Distribution and hosts
Introduced into the Pacific Northwest in the 1920s, this adelgid was first reported in eastern Virginia in the early 1950s. 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.
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.
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.
Application of insecticides is currently recommended for controlling the hemlock woolly adelgid. The best compounds are horticultural oils which smother the insects. A 1% solution is recommended from May through September, and a 2% 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.
Balsam Wooly Adelgid
The balsam wooly adelgid is a non-native insect accidentally introduced into the United States from Europe. The insect is very tiny and individual specimens are difficult to see as they cling to the bark of Fraser fir trees.
Hemlock Wooly Adelgid
This is the Asian cousin of the balsam wooly adelgid and is a prime threat to hemlock trees.