Invasive insect that kills hemlock trees found at Ann Arbor arboretum

Untreated trees infested with hemlock woolly adelgid die within 4-10 years

Infestations appear on the underside of tree branches, and look like tiny cotton balls. Photo courtesy of Michigan Invasive Species Program (WDIV)

ANN ARBOR, Mich. – Washtenaw County is now the seventh Michigan county to confirm an infestation of invasive woolly adelgid (HWA), which are insects that feed on tree sap and can kill eastern hemlock trees.

The Michigan Department of Agriculture and Rural Development verified the insect infestation at the Nichols Arboretum in Ann Arbor on May 9. The pest poses a problem for homeowners who have planted or previously had hemlock trees, but the infestation is not a significant threat to Washtenaw’s natural resources and environment, according to Mark Philip, director of MDARD’s Pesticide and Plant Pest Management Division.

The invasive insects weaken needles, shoots and branches over time by extracting sap from hemlock trees. Infested trees experience slower growth, take on a grayish-green appearance, and die within four to 10 years if left untreated.

Residents and park managers near the arboretum are being asked to examine their hemlock trees and report any HWA they find. Infestations appear at the base of hemlock needles on the underside of tree branches, and look like tiny cotton balls, according to the Michigan Invasive Species Program.

The insects do not travel far on their own, but can be taken to new locations by attaching to people, animals and vehicles.

Residents can prevent the spread of invasive species by taking the following steps:

  • Buying firewood where you burn it, and leaving it at home.
  • Staying on trails to protect natural areas.
  • Avoiding parking under trees or in fields.
  • Cleaning gear and vehicles before driving.
  • Reporting potential infestations by: emailing MDARD (; calling MDARD Customer Service Center at 800-292-3939 on weekdays between 8 a.m. and 5 p.m.; using the Midwest Invasive Species Information Network’s online tool; or using the MISIN’s smartphone app.

Learn more information about HWA and other invasive species on the state’s website here.

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