I have had several customers bring me Tussock Moth Caterpillars they found munching on their oak tree leaves.
The creamy white hairs on the caterpillar are long and bristle-like and spread out in tufts down the sides. Two long, sharp black pencil like hairs protrude near the front and rear and these hairs are connected to poison glands, which excrete venom on contact. Contact with the venom may result in itchy irritation.
You can spray your tree with Hi Yield Thuricide Concentrate or Natural Guard Spinosad Hose-end. Both of these products are organic and safe to use on all plants. Thuricide reacts with the cells of the stomach lining of the target pest causing them to not be able to feed within hours of ingesting. Spinosad is a contact kill, but still safe and organic for all plants.
(photo via ocala.com)