by Liz Potasek
As Arboretum Director of Operations (and naturalist/plantsman) Alan Branhagen took a quick break to stroll the Arboretum grounds on Thursday, he was surprised by how many early spring flowers were in bloom. He noted crocus, pasque flower and dwarf iris, and then he spied something truly stunning: a Mustard White Butterfly perched on a crocus in the Rock Garden.
The Mustard White Butterfly hasn’t been spotted at the Arboretum before. “We are on the southwestern edge of its range and it is a very unique butterfly that may actually be several cryptic species,” says Branhagen, who is the author of The Gardener’s Butterfly Book.
Mustard Whites are often confused with Cabbage White butterflies, an invasive species that causes problems in Minnesota gardens by feasting on vegetables in the cabbage family. But Mustard Whites are a native butterfly whose caterpillars feed on native mustard family plants found in woodlands and wetlands. “The adult butterflies nectar on white and yellow flowers,” Branhagen says. “This is not a butterfly one would expect to find in a garden unless it is surrounded by native woodlands and wetlands like here at the Arboretum.”
Mustard White are listed as “locally rare to uncommon” in the Swift Guide to Butterflies of North America by Jeffrey Galssberg. The Mustard White has two generations each year, one that emerges from chrysalis in early spring, and another that emerges in late summer. Branhagen notes that his particular Mustard White wasn’t in its typical spring form. “Spring butterflies that emerge from their chrysalises that overwintered usually have striking striations on their wing undersides that are very noticeable,” Branhagen says, noting that this butterfly appeared to be in its summer form. “This butterfly's next generation would emerge in late summer. The ‘summer brood’ are often almost pure white, and their offspring would overwinter as a chrysalis.”
While this particular butterfly’s markings are unusual for the time of year, it’s an exciting new discovery at the Arboretum. The Mustard White will be added to the Arboretum’s Butterfly Checklist, which can be found in the Oswald Visitor Center with other brochures.