The Grass Lake slough at Snail Lake regional park is teeming with butterflies, especially Monarchs, which may be congregating here for their southward migration. The Monarchs especially seem to like the meadow blazing star and don’t even mind sharing it with a lot of other butterflies, bees, etc.
But one of the many Monarch butterflies I photographed wasn’t a Monarch, but a Monarch mimic, the Viceroy, and these two unrelated species ARE difficult to tell apart. Can you spot the difference(s)?
The biggest difference in coloration of the two species is the bold black horizontal (sort of) stripe on the hind wing of the Viceroy, seen from above or below. The thick black lines on the hind wing of the Viceroy are similar to those of a female Monarch but are much bolder than the male Monarch’s, which also has a distinctive dot on each hind wing. In addition, Viceroy butterflies are smaller in size, only about 2/3 the size of a Monarch.
But where Monarch caterpillars grow up eating milkweeds containing poisonous cardiac glycosides which they sequester in their bodies (and wings), Viceroy caterpillars eat willow, poplar, and cottonwoods — not at all poisonous. Bird predators find Monarch butterflies extremely distasteful and will regurgitate or spit them out. Viceroy butterflies that most closely resemble their poisonous cousins in coloration are better protected from predation, and thus, the mimics survive to reproduce.
And then there are these two, apparently dissimilar butterflies, flitting around the same plants, often displacing each other from the same flowers.
Not at all look-alikes, in fact, color-wise, they couldn’t look more different, except for the pattern of white dots around the margin of the wings and the distinctive iridescent blue splashes of color on the back end of the hind wings. So, what’s going on here?
Male Eastern Tiger Swallowtail butterflies are almost always yellow with black stripes. Females on the other hand vary between yellow morphs and black morphs. And, the black morph is more commonly found in the southeastern U.S. where a similar-colored, poisonous and unpalatable Pipevine Swallowtail butterfly occurs. These are the “look-alike” models for the Tiger Swallowtail (and other Swallowtail butterfly species) to copy. Interestingly, the proportion of female black morphs of the Tiger Swallowtail is higher in southern populations because of genetic (sex-linked) process that makes black morph females produce mostly black morph females, and yellow females produce mostly yellow females!
Bottom line: you have to look closely when identifying a butterfly, because it might be a mimic!