Pandora moths have hogged the spotlight in Bend, literally and figuratively, but they weren’t the only flying insect to appear in massive numbers in Central Oregon this summer.

California tortoiseshell butterflies — orange butterflies with black spots and wings that stretch one to two inches in diameter — began appearing in high elevation stretches throughout Central Oregon in early June. Rob Flowers, entomologist with the Deschutes National Forest, said the insects appear in the area almost every year, but every so often, they arrive in large swarms, devouring plants, confusing motorists and covering large stretches of lakes and roadways in carpets of orange.

“I usually just tell people to enjoy the spectacle,” Flowers said.

Their numbers have tailed off since the height of the outbreak last week, but there are still butterflies at higher elevation locations like Newberry National Volcanic Monument and portions of the Cascade Lakes Highway. Flowers speculated that, at the height of the outbreak, the insects numbered “probably in the millions” in Central Oregon alone.

“It was impressive for a while,” he said.

Drivers traveling to some of Central Oregon’s public lands found themselves inundated with butterflies. Ralph Saunders, lead park ranger with Newberry National Volcanic Monument, said the butterflies began showing up at Newberry caldera a few weeks prior. He added that he’s heard from plenty of visitors who had windshields splattered with tens or hundreds of butterflies from the drive up to the caldera.

“With the Pandoras, you kind of know they’re here every other year,” Saunders said. “These just came out of nowhere.”

Moths and butterflies look similar, but have several small differences to their wing structure and behavior. Unlike Pandora moths, the palm-sized terrors that appear in Central Oregon every two years like clockwork, California tortoiseshell butterflies have massive outbreaks every 10 to 15 years, and no one knows precisely why, according to Flowers.

One possibility is that the insects’ natural predators — mainly beetles that feed on the butterflies while they’re still caterpillars — and parasites normally keep their numbers at lower levels. If those parasites and predators are temporarily lowered, that could allow the butterfly populations to explode from time to time, according to Flowers.

Additionally, Flowers raised the possibility that the butterflies could have genetic drivers that keep them from arriving in large numbers except during particular years, like cicadas, to keep diseases from spreading.

The butterflies begin their one-year life cycle as black caterpillars with yellow dorsal spines, which are fond of manzanita, willow and snowbrush, a flowering shrub found throughout the Western United States. Flowers said the butterflies can quickly strip trees and bushes of their leaves, but they rarely do any lasting damage to the environment, because of their short life cycles. Saunders added that they’re attracted to lakes and other bodies of water at the national monument, even gravitating toward puddles of water left by cars and boats.

Other than the danger of driving through a cloud of them, California tortoiseshell butterflies are not harmful to humans.

The butterflies have already begun to die out in lower-elevation stretches of Central Oregon, though they’re still visible in large numbers along the Cascade Lakes Highway. While they’ll most likely return next year in reduced numbers, Flowers said he couldn’t rule out another large outbreak.

“Nature always surprises you,” Flowers said.

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