California Tortoiseshell

On the afternoon of 25 August 2020, on a dragonfly search of the yard, I happened upon an unfamiliar butterfly—a California Tortoiseshell. This species is said to be enigmatic (Pyle 2002). They breed in much of the Rocky Mountain West and in southwestern Canada. Populations build up over the years and then numbers explode, their caterpillars defoliating host plants like Mountain Balm and Deerbrush. The butterflies spread out across the northern Great Plains. Butterflies reach Minnesota, Iowa, the Great Lakes, and even stray east to Vermont. During non-invasion years, these butterflies may be absent. What causes these fluctuations is unknown.