Eastern Hemlock are at risk in Michigan from pest

The Hemlock Woolly Adelgid could have on eastern hemlock if this pest becomes widely established in Michigan.

Eastern or Canada Hemlock is a valuable tree species in Michigan’s forests that could be negatively impacted by the Hemlock Woolly Adelgid if the pest becomes widely established. | Photo credit: New York Dept. of Environmental Conservation.
Eastern or Canada Hemlock is a valuable tree species in Michigan’s forests that could be negatively impacted by the Hemlock Woolly Adelgid if the pest becomes widely established. | Photo credit: New York Dept. of Environmental Conservation.

What’s that conifer tree with the small needles and tiny cones growing underneath those hardwood trees? While it might look like a “lone sentinel” amongst all those hardwoods, Eastern or Canada Hemlock (Tsuga canadensis (L.) Carr.) is a special and valuable tree. While its use in the wood products and leather industry has diminished, this tree species is quite valuable for its wildlife and ornamental value.

Unfortunately, a few small infestations of Hemlock Woolly Adelgid (HWA) were recently discovered in western lower Michigan. This tiny sap-feeding insect is an invasive forest pest that has killed hundreds of thousands of hemlock trees in several eastern states. Officials are currently conducting surveys to assess the situation. But the presence of HWA and its potential impacts on hemlocks in our Michigan forests are major concerns.

Eastern hemlock is a unique tree species. According to the USDA Forest Service, eastern hemlock is the most shade tolerant of all tree species and can survive with as little as 5 percent of full sunlight shining upon the forest floor. It is also a slow-growing, long-lived tree species that can take 250 years or more to mature on certain sites.

Eastern hemlock grows natively from Maine and through down to the southern Appalachian states and across southern Canada to the upper Great Lakes. According to recent MDNR inventory data, more than 173 million hemlock trees grow in Michigan.

Eastern hemlock grows as isolated trees in hardwood forests or in pure dense stands on moist sites. It can be found growing among various hardwoods such as sugar maple and American beech or with various conifers including white pine, balsam fir and others. In Michigan’s Upper Peninsula, it can be often be found growing as dense clumps of hemlocks on wetter soils.

In Michigan, the value of hemlock to wildlife for food and cover is a very important use of this species. For instance, dense stands of eastern hemlock provide critical cover for some wildlife species such as ruffed grouse, wild turkey and others that protects them from deep snow and cold winds in winter. In parts of the Upper Peninsula of Michigan, hemlocks stands are used by white-tailed deer as “yarding” or sheltering areas. In winter, this is where deer concentrate in large numbers and are considered critical areas of their habitat.

In addition, eastern hemlock is an important source of food and nesting habitat for many species of songbirds and small mammals. Species such as chickadees, crossbills, grosbeaks and others are known to feed on the seeds of this prolific cone-bearing tree. Many species of rodents such as such as red squirrels, porcupines, moles and others are known to feed on the cones and/or bark of hemlock.

Finally, due to graceful appearance and capacity to maintain its lower branches over time, eastern hemlock also has appeal as an ornamental tree in the landscape market. Many cultivars of hemlock are available that are compact or dwarf in size or exhibit other interesting ornamental characteristics.

The State of Michigan recognized the potential threat posed by Hemlock Woolly Adelgid (HWA) several years ago. A state quarantine was imposed more than 10 years ago to prevent the importation of hemlock nursery trees from areas known to have HWA populations. Because HWA is not a federally regulated pest, states may or may not conduct detection annual detection surveys for HWA.

The recent discovery of HWA populations in a few areas in western lower Michigan should be of concern to Michigan residents. If these small spots of infestations spread further, they will begin to threaten this important tree species in Michigan. Continued diligence by Michigan residents is needed to help ensure that HWA does not accidentally get introduced into any other areas of the state.

As part of the effort to reduce the chance of new forest pests from moving into Michigan, Michigan State University has launched a statewide effort to help residents learn about the risks and impacts of invasive forest pests. Funded by the Michigan Invasive Species Grant Program, the MSU “Eyes on the Forest: Invasive Forest Pest Risk Assessment, Communication and Outreach Project,” links research with outreach and communication projects through the MSU Department of Entomology and Michigan State University Extension.  

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