Hemlock woolly adelgid

The hemlock woolly adelgid has caused high mortality among eastern hemlock, making permanent changes to forests in many eastern states. The adelgid has reached Michigan several times but early detection has led to eradication.

The hemlock woolly adelgid (HWA) has killed much of the hemlock resource across eastern North America, both eastern hemlock and Carolina hemlock. Michigan is home to some of the largest stocks of old growth hemlock on the continent. It’s one of Michigan’s ten most common trees. Therefore, the HWA poses a significant threat to Michigan.

Adelgids are tiny insects related to aphids. They insert long stylets (sort of like straws) into the tissue of conifer twigs and feed on the sap. At this point, they become immobile and protect themselves by secreting a white, waxy substance. Colonies of these adelgids can be seen on the new growth of hemlocks. Unfortunately, infestations often begin at the top of tall trees and can be difficult to see.

Adelgids have peculiar life cycles, but the hemlock woolly adelgid can most easily be observed in the fall and winter. While the groups of white puffs may be difficult to spot, browning foliage in hemlock crowns may be a tell-tale indication. However, there are many other factors that could cause these symptoms.

Most of Michigan’s HWA infestations have been on imported landscape stock. In 2015, the first infestations of native trees were confirmed in Ottawa and Muskegon Counties. In all cases, alert citizens filed reports.

Most new infestations are found by non-professionals who are curious about tree health and stay informed about tree threats, such as the HWA. In Michigan, the “Eyes on the Forest” project is designed to help citizens learn about and find new infestations of particularly harmful exotic species. This is one more reason Michigan State University Extension recommends to avoid moving firewood around!

Should anyone believe they’ve seen HWA, it should be reported. Reports can be made on the MISIN website or through the Michigan Department of Agriculture and Rural Development (MDARD). People can also contact their county conservation district, most of which are informed about the HWA and other exotic species threats. 

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