Why are caterpillars invading Arroyo Grande? Think super bloom — and butterflies

Hordes of caterpillars in an Arroyo Grande neighborhood have startled residents who can’t seem to walk down the street without crushing a crawler. And they may be coming your way soon.

They’re wiggling up the sides of houses, marching across roads and chomping leaves in resident’s gardens filled with weeds and hollyhocks.

They’re likely the offspring of painted lady butterflies that hatched in the desert near the U.S.-Mexico border and are fluttering forward in a mass migration to the Pacific Northwest, Norman Smith, a retired entomologist and a Master Gardener with San Luis Obispo Ag Extension, told The Tribune.

Residents say the quantity of caterpillars is unusual.

“We took a walk this morning to walk our dog, and we just noticed there were hundreds of them walking in the middle of the street, all of them heading in the same direction. (They’re) all over houses in our neighborhood. It’s been a really neat thing to see,” said Sarah Smith, who walked with her children along Turquoise Street in the Halcyon neighborhood of Arroyo Grande on Thursday to see the spectacle.

“This is our second walk today to see them,” Smith told The Tribune. The kids have “been super interested.”

The caterpillars won’t be there long.

Soon, they’ll begin the weeklong process of metamorphosing into butterflies, with bright-orange wings decorated with a white stripe and a row of five tiny black dots, as described by University of Michigan’s Animal Diversity Web. Then, they’ll continue the migration north.

It’s likely the biggest boom in the population of painted lady butterflies since 2005, UC Davis Biology Professor Art Shapiro told Newsweek. That year in Sacramento, butterflies were passing in one’s field of vision at a rate of three per second, according to Shapiro’s website, which is dedicated to tracking butterfly populations across Central California.

The flurry of butterfly activity this year was stimulated by lots of rain that produced bountiful crops of host plants in the desert where painted butterflies winter along the U.S.-Mexico border, Shapiro said. Nearly a month ago, painted ladies swarmed Southern California then Central California.

In good years, they may migrate by the billions, according to Art Shapiro’s Butterfly Site.

Gardeners shouldn’t be too concerned, Norman Smith said.

“I suggest that nothing be done about it as they will disappear in a few days. Most of their host plants are weeds (like cheese weed), and so they are cleaning that problem up,” Smith said. “Adults that hatch out here will head on further north and do the same thing in the Bay Area or north of there.”

This story was originally published April 26, 2019 at 6:40 PM.