Hemlock Woolly Adelgid
Early Detection Saves Trees
Overview
Hemlock Woolly Adelgid (HWA) is an invasive insect that threatens eastern hemlock trees. The forest pest feeds at the base of the needles, draining the tree's stored nutrients and gradually weakening it. Without treatment, infested eastern hemlocks become unhealthy, their needles turn gray, and trees can die within 4 to 10 years. HWA can only be found on hemlocks. HWA has been spreading along the West Michigan coastline for several years, with confirmed infestations in nine counties, including Antrim County. Early detection is crucial—identifying an infestation quickly allows for timely treatment, which can save affected trees and stop the spread into new areas. Thanks to funding from the Michigan Invasive Species Grant Program and the U.S. Forest Service, we are offering free HWA surveys in high-risk areas. To schedule a survey please fill out our survey consent form - linked below.
Management Process
Managing HWA in Northern Michigan is typically a long process, with surveying, treatment, and evaluation that can stretch over several years. In a given area with potential for infestation, experts will survey every hemlock tree's branches for signs of HWA. If an infestation is confirmed, that tree as well as surrounding hemlocks will be documented and tagged for treatment. Treatment method depends on landowner preference and infestation location. Fortunately, some insecticides administered to hemlocks have proven effective in combating the spread of HWA, as seen in historical infestations of more southern Michigan counties. After treatment, evaluating the effect on a hemlock's health requires patience, as it takes around a year for insecticide to take effect and one to two more years to monitor the result. The length of time for which a tree is protected varies by the treatment applied, but follow up treatments are most often required.
Current HWA Distribution
Hemlock woolly adelgid are native to Japan and China, where they have naturally more resistant hemlock and predators that adapted alongside them in order to naturally sustain and manage the population size.
In Michigan, HWA have been spreading north along the coast of Lake Michigan since they were accidentally introduced in Michigan in Ottawa County in the early to mid 2000s. Their northernmost detected point in Michigan is currently in northern Antrim County. Hemlock woolly adelgid can be moved from tree to tree by birds, other wildlife, the wind, and on gear, equipment, clothing, infested nursery stock or other hemlock materials. Hemlock woolly adelgid can be moved when infested branches are trimmed and improperly disposed of, and also by vehicles or equipment brushing against infested branches.
The orange areas featured on the map highlight the areas in which HWA is most likely to occur and therefore are the highest priority area for us to survey. HWA infestations are often found along the coastline where distribution of eastern hemlock trees is highest.
Learn more about the distribution of HWA in Michigan here.
Learn more about HWA
Hemlock Woolly Adelgid (HWA) is native to Japan where it feeds on Asian species of hemlock and spruce trees. In its native range HWA is not a major pest issue because Asian hemlock species are fairly resistant to HWA and predatory insects are accustomed to feeding on the adelgids. In North America the Eastern Hemlock tree (Tsuga canadensis) is highly susceptible to HWA attacks in part due to little genetic resistance within the tree and also because of lack of predators. North American spruce tree species are not susceptible to HWA damage.
Identifying Eastern Hemlock trees
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Location: Found in shaded, moist environments across the northeastern U.S., including parts of the Great Lakes and Appalachian regions.
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Size: Eastern Hemlocks can grow 40–70 feet tall, with some reaching 100 feet. They have straight trunks up to 2 feet in diameter.
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Needles: Soft, flat, dark green needles with two whitish stripes on the underside. They are ¾ to 1 inch long and arranged in two rows along the twig. Needles will connect to the branch via a tiny "stem-like" appendage. (see fig. 4)
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Cones: Small, cylindrical cones, 1–1.5 inches long, hang down from branches and mature in the fall. (see fig. 2)
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Bark: Dark brown or gray, with shallow ridges that become scaly as the tree matures.
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Shape: The tree has a pyramidal or conical shape, with horizontal branches that may droop at the tips.
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Identifying HWA
HWA are tiny black insects that are difficult to see without magnification. However, as the adelgids feed, they secrete white, fluffy strands of wax from pores on their body. This wax — called an “ovisac” — covers the adelgids, protecting the insects and their eggs from desiccation and natural enemies. The white ovisacs are most visible from late fall through early summer and can be found at the base of needles, most often on the underside of the branch, as shown in the photo to the right. Their visibility during the winter months makes it easier to survey.
If you think you have identified HWA, make sure you report your finding to the Michigan Department of Agriculture and Rural Development (MDARD), or use the Michigan Invasive Species Information Network (MISIN) online reporting tool.
HWA Lifecycle
In North America, HWA completes two life cycles per year on hemlock trees. Reproduction is asexual — that is, all adelgids are female. In spring, a generation of adelgids called “progrediens” hatch from eggs. The crawlers (first stage) can move about on the tree until they find places to settle. Crawlers can be blown by wind to other trees and can also be transported by birds or other animals that perch on or brush against an infested tree. Most of the progrediens will settle at the base of needles on the previous year’s shoots. Once they settle and begin feeding, they will not move again. As the adelgids feed, they produce the strands of white wax from special glands on their bodies. The word “progrediens” comes from a Latin word meaning “to progress.” This generation of HWA does not require a long diapause period, and development “progresses” relatively quickly to the adult stage. A portion of the progrediens mature into winged adults, which will fly to spruce trees. In Japan, these insects feed on Asian spruce trees, causing round galls to form on the trees. In North America, however, HWA cannot feed on spruce. These individuals die without reproducing. The progrediens that remain on hemlock never develop wings, even as adults.
Deborah McCullough, Michigan State University Extension, 2015
Why are Eastern Hemlock Trees Important?
One of our most beloved evergreens, the eastern hemlock (Tsuga canadensis) also plays a critical part in our local ecosystems. Often found along coastal dunes, hillsides, and shady stream banks, eastern hemlock offer habitat for wildlife and provide shade for streams, effectively lowering stream temperatures and increasing oxygen for fish and other aquatic species. Hemlocks provide aesthetic value and are loved by homeowners. It is estimated that Michigan is home to 170 million eastern hemlock trees.
Water Quality
Eastern hemlock trees help improve water quality
Erosion Control
Hemlock tree roots hold streambanks in place and prevent erosion
Fishing
The unique conditions created by hemlock streams are important to many species of fish
Biodiversity
Hemlock trees are important for biodiversity
Wildlife Habitat
Dense hemlock groves provide habitat to wildlife year-round
Aesthetic Value
Hemlock trees are frequently used in landscaping
About our surveys
CAKE CISMA conducts hemlock surveys on public and private properties. You may submit the digital consent form above or request a PDF to print and mail back to us. Our surveys are free for the 2024-2025 season thanks to grant funding from the Michigan Invasive Species Grant Program and private donations. If you value our work and wish to support us please consider donating to CAKE CISMA.
When we arrive to survey we will call you to let you know we're at your property and to discuss any questions/concerns you may have about hemlock woolly adelgid. No news is good news--if we don't find anything during our surveys we will not contact you unless requested. If we do find a suspected HWA infestation we will collect a sample and send it to the Michigan Department of Agriculture for confirmation. You will be notified if the detection is positive, and CAKE CISMA will serve as a resource for you regarding treatment options. Please DO NOT collect samples yourself. CAKE CISMA staff have the required training and permits to collect and transport invasive species material.