The hemlock woolly adelgid, or HWA, is an invasive species that that poses a serious threat to hemlock trees. The easiest way to identify HWA is to look at the undersides of hemlock shoots for the white “woolly” masses they produce while feeding. Each little white ball of wool, called an egg sac, is actually wax secreted by an adelgid. The adelgids feed at the base of the needles, where the needles attach to the woody portion of the shoot.
Overwintering adult females are black, oval, and soft-bodied. They are usually concealed under the white woolly masses of wax they secrete. From March through May, these females lay 50 to 300 eggs in the woolly masses. The eggs are brownish-orange and very small. Depending on spring temperatures, eggs hatch from April – June.
Once hatched, juvenile hemlock woolly adelgids, known as crawlers, search for suitable sites on the host tree, usually at the base of the needles. They can be spread by wind, on the feet of birds, or in the fur of small mammals. Once settled, they insert their long mouthparts and begin feeding on the tree’s food storage cells, not the sap. The tree responds by walling off the wound created by the insertion of the stylets. This disrupts the flow of nutrients to the needles and twigs.
HWA remain in the same spot for the rest of their lives, continually feeding and developing into adults. Their continuous feeding severely damages the canopy of the host tree. Needles will dry out and lose color, turning gray and eventually dropping from the tree. Terminal buds will also die resulting in little to no new shoot growth. Dieback of major limbs and mortality usually occurs.
Michigan State University: Tiny Invasive Insect Attacking Hemlock Trees in Michigan