Some of the large migrations of Painted Ladies may contain up to 300 million butterflies. But the number of migrants fluctuates a great deal from year to year. Some years there are scarcely any to be seen up north. The heavy rainfall allows a luxuriance of plant growth in the desert, and the Painted Lady is not picky about host plants. Though thistle is preferred, the Painted Lady will lay eggs on more host plant species than almost any other butterfly.
More caterpillars survive, and many adults emerge within a short period of time.
Many of them migrate northward immediately, but those that stick around produce a second generation which accounts for the even larger second flight that is sometimes observed. Population crashes may occur because of host plant defoliation or parasite attack. The populations of American Ladies and West Coast Ladies also fluctuate, but these butterflies do not have mass migrations as impressive as those of the Painted Ladies. American Ladies lay eggs singly on everlastings, pussytoes, or other composites. The caterpillar makes a nest of silk, leaves, and other plant material to protect itself while feeding.