From "Garden Insects of North America" by Whitney Cranshaw
(p.314); Adelgids are a family of "woolly" aphids associated with conifers. Their life histories differ in some respects from those of "true" aphids, notably in that eggs are produced by all forms. Host alternation is common with many species, and some produce galls on the primary host (p.410).
Hemlock Woolly Adelgid (Adelges tsugae)
Hosts: Hemlock, with Canada and Carolina hemlock most seriously damaged.
Damage: Hemlock Woolly Adelgid is native to Japan & China and since it's accidental introduction in the 1950's, has become the most serious insect pest of hemlock in the eastern U.S. The insects feed on sap from twigs and concurrently introduce a saliva that is toxic to the plant. Foliage of infested plants yellows, and needles drop prematurely, particularly on interior areas of branches. Dieback of limbs is common, and trees have often been killed by this insect, particularly in New England.
Distribution: Generally distributed in areas east of the Appalachian Mountains, from South Carolina to Maine. Isolated infestations occur in parts of the Pacific Northwest and upper Midwest.
Appearance: Nearly black but often covered with white cottony wax. Newly hatched stages are tiny and dark reddish gray. Nymphs usually have a white fringe of wax around the body.
Life History and Habits: Winter is spent in the form of adults on twigs, with females beginning to lay eggs in late March. Eggs hatch in April, and this first generation is usually completed by early summer. A second generation follows and goes temporarily dormant during midsummer, resuming growth in fall.
OTHER ADELGIDS ON BARK AND NEEDLES
Balsam Woolly Adelgid (A. picea) is an introduced species now found in forest of both the northeastern and northwestern U.S. and areas of southern Canada. It is an important pest of balsam and Fraser Fir, with large colonies developing on needles and bark of trunks, branches, and twigs. Infestations commonly result in distortions of twigs and small branches, usually following death of terminal buds. Two generations are produced annually, with winter spent as a first-stage nymph on the bark.
VARIOUS WOOLLY PINE ADELGIDS (Pineus spp.) are associated needles , twigs, branches, and trunks of pin. Individual insects are densely covered with whitish wax. Foliage of heavily attacked trees becomes yellowish, and growth is stunted. Needle and shoot feeding can cause shoots to droop and die.
Species capable of developing dense colonies on trunks and branches are potentially most damaging. Perhaps the most important species in North America is pine bark adelgid (P. strobi) which colonizes the bark and needles of white, Austrian, and Scotch pine. All stages apparently occur on pine. Some other Pineus species alternate between pine and spruce.
pg. 410. ADELGID GALLS
The adelgids (Adelgidae) are the "woolly" aphids associated with pine, spruce, and other conifers (p. 314). Those that produce galls on plants have complex life cycles that usually involve alternate hosts. Galls are not produced on the alternate (summer) host.
Cooley Spruce Gall Adelgid (Adelges cooleyi)
Hosts: Spruce, Douglas-fir.
Damage: On spruce, distinctive cucumber-shaped galls develop on the tips of new growth, which usually kill the terminal. On Douglas-fir, Cooley spruce gall adelgids are conspicuous "woolly" aphids that suck sap from needles. When infestations occur on developing needles, symptoms may include yellowing and twisting of needles.
Distribution: Throughout the northern U.S. and southern Canada where spruce and Douglas-fir co-occur. Cooley spruce gall adelgid is native to North America.
Appearance: Exposed stages are typical "woolly" aphids, covered completely with long white waxy thread. Stages found inside galls on spruce are greenish gray aphids covered with a fine powder of wax. Winged forms that migrate between host plants are dark green, almost black.
Life History and Habits: Cooley spruce gall adelgid has a life cycle involving several different forms that alternate between spruce and Douglas-fir. On spruce, winter is spent as a partially grown female on the underside of twigs. Females grow rapidly in spring, producing a large mass of several hundred eggs which hatch in sychrony with bud break. The newly hatched nymphs migrate to the base of newly expanding needles. Their feeding induces enlarged cavities to form, in which they develop. During late June and early July the nearly full-grown adelgids emerge from cracks in the drying galls, molt on the needles to winged adults, and migrate to Douglas-fir.
On Douglas-fir, the nymphs overwinter as partially developed nymphs on the needles. They continue to develop in spring, producing a large egg mass around early May. At egg hatch, the nymphs move to tips of twigs to feed primarily at the current-season needles. They become full grown in July and produce a generation of adults that are a mixture of winged and wingless forms. The wingless forms remain on Douglas-fir and have a second generation. The winged forms fly to spruce and start the cycle on this host.
Eastern spruce gall adelgid (A. abietis) produces a pineapple-shaped gall on Norway spruce. A European species, it is widespread in eastern Canada and the northeastern U.S. Galls are not as large as those produced by Cooley spruce gall adelgid, and affected terminals often are not killed. All stages occur on Norway spruce, with winter spent as a partially grown female located near buds.
Some other species of Adelges cause galls on a spruce. Other galls on spruce may be produced by adelgids in the genus Pineus. Pine leaf adelgid Pineus pinifoliae) also makes galls in the terminal growth of spruce that resemble those of Cooley spruce gall adelgid but are of looser form. Galls remain green late in the season and are less conspicuous. Spruce is the primary (overwintering) host of this aphid, which infests pine during alternate stages of its life cycle.
… Cheryl Moorehead, 17 October, 2006