The butterflies just kept coming—at first thousands, then tens or even hundreds of thousands. Their wings were brown on the underside and vivid orange above, so as they flew by, they looked like chips of sunshine. The sight was marvelous, awe-inspiring, and more than a little disconcerting.
I encountered the butterfly cloud—technically, an irruption of California tortoiseshells —on a bright blue summer day in the Sierra Nevada. Along with Matt Forister, a biologist from the University of Nevada, Reno, I was hiking Castle Peak, a knob-shaped mountain northwest of Lake Tahoe. Castle Peak’s butterflies are one of the world’s most closely watched insect populations: Every summer for nearly 45 years they’ve been censused on a biweekly basis. Most of the data were collected by Forister’s mentor, Art Shapiro, a passionate lepidopterist and professor at the University of California, Davis, who recorded the information on three-by-five cards.
After Forister and his team computerized the surveys and analyzed them, they found that Castle Peak’s butterflies have been in decline since 2011. We were discussing why this was the case when we neared the 9,100-foot summit and were enveloped in an orange haze.