Examine Your Western Hemlock Trees for Woolly Adelgid

 by: Karen Ripley, DNR Forest Health Program Manager


May is a good time to examine your western hemlock trees for the hemlock woolly adelgid Adelges tsugae (HWA). 

Adelgids are insects that are close relatives of aphids.  Aphids and adelgids feed on plants by extracting juices and nutrients through piercing, sucking strawlike mouths.  The woolly adelgids exude filaments of a white, waxy substance that makes the insect itself look like a little tuft of cotton or wool.  This material protects the adelgid and an underlying cluster of eggs from extreme temperatures, dehydration, and predation. 

Adelgids cause minor wounding at their feeding sites and, if there are huge numbers of insects, can weaken a plant by heavy feeding activity. The most serious damage occurs when a host tree has a severe reaction to the adelgids’ anticoagulant saliva. 

HWA is not native to North America.  It was accidentally introduced from Asia to the western U.S. in approximately 1924 and was observed in the eastern U.S. in about 1951.  The western hemlock trees found in Washington (Tsuga heterophylla) do not seem to be affected by this insect, likely because western hemlock is closely related to hemlock tree species in Asia that are tolerant of the insects’ saliva.  However, the eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana) are highly sensitive to HWA.  These eastern tree species are rapidly killed by HWA infestations and, particularly because of its sensitivity to HWA and small population range in the Appalachian Mountains, the Carolina hemlock is currently at risk of extinction.

Early May is a good time to view HWA.  The insects themselves appear to be tiny, woolly balls that line the twigs on low branches of western hemlock.  Some trees don’t appear to have any insects.  Some trees are so heavily laden that the twigs appear “flocked” and no twig bark is visible.  As the new growth emerges from the buds along the twig tips, brown eggs that are within the tufty masses (the mother adelgid probably has died and shriveled up inside a cottony tuft) hatch and tiny “crawlers” walk out onto the new shoots, insert their mouthparts into the soft tissue, generate their own protective wool coverings, and live in that position for the rest of their lives.

Because western hemlock trees can support heavy infestations of HWA, Washington is a good place for researchers to seek predators and parasites that feed on HWA here and, after rigorous testing, might be released in the eastern US to provide biological control of HWA.  A beetle called Laricobius has already been tested and is regularly collected and distributed in eastern forests.  Recently two other insects, “silver flies” in the family Chamaemyiidae and genus Leucopis, have been identified whose larvae also eat HWA.  


Before any insect is purposefully moved and introduced into a new area, it must be tested to determine that, even if starved, it will only eat the target pest.

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