White balls of hemlock woolly adelgid ‘wool’ on a hemlock twig in Maryland.

If you work around hemlock trees, keep an eye out for signs of hemlock woolly adelgid (HWA), an aphid-like insect that was introduced to eastern North America and is fatal to most hemlock trees.

HWA produces distinctive, small balls of white wool at the base of hemlock needles. Recently, a number of infestations have been found in western Michigan and there is a risk that the pest could spread across Lake Michigan into Wisconsin. Hemlock woolly adelgid has not been found in Wisconsin. 

If you see this ‘wool’ on a hemlock tree it is important to take photos and report it to DNR forest health staff. Spider eggs sacs or other insects are often misidentified as HWA, so photos are important to identify the specimen of interest (don’t move the insect in case it is HWA).  

More information is available online.

Written by: Bill McNee, forest health specialist, Plymouth (Bill.McNee@Wisconsin.gov), 920-360-0942.