Other Common Names
Hemlock Woolly Aphid (HWA)
Explanation of Names
Adelges tsugae Annand 1924
tsugae - Latin for "hemlock trees"
Identification
Red-purple to purple-black. As they mature, they cover themselves with white, cottony wax, becoming much more noticeable.
Range
Native to Japan and possibly China, with two major North American ranges: in the west, northern California to Alaska; in the east, Georgia to southern Quebec.
Also established around the Great Lakes, in central California, and the Rocky Mountains in Idaho and British Columbia. [
Worldwide Map from Invasive.org]
Habitat
Anywhere with hemlock trees, including forests and ornamental plantings.
Season
One spring generation and a partial fall generation.
Food
Primarily hemlock trees; occasionally spruce and other conifers. They feed on new growth where the needles emerge from the stem.
Feeding damage causes needles to fall off, resulting in defoliation, limb dieback, and eventual death of the tree. A mature tree may die only 4 years after the original adelgid infestation.
Life Cycle
All Hemlock Woolly Adelgids are female and reproduce asexually. Adults appear in late winter and lay eggs from February to June. Eggs hatch in May and June. Newly-hatched nymphs, called "crawlers," seek out new stem and needle growth to feed on. They then molt into a legless form and remain stationary until maturity.
Nymphs estivate (hibernate) in late summer, then resume feeding in the fall. In mid- to late fall, they cover themselves with cottony white wax, and mature under this protective coating until adults emerge in late winter.
Remarks
Accidentally introduced to British Columbia and Oregon from Asia in the 1920's or earlier. The first eastern records are from 1951. By 2005 its range included 50% of eastern hemlock-growing areas. The eastern introduction is well documented but it is possible that the western population is native to the area (
Ann. Entmol. Soc).
Eggs and crawlers can be spread by the wind and on animals, especially the feathers of birds. Transplanting infested trees can also spread the insect's population.
The Eastern Hemlock (Tsuga canadensis) and Carolina Hemlock (T. caroliniana) are less resistant to feeding damage than the Western Hemlock (T. heterophylla).
Predators used for biological control include the native Tooth-Necked Fungus Beetle (
Laricobius nigrinus). Non-native lady beetles have been introduced from Europe (Pine Lady Beetle,
Brumus quadripustulatus) and Asia (
Sasajiscymnus tsugae and
Scymnus species).
Laricobius nigrinus has been introduced from Western US to Eastern US.
The most abundant predator of the HWA is Harmonia axyridis (Wallace and Hain 2000)
See Also
Balsam Woolly Adelgid (Adelges piceae) - always on true firs; seen on rough bark, branches, and tree trunks, not new growth and needles.
Print References
Wallace, M.S., and F.P. Hain. 2000. Field surveys and evaluation of native and established predators of the hemlock woolly adelgid (Homoptera: Adelgidae) in the Southeastern United States. Environmental Entomology 29: 638-644.
"The Hemlock Woolly Adelgid: Life Cycle, Monitoring, and Pest Management in New Jersey." © 2004 by Rutgers Cooperative Research & Extension, NJAES, Rutgers, The State University of New Jersey. Available online:
http://na.fs.fed.us/spfo/pubs/pest_al/hemlock/hwa05.htm.
"Pest Alert: Hemlock Woolly Aphid." 2005. U.S.D.A. Forest Service, Northeastern Area State and Private Forestry. Available online:
http://njaes.rutgers.edu/pubs/download-free.asp?strPubID=FS751.
