Below are two photos I took Saturday of Monarch caterpillars eating Milkweed leaves, their sole source of food. Monarch butterflies have declined precipitiously over the last twenty years, a direct result of declines in Milkweed, open space, and the degradation of their winter nesting grounds in Mexico.
In the Northeast, Milkweed is not considered a wetland plant by the USDA for wetland classification purposes, but is elsewhere in the United States it's status differs, as there are over 100 species of Milkweed in the United States. The swamp Milkweed found in the west thrives only in moist soils, and the photos I took were roadside, in a very dry environment. Due to the toxins found in Milkweed, which are also present in the caterpillar, birds and other predators steer clear from what would otherwise be a juicy snack.
Some helpful pages here:
Native Milkweeds - The Xerces Society
USDA Milkweed plant Profile
Wetlands Institute - Monarch butterfly
NRCS Monarch Butterfly Habitat Development Project
Monarch Butterfly Conference Report
A particularly well researched monograph on Milkweed from a Botanists perspective