Hemlock Woolly Adelgid Reaches Sleeping Bear Dunes

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A metal probe pointing to a white egg sac among hemlock needles under a microscope

A hemlock woolly adelgid egg sac under the microscope. National Park Service.

In February 2024, the National Park Service and the Michigan Department of Natural Resources found an outbreak of hemlock woolly adelgid (HWA) in the Old Indian Trail area of Sleeping Bear Dunes National Lakeshore. The insect was on more than a dozen hemlocks, and 80 more trees nearby might also be infested. It is the northernmost confirmed case of HWA in Michigan.

Because people, animals and the wind can all carry HWA from tree to tree, keeping it from spreading to other parts of the park, or home with visitors, is critical.

A small insect that starves trees

The hemlock woolly adelgid (Adelges tsugae) is a small, aphid-like insect native to Japan. After hatching it enters a mobile "crawler" stage and grows by feeding on sap flowing through a hemlock's new twigs and branches. Adults stop moving and lay eggs in sacs that look like white woolly clumps on the undersides of branches.

Because HWA feeds through winter and early spring, the tree cannot grow new needles in summer. With no new needles to replace the old, it eventually cannot make food, and it dies.

Crawlers are spread naturally by birds, mammals and wind, and people and vehicles can pick them up by brushing against hemlock branches. A single adult is enough to start an outbreak, since a female can lay eggs that are clones of herself.

How it spread

In its native range, HWA is kept in check by natural predators and by resistant host trees. Hemlocks in the eastern United States lack that resistance. Since it was accidentally introduced in the 1950s, HWA has spread from Virginia as far south as Georgia and as far north as Maine, and some Park Service sites in that range have lost 80–90% of their eastern hemlocks. It was first seen in Michigan in 2006, and the state's first infestation was confirmed in 2015.

Snow falling on a river with hemlocks along its banks

Hemlocks along a river in winter. National Park Service.

What is at stake

Michigan has more than 170 million eastern hemlocks. They give food, shade and cover to many birds and mammals, and matter most in spring, before other plants have leafed out. They often grow along rivers and streams, where their shallow roots hold the soil and limit erosion, and their shade regulates snowmelt and keeps water cool enough for fish and aquatic invertebrates.

At Sleeping Bear Dunes, 30,431 acres of forest, 65% of all the park's forest habitat, are at risk. Forests in the Crystal River watershed are a high priority for protection. Without intervention, HWA could devastate the species that depend on hemlocks.

A park scientist examining a hemlock branch through a microscope

A park scientist inspects a hemlock branch. National Park Service.

Fighting back

To treat HWA, park scientists first have to find it. They survey forest sites to see which areas are affected and how they might respond to treatment, usually when the insect is least mobile. Crawlers, about the size of a pepper flake, are hard to spot, so winter is the best time: the white egg sacs stand out, and the immobile adults are unlikely to be spread.

Once an infestation is confirmed, the trees are treated with a pesticide injected directly into the sap, which kills the insects as they feed. Each injection uses only about 2 mL, so the tree is unharmed, and injecting it directly spares surrounding plants. A treatment lasts 4–5 years.

Scientists also work to avoid spreading HWA themselves. They clean equipment, clothing and vehicles after working in infested areas, and may close trails there during the months when the risk of spread is highest.

How visitors can help

Obey trail closures and stay on trails.
After a hike, wash clothes, hiking gear and your vehicle with soap and water.
Learn to identify eastern hemlocks and HWA with Michigan's invasive species guide.
If you think you have found HWA in the park, send the park a photo with GPS coordinates. Report sightings elsewhere to the Northwest Michigan Invasive Species Network.