NORTH CAROLINA—An Eastern hemlock can live for 800 years, anchoring ecosystems from its roots to its branches. But the hemlock woolly adelgid—a bug the size of a mere speck by the eye—can kill these giants in just a few years. Foresters, entomologists, silviculturists, physiologists and other experts have been working together to keep hemlock trees alive and reduce the impact of this devastating insect.

A recent guide synthesizes years of research to provide best practices for controlling hemlock woolly adelgids. The guide is titled Integrating Chemical and Biological Control of the Hemlock Woolly Adelgid: A Resource Manager’s Guide.

“A goal of the guide is to prolong the health of some hemlock trees with insecticides applied conservatively and strategically in a way that ultimately reduces the amounts applied across the landscape,” said Bud Mayfield, USDA Forest Service researcher and lead author of the guide.

For the past six years, Mayfield has been part of a collaborative team examining the impact of Laricobius nigrinus beetles, which prey on hemlock woolly adelgids. Part of the strategy for controlling hemlock woolly adelgid is to establish outdoor insectaries, stands of hemlocks where predatory beetles are raised. Mayfield has worked closely with national forests and other partners across the Southern Appalachians to release the predator insects.

“It takes years for biological control populations to build up, and often trees cannot survive the wait,” said Mayfield. “This integrated strategy is proposed as a way to buy time for the hemlocks while predator numbers increase.”

Insecticides, particularly the neonicotinoids that are most often used to kill hemlock woolly adelgids, can harm other organisms. But the way insecticides are applied to hemlocks minimize this harm, as they are applied to the trunk or to the soil by the trunk. When taken up by the roots, the insecticides become part of the tree’s tissues and move into the branches and twigs. When the hemlock woolly adelgids suck the sap, they die. Hemlock is wind-pollinated, so its cones are not visited by bees or other pollinators.

Combining chemical and biological control should prolong hemlock health on some trees while allowing predator insects to establish on unprotected trees or on trees treated with low rates of insecticide.

The guide provides information on treatment timing, site selection, monitoring and more.