Invasive hemlock-killing bug spotted in Michigan

Foresters and landowners are urged to be on the lookout for the insect

(Photo: USDA Forest Service)

MADISON, Wis. -- The return of spring weather has the officials warning landowners to keep an eye out for infestations of hemlock woolly adelgid (HWA), an invasive insect native to east Asia and now raising concern for landowners and forest managers in Michigan.

HWA affects eastern hemlock trees (Tsuga canadensis) by sucking the tree’s sap and disrupting the flow of nutrients to needles and branches until the tree grows weak and eventually dies. This tiny insect is difficult to see, but it is identifiable by the cottony, white egg-containing sacs that form on the undersides of the tree’s branches. HWA was accidentally introduced to the West Coast of the United States in the 1920s, and by 1951 it had reached Virginia and spread to 20 states along the East Coast. In the 2010s, it was transported into Michigan on nursery stock. Since then, a quarantine with mandatory treatment of all incoming hemlocks was established to prevent future exposures. Thanks to HWA’s relatively slow spread rate and the diligent effort by forest managers, the spread has been limited to five counties within the state.

Stopping HWA does take considerable effort, though.

“We’re working on eradication, which is difficult because we keep treating, and it keeps spreading,” said Robert Miller, the Invasive Species Prevention and Response Specialist at the Michigan Department of Agriculture & Rural Development (MDARD).

Miller described a band of hemlock trees along the shore of Lake Michigan as currently facing the most immediate threat. Many efforts are being focused there to ensure that the infestation does not spread further into northern Michigan where there is a much higher concentration of hemlock. The state has an estimated 170 million eastern hemlock trees, which play critical roles in their ecosystems by providing habitat for other plants and wildlife and protecting waterways from run-off and erosion.

Trees infested with HWA can be treated with insecticide that contains either Imidacloprid or Dinotefuran as an active ingredient. Effective treatment may require reapplication in the following few years. HWA can also spread to nearby properties by wind, humans, birds and other animals, so it is recommended that neighboring property owners coordinate to develop a practical treatment plan. In many cases, individuals themselves can treat infestations on their property, but there may be some instances where a professional should be consulted, especially if there is danger of insecticide run-off entering water sources. Depending on location, there may also be other regulations about the amount of insecticide that can be applied per year, so it is important to research any restrictions before starting. Trees can be treated in the spring, and any new growth should be checked for fresh infestation signs in the late fall. Any previously infested trees should be monitored over the next few years to determine if it is necessary to reapply the treatment. Trees left untreated will likely die in 4 to 10 years.

With many hemlock trees being found on private property, individual landowners play an important role in controlling the threat of HWA.

“We rely heavily on the public to be our eyes and ears when dealing with invasive species,” Miller said.

Michigan landowners are asked to use the contact information below to report any infestations on their property.

Michigan has placed several eradicative and preventative measures in place to control the HWA spread. According to Miller, MDARD often works with Cooperative Invasive Species Management Areas (CISMAs) and other partners to survey and treat any infestations. They are currently collaborating with the West Michigan Conservation Network, which services the area of infestation near Lake Michigan. The work is largely funded by the Michigan Invasive Grant Program (MISGP) from the state and the Great Lakes Recovery Initiative (GLRI).

HWA has not yet been reported in Wisconsin, but an exterior quarantine is in place to restrict the imports of hemlock from areas of infestation.

For more information on identifying and managing HWA, visit or contact or 800-292-3939.

Injection 2 WMCISMA
A certified pesticide applicator prepares to inject an infested hemlock tree with pesticide.

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