Those are beech blight aphids, Fagiphages imbricator (Hemiptera: Aphididae).
This “woolly aphid” occurs throughout the eastern United States wherever American beech trees grow. It feeds primarily on the twigs and branches but occasionally it can be found on the main trunk and undersides of leaves. The body is covered with long waxy strands, which form a tuft of sorts at the rear end. During outbreak years, certain branches can be completely covered with the aphids, giving the appearance of a recent snowfall. In addition to the odd appearance of the affected trees, the aphids produce copious amount of honeydew (a euphemism for aphid excrement). Several people who have called the lab have complained this “wet stuff” is killing the plants beneath the affected trees. One interesting thing about this aphid is their alarm behavior when approached or disturbed. They vibrate their woolly rear ends in unison, or one might say they shake their little aphid booties when disturbed.
Beech trees appear capable of maintaining huge populations without noticeable harm. We are not recommending control of these insects with insecticides at this time. Problems associated with the honeydew may be minimized by spraying the ground and coated objects with a garden hose. The garden hose may also be used to reduce the number of aphids on smaller trees or lower limbs.