch beetle (ELB).

“Once the ELB builds in an area of tamarack as evidenced by initial pockets of dead trees, continued mortality is likely,” said Robert Heyd, DNR forest health specialist. “Those who wish to utilize the tamarack in affected areas are encouraged to harvest their tamarack quickly if ELB is active in their stand.”

This bark beetle, which only attacks tamarack, first became an epidemic more than a decade ago. Tamarack was stressed by two consecutive years of defoliation by the larch casebearer in 2001 and 2002. The combined stress of this defoliation with the repeated droughts of the last decade contributed to the buildup of ELB populations.

Heyd said that from a distance, the most obvious evidence of a current infestation is yellowing foliage that develops by late July or early August. However, often heavily infested trees will not fade before the appearance of the normal fall color change. This makes detecting currently infested trees via aerial surveys or from a distance on the ground difficult if not impossible. Heavily infested trees generally fail to leaf out the following spring.

When infested tamarack are examined at close range, small entrance holes, (averaging just over 1/12 inch in diameter), are the only external signs of an infestation. Under the bark, beetle galleries are etched on the surface of the wood.

In fall and winter, woodpeckers often remove some or all of the bark from infested trees when searching to feed on bark beetles. By late winter and early spring, the removal reveals beetle galleries and exposes the reddish-purple inner bark or white sapwood of the tree. Debarked trees make it easy to identify an infestation.

Eastern larch beetles infestations are fueled by stressing events such as defoliation, flooding, drought, fire, old age, damage from windstorms or snow breakage.

“As tamarack stands age, they become more susceptible to bark beetles,” Heyd said. “Because tamarack trees are intolerant of shade, stress from competition may increase as crowns close in older stands. It is important to note that not all outbreaks have been associated with obvious stressors.

“Eastern larch beetles appear to be capable of attacking and killing trees when no predisposing condition or factor is apparent.”

The DNR recommends prompt removal of logs and utilization of material larger than 4 inches in diameter to remove breeding material and help reduce infestations. Infested trees, logs and slash should be removed once evidence of an infestation is detected.

For more information, visit www.michigan.gov/foresthealth.