Milkweed Tussock Moth Caterpillar (Euchaetes egle) - Frass

I found this fresh, juicy pile of frass (poop) on a plant that had several milkweed tussock moth caterpillars feasting on it. They were decimating it, and when a caterpillar eats a lot, it also poops a lot!

Habitat: Rural garden with lots of milkweed. *Note, this frass (and the caterpillars) was NOT on milkweed, which I thought was odd because I didn't realize that the caterpillars had any other food hosts, but I guess they do. The milkweed has totally skeletonized already.