The hemlocks of Western Maryland are as ecologically important as they are iconic.
Unlike many of our other native conifers, which generally require plenty of sunlight, hemlocks are shade-tolerant trees that thrive in riparian areas and moist coves. In addition to filling this slightly unusual niche, they also provide countless resources for wildlife, including the shade that helps keep local streams cool.
Hemlocks also make wonderful landscape trees for properties that can accommodate their sometimes-large size. From the privacy they provide to the shade they supply to the elegant aesthetic they offer, it’s easy to see why they’re so popular.
But as you may know, these beautiful trees are currently threatened by an invasive and deadly pest known as the hemlock woolly adelgid (HWA). So, it’s important to give your hemlocks the protection and support they need.
We’ll explain exactly how you can do so below.
The Hemlock Woolly Adelgid Basics
You don’t have to be an entomologist to understand the best way to protect your hemlocks from this pest, but it is helpful to know a few basic facts.
Historically native to East Asia, hemlock woolly adelgids (Adelges tsugae) were accidentally introduced to North America during the middle of the 20th century. They were initially discovered in Virginia, but they soon spread throughout the Eastern U.S., including the western portions of Maryland.
Eastern hemlocks (Tsuga canadensis) are the only tree attacked by HWA in our area, though they will also feed on Carolina hemlocks (Tsuga caroliniana) in the Southern Appalachians.
Members of the order Hemiptera, HWA have piercing-sucking mouthparts, which they insert near the bases of hemlock needles to obtain nutrients. This feeding behavior causes needle loss, as well as bud and shoot damage.
As the infestation progresses, the canopy thins, preventing the tree from photosynthesizing properly. Significant infestations can ultimately lead to the tree’s death — typically within a few years’ time.
HWA are very small insects, and even mature adults are only about 2 millimeters long. This makes them difficult to see until they start producing cotton-like waxy material that helps to shield their eggs and prevent desiccation.
Did You Know?
Every hemlock woolly adelgid in the Eastern United States is female. While both sexes exist in their native homeland, the individuals living in our area reproduce parthenogenetically.
The Signs & Symptoms of Hemlock Woolly Adelgids
There are ways to protect your hemlock trees from HWA, but it’s critical that you take action early. Once a tree has suffered significant needle loss or canopy decline, recovery is very unlikely.
So, be sure to watch for the following signs and symptoms that may indicate your tree is under attack:
- White, Cotton-Like Masses: The classic sign of an HWA infestation, these woolly masses are often seen at the base of the needles in the late fall through spring.
- Needle Discoloration: HWA attacks may cause a tree’s foliage to turn grayish green, rather than the deep, emerald green that normally characterizes healthy hemlocks.
- Premature Needle Loss: Hemlocks under attack from HWA will often drop large numbers of needles, leading to a thinner, sparser canopy.
- Overall Loss of Vigor: Along with the signs and symptoms discussed above, severely affected hemlock trees often exhibit reduced growth.
If you notice any of these symptoms, be sure to contact the Baker Tree Service team to schedule an evaluation.
Protecting Your Hemlock Trees: Treatment Options
While HWA certainly fall into the bad-news category, there is a glimmer of hope — with prompt and proper care, many hemlocks can survive these attacks.
A few of the best treatment options available include:
- Soil Drenches & Injections: Systemic insecticides (including imidacloprid and dinotefuran) can be applied to the soil around your tree, so the roots can absorb it.
- Trunk Injections: Some insecticides can be injected directly into the trunk, which helps minimize pesticide exposure to the surrounding soil.
- Bark Treatments: Some systemic insecticides can be sprayed on the lower trunk, allowing them to penetrate the bark and move upward through the tree.
- Horticultural Oil Applications: Horticultural oils can kill HWA on contact, but they must be applied thoroughly and provide no residual control once dry.
- Insecticidal Soap Treatments: Insecticidal soaps can also kill HWA on contact, but as with horticultural oils, ensuring proper coverage can be tricky.
You don’t have to know the best approach for your specific hemlocks. Your Baker Tree Service representative will be happy to evaluate your trees and recommend the best approach for protecting them.
Avoid Fertilizing Hemlocks Under Attack
While many homeowners and property managers believe that a quick fertilizer application will help their hemlocks rebound from an HWA infestation, fertilizer can actually have the opposite effect. In fact, fertilizer application can cause HWA populations to grow.
Hemlocks & Hemlock Woolly Adelgids in Western Maryland: FAQ
It’s normal to have questions about HWA and your hemlock trees. We’ll try to help by answering some of the most common questions homeowners have in the Western Maryland area.
Will hemlock woolly adelgid kill a tree?
HWA can certainly kill hemlock trees, particularly when infestations are allowed to progress without treatment.
Can a hemlock tree recover from hemlock woolly adelgid?
With early treatment, many hemlocks can recover from an HWA infestation. However, they may be attacked again in the future, so many trees will need to be treated periodically.
How long does it take HWA to kill a hemlock tree?
The time it takes HWA to kill a tree varies, but many trees die within about four to 10 years of an infestation.
Can HWA spread from one hemlock tree to another?
Absolutely. This highlights the importance of early treatment, especially in landscapes containing multiple hemlock trees.
Should a heavily infested hemlock be removed?
Potentially. By the time a hemlock has become heavily infested, it may be beyond the point at which treatments will help. As the infestation progresses, the tree may become hazardous and present risks to people and property in the area.
Can cold winters kill hemlock woolly adelgid?
Yes — very cold winters do kill off large numbers of HWA. However, some almost always survive, so while cold winters can temporarily knock back HWA populations, they do not eliminate the threat completely.
Can I treat hemlock woolly adelgid myself?
Homeowners can administer some HWA treatments, such as horticultural oils and insecticidal soaps. However, Maryland state law restricts homeowner use of neonicotinoid insecticides such as imidacloprid and dinotefuran, which are some of the most effective options available.
Professional applicators also have the equipment needed to achieve thorough coverage when treating large trees.
Protect Your Hemlocks Before It’s Too Late
Left unchecked, hemlock woolly adelgids can cause serious damage to your trees, likely resulting in their eventual death. But effective treatments are available, provided they’re initiated at an early stage in the infestation.
If you’ve noticed white, cotton-like masses or thinning foliage, contact Baker Tree Service to schedule an evaluation. We’ll have one of our plant health care specialists evaluate your trees and recommend the best course of action.
Sources and References
- Maryland Department of Agriculture – “Hemlock Woolly Adelgid”
State agency overview of hemlock woolly adelgid (Adelges tsugae) in Maryland, covering its threat to eastern hemlock, survey and quarantine efforts, and biological control releases.
https://mda.maryland.gov/plants-pests/pages/hemlock_woolly_adelgid.aspx - University of Maryland Extension – “Hemlock Woolly Adelgid on Trees”
Extension guide to identifying HWA by its characteristic white woolly masses, understanding its damage to hemlocks, and homeowner treatment options.
https://extension.umd.edu/resource/hemlock-woolly-adelgid-trees/ - Maryland Department of Agriculture – “Hemlock Woolly Adelgid” (fact sheet)
Printable fact sheet summarizing HWA identification, life cycle, damage to eastern and Carolina hemlock, and management approaches.
https://mda.maryland.gov/plants-pests/documents/hemlockwoollyadelgid.pdf - Arbor Day Foundation – “Hemlock Woolly Adelgid”
Conservation nonprofit overview of HWA identification, its impact on hemlock trees, and basic treatment and prevention guidance for homeowners.
https://www.arborday.org/planting-your-tree/hemlock-wooly-adelgid - Penn State Extension – “Integrated Approach to Hemlock Woolly Adelgid Mitigation”
Extension guide outlining a combined strategy for managing hemlock woolly adelgid (Adelges tsugae), integrating chemical treatment, biological control, and silvicultural practices to conserve eastern hemlock.
https://extension.psu.edu/integrated-approach-to-hemlock-woolly-adelgid-mitigation