I walked out to the field behind our office yesterday and found this almost completely defoliated Common Milkweed (Asclepias syriaca).
Clustered on the few remaining leaves were dozens, if not a hundred or more, Milkweed Tussock Moth (Euchaetes egle) caterpillars.
The Milkweed Tussock Moth, also known as the Milkweed Tiger Moth, feeds exclusively on milkweeds and dogbanes. Like other species that feed on milkweeds - notably the Monarch Butterfly - the Milkweed Tiger Moth uses the cardiac glycosides present in the milkweed as a chemical defense against predators. The color combination of black, white and orange (or yellow) advertises this defense to potential predators. The long hairs (setae) help to deter predators and parasitoids such as wasps and tachinid flies. These setae are not present in early instars. This gregarious feeding approach by the caterpillars (especially early instars) may also help as a defense against some predators by protecting those caterpillars in the center of the cluster until they accumulate enough toxin and develop their setae.
By this morning, the two upper leaves of the plant had been fully consumed and many of the caterpillars had moved down the stem to the three remaining leaves on the plant. Many of the caterpillars had dispersed in search of other plants, the nearest of which was more than a meter away.