During the last week, I’ve seen several Eastern Tiger Swallowtail butterflies in the backyard. They seem to be especially attracted to the Cup Plant (the same plant the Goldfinches have been tearing apart).
The male color pattern is entirely yellow and black, but without the iridescent blue highlights), so why are there two color morphs for the female?
As I have described in an earlier post, several species of Swallowtail butterflies, and in particular, the female Eastern Tiger Swallowtail, imitate the color pattern of the distasteful Pipevine Swallowtail (a different species entirely).
Predators usually remember what tastes bad or makes them sick, so mimicking something that predators avoid is a good survival strategy. However, that requires that the predators actually have experience with the model (the one being copied), and so far, Pipevine Swallowtails are rare strays in Minnesota because the Pipevine food plants on which their larvae develop are absent here.
We now know that the melanistic (dark form) Eastern Tiger Swallowtail females produce almost all melanistic daughters, and the yellow color morph females produce almost all yellow daughters. So perhaps our melanistic females are offpspring of Tiger Swallowtail butterflies that hatched in nearby Iowa or Wisconsin, where Pipevine Swallowtails have been sighted along with their required food plants.
It’s interesting to note that there is no melanistic female color morph in the Eastern Tiger Swallowtail’s very similar sister species — Canadian Tiger Swallowtails, although the Pipevine Swallowtail has been found in several places in Ontario.