Swarms of any other insect might provoke fears of a coming apocalypse, but clouds of butterflies migrating through Southern California are captivating onlookers who are relishing the otherworldly spectacle.

The orange butterflies, called painted ladies, are known to travel annually from the deserts of Southern California to the Pacific Northwest. This month, people are taking notice because of the sheer size of the migration: Scientists estimate the teeming painted ladies number in the millions.

Substantial rainfall in the deserts near the Mexican border, where the North American painted ladies lay their eggs, is the reason for the unusually large swarms. The rain caused plants to thrive, giving the painted lady caterpillars plenty of food to fuel their transformation, said Arthur M. Shapiro, a professor of evolution and ecology at the University of California, Davis.