Keep hemlocks safe: Look out for hemlock woolly adelgid

We may be in the throes of winter, but these recent bouts of spring temps remind us all that spring is on its way! Soon, life will awaken from its slumber and feather, fur, fin, and fern will return to their annual activity. Included in this awakening are our welcome (and not so welcome) insect brethren, with whom bring all their plant-munching habits. One of these insects (hemlock woolly adelgid) feeds on our hemlock (Tsuga canadensis) trees – and though we’re not seeing it here in the Keweenaw yet, our trees are not totally safe from this invasive forest pest. And, this is the best time to look at the branches of your hemlock to see signs of this insect.

How to identify hemlock woolly adelgid (HWA)

The hemlock woolly adelgid (HWA; Adelges tsugae) is an almost microscopic brown insect with tube-like mouthparts used to suck sap, killing a large tree within the span of 4-10 years if untreated (https://www.michigan.gov/invasives/id-report/insects/hemlock-woolly-adelgid). The HWA reproduces clonally (no males required), creating small, round, white, “woolly” looking masses (1/16” to 1/4″), which are actually waxy encasements of adelgid eggs (ovisacs), and are the most visible identifier of an HWA infestation (see photo), before the hemlock tree begins to decline. The woolly ovisacs are found at the base of hemlock needles on the underside of branches and can be sparse or all over the tree branches, depending on the degree of infestation. The ovisacs are most noticeable between November to July. As each new generation hatches, drinks the sap from the hemlock tree, and reproduces, they gradually tap the tree dry. Whole forests of hemlocks have died when the HWA comes to town (see photo).

Think emerald ash borer; think dutch elm disease. Just as our ash and elm were largely removed from our forests, so have the hemlocks across New England and the Appalachian mountains due to HWA. Hemlock trees bring intense shade to our woods, which keep stream waters cold for fish in the heat of the summer and create insulated warm shelters for deer in the depths of winter. Large swaths of forests are dying, and because HWA eats both the largest tree and the smallest seedling, few hemlocks are coming back. Here in Michigan, HWA has been slowly making its way up the Lake Michigan coast since at least 2016 (see photo). As of just this past season, HWA has been confirmed further north than previously detected, now in Benzie County, MI, just south of Traverse City. They are projected to be able to spread even further north and are possible in the Upper Peninsula.

How is HWA spread?

The first incident of HWA in Michigan was from infested nursery stock. However, the insects can be spread from tree to tree by birds, other wildlife, the wind, and on gear, equipment, and clothing. Hemlock woolly adelgid can be moved when infested branches are trimmed and not fully disposed of, and also by vehicles or equipment brushing against infested branches.

What can you do to protect hemlock in the Keweenaw?

• Buy nursery stock that comes from an inspected, reputable source (demand that from your nursery) and inspect it yourself when you get it

• Do not move firewood

• Be aware of where you travel or hike (are there hemlock around?) and clean clothing, equipment, and vehicles before spending time in a new hemlock area.

• Keep an eye out for the telltale white, “woolly” bunches at the base of needles on the underside of branches of your local trees

While forest health managers work on combating this pest, we can be diligent in monitoring and reporting any signs or symptoms that could be confirmed by experts. This allows for early detection and response, decreasing the possibility that this insect will spread across the Upper Peninsula. Hemlocks make up a significant part of our forests with 170 million mature eastern hemlock trees in Michigan alone. We cannot afford to watch them die. Keep your eyes peeled and we may be able to stave off this pest!

Article and research provided by Abraham Stone, afstone@mtu.edu, undergraduate in Ecology; Dr. Tara Bal, tlbal@mtu.edu, Assistant Professor; Dr. Sigrid Resh, scresh@mtu.edu, coordinator, Keweenaw Invasive Species Management Area (KISMA) and Research Assistant Professor, Michigan Technological University


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