Hemlock Pests and Diseases

Hemlock Pests & Diseases

Hemlock Woolly Adelgid (HWA) is a powerful predator of these integral trees, but unfortunately it is not the only serious threat to hemlock ecosystems. Hemlock trees battle a variety of other insect pests, and can be beleaguered by diseases – here are a few of the more formidable.

Threats to Hemlock Trees:

Threats to Hemlock Trees - Dale Luthringer, Cook Forest State Park

Elongate Hemlock Scale

Elongate hemlock scale, a tiny insect with an oval hard cover, or “scale,” is brown or white, and attaches on the undersides of needles seemingly at random.  This pattern is different from HWA, which lines up near where needles are attached to the twig. It sucks out juices, weakening the tree.

The elongate hemlock scale is particularly devastating to hemlocks already affected by HWA or drought, and often arrives after HWA has been found in the area. This “one-two” punch of two invasive insects is extremely difficult to treat, requires a great deal of labor and costly chemicals, and ultimately reduces the numbers of hemlocks that can be saved. 

the Hemlock Looper

The hemlock looper moth (Lambdina fiscellaria), a/k/a the mournful thorn, is a native insect which has “very long pectinations resulting in a conspicuously feathery antenae.” This little bug can severely defoliate hemlock during high population phases, and also damages balsam fir, white spruce, oak, and other hardwoods.

the Spruce Spider Mite

These rapidly-reproducing, warm-weather-loving arachnids use “piercing-sucking mouthparts [to] withdraw sap containing chlorophyll from the needles” of conifers, which don’t recover once damaged.  “Infested needles become mottled and appear yellowish to gray in color,” and are commonly in groups low and on the inside of plants. They’re not terribly easy to kill (use miticide, not insecticide), so read this informative Penn State Extension article if you worry about a spruce spider mite (Oligonychus ununguis) infestation.

the Hemlock Borer

The hemlock borer (Phaenops fulvoguttata, f/k/a Melanophila fulvoguttata) prefers to attack when a hemlock has already been weakened by other pests, disease, or drought.  The best defense against these destructive bugs is maintaining healthy hemlocks.  

Click here for more information on the hemlock borer – USDA Forest Service “Pest Alert,” Aug. 2000.

hemlock borer insect
PA DCNR - Forestry , Bugwood.org

Signs of hemlock borer infestation include:

  • small oval holes in the bark, about 3mm in diameter, [from which] beetles have emerged;
  • larval galleries on the surface of the sapwood, beneath the bark;
  • woodpecker damage may indicate heavy infestation with larvae; and
  • bark chips piled at tree bases indicate the same.

Needle Rust

There are several types of rust affecting hemlocks.  No treatment is usually needed, but you can identify hemlocks infected with needle rust by spotting that “current-season growth is slightly swollen and curled, [and] orange-yellow spores coat the infected tissue.”  

Climate Change Threats

While neither pest nor disease, it’s worth noting here that hemlocks are also intolerant to drought.  Weather patterns of late have shifted, becoming less even and predictable.  Pennsylvania has witnessed several years of long dry periods punctuated by strong storms, the water of which falls too heavily for proper saturation into the soil.  Dry, warm winters benefit the HWA, while weakening the trees.  

The hemlocks of Cook Forest are less impacted than those of the Smoky Mountains primarily due to environmental factors:  the southern environment more strongly favors HWA.  While the southern hemlocks sickened more quickly, northern hemlocks held on while science worked on “vaccinations,” and our occasional cold winters knocked back HWA infestation levels. 

How to help hemlocks

Improve the health of all your trees:

  • plant and encourage a biologically diverse property – trees with unrelated but nearby neighbors are often healthier 
  • invite and protect pollinators, both bird and insect
  • plant natives 
  • insist on local stream and riparian health protections
  • reduce pollution:  gasoline, exhaust, consumer pesticides; minimize lawn
  • monitor your trees for stress or disease, and act early 
  • protect roots from soil compaction (e.g., don’t park or drive  under tree canopies)

The majority of this presentation is courtesy of Dale Luthringer, Environmental Education Specialist, Cook Forest State Park – with thanks for his gracious permission and his time, from the CFC.  

Do you think your hemlock is afflicted by pests or diseases?  For specific questions on hemlock health issues, please contact your DCNR Service Forester, or the PA Bureau of Forestry