Beech Wooly Aphid

These small, fluffy aphids feed off the sap of beech trees, and can form dense colonies on branches and the undersides of leaves. Waxy filaments extending from their posterior give them their wooly appearance. These aphids will bob their fluffy hindquarters up and down when they feel threatened. For this reason, they are also known as the boogie-woogie aphid.

Speaking from personal experience, one shouldn't stand under a branch of these things. They are constantly exuding honeydew and can create a small, sticky shower.