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What Can Pennsylvanians Do About Hemlock Woolly Adelgid?

This recent interview with a Penn State Extension forester provides an update on the invasive pest hemlock woolly adelgid.

Updated:

June 20, 2022

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Photo credit: Calvin Norman

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First found in Pennsylvania in 1967, hemlock woolly adelgid (HWA; Adelges tsugae) has infected, weakened, and killed an enormous number of our state tree, the Eastern hemlock (Tsuga canadensis). HWA was slowly moved from the eastern border through the Allegheny mountains to western Pennsylvania. As of 2019, it was officially in all but three counties in Pennsylvania. It is now, almost certainly, in all of them. Much has already been written about this pest (see resources, below), but a status report is in order.

To that end, I had the pleasure to speak with Calvin Norman about current research efforts and to ask him what individual Pennsylvanians can do about hemlock woolly adelgid. Mr. Norman is a forestry and wildlife expert who works for Penn State Extension. He is a forester who covers the southwest portion of the state, focusing on climate change and forest pests. At the nexus of climate change and forest pests is the HWA. As Pennsylvania winters have warmed, the killing freezes that reduce HWA populations have become less frequent, allowing for its spread westward and northward.

While many effective treatment options have been developed, it is impractical and unaffordable to treat all hemlocks in the millions of acres of public land in Pennsylvania. Trees that are considered to have high cultural or habitat value are treated. Research also continues in some of the remaining old-growth forests in Pennsylvania by the scientists looking to create and evaluate other avenues for control and eradication of HWA.

But there is much that the citizens of Pennsylvania can do to help prevent the spread and impact of this invasive pest on our remaining hemlocks.

First, all citizens can educate themselves on how to identify hemlock woolly adelgid and how infected hemlocks look. A plethora of online resources are available from Penn State Extension, the Pennsylvania Department of Conservation and Natural Resources (DCNR), and the Forestry Service of the United States Department of Agriculture (USDA).

Homeowners can play a unique role in interacting with this pest:

Keep the hemlocks on your property healthy. HWA is more successful in attacking a stressed tree. Provide supplemental irrigation during periods of prolonged summer or fall drought.
Inspect your trees regularly for this pest, that way you can find it before it has the time to spread and cause injury. HWA's tell-tale sign is the white, cottony egg sacs at the base of the needles. Infected branches die, then the whole tree. Mr. Norman emphasized that you can't really predict where it will show up, although the proximity of infected trees raises the odds of new infections. Be aware that this pest does not move on its own. Wind may carry it from tree to tree. It may also hitchhike on birds or other mammals, so new areas of HWA infestations may be found high in the tree or at ground level.
Treat infected trees. Many treatment options are available and detailed information has been previously published by Penn State Extension, see Integrated Approach to Hemlock Woolly Adelgid Mitigation. Depending on the location and extent of the infection as well as the size of the tree, homeowners may choose to hire a professional. Several treatment options are relatively easy and require no new equipment purchases, including the option of burying pesticide tablets in the ground. Remember to follow all directions on the pesticide label. As with any pesticide, more is not better. With chemical treatment, protection from HWA may last as long as four to ten years. Although some trees will experience secondary infections of mites, a tree treated for HWA will not necessarily experience a secondary infection.
Do not treat trees preventatively. If a tree is not infected, leave it be. The pesticide options for hemlock woolly adelgid are all broad-spectrum, meaning they will indiscriminately kill good bugs along with the targeted ones.
Do not use biological controls. While research scientists hope to someday confirm that certain species of insects will control HWA, there are no recommendations for this type of release at this time. More harm than good may come from doing this.
Plant locally-sourced native plants, as a way to stop the next insect or fungal "plague." HWA is an invasive exotic insect. Just like the emerald ash borer, Dutch elm disease, oak wilt, and chestnut blight—hemlock woolly adelgid was unintentionally introduced to our gardens and forests from outside the United States.
Plant diverse species throughout your landscape. This reduces reliance on a single species and also creates a distance between the same species.

Knowing all of this, I asked Mr. Norman if homeowners should still plant hemlocks. His answer was to plant them only if you are willing to undertake chemical controls. Research continues into developing disease-resistant hemlock cultivars, but this type of research takes a long time. Many native trees and large shrubs are readily available to act as substitutes for the Eastern hemlock, depending on the role that the replacement needs to play, be it as a windbreak, hedge, or specimen.

Concerned citizens and gardeners have a role to play as well. Become a contributor in identifying infected trees by using an app called iMapInvasives. This app can be downloaded to your phone for free. As you take hikes in the woods or walks in your neighborhood, you can be on the lookout for HWA, among other invasive pests. iMapInvasives is easy to use, provides valuable information for the research scientists, and allows citizens to become part of the solution to the problem.

Anyone who hikes or otherwise comes in contact with an area of hemlocks can take intentional steps to avoid becoming unwitting spreaders of HWA. When arriving home, clean off your boots and put your clothes in the wash. Humans are one of the many mammals that spread this pest.

Having closely watched the effects of hemlock woolly adelgid for several years, Mr. Norman feels that this is a very depressing pest. His biggest fear is that in the not-so-distant future there will be no remaining old growth stands of Eastern hemlock in Pennsylvania.

It is up to each of us to take a responsibility for the future of our own gardens as well as our shared natural areas. Become educated, treat your own infected trees, and try not to contribute to the spread of the invasive exotic hemlock woolly adelgid.