May–June 2024

Picky Pollinators

Many native Minnesota bee species depend on just one group of plants and sometimes a single plant. Bee surveyors are learning to find and catalog these specialized insects.

Emily Sohn

Before bee researcher Rachel Kranz left the Minnesota Department of Natural Resources central office in St. Paul on an early July morning in 2022, she checked the forecast, which was reassuring. It would be sunny and not too windy, with temperatures above 60 degrees: perfect for bee spotting.

Her destination was Savanna Portage State Park, about an hour west of Duluth, where she planned to look for a specific bee species, Dufourea novaeangliae. The species, a specialist that collects only the pollen of pickerelweed flowers to feed its larvae, hadn’t been spotted in Minnesota since 1934, when researchers observed it at a lake in the park. Kranz, along with intern Clara Brown, arrived at the same lake around 10 a.m.

When they got out of the car, Kranz says, they could see small specks flying around a shoreline that was dense with pickerelweed in full bloom—its distinctive tubes of lavender flowers topping bright green, leafy stalks that poked high out of the water. After scrambling to pull on hip waders, the two researchers forged into the thick of the pickerelweed, where they netted a few bees. They put them into vials and took a closer look.

The insects were tiny, just 1 centimeter long with elongated faces and long tongues, and they were darting around—all characteristic signs of Dufourea. Kranz knew it would take analysis in the lab to confirm, but she eagerly texted photos to several colleagues, who also thought the bees looked like Dufourea

“We were shaking in our waders,” Kranz says. “We were really excited to see them.”

It was an enthralling moment, not just because this was the first time anyone had seen Dufourea novaeangliae in Minnesota in some 90 years. The find also represented an expanding tally of bee diversity in the state, which has grown to include a large number of specialist, or oligolectic, species that, like Dufourea, depend on just one genus or group of plants—sometimes even a single species of plant when it’s the one that’s available locally. As part of a decade-long effort, Kranz and colleagues at the DNR and the University of Minnesota have so far counted 508 species of bees statewide. About a third of them are specialists. Some of these species have small or isolated ranges, but specialists live all over the state. 

Besides highlighting the often-overlooked variety of wildlife in Minnesota, the work has important conservation implications, says Nicole Gerjets, a bee biologist for the Minnesota DNR.

“There is a lot of talk about pollinator conservation and ‘Save the Bees’ and all this, but it’s really hard to know what actions to take if we don’t even know what species are here,” she says. “To know where they are and what plants they use will help us make some decisions on what areas we should protect, and how we can help these species in the future.”

A Big Bee Survey. If people think about bees, it’s usually honeybees or bumblebees that come to mind, Kranz says. With yellow and black stripes and stingers on their plump rear ends, these are the varieties that cartoon bees are modeled after. Honeybees, in particular, have gotten the bulk of bee publicity in recent years. Important for pollinating some agricultural crops, they have been threatened by a syndrome known as colony collapse disorder. But honeybees, which are not native to Minnesota, and bumblebees make up only a small portion of the world’s 20,000 bee species.

For a long time, nobody knew how many of those species lived in Minnesota. The first attempt to make a state list came in 1918, when entomologist Frederic Washburn compiled state data. He came up with a list of 66 species. And while he acknowledged that this was likely an undercount, it wasn’t until 2014 that biologists at the DNR’s Minnesota Biological Survey launched the first comprehensive effort to assess the state’s bees. The gap is likely because there aren’t enough trained entomologists and taxonomists to collect this kind of data on all sorts of insect species, says Jessica Petersen, an invertebrate ecologist at the DNR. 

They began with a look at specimens in the University of Minnesota Insect Collection, among other museum and academic collections from other states that included Minnesota bees, which generated a potential list of 427 species. Next came fieldwork to see which of those still live here and whether there were others that hadn’t yet been documented. On state-managed lands, the search took place each year, from May to October, starting in 2015.

The initial push to find bees was fruitful but also had limitations, Petersen says. To catch bees, the team was using both hand nets and bowl traps—colored bowls that attract bees to soapy water where they get stuck and die. But by about 2021, it had become clear that bowl traps were catching the same species again and again. From her previous experience with a bee species that specializes in pumpkins, Petersen knew that specialists could be so focused on finding their flowers that they often ignored passive capture techniques like bowl traps.

Concerned that they were also unnecessarily killing bees that had already been identified, she looked carefully at the species that still hadn’t been documented in Minnesota in at least 50 years, and she noticed something: Many of them were, like her pumpkin bees, specialists. That led to a shift. The team ditched the bowl traps in favor of hand-netting only, and they began organizing field visits to look for specialists.

Because these bees often live for just a few weeks as adults, buzz around for only a few hours a day, and live only in one part of the state, looking for them required careful planning—and a spirit of adventure. In the summer of 2023, Kranz and Gerjets climbed into a canoe on a lake in Minneapolis’ Theodore Wirth Regional Park and looked for shoreline-dwelling bees as a colleague paddled them slowly around.

“Nicole and I are on our knees in the front of the canoe, trying with shorter nets to grab these bees off of these floating plants, and trying not to tip the canoe or whack somebody else with our nets while swinging,” Kranz says. “It was definitely a new skill to learn.”

Zeroing in on Specialists. Catching bees was only part of the challenge. Identifying them can also be hard, says Zach Portman, a bee taxonomist in the University of Minnesota’s Department of Entomology who analyzes the specimens that DNR researchers collect in the field. Many of the insects are extremely small, requiring analysis by microscope. Some have never been studied, so there is little information to go on.

To distinguish one bee species from another, Portman says, he looks at subtle differences, like body shape, relative lengths of different segments, coloration, and reproductive structures. For many species there is no published information on how to identify them. Sometimes, he compares his samples with original specimens that a species was described from.

After years of driving, paddling, scoping, netting, and analyzing, the researchers published their findings in 2023 as a checklist of all species identified so far in Minnesota. They counted 508 in total. About 30 percent, 147 of them, were specialists. The finds included 23 species that had never been documented in Minnesota. The researchers also found some species that are known to be in trouble nationwide, such as the endangered rusty patched bumble bee.

Many of the newly identified specialists have unique life histories. Macropis steironematis, for example, collects oil from blooming loosestrife plants (Lysimachia), which it feeds to its young. This bee hadn’t been documented in the state since 1970 until the new push found it again. The search for Macropis turned up another surprise, Petersen says: a kleptoparasitic bee (genus Epeoloides) that feeds on the same flowers and targets Macropis nests.  The genus had never been seen in Minnesota before.

There may be more species yet to find. The report noted 38 species that were seen at least 50 years ago but haven’t shown up in recent searches, either because they’re gone or because they just haven’t been seen yet. As the work continues—with funding from the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources—the researchers are zeroing in on specialists.

One hope is to assess how they’re doing. Globally, scientists have estimated that 40 percent of bee species are at risk of extinction. With their dependence on specific plants, specialists are particularly vulnerable, and there are plenty of known threats, Portman says, including habitat loss, disease, and climate change.

But it is notoriously difficult to assess sizes of bee populations, and it’s an especially challenging task to monitor specialists, which are already hard to detect in the first place. Establishing a baseline is the first step toward protecting what’s there. “We don’t know how to conserve it if we don’t even know what it is,” Petersen says.

Be Bee Aware. Raising awareness is another goal. By sharing information about Minnesota’s native specialist bees, researchers want to inspire people to care about bees like they do about other creatures. Like birds, Kranz says, bees can be fun to watch, especially because different varieties show up on different species of flowers at different times of year. In spring, blueberries, willows, Jacob’s ladder, and leatherleaf attract one crop of bee specialists. By midsummer, another set flocks to asters, bee balms, and alumroots. Later in the season, goldenrods, sunflowers, and asters become prime targets.

Looking for bees can make a difference. Citizen scientists have already signed on to observe and report sightings of bees as part of the Minnesota Bee Atlas and a statewide Bumblebee Atlas project. Keeping an eye out for specialist bees could also aid scientists in their effort to understand these understudied creatures.

To give the state’s bees a helping hand, experts say, people can plant flowers in their yards to attract specialist bees, avoid spraying pesticides, and leave piles of leaves and sticks in untouched areas on the ground, where the insects often build their nests. (Contrary to common misconceptions, the researchers add, most bees don’t sting.) 

Watching for bees has a way of making people slow down and take notice of the world around them, Gerjets adds.

“Our main message is just, ‘Look. Look at these,’” Petersen says. “And, ‘Here are some really cool ones.’”