Working together to save Michigan’s valuable hemlock trees

LANSING — The hemlock wooly adelgid is an invasive species in Michigan from Japan that damages eastern hemlock trees.

The defoliated eastern hemlock trees in the center of the photo show damage from hemlock woolly adelgids in the Great Smoky Mountains.

The defoliated eastern hemlock trees in the center of the photo show damage from hemlock woolly adelgids in the Great Smoky Mountains.

Though not a standout, hemlock is an important part of the mesic northern forest, providing shelter for deer and nesting birds, and keeping forest streams cool and clean.

Now, the state’s hemlock resource, estimated at 170 million trees, is threatened by a tiny invasive insect – the hemlock woolly adelgid.

Nearly invisible to the naked eye, the black, aphid-like bug pierces branches and feeds on sap, slowly sucking the life from the tree.

To protect its eggs, the adelgid spins a cotton-like, waxy white ball. These “ovisacs,” resembling the tips of cotton swabs, are visible on the underside of hemlock branches, near the base of the needles. It is the woolly appearance of these ovisacs that help give the hemlock woolly adelgid its name.

Despite a 2001 external quarantine restricting the shipment of hemlock to Michigan from states infested with the adelgid, the insect was detected in Emmet County, just south of the Mackinac Bridge, in 2006.

Reports were then later confirmed in Macomb and Ottawa counties in 2010, in Berrien County in 2012 and in Allegan County in 2013.

These small, localized infestations were managed by surveying and removing infested trees and treating nearby trees with insecticides. By 2015, just when these sites were receiving an “all-clear” designation, reports of hemlock woolly adelgid were confirmed in new areas of Ottawa County and in southern Muskegon County.

Surveys, and reports from the public, revealed infestations in northern Muskegon County in 2016, and in Ottawa, Allegan and Oceana counties in 2017. Not only private lands were affected, but also state parks in these four western Lower Peninsula counties were found to have severe infestations.

“Given the checkerboard pattern of hemlock woolly adelgid across the western counties, it is likely that multiple introductions of infested tree stock are responsible,” said Scott Lint, a forest health specialist with the Michigan Department of Natural Resources.

Once it is introduced, the adelgids can be spread by wind, wildlife and vehicles that brush against infested trees.

Along with the 2001 external quarantine, the department issued an internal quarantine in 2017, restricting the movement of hemlock tree nursery stock and unprocessed hemlock products from, or within, Allegan, Muskegon, Ottawa and Oceana counties.

A recent grant from the U.S. Forest Service’s Landscape Scale Restoration Program will expand outreach to local units of government in affected areas and provide training to their staff.

Knowing how they will respond to a newly encountered environment, what they need to survive and whether they develop new behaviors are important considerations in determining how best to control them.

Deborah McCullough, a professor in Michigan State University’s departments of Forestry and Entomology, is at the center of a multifaceted effort to understand the hemlock woolly adelgid’s life cycle in Michigan, its response to insecticide treatments and the effects of Michigan’s winter temperatures on its survival.

McCullough and her colleagues have already completed an adelgid risk map, layering hemlock stands identified by satellite imagery over climate data indicating temperatures favorable for adelgid survival.

The map directs survey crews to the most likely places hemlock woolly adelgids might be found. Preliminary findings from treatment studies are communicated with partners and contractors to improve results in the field.

Silver Lake State Park, in Mears, along the Lake Michigan shoreline in Oceana County, is the most-northerly-known location of hemlock woolly adelgid in Michigan.

Emma Fojtik and Katie Knapp, crew members with Ottawa Conservation District’s adelgid project, perch halfway up the slope of a forested dune on private property just south of the park.

“Once we find an infested tree, every hemlock within 800 feet of the tree will be treated,” Knapp explains, as she gestures toward a seedling full of white masses. “Basically, all of the hemlocks on this property have hemlock woolly adelgids.”

Nearly identical work is happening at Silver Lake State Park, where DNR staff is surveying and preparing for hemlock treatments.

“Our current strategy is based on the knowledge we have now,” said James Wieferich, a technician with DNR Forest Resources Division. “If adelgid infestations are limited to the areas we have surveyed, we can create a barrier to sever the infestation from areas farther north that are not infested and then stair-step treatment down to the southern limits (of the infestation).”

North of the designated barrier, the Nature Conservancy – in partnership with the Michigan Dune Alliance – will soon begin detection surveys in coastal areas not known to be infested with adelgids.

Armed with effective treatments and a coordinated management strategy, Michigan hopes to be able to contain its hemlock woolly adelgid infestation.

The Nature Conservancy’s Shaun Howard, project manager for Eastern Lake Michigan, is cautiously optimistic.

“(Working together) we have more data to make decisions on a broader scale,” Howard said. “Treatments are available and effective. Once trees are infested, tree mortality could take four to 10 years, so we have time to save the trees – but I can’t say whether this will be eradication (of the infestations) or just the beginning of a long-term effort.”


More information about hemlock, hemlock quarantines and identifying and treating hemlock woolly adelgid is available at


Posted by Ken Grabowski

Ken is News Advocate’s education reporter. He coordinates coverage for all Manistee County schools and West Shore Community College. He can be reached by phone at (231) 398-3125 or by email at

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