The Hemlock Woolly Adelgid (Adelges Tsuga Annand)

The hemlock woolly adelgid (HWA) is a serious pest of Canadian hemlock, Tsuga canadensis, in the northeastern states. This insect was discovered in Connecticut in 1985 and has spread rapidly to both cultivated and forest hemlocks.

Adelgids are small, soft-bodied insects that are closely related to aphids. They have piercing-sucking mouth parts, which are inserted to remove plant sap for food. The HWA feeds primarily on young branches causing cessation of tree growth, discoloration and premature drop of needles, the dieback of branches and possible death of the tree in as little as one year.

Description and Life History

The females are oval, blackish-grey, about 2mm in length and serve as the overwintering stage. The brownish-orange, very small, oblong eggs are laid in cottony white egg sacs (about 50 eggs per sac) on young twigs from late march to May. The presence of the egg sacs offers the most visible diagnostic evidence of an HWA infestation. The eggs hatch into reddish-brown crawlers (nymphs) from early April through early June and begin feeding on the sap of young twigs, maturing into adults in a few weeks. Some of the adults are wingless and remain on hemlock for a second generation, while the winged forms may fly to nearby hemlocks or spruces.

None of the common native and exotic spruces appear to be suitable hosts for the winged HWA to complete its life cycle. Second generation crawlers initially feed on new twigs during July but become dormant in late summer through early autumn. Crawlers resume feeding in mid-October and develop into the overwintering adults.

Pest Management and Control

Infestations of the HWA can be detected early by periodically examining young twigs for the presence of the egg sacs. They are readily observed in the spring before the eggs have hatched. Keep in mind that remnants of old egg sacs may remain on twigs long after the eggs have hatched and the insect has been controlled. Early detection is very important because injury to hemlock may develop quickly. That is why periodic examinations are an important practice in the control of the HWA.

Several insecticides are available that will provide excellent HWA control. Insecticides should not be applied on a preventive basis. Many of the hemlocks in Connecticut will not be infested and will not need treating. Controls should be applied only when HWA infestations are observed.

An excellent time to control an HWA infestation is from July through September after the eggs have hatched and the young adelgids are relatively unprotected. Summer oil and insecticidal soap have been found to provide excellent control with one application, provided complete coverage is made in this time period. Both insecticides kill only by direct contact. Thorough coverage with a drenching spray is essential for control. Dormant oil is effective for HWA control when applied in April to mid-May. It will also control scale insects and mites at the same time.

Other insecticides capable of controlling HWA include acephate, dimethoate and malathion. Thorough coverage of the insect is essential for effective management. More than one application may be needed for effective control where it is difficult to achieve complete coverage of the plant. Unfortunately, natural enemies, such as disease, parasites and predators, have not been numerous enough to achieve significant management, but they may offer hope in the future.

Pesticides for HWA Control
Common Name Trade Name
acephate Orthene
dimethoate Cygon
insecticidal soap Safer's, M-Pede
malathion Malathion, Cythion
horticultural oil Volck, etc.
Revised by: Latif Lighari, Extension Educator, Agriculture
Edmond L. Marrotte, Consumer Horticulturist
Originally written by: Latif Lighari and Roger Adams, IPM Coordinator

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