Wave of Butterflies Slows to a Trickle

The number of butterflies migrating through California has fallen to nearly a 40-year low as populations already hurt by habitat loss and climate change encountered a cold, wet spring, researchers said.

“Some of them were already in decline, but this weather really added insult to injury, kicking them when they were down,” said Arthur Shapiro, a UC Davis entomologist.

Shapiro, who over 35 years of tracking the insects has developed one of the world’s largest butterfly databases, monitors 10 observation stations from the Bay Area to the eastern slopes of the Sierra Nevada.

About half of the usual species haven’t shown up, while others -- such as the drab-colored sooty wing or the iridescent eastern tailed-blue -- are fluttering in at one-fourth or less of their usual numbers, he said.

The change was particularly dramatic for the red and black painted ladies, which last year enjoyed a possibly record-breaking migration after feeding on vegetation nurtured by abundant rain in Southern California’s deserts.

Last spring, millions of them migrated through the state and into Oregon, passing Shapiro’s Sacramento site at a rate of four per second. This spring, he had reports of four painted ladies a month in the same area.

Erratic weather was part of the problem. A mild winter, warm February and wet March upset the usual cues that tell butterflies when to emerge from dormancy, Shapiro said.

As difficult as the soggy March was for butterflies, it may be only one of many factors contributing to their steady decline, researchers said.

Long-term changes in rain patterns linked to global warming and the paving over of habitat could be playing a role, said Jessica Hellmann, an entomologist with the University of Notre Dame who has examined Shapiro’s data.

“If you whack a population and whack it again and again, it’ll go extinct in that area,” Hellmann said.