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October 01, 2018
If you travel past Michigan's cities, past the farms, there's a point where the billboards give way to forest land as far as the eye can see.

This is the home of eastern hemlock – spanning from West Michigan's dunes, across the northern Lower Peninsula, into all but two counties in the Upper Peninsula.

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.

The Threat

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.

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Eastern hemlock trees growing at Porcupine Mountains Wilderness State Park in Ontonagon County are shown. The old-growth hemlocks here are part of the reason the forest was preserved as a park. MDNR photo
A native of Asia, the adelgid probably arrived in the U.S. on a shipment of hemlock from Japan. It was first identified in Richmond, Virginia in 1951 and by the 1980s had spread to large tracts of forest in the Appalachian Mountains. On the move 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.

Coordinated response

As the map of new infestations grew, the need for a coordinated plan of action to battle this invasive species was clear. Staff from the Michigan departments of Natural Resources and Agriculture and Rural Development, Michigan State University, the U.S. Department of Agriculture's Forest Service, and the Ottawa County Parks and Recreation Department formed the Hemlock Woolly Adelgid Coordinating Committee, which cooperatively completed a statewide strategy document in August 2017.

Prevention

The Michigan Department of Agriculture and Rural Development spearheads the first line of defense – prevention. 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.

Staffers provide education, certification and inspection services to nurseries and producers handling hemlock in the quarantined counties, and they train certified pesticide applicators on the proper use of insecticides to treat hemlock wooly adelgids. The agriculture department staff also verifies reports of adelgids detected in new locations.

Detection and response

The DNR's Forest Resources and Parks and Recreation divisions are finding and treating hemlock woolly adelgid infestations on state lands, including at Silver Lake, Duck Lake, Muskegon, P. J. Hoffmaster and Grand Haven state parks – spanning the shoreline along these four affected southwestern Michigan 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.

Infestations on private and municipal lands in the four-county area are being surveyed by the Ottawa Conservation District, supported by funding from the Great Lakes Restoration Initiative, the Michigan Invasive Species Grant Program and Ottawa County.

To keep information organized and efforts coordinated, all partners use the same software for data collection. All survey and treatment information is housed in one database managed by the DNR that can be used by partners to inform decision-making and work flow.

Investigation

When invasive species arrive, they don't come with a set of instructions. 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.

"There are so many parts to managing an infestation – research, funding, partnerships, survey, treatment," McCullough said. "Working together means we're sharing information and moving each other forward, but at the same time each of us is able to focus on our part of the task."

The Northern Line

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, say infected trees perch halfway up the slope of a forested dune on private property just south of the park.

They are mapping the location of every hemlock on the property, recording each tree's diameter and attaching a numbered tag to the trunk. This prepares the site for chemical treatment to be applied by contractors in the fall or spring.

"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.

Detection surveys are broadscale and quick, examining no more than 30 trees per acre on selected plots to determine whether hemlock woolly adelgids are present. These surveys will be conducted by Cooperative Invasive Species Management Area staff – local partners who also assist by providing outreach to communities affected by invasive species.

Signs of Hope

At a campground in Norton Shores in Muskegon County, an early infestation site and ground zero for McCullough's research, stands of hemlock look gray and thin against the background of maples in full summer flourish.

A closer look reveals bright, vibrant hemlock trees among the maples, tied with assorted colors of plastic marking tape. Another hemlock has fresh, green growth at its tips. These trees are part of a study, funded by MSU's Project GREEEN, to improve treatment success for the insecticides Imidacloprid and Dinotefuran.

Dinotefuran is fast-acting but short-lived, protecting trees from adelgids for one to two years. Imidacloprid takes up to one year to show results but provides protection for at least four years.

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."

McCullough is investigating the effects of temperature on the adelgids – another factor that may improve the odds of success in the battle against these invasive insects.

After an extremely cold night in Muskegon in January 2018, 80 percent of the hemlock woolly adelgids on a sample tree at the campground in Norton Shores had died. Warmer temperatures on the same night at a site in Ottawa County showed far less adelgid mortality.

"Michigan's known infestations are along the lakeshore, which has its own micro-climate," McCullough said. "The lake effect means more snow and generally warmer winter temperatures than our inland areas, which may have an effect on the adelgids' ability to survive and spread."

Knowing what's at stake – the significant environmental, recreational and economic costs of losing Michigan's hemlock trees – keeps the team committed to working together to protect this valuable resource.

More information about hemlock, hemlock quarantines and identifying and treating hemlock woolly adelgid is available at www.michigan.gov/hwa.

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