Hemlock woolly adelgid arrived on North American soil in the 1920's, an accidental introduction to the Pacific Northwest from its native Asia. Three decades later, Adeleges tsugae also landed in the eastern U.S. These aphid-like insects suck the life out of our native hemlocks, free of natural enemies that keep the pest under control in its native range.
Description:
Hemlock woolly adelgids are quite tiny, so you'll need a hand lens to get a good look at this pest. You're much more likely to notice the waxy secretions adelgids produce, which look like bits of cotton on the hemlock needles. These "woolly" secretions disguise the adelgid adults and eggs from predators.
Adelgid eggs are rust-colored, but hidden in their woolly egg cases. Young nymphs can be seen with the aid of a magnifier, appearing as tiny black ovals at the base of needle stems. Reddish-brown adults reach less than 2mm in length.
Classification:
Kingdom - Animalia
Phylum - Arthropoda
Class – Insecta
Order – Hemiptera
Family - Adelgidae
Genus - Adeleges
Species - tsugae
Diet:
Both nymphs and adults suck hemlock sap from the base of needles. The adelgid may also introduce a toxic saliva that further damages the tree. In North America, hemlock woolly adelgid infests both eastern hemlocks and Carolina hemlocks. Western hemlock species seem to tolerate adelgid attacks better than these two eastern species, which may die within a few years of adelgid infestation.
Life Cycle:
Hemlock woolly adelgid undergoes three stage of metamorphosis – egg, nymph, and adult. Woolly adelgids in North America reproduce asexually, and all are females. In some parts of its range, Adeleges tsugae completes two generations per year.
Eggs – An adelgid female lays anywhere from 50 to 300 eggs on the undersides of hemlock branches.
Crawlers – The immature nymphs migrate to feeding sites, usually in the newer growth on the hemlock tree. Once a crawler chooses a location at the base of a hemlock needle, it molts, losing its legs and becoming immobile.
Nymphs – Nymphs secrete wax ("wool") as they feed on the hemlock sap. After four instars, nymphs molt into adulthood.
Adults – Wingless adults remain on the hemlock host, laying eggs. Some winged adults migrate to spruce as an alternate host. In North America, adelgids do not colonize spruce.
Special Adaptations and Defenses:
In late summer, adelgid nymphs go into a period of dormancy, or aestivation. The nymphs resume feeding in the fall, usually around October. Adelgid eggs are laid within sticky envelopes called ovisacs. This destructive pest disperses as birds and other animals come in contact with the ovisacs and crawlers, and carry them to other trees. Wind also blows hemlock woolly adelgid from one tree to the next.
Habitat:
Hemlock woolly adelgid lives in hemlock trees, both in forests and in the home landscape.
Range:
Hemlock woolly adelgid is native to parts of Asia, including Japan, India, Nepal, China, and Taiwan. In North America, two ranges occur. West of the Rocky Mountains, Adeleges tsugae inhabits hemlock stands from northern California to southeastern Alaska. In the east, adelgid populations exist from northern Georgia to southern Maine.
Other Common Names:
Woolly adelgid, HWA.
