Invasive Species | Forest Pests

Hemlock Woolly Adelgid

Chris Evans, University of Illinois, Bugwood.org

Hemlock Woolly Adelgid

(Adelges tsugae)

French common name: Puceron lanigère de la pruche

Hemlock Woolly Adelgid (HWA) is an aphid-like insect that attacks Eastern Hemlock trees (Tsuga canadensis); it feeds on the base of hemlock needles, causing premature dieback, needle loss, foliage thinning, and tree death within 5-15 years. HWA is easily spread by the wind, birds, other wildlife, and by humans moving infested wood products.

Characteristics

Life cycle: All HWA are female and reproduce asexually. There are two generations throughout the year: one that overwinters and is present from early summer to mid-spring (sistens) and a spring generation present from March to April (progrediens). Each generation has 6 life stages: egg, mobile ‘crawlers’, 3 ‘settled’ nymphal instars and adults.

Host tree: Primarily Eastern Hemlock, a coniferous tree identifiable by its flat, short, and needle-like leaves that have two white stripes on the underside and are attached to the twig by a small stem.

Eggs: Clusters of amber-colored eggs (0.3-0.4mm) found in white woolly ovisacs that are slightly sticky; 75-300 eggs per sac.

Crawlers: Only mobile stage; brownish-orange in color and less than 0.5mm in size.

Additional Nymphal Stages & Adults: Very small (less than 1.5mm); black with heavy waxy coats and wingless. Found attached to the base of the needles. They stay in one spot for the rest of their lives, feeding on the sap of the tree.

John M. Randall, The Nature Conservancy, Bugwood.org

Bruce Watt, University of Maine, Bugwood.org

Eastern Hemlock Tree Identification

Type of tree: coniferous; has needles and cones rather than leaves; needles are present year-round

Needles: short (up to 1/2 inch long), flat, blunt-tipped, and bottom has two parallel white stripes underneath (sometimes called “racing stripes”). They are attached to the twig by a short stem and are usually arranged in flat rows along the twig.

Cones: small (up to 3/4 inch long), green to brown in color, delicate and papery. They hang downwards from the branches.

Bark: color varies from gray brown (young tree) to cinnamon brown (mature tree); texture ranges from smooth (young tree) to thick with ridges (mature tree).

Tree shape or silhouette: loosely coned-shaped, branches are described as dropping or feathery

Rob Routlede, Sault College, Bugwood.org

Keith Kanoti, Maine Forest Service, Bugwood.org

Pennsylvania Department of Conservation and Natural Resources - Forestry , Bugwood.org

Rebekah D. Wallace, University of Georgia, Bugwood.org

For more information about distinguing Eastern Hemlock trees from other tree species (Balsam Fir, Douglas Fir, Canada Yew), check out this resource (click here)