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
Brachycaudus cardui "B. cardui" are usually attended by ants which feed on the honeydew the aphids produce and drive away predators. Researchers found that in a damp meadow which contained ragwort ("Jacobaea vulgaris") but no ants, although winged "B. cardui" arrived at the site, they failed to establish colonies despite an abundance of the host plant. In an experimental habitat, colonies of "B. cardui" on "J. vulgaris" flourished when ants were present but died out if ants were excluded. Infestation with "B. cardui" causes the margins of affected leaves to roll up. This aphid is a vector of plum pox, a serious