This is the dreaded gypsy moth caterpillar, Lymantria dispar, which can multiply out of control and strip entire oak trees down to the branch. In some cases, whole forests lose their leaves to hordes of these caterpillars. Walking into a forest under siege from gypsy moth caterpillars, you can hear the sound of millions of tiny jaws working away, eating every leaf in site. Attempts have been made to control this caterpillar by spraying infested forests with a kind of bacteria that kills the caterpillars. While this can be effective, the bacteria is known to kill many other species of caterpillars in addition to the gypsy moth. It's a high price to pay to rescue trees that will likely eventually survive anyway!