Even though the Painted Lady cannot overwinter in any stage in the northern part of its range, it is perhaps the most widespread butterfly in the world. It is found throughout Africa, Asia, Europe and North America. Most of North America has no Painted Ladies at all between the first hard frosts of winter and the advent of spring. In February and March, they begin emigrating from their wintering grounds in the Southwest and perhaps other warm regions; by late spring they have repopulated the entire continent. Unlike the Monarch butterfly's round trip migrations, the Painted Ladies only travel one way. The number of butterflies varies greatly from year to year, although no one knows why. Various theories have been advanced: parasite population swings, host plant variations, nectar availability due to abundance of rain or lack thereof. 

September 11, 2003 - The Chicagoland area is experiencing a population explosion of Painted Lady butterflies, just as posited by the above paragraph. They are everywhere, by the hundreds of thousands. I found dozens of them grazing in just one small patch of goldenrod, more butterflies than I'd ever seen in one place at one time.*

*Footnote March 2012: Northern Illinois has experienced a 'population explosion' of Red Admiral butterflies that puts the one above to shame. However, this sudden abundance was credited to the warmest March on record - these butterflies migrated north in record numbers. I was walking at Castle Rock State Park near Oregon, Illinois the other day, and on every forest trail, every meadow there were hundreds of them.