Animal of the Month: California Tortoiseshell Butterfly

You may have noticed the ground around Bend has been covered in a sea of orange and black over the past few weeks. Maybe you've had to wash your windshield more frequently, or maybe you've been dodging these flying beauties while out on a trail. Well, the reason for this explosion of orange and black also happens to be our animal (or rather, insect) of the month: the California Tortoiseshell Butterfly. 

Photo courtesy of Frank Townsley,

Photo courtesy of Frank Townsley,

The Nymphalis californica, or California Tortoiseshell, was first identified by entomologist Jean Baptiste Boisduval in 1852. The top side of the wings are orange-brown with large black spots and dark wing borders. The underside of the wings are dark mottled brown with darker wing bases. This coloring serves as camouflage for the butterflies - when the wings are closed the butterfly looks like a dead leaf. Their wingspan measures between 3.2 and 7 cm. 

The "Tortie" overwinters as an adult and on mild midwinter days, can be found enjoying the sunshine outside of their overwinter location. Adults emerge in late May to early June and emigrate soon after, going north or east to higher elevation. This species is known for having population explosions. They are currently making their way through Oregon and the PNW, which explains why we're seeing such an abundance as of late! 

The cause of these population explosions is not as random as one might think. Entomologist and UC Davis Professor Arthur M. Shapiro, Ph. D., explains the reason for the California Tortoiseshell's massive population boom: 

"Occasionally the upslope movement encounters the retreating snowline, forcing females to lay only on Ceanothus that have melted out. This can result in enormous larval densities, total defoliation of the hosts and mass starvation -- while close at hand, plants that were still under snow when the eggs were laid sit absolutely untouched!" 

The range of the Tortie remains mostly on the west coast of the United States. From British Columbia south along the Pacific Coast to Baja California Norte, east to Montana, Wyoming, Colorado, and New Mexico. Rare migrants to Michigan, Pennsylvania, New York, and Vermont after periodic population explosions in Mexico.

California Tortoiseshell Habitat

Various species of wild lilac (Ceanothus) provide habitat for the butterflies to lay eggs, and food for caterpillars. Adults visit flowers of many kinds, aphid and scale honeydew, damaged fruit, sap. A particular favorite of the California Tortoiseshell is a mud puddle. The insects enjoy the damp surface, and it is possible to see hundreds or even thousands packed side-by-side enjoying the mud.

If you thought this year's population explosion was bad, just wait for next year! Following wildfires, buried Ceanothus seeds germinate and surviving Ceanothus resprout profusely, growing more rapidly than young conifer seedlings. This means we will likely be seeing a lot more of these guys next summer!