Winter 2004

At some sites in Chicago Wilderness, members of the
Butterfly Monitoring Network recorded 300 to 400 Painted Ladies
per hour — they normally spot 15 to 20.

Explosion of Painted Ladies

I first witnessed the massive infiltration of painted ladies on the Field Museum Campus in downtown Chicago, where I walked outside to find hundreds of these small, orange-and-black butterflies flitting over the landscaped grounds. These butterflies, I discovered, were only a fraction of a larger "invasion" that swept the region for two weeks in mid-September.

Of recent "painted lady years," this was by far the largest. At some sites in Chicago Wilderness, members of the Butterfly Monitoring Network recorded 300 to 400 individuals per hour — they normally spot 15 to 20. Painted ladies (Vanessa cardui) are one of several species that each year repopulate our region from their home turf across the American Southwest. Short-lived and unable to survive winter in our region in any form, in good years hundreds of them float through the spring skies of Chicago Wilderness to re-establish summer breeding populations here. These populations often spike again later in the summers after successful spring migrations — but this spring was not good. My quest was to find some explanation for this year's unusually large "late show."

As I studied records from butterfly monitors and scientists across the country, a definite pattern emerged. Over the course of the summer, the sightings of painted ladies clustered in a band across the lower Midwest, with the most sightings in Iowa and Nebraska. These areas had reported hefty numbers of Vanessa cardui as far back as July, while here in Chicago Wilderness, painted ladies were few and far between. Looking back to our unusually long, cold spring with its endless northeasterly winds, followed by several high-velocity summer storms, it's easy to see why butterfly populations of most species were down this year.

I wondered, though, whether our painted ladies were moving into Chicago Wilderness en masse or just breeding madly here? From most outward appearances, our area experienced an "influx" as opposed to an "irruption." First, a late irruption seemed unlikely without evidence of a massive spring migration. Additionally, none of my sources recorded any courtship displays, although I did receive several documented observations of painted lady caterpillars as well as reports of the butterflies' "fresh" appearance.

Though most recorded sightings indicated that the painted ladies weren't headed in any one direction, the general consensus favored a northward migration fanning from the epicenter, with smaller numbers of the butterflies moving south and east. In the end, I decided that we had likely experienced an influx due to a southern population explosion. In other words, the expected spring migration, held back by difficult spring conditions, finally thrust north when the soy-nourished southern population suddenly exceeded its bounds.

The problem with all these conjectures is that nobody actually witnessed the arrival of the painted ladies — they just appeared without any obvious migration. And many questions about painted lady migration have yet to be explored. For instance, can they migrate in the evening? Can they soar on thermals high out of sight, travelling long distances with little wear and tear?

Alas, even an exceptional year must come to a close. Increasingly cold nights and dwindling nectar sources brought a rapid demise to the spectacular, unexpected flight of Vanessa cardui. For this year, "The Case of the Painted Ladies" has officially CLOSED.

— Mary Beth Prondzinski
Collections Assistant – Zoology, The Field Museum