How to make a city welcoming to butterflies

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It’s a rare clear, sunny May day in Golden Gate Park, 65 degrees by ten o’clock in the morning – a promising prospect for spotting butterflies at Strawberry Hill, a designated urban butterfly habitat in the middle of Stow Lake.

A group of garden volunteers walk up the hill following Andy Stone, supervisor of the parks section of the San Francisco Department of Recreation and Parks. He outlines what needs to be done for the day: pruning the Cape ivy so that a few young oaks growing on the hillside can breathe a little, pulling up the brome along the hiking trail and planting new ones. flowers for the butterflies and bees that frequently visit the hill.

“Gardening is just an illusion of control. It’s kind of like living that way,” says Stone, who also oversees the volunteer gardening day at Strawberry Hill. Stone has been a gardener at Golden Gate Park for 35 On Strawberry Hill Volunteer Day, the second Saturday of every month, rain or shine, he moves quickly around the plants, picking the edible parts and leaves of mallow, radish and a plant in the broccoli family. He urges volunteers of all ages to try some. While Stone is an impressive flora expert, he admits his identification of butterflies isn’t up to scratch.

“Ninety percent of the butterflies become bird food,” Stone says, but he concludes he’s seeing more butterflies this season than last. “Probably because we plant more things they like.”

Golden Gate Park gardener Andy Stone leads a group of volunteers for the restoration of Strawberry Hill in May 2022. (Photo by Carly Nairn)

And what do butterflies like? It depends, says Arthur Shapiro, a lepidopterist and professor of evolution and ecology at the University of California, Davis. Variables include species, life stage, time of year, and where nectar is available. Whether or not a pupa or adult overwinters in a particular location. Monarchs are well known for preferring milkweed, while pipevine plants host their namesake pipevine swallowtails.

Sun exposure for a significant portion of the day is mandatory, Shapiro says, a requirement sometimes difficult to meet in Strawberry Hill, with San Francisco’s ubiquitous summer fog. Yet the butterflies still decide to visit the habitat, due to its abundance of flora throughout the seasons and its dedicated stewards who, over generations, have made Strawberry Hill, an island in the mist, a paradise for the butterflies.

It’s also a matter of biology, notes Shapiro – when collecting nectar, an essential food source, an adult butterfly needs a platform on a flower or plant to place its feet on and have the right dimensions to insert its proboscis into a flower’s nectar source. So, for example, “Yarrow is appropriate for streaks of hair, but does nothing for painted ladies and monarchs,” Shapiro says.

Certain flora give a variety of butterflies a good meal, or a good place to lay eggs or shelter, such as lupine, some species of asters and goldenrod, and pennyroyal. For urban butterfly habitats, whether it’s a community garden in Oakland, a garden in San Ramon, or a public park, like Strawberry Hill, the goal is to “plant smartly to provide them with the resources they need,” says Shapiro.

Within the group of people dedicated to planting flowers to support pollinators, there is a long-standing debate over which species to favor.

Stone prefers to plant native California plants, when possible, in Strawberry Hill. Matthew Shepherd, director of outreach and education for the Xerces Society for Invertebrate Conservation, a nonprofit pollinator advocacy organization, agrees. “Native plants support the caterpillars,” says Shepherd. “All you have to think about are the native plants.”

pipevine swallowtail caterpillar
A caterpillar of the pipevine swallowtail (Beaten philenor) does quick work on a Dutchman’s pipe sheet (Aristolochia macrophyllum) at Strawberry Hill in San Francisco. (Photo by Carly Nairn)

Shapiro has often said otherwise. He advocates what he calls a “what works” approach to butterfly habitats. Shapiro says he has seen butterflies and plants continually evolve to change their relationships with each other, allowing butterflies to find new hosts on non-native plants. And that’s not necessarily a bad thing. “I’m sick of being told to use natives,” he says, “especially if a butterfly isn’t more of a use for it than a fire hydrant.”

In some cases, Shapiro says, native plants did not recover, and the more common non-native plants that replaced them became essential to butterfly health. yellow star thistle (Centaurea solstitialis) is native to the Mediterranean and poisonous to horses, but also provides a much-needed source of nectar for butterflies in summer. Although generally considered a widespread noxious weed, some butterflies have come to depend on it, due to its hardiness and long flowering season, Shapiro says. “You shouldn’t limit yourself to native people,” he says. “Personally, I think [planting nonnatives] Is it worth it. »

California wildfires in recent years have accelerated “chemical-mediated” evolutionary adaptation to some non-native species when a butterfly’s native plant host is not available. For example, the butterfly bush (Buddleia davidi) is not native to California, but butterflies are attracted there because a chemical receptor it emits triggers egg laying. After a few generations, butterflies will spontaneously lay eggs on it. “There is a better life through chemistry,” Shapiro says.

Strawberry Hill is one of many urban butterfly habitats that tick many variable boxes for a robust butterfly population, where many species live out their lives. There can be many different “hotspots” in the same urban area, but a few factors are critical.

The place should support the four life stages of a butterfly – egg, larva, chrysalis and adult. And what may be a butterfly’s host plant for eggs is not the same one it uses for foraging. Equally important is shelter from the cold and a safe, undisturbed place for the pupa. In addition, there must be free air circulation for the butterflies to fly. While the streets can act as a wind tunnel, a mass-migrating species needs clear skies. Finally, the site must be free of pesticides, which makes many urban pollinator habitats unique compared to garden plots or farmland.

“San Francisco is ahead of the curve in terms of pesticide use,” Shepherd says. In most public parks in San Francisco, pesticides are not used, especially insecticides based on neonicotinoids, chemicals that cause flight and taste problems for pollinators. Shepherd says many public park districts are moving away from pesticide use altogether.

However, what Shepherd says he finds most rewarding is transforming attitudes towards protecting pollinators and the places they call home. The norm for a large public park with lots of green space and pollinator support no longer feels like an Olmsted Brothers project, with mowed grass and well-distributed trees. Instead, says Shepherd, the city’s parks department deliberately creates meadows of wildflowers that can look a little messy.

At Strawberry Hill, volunteers are told to plant new flowers wherever there is room for them, once the weeds have been pulled out. “The wheel is turning and people are looking to rebuild the ecological value of spaces,” says Shepherd.

The increased awareness of the importance of pollinators, within ecosystems and the food web, has not gone unnoticed at the park district level. Due to a wave of public engagement, Shepherd says, park services are hiring people whose studies included supporting pollinators and are able to create change at the park district level.

“It’s exciting to see it growing,” he says. “Climate change is a motivating factor for some people. [It’s a] push for change.

After a few hours of pulling weeds and trimming ivy, the Strawberry Hill volunteers head out for the day, while Stone continues his endless work. Beside him, a West Coast butterfly lady gently flutters from one seaside daisy to the next. It’s having a lazy day at home.

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