Painted lady butterflies migrate only occasionally, and migrations occur every 8 to 10 years on average. When these migrations occur in the autumn, millions of butterflies form streams of flying individuals that sweep from Canada and the contiguous U.S. to fly southward and into Mexico. There, they find acceptable host plants that are repositories for new egg laying. This begins a new generation of butterflies that, in spring, will begin another migratory flight northward into California, Arizona, New Mexico, and Texas, and, from these states up into the northern band of the United States and Canada as host plants become available. The genetic mixing that occurs during these migratory flights and with the over-wintering stay in Mexico keeps the gene pool well homogenized and uniform.