Having overwintered as an adult, the Mourning Cloak butterfly (Nymphalis antiopa) is one of the first butterflies seen in early spring. Mating takes place and eggs are laid in a cluster encircling a twig of a willow, cottonwood, elm, birch or hackberry tree. The hatching caterpillars stay together until they move off their host plant to pupate, which is what is happening right now and why you can find these distinctive spiny black and red caterpillars at this time of year. Adult Mourning Cloaks emerge in midsummer, enter a state of dormancy until fall and then seek a sheltered spot in which to hibernate until spring.
Looking every bit like the Golden Alexander (Zizia aurea) flower buds on which they were laid, the pale yellow eggs of a Black Swallowtail (Papilio polyxenes) are next to impossible to find unless one is fortunate enough to see them in the act of being laid. Members of the parsley family (Golden Alexander, Wild Parsnip, Queen Anne’s Lace, Dill, Carrot) are host plants for most ravenous Black Swallowtail larvae, and thus that is where you will find their eggs. As they eat, the caterpillars absorb toxins from their host plant, which does not harm them but makes them distasteful to avian predators.