The hemlock woolly adelgid, or HWA, is a small invasive insect that is killing eastern and Carolina hemlock trees in eastern North America. This article will cover the biology of this pest, the impact that it’s had, and what can be done to control this insect.
HWA is native to China, Japan and western North America, and was likely introduced to the eastern U.S. from Japan. Since its introduction more than 50 years ago, HWA has killed thousands of acres of eastern hemlock and Carolina hemlock in eastern North America. HWA feeds by sucking sap from hemlock trees and attaches itself at the base of hemlock needles. The insect stays at the needle base for most of its life and surrounds itself and eggs with a protective white wool, where HWA gets its name. HWA reproduces quickly and is difficult to control because it has two generations per year, females do not need to mate to reproduce, and each female lays up to 175 eggs. By feeding on the sap of the tree, HWA makes it so the hemlock can’t absorb water and nutrients, killing the tree in as little as 2-4 years.
Hemlock trees are important because they provide habitat, nutrients and shade, which lowers temperatures so all kinds of plants, wildlife and insects can survive in eastern forests. Hemlock death also effects property value and personal safety in residential areas. There are no other evergreen tree species that can fill these roles, so hemlock death permanently damages eastern forests. For these reasons, it is important to find the best management solution for you.
Before you can treat, you need to know what to look for. Hemlock trees have small, flat needles with a white stripe that runs down the center of the needle. To identify HWA, turn the hemlock needles upside down and look for white woolly balls at the base of the needle, where the needle meets the twig. If hemlocks are heavily infested with HWA, the needles will yellow and drop and branches may die.
Options for hemlock management for homeowners currently include contact insecticides, systemic insecticides, or removing the infested branches and/or trees. Choosing the right insecticide and application method should only be done by consulting a trained professional. Contract your local state forester or university extension office if you think you have an HWA infested hemlock tree.
Links to other factsheets for more information: