Hemlock Woolly Adelgid

Cobblestone Bridge
Hemlock woolly adelgid was first spotted from Cobblestone Bridge over Jordan Stream near the boundary between Acadia National Park and the Land and Garden Preserve.

Ashley L. Conti/Friends of Acadia

In July 2022, NPS confirmed the presence of hemlock woolly adelgid (HWA) in Acadia National Park.This small, invasive insect pest has wreaked havoc in the forests of the eastern U.S. and Canada, damaging Eastern hemlock (Tsuga canadensis) populations and in turn affecting entire ecosystems. This page provides a general overview, resources, and contact information to help protect Acadia’s forests.
Person inspecting a hemlock branch covered in white wool, sign of HWA
HWA wool can be seen with the naked eye from late fall to early summer, but other life stages are harder to recognize.

Sam Mallon/Friends of Acadia

Species Profile

The hemlock woolly adelgid (Adelges tsugae) is a non-native, invasive, aphid-like insect. Its life cycle consists of an incomplete metamorphosis in which two generations per year grow from eggs to crawlers to adults.

HWA egg sacs are easily identifiable as small, round, woolly orbs on the underside of the needles. When the eggs hatch, adelgids emerge as crawlers, the first mobile instars or developmental stage. Crawlers do the most damage to Eastern hemlocks because they require the sap of the tree to grow. They feed on new branches and twigs during winter and early spring, preventing the production of new needles in the late spring and summer. This is what eventually causes the death of the tree.
HWA closeup
Close-up view of HWA wool from a sample collected at Jordan Stream.

Sam Mallon/Friends of Acadia

Arboreal species of birds and mammals spread HWA crawlers from tree to tree. People, vehicles, and wind can also spread HWA during their crawler stage (March-July). Adults remain on or near the tree where they hatched to lay their eggs. Although these adelgids do not directly affect other animals or plants, the loss of hemlocks has an impact on the ecosystem as a whole.
Map showing HWA infestations spreading across the Eastern US from 1951-2021
While HWA was only recently found in Acadia National Park, it has been spreading across eastern North America for over 70 years.


Origin of HWA

HWA was accidentally introduced to the United States from Japan, where natural predators and host tree resistance kept the population of A. tsugae in check. It was discovered in the Pacific Northwest as early as the 1920s but has not posed a significant threat to the more resistant western and ornamental hemlocks. On the East Coast, the first recorded sightings occurred in Virginia and Southern Pennsylvania in the 1960s. Until the 1980s, HWA wasn’t recognized as a large-scale problem, but it now covers the extent of the Eastern hemlock’s range from Georgia to Maine.
Jordan Stream
A canopy of Eastern hemlock maintains the cool water temperature of Jordan Stream.

Ashley L. Conti/Friends of Acadia

Changing Ecosystems

The Eastern hemlock, Tsuga canadensis, covers an estimated 2.5 million acres of eastern forests. In the southern Appalachian region, where initial infestations of HWA took hold beginning in the 1980s, some NPS sites and national forests reported losing 80-90% of hemlock trees. Mild winters and high population density of susceptible trees contributed to the rapid spread of HWA in this region. The significance of the loss could be seen throughout the ecosystem; when the hemlock forest canopy began to deteriorate, the decline of overall forest health, wildlife habitat, and water quality soon followed.

Eastern hemlocks tend to grow near freshwater streams, ponds, and lakes. During the hottest months, the hemlock canopy maintains cooler, more constant water temperature by providing dense cover from the sun. This creates an ideal environment for small invertebrates and cold-water fish. These invertebrates are the major food source for Acadia’s many species of freshwater fish, including brook trout. As hemlocks decline due to an HWA infestation, so do the species that depend on them.
Managing HWA
Jesse Wheeler, Vegetation Program Manager at Acadia National Park, examines HWA on an Eastern hemlock branch.

Sam Mallon/Friends of Acadia

Managing HWA

The first step to protecting Acadia’s forests from HWA is to locate the areas of hemlock that are most important to protect, such as stands that contain older, dominant canopy trees. Surveying new areas and closely monitoring existing infestations is crucial to understanding the extent of the problem. Park visitors and local landowners have been a great help in identifying and reporting HWA in our area—so know what to look for and who to tell if you have a possible HWA sighting (see contact information below).

The Invasive Plant Management Team (IPMT) has also begun cutting back hemlock branches that may come into contact with vehicles, bikers, or pedestrians along roadways and trails. Visitors should continue to buy firewood locally. Slowing the spread and stopping new introductions gives forest managers more time to respond.

Since HWA is a non-native insect, it has no predators native to North America. Predation is one form of biological control that maintains the population at a manageable level. Beetle species that feed on the winter generation (Laricobius osakensis) and summer generation (Sasajiscymnus tsugae) of HWA have been introduced to manage infestations in other areas. The USDA found these beetles pose no significant risk to native species and are now reared for release in natural areas. In October 2022, the Land and Garden Preserve released Laricobius beetles along Jordan Stream in an area bordering Acadia National Park.

Other national parks such as Shenandoah and Great Smokey Mountains have successfully used chemical controls, or pesticides, to treat individual trees for HWA. Integrated Pest Management, or using a combination of these tools, may be the best approach to help Acadia’s hemlocks survive HWA. Now that the adelgid is here, it will never be completely eradicated—but if we act quickly and thoughtfully, we can prevent the devastating loss that other regions have experienced.

How You Can Help

This page was adapted by Emma Lanning from a web article by Sophia Cameron and Jesse Wheeler

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    Last updated: February 2, 2023

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