Vanessa cardui migrations take place at high altitude, which explains why these migrations are seldom witnessed. "V. cardui" is known for its distinct migratory behavior. In California, they are usually seen flying from north to northwest. These migrations appear to be partially initiated by heavy winter rains in the desert where rainfall controls the growth of larval food plants. Painted lady migration patterns are highly erratic and they do not migrate every year. Some evidence suggests that global climatic events, such as el Niño, may affect the migratory behavior of the painted lady butterflies, causing large-scale migrations. Based on experimental data, the
Urophora cardui arvense", during the host's growing season. After hatching, the larvae burrow into the stem of the plant and form a gall (or swelling). During maturing, the larva will be 98% of its full body weight, and appear as an adult. The larvae pupate in early spring for 24 to 35 days, and appear reddish brown, but are still inside the gall. The fly goes through a tunnel formed in the gall during June to October. Urophora cardui Urophora cardui or the Canada thistle gall fly is a fruit fly which, contrary to its common name, is indigenous to Central Europe