Plants > Plant Pest Information > Hemlock Woolly Adelgid  

Adelges tsugae (Annand) - Hemlock Woolly Adelgid

BACKGROUND

In its native range within Asia, hemlock woolly adelgid (HWA), Adelges tsugae, is not a very serious or destructive pest. Populations 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.

HOSTS

Spruce (Picea spp.), 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.).

DISTRIBUTION

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

BIOLOGY

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. The eggs hatch in June and July. The newly hatched first instar nymphs are often called "crawlers" because they crawl around the plant as they look for a place to feed.   These first instar nymphs will eventually attach themselves at the base of needles where they will 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 waxy 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).

DETECTION & IDENTIFICATION

Symptoms

• 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. The first attacks often occur on the lower branches.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.

Identification

• Egg: Oblong and amber in colour. Eggs which develop into sistens are approximately 0.36mm 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.

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Text: Plant Pest Surveilance Unit.
Photos: USDA, Forest Service, Hamden, CT, USA