Most Un-Wanted: Hemlock Woolly Adelgid

by Thayer Nursery

The hemlock woolly adelgid (HWA) is an insect native to East Asia that feeds by sucking sap from hemlock trees. It is a destructive pest that poses a major threat to both the eastern and the Carolina hemlock. Hemlocks are a vital component of the New England forest system, providing erosion control along stream banks, as well as food and shelter for deer and wildlife. Major changes to ecosystem structure and function, including hydrologic processes, are expected with the loss of hemlock.

The presence of HWA can be identified by its egg masses, which resemble small tufts of cotton that cling to the underside of hemlock branches. Hemlocks stricken by HWA frequently turn grayish-green, rather than the dark green of healthy hemlocks. 100 to 300 eggs are laid in HWA egg sacs and larvae emerge in spring and can spread on their own or with the help of wind and birds.

The HWA feeds on the sap of tender hemlock shoots, causing the tree to lose needles and to stop producing new growth. Death of the hemlock typically occurs four to ten years after infestation. Trees that survive the direct effects of the infection are usually weakened and may die from other causes.

Few options are available for control of HWA. The adelgids can be physically removed from the tree, either by vigorous washing or by cutting off infected branches. An organic approach to treating HWA is the use of horticultural oil and insecticidal soaps. The most vulnerable stage of HWA is the immature crawler stage (nymph) that moves out of the protection of the egg sac. These immature insects are exposed on the new growth, making it important to thoroughly apply the oil or soap throughout the tree.

Fertilizing HWA infested hemlocks with nitrogen only leads to a quicker demise. Insects seek certain building blocks of life from nature in their food sources. A hemlock that is very succulent and high in particular nutrients greatly enhances the success of HWA populations. Research shows that hemlocks that are infested with HWA should not be fertilized with nitrogen. Mulching with organic compost at the base of the hemlock is a better way to keep the hemlock healthy.

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