New Manager’s Guide for Controlling Hemlock Woolly Adelgids

Hope for the hemlocks

A hemlock branch, covered in adelgids
A healthy, hemlock branch with a high density of HWA. Such branches are desired locations for releasing adult predator beetles. Photo by Scott Salom, Virginia Tech.

An Eastern hemlock can live for 800 years, anchoring ecosystems from its roots to its branches. But a bug that’s a 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, the hemlock woolly adelgid.

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.

“The goal of the strategy is to prolong the health of some hemlock trees with insecticides, while, on other trees, establishing adelgid-eating insects,” says Bud Mayfield, USDA Forest Service researcher and lead author of the guide.

In forests, the researchers propose creating four classes of hemlock trees ranging from full protection to none. Full protection means the full rate of insecticide, and trees are retreated every 5-7 years. Typically, this class would include the most beautiful or valuable hemlocks. Two other classes would provide some protection with a reduced amount of insecticide for temporary protection. Trees in these two classes will eventually support woolly adelgids – and their predators – after the insecticide treatment wears off.

The final class of hemlocks are not treated with insecticides. If not already infested with hemlock woolly adelgids, they will be soon. The miniscule adelgids will suck the sap out the tree’s young branch tips. But these adelgids will become food for the adelgid-eating predators – which strategically should be released on infested trees that are still healthy.

A diagram shows a hypothetical forest and marks the trees that are in each of the four protection classes with different symbols
Predators are released on Class 1 trees, eventually colonizing class 2 and 3 trees when chemical protection wears off. High-value class 4 trees remain perpetually protected.

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,” says 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.

“It is not practical nor ecologically responsible to chemically treat all hemlocks indefinitely,” says Mayfield. “A goal of this guide is to use insecticides conservatively and strategically in a way that ultimately reduces the amounts applied across the landscape.”

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.

The guide is most relevant to stands of at least 10 acres, in which hemlock makes up at least 20 percent of the basal area and where hemlock trees are not too widely spaced. The stand guidelines are important because stands that are too small, or stands where hemlocks are too far apart, might not support predator populations.

A woman looks up at a hemlock tree to evaluate crown health
The guide include information on evaluating hemlock crown health, monitoring predator populations, and much more. Photo by Bud Mayfield, USFS.

A growing body of research suggests that extra sunlight helps hemlock trees resist the adelgids. Mayfield has contributed to this research, which began in a controlled environment and recently moved to the field. Results from both environments have been very encouraging.

“Management recommendations based on this research are still in development,” says Mayfield. “As silvicultural treatments become available, they can be integrated into the chemical and biological control strategy we propose.”

“In the meantime, hemlocks in small canopy gaps, along forest edges, or in other relatively sunny areas could be good candidate trees on which to release predator insects – assuming the hemlock trees are infested.”

Read the full text of the guide.

For more information, email Bud Mayfield at albert.e.mayfield@usda.gov.

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