The milky white sap in Milkweeds is toxic, which is a defense mechanism utilized by the Monarch. The yellow and black markings of the caterpillar are a warning sign to predators not to eat them. Black Swallowtail Butterfly (Papilio polyxenes) have used this tactic to their advantage. Though not identical to the Monarch, the caterpillars of Black Swallowtails have similar green, yellow, and black markings to fool predators. But these caterpillars prefer plants in the carrot family, like Queen Anne’s Lace, Dill, Fennel, Parsley, and of course Carrots. When you are planting the herb garden in spring, pop some extra Parsley plants around the yard (or Dill if you can keep it from reseeding all over the yard). You’ll be rewarded with seeing more of the dark beauties flitting about the yard.
Eastern Tiger Swallowtail
Closely related, and also a common Wisconsin native, is the Eastern Tiger Swallowtail (Papilio glaucus), though with a less attractive-looking caterpillar. The green larvae have two yellowish spots, like eyes, that seem to stare you down if you find them eating the leaves of Cherry (Prunus), Birch (Betula), Ash (Fraxinus), Linden (Tilia), and Tuliptree (Liriodendron tulipifera).