Navarro Ridge Butterfly Explosion
On a lonely logging road on the east end of Navarro Ridge in Mendocino County CA, 10 miles from the Pacific coast, environmental conditions combined to produce millions of butterflies that hatched and swarmed out over surrounding communities.
The butterflies are California Tortoise Shells (Nymphalis californica). Like Painted Ladies, they have population explosions somewhere every summer.
2006: the explosion blew away, and California Tortoise Shells are scarce again. But a long wet cold spring reduced many other California butterfly populations too. So stay warm.
2007: May 10: They're back & there's millions of 2nd instar caterpillars again.
Nymphalis californica breeds in CA, OR, WA, BC, AL, ID MT, NV, AZ, UT, NM, and CO. Migrations sometimes extend as far east as New York State. The habitat they prefer is Transitional zones like where woodland and Chaparral vegetation changes to tall timber (Hudsonian). At lower latitudes and warmer areas there are usually 3 flights per year. Adults hibernate over winter and become active the next year as early as Feb. Pupa are bluish gray, sometimes green, orange, or nearly purple/black. All have a tan abdomen w/ dorsal bumps which are a lighter color and have a dark middle and orange tip.
When natural population controls (temperature, predators and food supply), fell out of sync, so many butterfly pupae were produced that bears quickly learned to knock down the host plant bushes (Ceanothus thrysiflorus) to eat the pupae (cocoons) off the upper branches. That doesn't make much of a dent in the butterfly numbers though, because the affected area covers several hundred acres in patchwork along the ridge, often with pupae hanging from branches as thick as cherries.
Ceanothus is also called California Lilac, or Tick Bush. There usually are ticks on it because deer like to hide in the tangle and shade, and deer ticks crawl onto the bushes. Then people walk along the same trails through the bushes, and the ticks crawl onto the people.
Here's a picture of some branches that a bear broke down. -->
This picture is evidence that the same area had a previous population explosion last year (2004), or the year before. These bushes are dead & bleached gray. 
This years (2005) bushes still have green wood even though the leaves have all been eaten by caterpillars.
<--- This picture shows 8 adults perched on a bush, waiting for the right breeze like surfers waiting for a wave.
Here's some 2nd instar caterpillars (larvae). An instar is a stage between molts when they shed their skin. They shed 5 times. The last one is the pupa, when they shed that skin, they hatch into butterflies. --->
<--- Life's hard for caterpillars though, & not all the larvae survive to make pupae. Besides being eaten by bears, skunks, raccoons, possums, mice, hornets, and birds, some starve when they run out of leaves, and some catch diseases or get fungal infections during wet weather.
Some are parasitized by little wasps and Tachnid flies. Here's a picture of a pupa that hatched out a Tachnid fly parasite. Tachnids lay their eggs on the butterfly larva, then the fly larva hatches and eats the butterfly larva inside its pupa. --->
<--- When butterflies emerge from their pupa, they split open the chrysalis by gulping air & blowing themselves up like a balloon, till the pupa shell cracks, then they crawl out of the split side, like this & they don't leave a gaping hole. 
Here's some Tachnid pupae & a Tachnid larva. --->
And a general ID for adult Tachnid flies (in all their evil glory) --->
But even with all these natural population controls, there's still thousands and probably millions of butterflies that hatched to fly away and lay eggs somewhere else. The worst thing that can happen to limit butterfly numbers is bad weather, disease, and lost habitat. The best thing for these butterflies is logging because it lets sunlight into the forest floor where ceanothus bushes are quick to grow. (Well OK, sort of quick, like about 5 or 10 years.)
Host Plants: Shrub Rhamnaceae : Ceanothus thrysiflorus, integerrimus, velutinus, cordulatus, fenderi, sanguineus, cuneatus. (Legit: Butterflies of N. America, by James A Scott)
