Navarro Ridge Butterfly Explosion

June 8 2005 to 2015
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Navarro Ridge Road View Larger Map

2005: 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.

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 Ceanothus trunks that a bear broke down. The big one's about 3 inches in diameter -->

The butterflies are California Tortoise Shells (Nymphalis californica). Like Painted Ladies, they have population explosions somewhere every summer.

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 because the leaves were all eaten.

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. I counted 12 before the picture was taken, but I can't see them now, maybe you can.

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. The number of poops visible in this picture gives an idea of the population density, and the competition for food. 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. This picture shows leaf litter under one of the bushes, and some of larvae that fell off the branches and starved (some of them are only molted skins though).

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.)

I collected about 200 pupae off the bushes, gave some to the Albion grammar school, and kept 120.
Of the 120, 71 did not emerge.

    Of these 71:
  • 11 (obviously) were damaged from handling, or from wriggling so energetically that they broke. When the pupae are disturbed on the branch, they wriggle vigorously to intimidate predators that might be climbing up the bush. When many pupae shake at the same time, the whole bush rattles with a life of it's own like a rattlesnake.
  • 9 hatched out tachinid larvae.
  • 51 had unidentified ailments, due possibly to:
    • unseen injuries,
    • bacterial, viral, or fungal infection from moldy, soiled, or dusty food.
    • lack of vigor from inbreeding,
    • lack of food as a larva.
    The rest (49) flew away in the span of one week.

2006: the explosion blew away, and California Tortoise Shells are locally 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.

2008 June 8: There's no butterflies this year, I saw one hatched out chrysalis, and a few partially chewed leaves, no adults, eggs or caterpillars. The weather's been dry and the winter not bad, but the logging has returned, with lots of dust, noise, and uprooted ceanothus. The road is graded & the Orks have been expelled from the ruts, but there'll probably be nothing flying there till the logging ends again.

2009: may 18: Logging has moved down the ridge, I can hear it in the distance. Access to the skid roads has been blocked. Plenty of small caterpillars though. The small ones hang out in bunches & wads so they look more intimidating to birds

2010: June 12: Absolutely nothing, no adults, pupae, larvae, or chewed leaves. But it's been raining a few days a week for the last few months, and it's probably safe to guess that they dislike wet weather, because 2005 was was a bumper year here for N. californica, weather was dry and the roads were dusty in April. '07 was not as dry (as '05) but not rainy like 2010 and 2006. The logging appears to be finished for the present. A lot of the effort to block access to the skid roads with berms seems to have been wasted though, some of the berms cut off drainage so that ponds develop which flood the county road, other (berms) have simply been skirted by local citizens driving 4x4's through less dense thickets. I'm glad it's not my problem. I understand it's a good year for Red Admirals (Vanessa atalanta) in the eastern United States. I doubt if they like wet weather much more, but their host plant (nettle) does, and wherever it's abundant and other conditions conspire, local population explosions take place. The adults migrate northward like Painted Ladies (another Vanessa).

2011: June 22: Another long wet spring and again nothing. So I looked up this graph ( which shows california rainfall between 1999 and 2011. Local rainfall variations do occur though because Sonoma County (just south) reports a drought during 2007-2009 when the state total actually went up for 2008, so there should have been another explosion in 2009 except for the logging. It's possible that a number of consecutive dry seasons are necessary to create really big flights. The fantastic number of pupae seen in 2005 can probably be explained by the previous 3 year drought seen in the graph above.
More info: L.E. Gilbert. 1985. Ecological factors which influence migratory behavior in two butterflies of the semi-arid shrub lands of South Texas (.pdf, 25 pages, 2.3 MB). Short version: droughts reduce the number of predators, and rain reduces the number of butterflies.

2012: February 24: Rainfall has been about 1/2 normal for this time of year, and logging has ended, so I'm predicting a jump in numbers, probably not huge, but at least notable. Wait till around May if you want to check them out, the access gate on E. Navarro Ridge Road is still closed for winter.
2012: May 11: OK I got that wrong, it rained 3-4 days a week from March 1 through April. All the Ceanothus on Navarro Ridge looks great; healthy green & blooming, but nothing's eating it. --->
No Tortoise Shells.

2013: Mar. 20 and May 26: Nope

2014: Jan 8: I'm guessing again this will be another explosion year because there's only been a few days of rain since fall (our well is completely dry).
2014: June 17: Nope, only one branch had a few chewed leaves but no larvae. Other butterflies flying were: 4 Calif. Sisters, 3 Painted Ladies, 1 Buckeye, 1 Pine White, 1 Pale Swallowtail, within 15 minutes.

2015: May 10: Nothing but more logging --->.
But I found an American Chestnut tree growing on the ridge. Probably from a homestead. A grove was planted near Yorkville (about 30 miles away) around 1910 by some loggers who felt responsible to spread seedlings so they wouldn't become extinct from a blight that wiped them out east of the mississippi between 1880 and 1920.

2016: May 1: Same as last year but no logging yet. The Chestnut has dropped seeds and some have sprouted. I've heard Chestnuts will self pollinate under some conditions. During a visit in june several years ago, there were male flowers. Female flowers bloom later and need to overlap blooming to self pollinate, -OR- there might be more chestnut trees around, -OR- the logging company's forester might be pollinating it artificially, and he might have access to a Chestnut blight resistant strain for fertilization (lot'a maybe's).

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.
Host Plants: Shrub Rhamnaceae : Ceanothus thrysiflorus, integerrimus, velutinus, cordulatus, fenderi, sanguineus, cuneatus. (Legit: Butterflies of N. America, by James A Scott)

Here's a video of a different butterfly explosion in Oregon. Pine White (Neophasia menapia):

Raising Butterflies, YouTube

Why People Collect Butterflies, an Irish folk tale.