One of the fun things about regularly tending the same garden plot throughout the year is seeing the cycles of insect life that thrive on our vegetables and other plants. Of course, not all of these creatures are welcome, as anyone whose fava plants have been smothered with black bean aphids can attest. But most of us see the beauty in our native butterflies and are curious about the occasional strange moth we encounter.
We seem to get quite a range of butterflies and moths visiting the garden, many of which lay their eggs here, resulting in the caterpillars we find munching on our plants. Possibly, the garden’s location between the park and Mt Sutro makes it easy for insects to stumble upon it in their travels. And certainly the broad range of San Francisco native plants that Greg Gaar has planted in the sidewalk beds and nearby at Kezar Triangle make the area attractive for native insects.
The most common butterfly in our garden, as in many others, is the non-native cabbage white (Pieris rapae), which is a global pest of brassicas (mustard family plants like cabbage, kale, collards and broccoli). It spread rapidly across North America in the 19th century, apparently after being introduced to Québec around 1860. [Note: The
two photos here are from Wikipedia, as for some reason cabbage whites haven’t inspired me to pull out a camera or smartphone.]
If you’ve found your kale plants shredded and little bluish-green caterpillars hiding on the undersides of the leaves (or the much chubbier later instars), then this is the work of cabbage whites.
But the cabbage white is the exception. The vast majority of butterflies and moths in San Francisco are native species that are not especially interested in eating our vegetables. In fact, many species have a very limited range of “host plants” on which they will lay their eggs and which will be consumed by their larvae (i.e. caterpillars).
A great resource to identify these other species is Butterflies of San Francisco, a $5 fold-out guide produced by Liam O’Brien for Nature in the City, and available from their web store. Liam’s guide details the 36 species that have been found in SF, along with their host plants, the months they are active and, most importantly, Liam’s life-size illustrations of each one.
So what else might you see around our garden? Well, Swallowtail butterflies are the largest and probably most dramatic visitors, cruising the plots in mid-summer. The western tiger swallowtail (Papilio rutulus) above was feeding on a Dahlia in the garden in July 2015. The related anise swallowtail (Papilio zeliacon) is common on hilltops, such as Twin Peaks, but also appears in the garden from time to time. Here’s one from Kezar Gardens (precursor to the current garden) in August 2012.
Both species have fearsome-looking caterpillars. Here are a couple of color variations of anise swallowtail caterpillars on leaves at Kezar Gardens in October 2012.
While the anise and tiger swallowtails have largely yellow wings, their cousin the pipevine swallowtail (Battus philenor) has iridescent blue-black wings. This rare species feeds only on the uncommon Dutchman’s pipe vine (Aristolochia californica). The best place to see pipevine swallowtails locally is at the park’s botanical garden.
Other showy butterflies appear to like dahlias, as evidenced by this monarch (Danaus plexippus) in September 2015. Monarchs are one of a few butterfly species that spend the winter in San Francisco—many others just come here to feed and breed in the spring and summer. The monarchs that spend the winter in California come from across the western states and as far away as British Columbia in Canada. One monarch tagged and released in Corvallis, Oregon, on August 30 this year turned up two-and-a-half weeks later at a rooftop garden in North Beach.
San Francisco’s smaller butterflies also stop by the garden at times. This is a gray hairstreak (Strymon melinus) seen on a sunflower at the garden in July 2014. The gray hairstreak is relatively common, unlike its cousin the green hairstreak (Callophrys dumetorum) which is restricted to coastal hills and dunes. The butterfly was thought to have been extirpated from San Francisco until it was rediscovered by Liam O’Brien in April 2006. In the 10 years since then, the Green Hairstreak Project has been working to secure and restore habitat for this butterfly in the Golden Gate Heights area, from Grandview Park south to Hawk Hill. Green hairstreak caterpillars feed only on coast buckwheat (Eriogonum latifolium) and common deerweed (Acmispon glaber).
Getting close enough to a stationary butterfly or moth to identify it or even take a photo can be a real challenge. That’s not the case with caterpillars. They’re slow enough to allow careful focusing, which allows you to try to identify them later with the benefit of all the resources that the web provides.
Here, for example, is a caterpillar of the American painted lady butterfly (Vanessa virginiensis) which turned up on the compost bins earlier in September 2016. Its host plant is reportedly western pearly everlasting (Anaphalis margaritacea), which I don’t think I’ve seen around our garden.
Caterpillars, like many other insect larvae, can be very tasty snacks for local birds to feed their growing fledglings. In general, evolution has pushed butterfly and moth species down two routes to ensure the survival of enough caterpillars to form the next generation of adults. One path is to evolve defenses, such as poisonous compounds and irritating hairs, and then advertise these dangers through outlandish color schemes, such as the painted lady caterpillar above.
The alternative is to use camouflage techniques to appear as inconspicuous as possible. That’s clearly the tactic employed by the cabbage white, and it’s also used by quite a few other caterpillars that are indistinguishable from leaves and twigs without close inspection. Here, for example, is an unknown moth caterpillar from the family Geometridae (inch worms) that appeared on a plant of farewell to spring (Clarkia rubicunda) in August 2015.
In case you don’t see a caterpillar in the photos, it’s the reddish green “stick” running from bottom-left to top-right of each frame. The plant had about half a dozen of these caterpillars, which looked almost identical to the maturing flower buds. I was intrigued enough to leave them on the plant for a week, by which time they had stripped most of the foliage. Ironically, with the plant much less leafy, the caterpillars’ camouflage was less effective and they were swiftly picked off by the birds.
Identifying caterpillars, butterflies and moths (or any organism) can be quite a challenge. Here’s a couple that turned up recently for which I need to find IDs.
In addition to Liam O’Brien’s butterfly guide and the many excellent Internet sites, another approach is to upload your findings to iNaturalist, via their smartphone apps or website. iNaturalist can be used by anyone interested in wildlife, from those just starting out to knowledgable amateurs and professionals. It automatically captures the location of your wildlife photo and guides you through providing as much identifying detail as you have. Then other members can volunteer their identifications and help you discover exactly what type of bug, beast or plant you encountered.
You can choose to add your observations to one or more projects, such as San Francisco Community Gardens. iNaturalist also helps validate your hunches as well. You can search just for certain organisms within a particular geographic area, like butterflies and moths in San Francisco, to see whether what you found is something others also reported.
The team that developed the app is now based in the park at the California Academy of Sciences, so there’s a very good chance that someone will help ID your observations from the garden.
If you’ve discovered other butterflies, moths or their caterpillars in the garden, please respond in the comments. And if you have pictures, we’d love to feature them in a follow-up post.