... if you want to know which flowers are best to help bumble bees
A male yellow head bumble bee, Bombus flavifrons, feeds on white heath aster in Montana during autumn." (Photo by Clay Bolt)
IN A FIELD OF WILDFLOWERS just outside Baltimore, Maryland, a female common eastern bumble bee makes her way methodically from bloom to bloom around a bright yellow clump of cup plant in late August. Once the pollen baskets on her back legs are filled, she flies off to deliver the food to her colony nestled nearby in an abandoned underground rodent hole. She then sets off on a new foraging foray.
It’s a summer scene that’s been playing out much the same way for millennia, but with one significant difference: Stationed not far from the cup plant, a clipboard-toting volunteer naturalist carefully records the bumble bee’s activity, focusing on each kind of plant she visits—and each kind she does not visit—throughout this 3-acre meadow housing dozens of different flowering plant species. That evening, the volunteer will enter his observations into the database of Ask A Bumble Bee, a community science project launched last spring by the U.S. Geological Survey’s Wild Bee Lab. The survey will be one of more than 800 similar surveys conducted by more than 100 volunteers who fanned out across 13 mid-Atlantic and northeastern states and the District of Columbia from June through late October 2022.
Funded by the U.S. Fish and Wildlife Service, Ask A Bumble Bee was the brainchild of biologist Sam Droege, who heads the federal bee lab. Once the data from all surveys are analyzed, he says—by the start of this year’s planting season—“we will be able to say what the favorite bumble bee plants are and, more importantly, what they are not.” He adds that results from a 2021 pilot project “already indicate mismatches between lists of plant species recommended for bumble bees and what plants they actually use.”
Minimizing such mismatches is important, Droege says, because bumble bees need all the help they can get. In the past two decades, scientists who monitor the insects—critical pollinators of both crops and native plants—have reported troubling declines, including five once-common and widespread North American species (the western, rusty patched, yellow-banded, American and Franklin’s bumble bee) that have vanished from large parts of their ranges. A report published by the Xerces Society for Invertebrate Conservation and the International Union for Conservation of Nature (IUCN) concludes that 28 percent of the continent’s nearly 50 bumble bee species “face some level of extinction risk.”
The causes of bumble bee declines are many and complex, ranging from habitat loss and pesticides (including backyard mosquito spraying) to climate change and introduced pathogens—“a classic case of death by a thousand cuts,” says Rich Hatfield, senior conservation biologist for the Xerces Society. While a typical homeowner may feel powerless in the face of many of these threats, Droege says anyone with a yard, patio or other outdoor space can provide bumble bee habitat. “Bees are not bison,” he says. “They can do perfectly well in a small backyard as long as they have good, pesticide-free habitat.”
Because bumble bees rely exclusively on flowering plants for food—with adults consuming nectar for energy and feeding their larvae pollen for protein—providing such plants is key to good habitat. But which plants? According to Droege, most planting guides for the insects rely on photographs that show bumble bees feeding on a particular kind of plant—a methodology he calls “not very scientific” because the bees are generalist feeders that will gather just about any kind of nectar and pollen to feed their hungry broods if it’s the only food available.
To really help the insects, Droege says we need to know which plants they prefer. Ask A Bumble Bee is revealing those floral preferences by comparing all plants in all survey sites head-to-head with one another. “It’s just as important to find out which flowers bumble bees are ignoring,” he says.
According to Jenan El-Hifnawi, the project’s coordinator, last year’s participants turned in a total of 820 surveys conducted from Maine south to Virginia. Though she and Droege are still analyzing the data, they’ve already seen surprises. Cup plant, for example, is a native species in the family Asteraceae that’s not usually included on bumble bee plant lists, and many wildlife gardeners are unfamiliar with its flowers. Yet cup plant looks like it will rank high on the project’s final list of flowers most favored by the bees.
By contrast, gardeners frequently plant black-eyed and brown-eyed Susans for bumble bees, but the insects tend to shun these plants when better alternatives are available. Similarly, white clover, a nonnative that sprouts spontaneously in suburban yards, has long been considered among the bees’ springtime favorites—one reason for the recent popularity of “No Mow May.” Yet “we didn’t see visitation to white clover in spring, though we did see it in fall,” says El-Hifnawi.
The project’s final list of favorite bumble bee plants should be “straightforward and useful to any homeowner or land manager with an interest in supporting the insects,” she says. In addition to general recommendations, El-Hifnawi and Droege plan to break down floral preference by bumble bee species. “We know that morphologically there are significant differences between species, some with longer faces or tongues, for example, that presumably prefer flowers with longer corollas,” she says. “But that’s not something that’s been super well explored.” Such species preferences matter particularly in the case of bumble bees that are threatened or declining, El-Hifnawi adds. “We want to inform people on how best to target those species.”
That includes people such as refuge managers who are responsible for conserving and restoring large tracts of land. Though El-Hifnawi hopes the project’s recommendations will be helpful to backyard gardeners, “our ultimate goal is to get the list to those kinds of land managers,” she says. Hatfield agrees that “it is how we manage our large rural landscapes that will ultimately decide the fate of North America’s most vulnerable bumble bees.”
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