Swamp milkweed (Asclepias incarnata, Asclepiadaceae) is a rather common herb in southeast Michigan floodplains. The lavender to pinkish-purple flowers, narrow leaves, and preference for wet soil set this milkweed apart from others found in this area. Although several milkweed species can be found here, only two others bear a resemblance to this one. Common milkweed (A. syriaca) has broader leaves and is found in drier fields, meadows, and on roadsides. Purple milkweed (A. purpurascens) has much darker reddish-purple flowers, and is found in open spaces in forests (Brandenburg 2008).
Like most milkweeds, swamp milkweed produces a thick, sticky white sap that contains toxic chemicals. These toxins prevent most animals from eating this plant, but some insects have adapted to feed on it. Some are not only resistant to the toxins, but incorporate them into their own physiologies to make themselves toxic to predators. Perhaps the best-known example of this is found in the monarch butterfly (Lepidoptera: Nymphalidae: Danaus plexippus), but is also seen in other insects like the large milkweed bug (Hemiptera: Lygaeidae: Oncopeltus fasciatus). Insects that acquire toxicity from milkweed typically exhibit striking colors as a warning to predators.
The sticky nature of the sap can create another problem for insect herbivores. This substance can gum up their mouthparts, preventing them from feeding. Some insects get around this problem by making incisions in the leaves in strategic locations, causing the sap to leak out. This decreases sap flow in other locations, where the insects then feed. Since the sticky sap merely trickles out in these secondary areas, they can then feed without being overwhelmed by the gummy substance. Adult swamp milkweed leaf beetles (Coleoptera: Chrysomelidae: Labidomera clivicollis) use this approach. The larvae, however, seem to feed more on flower buds that exude much less sap.
Brandenburg, D.M. 2010. National Wildlife Federation Field Guide to Wildflowers of North America. Andrew Stewart Publishing, Inc., New York, NY.