Adelges tsugae (Hemlock Woolly Adelgid) - Fact Sheet


In its native range within Asia, hemlock woolly adelgid (HWA), Adelges tsugae, is not a very serious or destructive pest. Population levels are controlled by natural enemies and by host resistance. The HWA was first reported in western Canada (British Columbia), in the 1920's and in the United States (Virginia), in the 1950's. In the eastern US, all sizes and ages of eastern hemlock, Tsuga canadensis, and Carolina hemlock, T. caroliniana, are susceptible to HWA infestations. In British Columbia, damage to western hemlock, T. heterophylla, has been minor.

Plant Pest Credit Card- Hemlock woolly adelgid PDF (1.3 mb)


Yeddo spruce (Picea jezoensis hondoensis (Mayr) Rehd.), Tiger-tail spruce (Picea polita (S. & Z.) Carr.), Eastern hemlock (Tsuga canadensis (L.) Carrière), Carolina hemlock (Tsuga caroliniana Engelm.), Chinese hemlock (Tsuga chinensis (Franch.), Pritz.), Japanese hemlock (Tsuga diversifolia (Maxim.) M.T. Mast), Western hemlock (Tsuga heterophylla (Raf.) Sarg.), Mountain hemlock (Tsuga mertensiana (Bong.) Carr.), Southern Japanese hemlock (Tsuga sieboldii Carr.). Himalayan hemlock (Tsuga dumosa (D.Don) Eichler)


  • Asia: China, India, Japan, Taiwan
  • North America: Canada (British Columbia), United States (Alaska, California, Connecticut, Delaware, District of Columbia, Georgia, Idaho, Kentucky, Maine, Maryland, Massachusetts, Montana, New Hampshire, New Jersey, New York, North Carolina, Ohio, Oregon, Pennsylvania, Rhode Island, South Carolina, Tennesse, Vermont, Virginia, Washington, West Virginia)


The Hemlock Woolly Adelgid (HWA) has a complex life cycle. In North America and in Japan there are three life generations per year: Sistens, which overwinter, progrediens which remain on hemlock, and sexuparae which migrate to spruce. In May-June, "winged" sexuparae and the "wingless" progrediens develop simultaneously. In June, the sexuparae females fly to a spruce host on which they lay eggs. These "unfertilized" eggs hatch into sexuales which, in North America, fail to develop successfully regardless of the spruce species used as a host. The "wingless" female progrediens oviposit on hemlock. These sistens eggs hatch in June and July. The newly hatched first instar nymphs are called "crawlers" because they crawl around the plant as they look for a place to settle and insert their stylets (mouthparts) to feed briefly before becoming inactive. This period of inactivity will last until mid-October at which time feeding will resume and the nymphs will mature during the autumn and winter. The nymphs will slowly develop a white "woolly" overcoat as they mature into the final adult stage. This sisten generation matures into adults in February after four nymphal instar stages. From March to May, the sistens each produce a single ovisac containing up to 300 eggs which will develop into sexuparae and progrediens adults four weeks after the eggs hatch. The number of eggs that become sexuparae increases with adelgid density. This response could be due to declining host nutrition. Weather conditions between October and March can hasten or delay the development of the sistens generation by as much as two weeks. In Connecticut and Virginia there are significant overlaps of all the HWA life stages, especially during late spring when 12 life stages can be recorded on the same day (nymphs, adults of both progrendiens and sexuparae and sisten eggs).

HWA is a separate endemic lineage in western North America. Western hemlock species are tolerant to the western biotype of HWA. The source of HWA in eastern North America has been reported to be a lineage of adelgids living predominantly on Tsuga sieboldii at low elevations in southern Japan. Foresters in western North America are concerned about the potential introduction of the eastern North American biotype of HWA in the west, as it is not known if trees would succumb to tree death as in the eastern United States.

Detection & Identification


  • White "woolly" sacs can be seen at the base of needles, particularly on the younger growth, throughout the year but are most abundant in the spring. The "woolly" appearance is due to a fluffy wax coating that covers the body of the adults. Dieback of twigs and discolouration of the foliage is the result of nymph feeding. Preference is given for maturing trees on stressful sites. Complete defoliation and death can occur within 4 years.
  • During the first year that follows the initial colonization of a hemlock stand, HWA populations expand rapidly reaching a peak density. The second year, there is a considerable decline in the HWA populations because trees fail to generate new growth which is the preferred food of the adelgids. The following year, when some new shoots appear, the number of adelgids rises again but the fourth year, when most trees die, only non-viable sexuparae are produced. Thus, the population dynamics of HWA vary according to density-dependent factors.


  • Egg: Oblong and amber in colour. Eggs which develop into sistens are approximately 0.36 mm long and 0.23 mm wide, those which produce progrediens and sexuparae measure around 0.35 mm in length and 0.21 mm in width. Eggs from which sexuales emerge measure 0.37 mm long by 0.25 mm wide. They are produced by the sexuparae adults which deposit up to 15 eggs beneath their folded wings. Eggs of adult sisten are laid in a single batch of up to 300 in a spherical "woolly" ovisac made of white wax threads. Progrediens adults also lay their eggs (up to 250) in similar but smaller cottony ovisacs.
  • Nymph: Sistens and Progrediens: Very similar in size and appearance. Crawlers (first-instars) are about 0.44 mm long and 0.27 mm wide, brownish-orange in colour. Second-instars measure around 0.57 mm in length and 0.34 mm in width. Legs are short and thick. Third-instars are approximately 0.67 mm long by 0.43 mm wide, and fourth-instars 0.74 mm by 0.47 mm. Sexuparae: First-instars similar in appearance to those of the sistens and progrediens. Second-instars measure about 0.60 mm in length and 0.35 mm in width whereas third-instars are approximately 0.77 mm long by 0.47 mm wide and fourth-instars, 0.89 mm by 0.49 mm.
  • Adult: Sistens: About 1.41 mm long by 1.05 mm wide. Possess a heavy waxy-coat. Progrediens: Approximately 0.87 mm in length and 0.63 mm in width. Sexuparae: Measure around 1.09 mm by 0.51 mm. Dark brown in colour with long (five-segmented) antennae, compound eyes and four textured wings.
Figure 1, shows varying levels of decline and mortality on the landscape. Hemlock woolly adelgid feeding causes dessication of the needles and trees will eventually take on the grayish-green cast before mortality occurs. (Photo Credit: William Ciesla, Forest Health Management International)
Figure 1 - (Photo Credit: William Ciesla, Forest Health Management International)
Figure 2, shows the woolly egg masses located on the underside of hemlock branches. Immature crawlers are emerging from the egg masses and settling at the base of the hemlock needles on the underside of the branch, where they will insert their stylet into the plant tissue and begin feeding. (Photo Credit: Lorraine Graney, Bartlett Tree Experts)
Figure 2 - (Photo Credit: Lorraine Graney, Bartlett Tree Experts)
Figure 3, shows a close-up of the hemlock woolly adelgid adult, covered in white, wool-like wax filaments resembling small tufts of cotton. (Photo Credit: Lorraine Graney, Bartlett Tree Experts)
Figure 3 - (Photo Credit: Lorraine Graney, Bartlett Tree Experts)
Figure 4, shows the most obvious sign of a hemlock woolly adelgid infestation, with the white, woolly ovisacs located at the base of the needles on the underside of the hemlock branch. (Photo Credit: Chris Evans, Illinois Wildlife Action Plan)
Figure 4 - (Photo Credit: Chris Evans, Illinois Wildlife Action Plan)
Text: Plant Pest Surveillance Unit.
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