Millions of painted lady butterflies that fluttered into California's Central Valley in the last week of March could be just the advance guard of one of the largest migrations of the species on record, said Arthur Shapiro, a professor and expert on butterflies at UC Davis.

"This may be the biggest migration of modern times," Shapiro said.

Shapiro said he is getting reports of "billions" of butterflies around Trona, near Death Valley, and in the San Fernando Valley. More waves of butterflies are likely to appear in central California over the next few weeks as the insects take wing.

Painted lady butterflies, known by the scientific name Vanessa cardui, spend the winter in the desert. As caterpillars turn into adults in the spring, they migrate north in search of fresh food and breeding grounds, powered by a supply of yellow fat they have built up over the winter.

Painted ladies migrate every year, but usually less conspicuously and in far fewer numbers. This year, however, exceptionally high winter rainfall in southern California has created a bumper crop of plants for the caterpillars to eat, fuelling a population boom, Shapiro said.

The butterflies take about three days to reach the Central Valley, and the current generation will fly as far as southern Oregon. Their offspring will fly on to reach British Columbia by summer, before heading south again in the fall.