Looks like the yellownecked caterpillar, Datana ministra. Here's a good way to identify them. If disturbed, the caterpillars lift their head and tail to form a "U"-shape. If so, then natural predators usually keep them under control.
We had gypsey moth and oak worm caterpillars back in the 80"s, but these are not them. The oak worms came out in Sept - Oct timeframe so we never sprayed them. It made leaf cleanup easier.