Butterflies by the Number a Sight to Behold on Yearly Trek

A massive wave of brightly colored butterflies passed through SanDiego County on Friday, on their yearly migration northward.

Millions of orange, black and white paInted lady butterflies have flooded skies over San Diego and Orange counties the past two days. Usually, they number in the thousands. But heavy rains this year caused a population explosion.

The butterfly usually takes flight from habitats in Mexican or San Diego County deserts in late April, but heavy spring rains and increased desert vegetation spurred an early departure this year, said Herb Field, a San Diego entomologist. The butterflies travel in several waves; the one that left San Diego County on Friday is the largest.

The increased vegetation also provided more food for the caterpillars this year and increased their numbers, Field said.

Soon after emerging, the butterflies start an annual migration northwest toward cooler climates and more abundant food.

The colorful swarms are expected to dwindle as the insects succumb to fatigue, car windshields and temperatures that eventually turn too cool for them. Hardier butterflies may continue north as far as Canada, Field said. Then they die, at the age of 2 months. But, before they left the desert, they laid the eggs that will make up next year’s traveling swarm.

The yearly cycle begins after desert caterpillars gorge themselves on weeds and thistle, then bind themselves into cocoons, Field said.

The plain caterpillars then emerge as brilliant butterflies, but what pushes them northward is still a mystery.

“They somehow know the weather will change, and it will get too hot for them, and so there is this mass flight in a northwest direction,” Field said. “It is just one of those things scientists just don’t understand yet.”

Motorists in San Diego County didn’t seem to understand either as the butterflies splattered on windshields.

A separate wave of the butterflies caused headaches for motorists south of Bakersfield on Thursday, particularly on California 99 near Weed Patch, Pumpkin Center and Greenfield.

“Your windshield got so doggone full of bugs that it was hard to see,” said Lupe Gonzales, manager of Weed Patch Market. “I had trouble getting to work. There were just loads of them everywhere.”

But birds that were spotted swooping into the quickly moving clouds of insects for dinner seemed happy.

Painted ladies do not bite or sting and spare farmers grief by eating weeds and roadside vegetation, Field said.

The butterflies rest in eucalyptus trees or leafy bushes by night and travel by day, pushed by gentle winds, he said.

“It is a spectacular and beautiful sight,” he said.

Times staff writer Barry Horstman contributed to this report.