Science of Saving Hemlocks

Threat of the HWA:

The tiny pest known as the Hemlock Woolly Adelgid (HWA) is a sucking insect native to Japan and is believed to have been introduced to the US, near Richmond, Virginia. They feed only on hemlocks and depend on the tree to complete their life cycle.

An up-close view of a HWA. Image Credit: Kelly Oten, N.C. Forest Service

In eastern North America, HWA reproduces asexually. Every individual is essentially genetically identical and capable of reproducing. They have two generations each year, and with abundant hemlock hosts in eastern forests, HWA has reproduced rapidly.

Map showing established HWA populations. Credit: USFS

Incapable of moving far on their own, these little bugs disperse via wind, birds, animals, people and traffic. HWA was first detected in Shenandoah National Park in the 1980s; by the early 2000s, its reach was significant and devastation was widespread.

Map showing the rapid spread of HWA infestation by county and year. Credit: HRI

The HWA kills trees slowly, affixing itself to the base of the hemlock needle where it feeds on the tree’s sap and starch reserves. HWA feeding interferes with the tree’s ability to take up water and nutrients. As a result, the hemlock’s needles take on a grey and dusty appearance (hence the nickname “grey ghosts”) and begin to drop.

dead hemlock
Grey ghosts line the river in the Linville Gorge area of the Pisgah National Forest. Image Credit: Steve Norman, USFS

Increasingly unable to photosynthesize, the tree slowly dies from the bottom up. Trees can succumb to the pest in as little as four years, but in some cases this takes much longer. Sadly, the largest trees, which require the movement of more water and nutrients to their crowns, appear to be the most vulnerable.

HWA egg sacs
“Woolly” egg sacs that give the HWA their namesake. Image Credit: Robert L. Anderson USFS



Many of our remaining mature eastern hemlock trees are only alive today because they have received chemical pesticide treatments that have acted as an immediate life-support system. Chemical treatment is still the only completely reliable way to save an individual tree from mortality due to HWA.



Soil injection and drenching both apply liquid chemicals to the soil around the base of trees. While this is a common practice among both public and private landowners, proper training, licensing, and permitting may be required. These chemicals are water soluble and can easily be taken up by the trees roots to ward off HWA and other pests. An alternative form of insecticide used by PHHAT volunteers is known as CoreTect.

The reason PHHAT is using CoreTect instead of other products applied to the soil is because there is less risk of unintended exposure when transporting and handling the product, which is important because of the boat-based aspect of this project and volunteers are coming into direct contact with the river.

The main ingredient in CoreTect is imidacloprid, which is used against a host of pests, including aphids. All PHHAT volunteers have been trained by professionals and operate under the permit of one or more licensed applicators present on the river.

Screen Shot 2018-07-26 at 2.48.39 PM
A PHHAT volunteer applies CoreTect pellets to the base of a hemlock tree during a workday. Image Credit: Levi Rhodes

CoreTect comes in solid, slow-release tablets that are inserted in the soil at the base of hemlock trees. The number of tablets volunteers use is based on how large each tree is – typically 2-3 tablets (sometimes less) per inch of trunk diameter measured at breast height (DBH).

PHHAT volunteers keep these chemicals safely strapped inside their kayaks in double drybags. Thanks to American Whitewater and Watershed Drybags, volunteers were able to acquire customized gear just for this project.

Screen Shot 2018-07-26 at 2.51.00 PM
Measuring out the proper amount of pellets. The chemicals stay safe and dry inside a custom American Whitewater Watershed Drybag. Image Credit: Levi Rhodes

While chemical treatments last a number of years, we shouldn’t depend on them as permanent solutions. They are critical, however, in helping us buy time while we research a long-term response. In addition to chemical treatment, other methods of hemlock conservation include biological controls, genetic research, and silvicultural strategies.