The ongoing Painted Lady migration is well underway, after a seeming delay in some areas due to cool weather. Large numbers were passing through Palm Springs, California on and before March 2, 2019, and some of these butterflies were observed flying westward. They also became abundant in the Las Vegas area around March 4, and continue to be present there in large numbers. Most recently, an enormous swarm has been flying northward through the Los Angeles area, both yesterday and today, March 12. Painted Ladies are now widespread throughout southern California, including in Borrego Springs, where an extensive northward migration was underway on March 11. In Yuma, in the southwestern corner of Arizona, Painted Ladies have likewise been flying in huge numbers for the last two to three weeks, mainly in a northwestern direction. The entire outbreak was likely triggered by considerable local rain associated with the current El Niño event, followed by prolific blooms of desert annuals and growth of larval food plants.
Although the butterflies have often been reported as flying generally northward, there seems to be substantial local variation in flight direction in much the same way as occurred during a similar large and well-observed migration from February through April 2005, as you can see by looking at our flight direction map for that interval. Once there, check the Painted Lady box, enter that range of dates, and zoom in to see the directions of the arrows and get an impression of how the migration proceeded and spread during spring 2005.
-- Royce Bitzer, March 12, 2019