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I didn’t intend to raise butterflies when I planted the fennel. It was decorative enough, and figured in a few Italian recipes I had. It did entirely too well, propagating like crazy and muscling into the rest of the garden; and it tended to get woody and inedible before I harvested it. Weeding was complicated by its tenacious rootmass. But every year it produced a fine crop of anise swallowtails. 

The green-and-black-banded caterpillars showed up early in the summer, chomping away on the fennel plants. I don’t recall having seen the chrysalis stage, but at least some went through their metamorphosis in my garden: I’d see freshly emerged adults clinging to the living room windowsill while their black-and-yellow wings dried and expanded. Then they’d be off in search of mates, which is pretty much the whole agenda of a butterfly’s brief airborne existence. 

Hilltops are an anise swallowtail’s equivalent of singles bars. Males claim a likely spot and wait for females to wander by. They’re picky about their location; some have found their way back to their preferred summit from up to three miles away after being experimentally transferred by entomologists. “Hilltopping” males pursue passing females but ignore other males, who have a distinctive perfume-like smell. 

After mating, the females head for the nearest fennel patch to lay their round, cream-colored eggs. There are lots of those patches to choose from. A Mediterranean native, fennel (also called finocchio, like the late North Beach club) came to California with Italian kitchen gardeners and found the climate congenial. You’ll see it in head-high stands in vacant lots and on open slopes and roadsides. 

What did the swallowtail larvae do for food before there was fennel? They ate its wild relatives, plants called umbellifers in the family that includes parsley, celery and carrots: plants like cow parsnip, lovage, rangers button, yampah and lomatium. The native umbellifers grew in a variety of habitats, and there wasn’t a lot of competition for them from other plant eaters. 

In the ancient arms race between plants and insects, the umbellifers evolved chemical defenses. The plants contain toxins called furanocoumarins, lethal to most insects. But a few species of swallowtail butterflies developed an enzyme that detoxifies the chemicals. Other substances in the plants actually stimulate the caterpillars’ appetites: a drop of anisic aldehyde will give them the raging munchies. 

So when fennel came along, it tasted right and the butterflies switched to the new host. Cultivated fennel has less of a chemical load than wild umbellifers, and the caterpillars, not having to counter the toxins, grew up to 25 percent faster. And the rapid spread of fennel gave the swallowtail access to new habitats, like urban gardens and weed patches. It’s one of the rare cases of a native species benefiting from changes we’ve made to the environment. 

There’s a downside, though. To a caterpillar, the anisic aldehyde in a fennel leaf and the methyl chavicol in a citrus leaf taste just about the same. Anise swallowtails hopped from fennel patch to fennel patch in the coastal lowlands until they hit the orange groves of the San Joaquin Valley. Here was a bonanza of tasty stuff. Although they haven’t reached plague proportions, swallowtail caterpillars are now considered a pest by citrus growers.  

In the Bay Area there’s definitely fennel to spare, so we can enjoy these big showy butterflies without worrying about their economic impact (unless there’s a Meyer lemon or other hardy citrus in your yard). I’m not in the swallowtail business any more, having moved away from my ineradicable fennel. And the butterflies haven’t found my kaffir lime tree yet.