Columns

Wild Neighbors: Matters of Taste

By Joe Eaton
Tuesday July 05, 2011 - 09:04:00 AM
Anise swallowtail: a fennel specialist.
Eugene Zelenko (Wikimedia Commons)
Anise swallowtail: a fennel specialist.
Western tiger swallowtail: a generalist.
Mila Zinkova (Wikimedia Commons)
Western tiger swallowtail: a generalist.

On our way to a Fourth of July block party this afternoon, Ron and I witnessed a mass emergence of gulf fritillaries. Ten or a dozen of them were fluttering around a passionvine-covered fence, getting accustomed to their wings. Some checked out the passionvine flowers, but with minimal interest. One had worked its way a few doors down the street and was filling up at a buddleia. By the time we passed that corner again, a couple of hours later, the butterflies had all dispersed. 

Gulf fritillaries—not true fritillaries, they’re members of a mostly tropical American family—are obligate pipevine specialists. Adults can nectar wherever they feel like; larvae require pipevine leaves. They sequester the plant’s toxins, as monarchs do with milkweed. The adults’ orange wings are a badge of unpalatability. Originally native to the Gulf Coast, gulf fritillaries followed ornamental pipevine plantings west to California, where they are now one of the most common urban butterflies. 

In terms of larval food plants, butterflies and moths run the gamut from narrow specialists to broad generalists—sometimes in the same genus or family. Their feeding preferences represent various outcomes of what James Scott (The Butterflies of North America) called “a perpetual ‘war’ between insects and plants.” As plants evolved chemical defenses against leaf-eating insects, butterflies and other insects countered with enzymes that detoxify those chemicals, and other adaptations. Scott again: “Few insect species can detoxify or tolerate the numerous plant poisons residing in a great array of plants; and because most insects can detoxify only a few poisons, most eat only a few plants—generally plants that are closely related.” 

The swallowtail family is a case in point. The blue-black pipevine swallowtail uses only pipevines, either the native California pipevine or exotic cultivars. Again, the larva acquires a chemical defense—aristolochic acid in this case—from the host plant. 

Anise swallowtails, patterned in black and yellow, rely on plants in the celery/carrot/parsley family that contain essential oils like anisic aldehyde. UC Davis entomologist Arthur Shapiro writes in his Field Guide to Butterflies of the San Francisco Bay and Sacramento Valley Regions that the larvae will eat filter paper if it’s coated with the proper oil. Formerly, they fed on native plants like yampah, biscuitroot, and water hemlock. But most populations have switched to two exotic species, sweet fennel and poison hemlock. Pierre Lorquin, who collected butterflies in California during the Gold Rush, noted that the swallowtails were already eating fennel. The larvae also have a taste for citrus leaves, but their economic impact has been limited. 

A third member of the family, the pale swallowtail, favors coffeeberry and ceanothus. According to Shapiro, females will sometimes lay their eggs on white sweet clover, which somehow smells right. Their offspring won’t eat it, though. 

The western tiger swallowtail is more eclectic in its diet, which includes sycamore, ash, cottonwood, willow, cherry, peach, almond, privet, lilac, and sweet gum. Adult western tigers are regulars in my back yard, and I keep wondering where they’ve parked their larvae. We’re surrounded by ashes and sweet gums, but there aren’t that many in the immediate block. 

Lately I’ve been seeing some kind of blue in the yard: Vladimir Nabokov’s favorite butterflies, although the species that was named for him is, inevitably, a satyr. This blue flies high and fast, and never sits down. Blues in general prefer plants in the legume family: lupine, lotus, vetch, locoweed. Notable exceptions are the western pygmy blue (goosefoot, pickleweed, Russian thistle) and the Sonora blue (dudleyas.) Lupines are not locally common. Maybe the blue’s larvae are on my neighbor’s sweet peas. 

Some blues, including the Eurasian large blue, have diverged from the typical lepidopteran vegetarian habit. Their larvae, tended by ants as a source of honeydew, are sometimes taken into the anthill where they repay their hostesses’ hospitality by eating the ant grubs. Caterpillars of the harvester, a small black-and-orange eastern butterfly, prey on wooly aphids. And of course there are the notorious killer inchworms of Hawai’i, moth larvae that have become ambush predators. It’s a strange world. 

Red admirals—a name Nabokov was cranky about, preferring the traditional ‘red admirable’—are regulars in the yard, likely because we’ve refrained from weeding out all the pellitory, their preferred host plant. These butterflies will also eat stinging nettles, which must have been their primary food before pellitory was introduced from Europe, as well as baby’s tears. 

Then there’s the generalist’s generalist, the painted lady. A concise version of its menu would include fiddleneck, borage, comfrey, thistle (including artichoke and cardoon), sunflower, plantain, nettle, lupine, sweet pea, mallow, and checkerbloom. Females never lay their eggs on woody plants like tree mallows, but the larvae will happily consume their leaves. 

We do our best to provide caterpillar chow for whatever is around. It’s a crapshoot; the pipevine swallowtails have ignored our offerings for years. Even so, we just planted a passionvine out front. Call it an act of faith.