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WSU/Pend Oreille Extension introduced the Sense of Place series in 1999, with a focus on local landscape and natural history of Pend Oreille County, Washington. A partnership with the Kalispel Tribe of Indians Natural Resources Department has allowed us to expand this program through EPA funding to include more classes, a newsletter and this website.

An Outbreak of Orange

by Carol Mack


A natural phenomenon is always exciting. My daughter and I encountered one this summer—on a sunny mid-July day, driving down our lane. Butterflies. By the thousands. They darkened the surface of the road for a mile, and rose about us in orange clouds as we drove through. We extended our arms out the windows trying to snag a few on the wing, laughing at the sheer spectacle of it all. The radiator grill turned out to be more successful than we were—and examination of one of the victims confirmed these were California tortoiseshells. The uppersides of their wings are bright orange with black borders. In contrast, the underside is camouflaged to look like ragged old bark or a leaf when they are perched with wings closed. This is classic “startle-coloration” strategy—that unexpected flash of color may allow a moment to escape a hungry bird. (For a color picture of the adult and caterpillar, try the website or field guides listed below.

The most common food plant for these butterflies is snowbrush, Ceanothus velutinous, a sprawling evergreen spicy-scented shrub. It is also called buckbrush, spicebush, wild lilac, and mountain balm. It has dense pyramidal clusters of tiny white flowers in early summer and shiny, sticky leaves that glisten in the sunlight. But neither it, nor the closely related redstem ceanothus, showed any damage, anywhere I looked in the Newport area. These butterflies came from somewhere else. A partial answer showed up the next day when two friends (who were working in the Ruby Creek drainage) reported witnessing a ceanothus bush violently twitching around, although the day was perfectly still. When they investigated, it was hanging with tannish-gray chrysalides that were writhing forcefully, causing the whole branch to shake. In spots through the area at higher elevations, the ceanothus had been stripped down to sticks.

We normally think of a butterfly’s chrysalis stage as a resting time, but this particular species is uncommonly active. Robert Pyle (in The Butterflies of Cascadia) describes an area heavily infested with California tortoiseshell larvae and chrysalids this way: “their corporate shimmy made the entire hillside look alive.” He speculates that the twitching behavior may have evolved to deter the small parasitic wasps attempting to lay eggs in them. He goes on to say:

This enigmatic butterfly builds up its numbers for years until it bursts out in phenomenal mass movements. In such years the mountain balm and deerbrush are defoliated over wide areas, tortoiseshells show up a long way from their points of nativity, and they become the most abundant butterflies along the mountain streams. Then the numbers crash, and scarcely a tortoiseshell will be seen in the entire region for the next several years. Our previous great peak lasted about from 1985-1990, peaking in 1987.

These periodic mass migrations (“irruptions”) have sometimes closed down highways because of their magnitude, and the butterflies are often mistakenly reported as migrating monarchs. Theories for these population explosions and crashes range from sunspots (same periodicity) to depletion of food plants and/or increase of parasites such as the wasps.

Will the tortoiseshells continue to build up numbers for the next few years until most of the ceanothus is consumed, or was this summer the peak? Stay tuned. And when you travel, keep an eye out for orange fellow travelers—some of them may have started out in Pend Oreille County too.

Butterflies of Pend Oreille County, WA website:
http://www.npwrc.usgs.gov/resource/distr/lepid/bflyusa/chklist/states/counties/wa_51.htm

Glassberg, Jeffrey. Butterflies through Binoculars (The West). Oxford University Press, 2001

Pyle, Robert Michael. The Butterflies of Cascadia. Seattle Audubon Society, 2002

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