Pipevine Swallowtail Butterfly – Source: Public Domain, Wikimedia Commons

Each year, in June or July, a group of volunteers goes out and counts butterflies.

This year’s butterfly count in San Francisco was on June 8th, and yielded 751 individual butterflies, of which 703 were identified by species. (The spotters ID’d the remaining  48 only by family.) It’s down quite a bit from last year and the year before. It added one butterfly not recorded here before: the Rural Skipper.

But the real surprise was the butterfly that topped the list.  This is the first year since 2010 (when we started following the San Francisco Butterfly count) that Cabbage Whites did not top the list.

Instead, they were overtaken by the Pipevine Swallowtails from Angel Island, where a count of 140 butterflies of this species added to the 15 spotted on the mainland to bring the total to 155.

THE TOP THREE BUTTERFLY SPECIES

Cabbage White sitting on Oxalis

The top three butterflies of 2013 were the Pipevine Swallowtail, the Cabbage White, and the Echo Blue, AKA the Spring Azure, and they accounted for just over half the identified butterflies. (There were a few that were id’d by family but not species.)

• Last year, 2012, it was Cabbage White in first place, with the California Common Ringlet and the Sandhill Skipper tying for second place, and the Common Buckeye in third place.
• In 2011, the Cabbage White, Anise Swallowtail, and Echo Blue.
• In 2010, the Cabbage White, the Umber Skipper, and the Anise Swallowtail.

Echo Blue Butterfly. Source: Katja Schultz, Wikimedia Creative Commons

• The top three butterflies usually account for around half of the butterflies identified. This year, it was 50%. In previous years, it’s varied between 44% and 58%.

• The ten most seen species accounted for 85% of the total butterflies spotted, around the same as in previous years, when it was 82-85%

WEATHER AND BUTTERFLIES

This is an annual count, and it’s nationwide. The North American Butterfly Association (NABA) sponsors a “July 4th” butterfly count; it’s a count of all the species volunteers spot within a 15-mile radius on a particular day in the 4 weeks before or after July 4th. In San Francisco, it’s usually organized by Liam O’Brien who keeps the www.sfbutterfly.com website and blog.

The problem for San Francisco is that June and July weather is notoriously variable, largely because the fog can roll in at any time and create completely different weather conditions. This year’s count on June 8th took place over a month earlier than last year’s Count That Nearly Wasn’t (July 24th). The weather started out warm, but the fog arrived around noon, and that would have affected the numbers.

In 2010, it was held on June 7th on a foggy day; in 2011, it was held on July 3rd, a month later and the weather was bright and sunny. In 2012, it was foggy but clearing to sunshine on July 24th. As the timing and weather changes, so do the butterflies seen. 

The number of people seem to matter less… there’ve been only 16 people in 2012 and 2011, compared with 34 in the two earlier years. But the number of butterflies spotted didn’t seem directly correlated. Neither did the number of parties – 9 this year and last year, 10 in 2011, and 14 in 2010.

The other factor that could affect the butterflies is, of course, the preceding rainy season. If it’s too dry for their favored plants to do well, fewer eggs are likely to make it through the larval stage and pupation.

THE PIPEVINE SWALLOWTAIL

Pipevine Swallowtail Butterfly – Source: Wikimedia Commons, Public Domain

According to the wonderful butterfly website from UC Davis’s Professor Art Shapiro, the pipevine butterfly has an interesting relationship with its nursery plant, a vine called Dutchman’s Pipe, (Aristolochia californica). The plant contains aristolochic acid, which makes the caterpillars poisonous, so they don’t get eaten.

But it also means that the little caterpillars have to eat the youngest and most tender leaves that don’t have enough of the stuff to kill them. The larger leaves are okay for the larger caterpillars, but only in small doses; as they eat it, the plant responds by increasing the aristolochic acid content, so the larvae are forced to move on to other leaves. They sample all the leaves, but don’t eat enough to kill the plant. (They can, however, eat so many of the seeds that the plant fails to reproduce that year.) The adult butterflies nectar on a variety of flowers, but they apparently only reproduce on aristolochia plants.

DETAILED RESULTS

If you want to read back, the articles for previous years are here:

Click here for: Article on the 2012 San Francisco Butterfly Count results.

Click for: Article on the 2011 San Francisco Butterfly Count results.

Click here for the article on the 2010 San Francisco Butterfly Count results.