Yesterday afternoon I noticed a number of milkweed tussock moth larvae (Lepidoptera: Erebidae: Euchaetes egle) feeding on a common milkweed (Asclepias syriaca, Apocynaceae) near the edge of my yard. As with a number of other insects, these caterpillars have learned to overcome the chemical defenses milkweeds use to deter herbivory.
Milkweeds produce sticky white latex that tends to gum up the mouthparts of most insects. Milkweed tussock moth larvae get around this problem in two different ways. Smaller, younger larvae simply avoid the latex-bearing leaf veins, feeding instead on leaf tissue between the veins. This results in a “skeletonized” look to the leaves. Older larvae make incisions in the veins upstream from a given area, thereby cutting off the flow of latex to the spot where they want to eat.
Milkweeds also produce toxic cardiac glycosides that are harmful to most insects, but a few like these moths have developed a tolerance. Like some other insects they even incorporate it into their own tissues to make themselves toxic to predators. Although the defenses milkweeds have evolved to prevent herbivory by insects have helped them a great deal, some specialist insects have found an ideal niche feeding on these generally unpalatable plants.