Yesterday I visited a floodplain forest here in southeast Michigan to see what was still alive and kicking after our first freeze. In the warm afternoon sun I found many of these oleander aphids (Hemiptera: Aphididae: Aphis nerii) covering the stem of a swamp milkweed (Asclepias incarnata, Asclepiadaceae). Like the majority of the true bugs, aphids use their piercing-sucking mouthparts to feed on the sugary sap of plants. As the name suggests these aphids prefer oleander, but also commonly feed on milkweeds (and are often called milkweed aphids).
Aphids are tiny, soft-bodied bugs that are easy pickings for predators like lady beetles and lacewings. These particular aphids, however, acquire some defense from their host plant. Milkweeds produce toxic chemicals called cardiac glycosides that most animals find distasteful or even deadly. Some insects have evolved to feed on milkweed, tolerate these toxins, and incorporate them into their own bodies. Such adaptations make the insects themselves toxic to predators. These insects are often brightly-colored, typically with a black-and-orange color scheme, to warn predators of this toxicity. In addition to these aphids, other insects like monarch butterflies, swamp milkweed leaf beetles, and large milkweed bugs exhibit the same feeding behavior, toxicity, and coloration.