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Hemlock Woolly Adelgid

A mass of hemlock woolly adelgid egg masses lined up along the base of the needles. This is the most noticeable stage for this pest. (Photo: R. Childs)
A winged adult hemlock woolly adelgid which is rather rare. These fly to seek a spruce species. However, in the USA, there are no spruce species that support this pest so any winged HWA that fly to a spruce species here will die. (Photo: R. Childs)
These nymphs are the dormant phase that occurs during most of the summer months. Note that they are tiny, flat, black and oval with a ring of white "fringe" around their perimeter. (Photo: R. Childs)
A close up of the red eggs found within one of the cottony egg masses. Each cottony mass contains about 30 eggs. (Photo: R. Childs)
Maturing hemlock woolly adelgids soon after coming out of dormancy. (Photo: R. Childs)

Pest: Hemlock Woolly Adelgid (Adelges tsugae Annand)
Order: Homoptera
Family: Adelgidae


The hemlock woolly adelgid was introduced into Massachusetts in 1988 from an already existing infestation in Connecticut. It attacks both the Carolina and Eastern hemlock and is capable of severely weakening and killing its host plants. Healthier plants, prior to infestation, may endure longer, but previously stressed plants may die in 3-5 years. The key to its management is to recognize it early and implement the proper management strategies.

The Pest:

The hemlock woolly adelgid (HWA) is a tiny insect that is closely related to the aphids. Adelgids, in general, are associated with conifers and produce galls and or woolly masses. The HWA only produces woolly masses.

This insect is atypical of most insect species in the Northeast in that it is inactive for much of the growing season and very active throughout the winter. From mid-July until mid-October, they are immatures (nymphs) and are flat, black, oval, and ringed with a fringe of white waxy strands. These very tiny nymphs can be found at this time settled on the stems of the host at the base of the needles. They neither feed nor develop during this time period. In mid-October, they resume feeding by using their piercing-sucking mouthparts to remove plant sap from the twigs of the host plant. While they feed, they develop into adults with new egg-masses appearing beginning in March. These are the most noticeable stage of the pest and have the appearance of small, white cotton balls lined up at the base of the needles. From then until June there can be many life stages present at one time and much feeding and injury can occur. There are two generations per year. Magnification is required to see all of the individual life stages.

Plants growing in stressful sites (compacted soils, ledgy soils, poor drainage, drought prone, etc.) are much more apt to succumb within 3-5 years from this pest's invasion. Hemlocks in more natural (healthier) settings, such as the forest, have been shown to withstand infestations longer. Once found in the landscape or nursery, management is usually necessary. Forest trees may present a management challenge, although options do exist. For more information regarding management of HWA in New England Forests, visit:

Management Strategies:

The HWA, in the landscape and nursery, is very manageable if the problem is addressed early and aggressively. One option that poses reduced risk to non-target insects is the use of horticultural oils. These can be applied as either a dormant oil spray and/or as a summer spray. In order for these to be effective, the application must be thorough throughout the entire plant and may need to be reapplied later in the growing season. It is essential to kill virtually all of the HWA on a plant or the population will soon reappear at unacceptable levels.

Other active ingredients that have been shown to effectively manage the hemlock woolly adelgid include imidacloprid and dinotefuran. Imidacloprid has been shown to take several months to take effect after application, but research suggests it may protect trees from hemlock woolly adelgid for up to 5 years. Imidacloprid has not been shown to effectively manage the elongate hemlock scale (Fiorinia externa). In cases where a hemlock is infested with both the elongate hemlock scale and hemlock woolly adelgid, dinotefuran is an active ingredient that can manage both insects. Dinotefuran may be effective within weeks of application. Dinotefuran may protect trees from HWA for approximately 2 years, according to research. Products containing dinotefuran are currently listed as state restricted use products in Massachusetts. For more information, visit:

Both imidacloprid and dinotefuran work best as a systemic application and can be introduced into the plant via soil or trunk injection. Other application methods include soil drenches and basal sprays. The basal spray application is made when the product is applied to the bark of the tree from the root flare up to approximately 4 ft. high on the trunk. These active ingredients can also be used in a management program with horticultural oils; if the time of year is not conducive for an application of imidacloprid, then an oil can be applied with a follow-up application of the other. Special care must be taken when using imidacloprid or dinotefuran due to toxicity to bees and other pollinators. However, with some of the aforementioned application methods (such as injections or basal sprays), risks to pollinators can be greatly reduced as these application methods greatly reduce the risk of drift during application onto plants in bloom. (Which is to be avoided anyway.)

Once this pest is in any given area, it will constantly pose a threat to all hemlock, even those that have been treated. Therefore, all hemlocks in these areas should be monitored carefully several times a year and treatments applied when it is found, as necessary. Wind and birds are the primary means for moving this pest from one area to another but humans can also move this pest on plants if care is not taken. Certain species of Asian hemlocks are resistant to this pest and nurseries are beginning to grow and sell those in the United states. The western hemlock (Tsuga heterophylla) is a native species and is highly resistant to this pest but it does not grow well at all outside of its native range of the northwestern USA.

Also see our Hemlock Woolly Adelgid FAQ

Robert Childs (revised by Tawny Simisky)
Last Updated: 
May 2018