A Copper Underwing Caterpillar, Amphipyra pyramidea, was nibbling on our native plum tree. It's a pretty blue green color that stands out against the foliate. It's a hefty sucker too. Perfect for fishing, making some bird happy, or just turning into a boring brown moth. They feed on assorted fruit trees, and hardwoods. I read they only have one generation each year and over winter in the egg stage. Considering it's May I guess one generation means females lay continuously over the summer, because it's hard to see the adults just waiting for fall.
This ID is brought to you by. Caterpillars of Eastern North America: A Guide to Identification and Natural History (Princeton Field Guides)