Hemlock Woolly Adelgid
Contact: Eric Day, Manager, Insect Identification Laboratory
ENTOMOLOGY PUBLICATION 444-244, August 1996
Hemlock Woolly Adelgid
Distribution and Hosts
Introduced into the Pacific Northwest in the 1920's, this
adelgid was first reported in eastern Virginia in the early
1950's. Since then it has spread primarily northeastward and
now occurs as far north as Connecticut and Rhode Island. The
primary host is hemlock, with spruce being a possible secondary
(alternative) host.
Description of Damage
Immature nymphs and adults damage trees by sucking sap from the
twigs. The tree loses vigor and prematurely drops needles, to
the point of defoliation, which may lead to death. If left
uncontrolled, the adelgid can kill a tree in a single year. When
not at serious risk to the tree, presence of the dirty white
globular masses of woolly puffs attached to the twigs or base of
needles reduces the value of ornamentals.
Identification
These small insects display several different forms during their
life history, including winged and wingless forms. Generally,
they are brownish-reddish in color, oval in shape, and about 0.8
mm in length. Crawler stage nymphs produce white cottony/waxy
tufts which cover their bodies and remain in place throughout
their lifetime. The white masses are 3 mm or more in diameter.
The presence of these masses on the bark, foliage, and twigs of
hemlock is a sure sign of hemlock woolly adelgid.
Life History
There are four forms of this insect. Each form goes through six
life stages (egg, four nymphal instars, and adult). As a cool
weather species, most development of these stages occurs between
October and June. As temperature rises thereafter, the first
instar nymphs go into a dormant stage. Eggs are laid by adult
adelgids the following February or March. Half of these eggs
develop into a winged, migratory, asexual form that migrate to
spruce. The other eggs develop into wingless adults that remain
on the hemlock tree.
Control
Application of insecticides is currently recommended for
controlling the hemlock woolly adelgid. The best compounds are
horticultural oils which smother the insects. A 1 percent
solution is recommended from May through September, and a 2
percent solution from October to April. Complete coverage of
the tree is necessary and can result in 100 percent mortality of
the adelgids. Only one complete application of oil is
necessary. Soap can also be used, but may be toxic to the
trees. Following treatment, monitor the situation. Tree
fertilization can result in more damage, as adelgid populations
are known to flourish on such trees.
It is believed that this species originally came from Japan.
Currently, researchers are investigating the prospects of
identifying and importing natural enemies for use against this
pest.
Further Reading
Johnson, W.T. and H.H. Lyon. 1991. Insects That Feed on Trees and
Shrubs. Cornell Univ. Press, N.Y. 560 pp.
McClure, M.S. 1989. Evidence of a polymorphic life cycle in the
hemlock woolly adelgid, Adelges tsugae (Homoptera: Adelgidae).
Annals Entomological Society of America 82:50- 54.
Prepared by S.M. Salom, Department of Entomology, Virginia
Polytechnic Institute and State University, Blacksburg, VA
24061-0319.
