On a sweltering summer day in Madison, Wisconsin, Jade Kochanski wades through a prairie bursting with wildflowers. Insect net in hand, she carefully scans the colorful blossoms. She’s on a mission to capture and identify the bumblebees that live there. If she’s lucky, she’ll discover a rare find—an endangered rusty patched bumblebee, named for the red-brown smudge that workers and males of the species sport on their backs.
Kochanski, a PhD student in integrative biology at the University of Wisconsin-Madison, meticulously measures the bumblebees she captures and snips off a tiny piece of leg, a common, non-lethal way to obtain genetic material from these creatures. Each sample she collects from rusty patched bumblebees is destined for a lab in Utah to help researchers uncover the genetic secrets critical to the bee’s survival. DNA analysis will help scientists estimate the number of colonies, identify patterns of inbreeding within those colonies, and understand overall genetic diversity and health of the rusty patched bumblebee populations.
Once common across the eastern United States and Canada, rusty patched bumblebee (Bombus affinis) populations have plummeted by nearly 90 percent over two decades due to habitat loss, pesticide use, pathogens and climate change, according to the U.S. Fish and Wildlife Service. Now, they are found in only a fraction of their former range, with scattered populations in the Upper Midwest and Northeast United States. In 2017, the rusty patch became the first bumblebee to be listed as endangered in the United States.
In response to listing, Kochanski joined a network of scientists who began collecting tissue samples from rusty patched bumblebees across its range. The initiative started in 2020 when Tamara Smith, the U.S. Fish and Wildlife Service’s lead biologist for the rusty patched bumblebee, organized a meeting with bumblebee researchers in Minnesota. Their goal: to identify conservation tools that could help save the species. “Genetics was the No. 1 question that came out of that [meeting],” says Smith. “We needed to get a handle on what was going on with this species if we wanted to move forward with some of the other tools that would help with conservation.”
Their research was published this past spring in the Journal of Insect Science and provides the first range-wide genetic study on the species. The findings offer key insights that can aid conservation of the species, but also raise questions and concerns about the ability of the species to recover. “Genetically, they’re not doing as well as we had hoped,” says Smith.
The study revealed a surprisingly low number of rusty patched bumblebee colonies, even in places where the bee seems prevalent. “We’ve seen the Upper Midwest as the stronghold of the species,” says John Mola, an ecologist at Colorado State University and lead study author, “but what we’ve seen from the genetic data is that even within these strongholds for the species, they are still far fewer colonies than we might have expected.”
Since only queens produce offspring, bumblebee populations are measured by colonies, not individuals. Each spring, queen bumblebees emerge from hibernation to create nests where they produce non-reproductive female workers that forage, nurse larvae and defend the nest. Later in the summer, queens switch to producing reproductive individuals, including males and new queens, which will form their own colonies the following year. Each colony can contain over 100 individuals. Thus, even if several worker bees are seen in an area, they could all come from just one or two large colonies.
According to the study, having few colonies poses significant risks, including making them vulnerable to local extinction from unpredictable events like fires on the prairies where they live. In light of the study, land managers will need to be more strategic with using prescribed burns for managing prairies within the bee’s range. To assist these efforts, as part of her dissertation, Kochanski is developing burn recommendations that balance the need for prairie maintenance with protecting bumblebees.
Few colonies also mean that removing even a single queen from an area could lead to a local extinction, the study reports. That finding has clear implications for future captive breeding programs. While breeding endangered species in captivity can help boost their numbers, increase genetic diversity and facilitate reintroductions, removing rusty patched bumblebee queens from the wild is risky due to their incredibly low numbers. Nevertheless, Smith emphasizes that captive breeding for the species is still an option if protocols are established. “While there may be risks with taking a queen out of a population, you have to look at what that might mean in terms of a benefit to the species,” she says. Since survival rates can be low in the wild, even a slight increase in survival in captivity can make breeding a viable option, she explains.
The study also found a clear sign of inbreeding in the bumblebees, which could make them more susceptible to diseases and environmental change. Genetic analyses revealed that 15 percent of males were diploid with two sets of chromosomes. Male bumblebees are typically haploid with one set of chromosomes, while females are diploid with two sets, Mola explains. A queen lays fertilized eggs for females and unfertilized eggs for males. However, insufficient genetic diversity caused by inbreeding can result in fertilized eggs intended to be females developing into diploid males instead, he notes.
Though the level of inbreeding observed in the bees seems high, the researchers stress that it is unclear whether this is abnormal. “It is interesting to know that inbreeding is happening,” says Jonathan Koch, a research entomologist at the U.S. Department of Agriculture and senior study author, but “is this something that is alarming? Because honestly, we don’t quite know yet.”
Mola says that while 15 percent feels significant, “How that compares to other species or other populations, we’re not really sure, because despite knowing this is a good way of estimating inbreeding in bumblebees, it’s not widely done.”
To find out whether the rate is abnormal, Mola, Koch and colleagues are using museum specimens collected before the 1990s to identify inbreeding rates when the bee’s numbers were higher.
Genetic analyses also identified three distinct populations of rusty patched bumblebees—in Minnesota; the central Midwest (Wisconsin, Illinois and Iowa); and the Appalachians. Koch says this is a critical finding and somewhat surprising since studies on other bumblebee species in eastern North America have found little genetic differentiation across populations. This knowledge will impact how the species is managed, says Jay Watson, a conservation biologist at the Wisconsin Department of Natural Resources who works with the imperiled species. “We can’t just rear bumblebees in Wisconsin and ship them to other parts of the range,” he explains.
Rich Hatfield, a senior endangered species conservation biologist at the Xerces Society for Invertebrate Conservation who was not involved with the study, hopes the information generated will influence the approval of future development projects that could harm rusty patched bumblebees. He notes that in the past, some development projects were approved despite potentially harming rusty patched bumblebee colonies, which, based on this study, may have threatened the species’ long-term survival. We now know that every colony of the species is important to maintain genetic diversity, he says. “Before this paper, we were sort of hand-waving and suggesting this might be the case.”
While the study generated valuable information to inform policy and management decisions, it is just a first step, says Koch. He and colleagues have begun to use new genetic markers that allow for more fine-scale evaluation of rusty patched bumblebee populations, which will help determine how long the species’ decline has been occurring. They have also started long-term monitoring of the species at nine sites across its range, which will provide a better understanding of how the species’ demographics change over time.
Although the rusty patched bumblebee has become a poster child for pollinator conservation and attracted significant interest from scientists and the public, Smith notes that the species still needs substantial help. “What we’re seeing right now, with the information we have, [their populations] are either stable or going down,” she says. “They’re definitely not going up.”
Despite the challenges, Smith remains hopeful. She has seen other species in worse shape make significant progress toward recovery. “With endangered species work, you have to be optimistic,” she says.
Editors’ Note, August 29, 2024: We have removed a photograph from this story that misidentified the depicted specimen as a rusty patched bumblebee. It was instead a red-belted bumblebee.