Scientists are working hard to slim them down.

Before the 1990s, the rusty-patched bumblebee could be found in 28 states throughout the Midwest and the New England region. Then, it vanished mysteriously. Within a few decades, 90 percent of rusty-patched bumblebees were gone in an ecological poof . Even more troubling, several other closely related bumblebee species also died-off. Now it’s almost unheard of to see the rusty-patched bumblebee. Last month it became the first bee in the continental U.S. listed under the Endangered Species Act.

Pesticides, climate change, habitat destruction, stress from competition, or a combination of these might all be to blame. Most recently, however, entomologist have been investigating a parasitic fungus found on the bumblebee called Nosema bombi, and the more they learn the more they’ve become concerned.

Some of the most insightful research into the parasite is done in northern Utah, in the lab of James Strange, a research entomologist with the U.S. Department of Agriculture. Lately, Strange has studied a close relative of the rusty-patched bumblebee, the Western bumblebee, rearing colonies in his lab, exhausting himself to get them healthy, then infecting them with a parasite that bloats male bees until they’re impotent.

A queen bumblebee can birth males—called drones—on her own. But only after a male fertilizes her eggs can she produce female bees. This is crucial, because drones are essentially layabouts. It’s the women, the worker bees, that do all the foraging to sustain a colony. So inside a mating cage, Strange places a queen bumblebee, and on the other side a drone infected with Nosema bombi. Already this fungus has spreads down the bee’s throat, has rooted itself in the gut where the spores “proliferate like crazy,” according to Strange.

There the fungus swells in the soft tissue between the bumblebee’s organs until the drone grows so plump it can’t bend its abdomen to mate with the queen. Without fertilization, the queen can only birth more males. Without females, future colonies starve.

It’s devastating for bees, and perplexing for scientists, because the oddest part of Nosema bombi is that it’s lived alongside bumblebees for centuries.

“We still don’t know why this fungus went from being not so bad, to really bad for certain bees,” Strange told me.

Answering that question might determine if the rusty-patched bumblebee survives. And because this is the first bee in the continental U.S. to get federal protection, whatever plan entomologist come up with will undoubtedly shape how conservationists protect other pollinators, even honeybees, which have been ravaged by similarly mysterious die-offs. In the case of the honeybees, it’s estimated that the U.S. lost 44 percent of the population last year, caused by the not-yet-fully understood colony collapse disorder. There are several theories about why the bees leave seemingly healthy hives and never return, and some of these theories include the spread of disease or parasites, similar to what may have devastated the rusty-patched bumblebee. So many entomologists and conservationists see protecting the rusty-patched bumblebee not as a fight for one species, but the beginning of a much larger battle to rescue all pollinators. Drawing a protection plan like this has never been tried before, and it presents several novel problems. Right now, the most perplexing is why—if Nosema bombi is to blame for the decline of the rusty-patched bumblebee and its close relatives—did it suddenly became so lethal?

Collectively bees pollinate about 30 percent of the world’s food. Much of that is done by honeybees, which is why most of the news and panic about the massive bee losses has focused on European honeybees that are not indigenous to the U.S.

Bumblebees do not make honey. The some 50 American species mostly pollinate wildflowers, as does the rusty-patched bumblebee. It gets its name from a rust-colored square on its back, which is distinct to the species, but it generally shares common bumblebee traits: They’re more plump than honeybees, their bodies often covered in a thick fuzz, and they’re one of the few insects able to regulate  body temperature. All of this allows them to live in higher altitude alpine environments, which makes them critical to the plants and animals that live there. Bumblebees are also distinct in that each year nearly their entire colony, comprised of hundreds of bumblebees, dies.

In the early spring a queen emerges from her hibernation hole dug beneath a few centimeters of soft soil and searches for an unused rodent den, a dead tree, even a heap of grass clippings to begin her colony. To build it she gathers pollen and creates waxen pots laden with eggs. The first born are previously fertilized worker bees that find food and build up the colony until survival efforts turn to needs for  expansion sometime in the late summer. Now the queen lays the colony’s reproductive members: more queens and the male drone bumblebees. The males will leave the colony in search of other queens, never to return, and when winter comes again every bumblebee but the queens die.

It’s a delicate balance on its own. And with overstuffed fungi-infected drones too fat from spores to mate, it has thrown the rusty-patched bumblebee into near-extinction, along with several closely related species. Franklin’s bumblebee, which hasn’t been spotted for about a decade, is believed extinct now.

Honeybees in America have been commercially grown practically since European colonists brought them across the Atlantic Ocean. But large-scale bumblebee breeding  only took off in the 1990s. Bumblebees are the only bee large enough, and with wing muscles powerful enough, to pollinate the tomato plant’s sticky pollen. Twenty years ago the pollination industry exported thousands of Eastern bumblebee queens to raise in Europe, then imported them back to farms and greenhouses all over the U.S. An entomologist at the University of California, Davis, Robbin Thorp, as well as Sydney Cameron, at the University of Illinois, noticed this commercialization timeline matched suspiciously close to the decline of several bumblebees.

Many scientists thought the rise of neonicotinoid pesticides, which occurred around the same time, were to blame. “But people were just making these wide-ranging declarations about bumblebee decline around the United States, and then making guesses about why,” Cameron told me. The pesticides are definitely bad for bees, and many entomologists believe they impair the immune system and decrease a queen’s fertility. But if pesticides were the only cause, Cameron said, all bumblebees and honeybees would likely experience similar devastation. So working off the Nosema bombi hypothesis, Cameron examined eight bumblebee species, taking samples from museums reaching 100 years back.

She found that four of the tested species showed declines of up to 96 percent in just the past two decades, and, while a similar strain of the fungus had always been present, the four worst hit species had low genetic diversity. These bees had likely never been exposed to such high concentrations of Nosema bombi. When bumblebee commercialization took off in the 1990s, breeders could have unknowingly exposed queens plucked from the U.S. and packed in European warehouses to this fungus. Raised so closely together, the bumblebees basted in the parasite, became super-carriers, or perhaps by dint of so many generations born under the same roof the pathogen altered its genetic make up, becoming more deadly, especially to certain species.

When breeders shipped the colonies to the U.S. where they would pollinate the country’s food, the infected bumblebees spread the parasite. In a way, it’s an ancient problem: disease-laded colonizers journey to new land, infecting the indigenous.

If true, what this means for a bumblebee protection plan varies. The most proven policies would include plans that focus on making bee habitat healthy. This includes growing flowers and plant species the bumblebees feed upon, because so much of the land nowadays has been developed, the wildflowers cleared, often to make way for monoculture farmland.

These giant farms should also stop hosing their plants with pesticides, said Scott Hoffman Black, executive director of The Xerces Society,* an invertebrate conservation group. Black told me in many cases big farms douse their seeds, soil, plants, in pesticides even if there’s no direct threat from a pest. He calls it the prophylactic, or “silver bullet,” approach. The strategy dates back to the development of neonicotinoids in the 1990s, which were pushed by the pesticide industry as an all-encompassing pest killer safe for mammals and birds. Neonicotinoids are indeed safer for mammals and birds, but not necessarily insects. These pesticides are water soluble, are absorbed into the plants, into the plant’s pollen, and have been linked to bee and bumblebee deaths, reduced queen fertility, and generally decreased insect health. Europe and Canada have already limited their use, but regulation has not come to the U.S., and Black blames this on the lobbying power of the pesticide industry.

“Many of our farmers get it,” Black said of his organization's push to switch onto a more targeted pesticide approach. “They understand that if we ruin the environment around the farm they may not get the benefits they need.”

Some corporate farms are signing on with Xerces’ plan, like General Mills, which is spending millions to change the way it uses pesticides. But pollinator decline is not just a rural, or corporate problem. Per-acre, more pesticides are found in the suburbs. Homeowners generally don’t know what they’re doing when they buy pesticides from a big-box store. They end up overspraying.

As for Nosema bombi, one plan all the entomologists I spoke to agreed with is that the federal government must regulate bee movement. Not among the bees themselves, but bee breeders. Right now the U.S. Department of Agriculture oversees the international importation of bees, and something similar needs to be done among states. The need to feed a rising population has created agricultural production in sizes beyond the pollination capability of wild bees, so as American farmers rent-out bee colonies to do their pollinating, more attention must be paid to what bees are going where. Ideally, this would restrict  commercial bee movement to areas where the species is indigenous. No more Eastern bumblebees grown in European factories and shipped over ocean to U.S. states from Pennsylvania to California.

It’s going to be difficult, as T'ai Roulston, a professor in the Department of Environmental Sciences at the University of Virginia, told me, because “we don’t have any history at managing disease in wild insects.”

It may be that the plague of Nosema bombi has spread beyond the point of human intervention. If that’s the case, there’s only a couple things scientists can hope for: One, that the rusty-patched bumblebee colonies alive now have developed a genetic resistance to Nosema bombi, which is why they’re still alive. And two, that entomologists can rear captive colonies of parasite resistant rusty-patched bumblebees. If that happens, then like conservationists did with the California condor, entomologists could  return healthy bumblebee colonies to their former habitats. Some scientists believe this crosses a line. It represents too much meddling. But even if it could be done, Roulston told me, he doubts anyone is capable. “There aren’t many people who could do it,” he said. Then a moment later he remembered one person.

James Strange, Roulston said, the entomologist in northern Utah, “If anybody is trying it, it’s probably him.”

So I asked Strange, is he up for it?

“I’m up for trying anything,” he said, with a nervous laugh.

*This article originally misstated the name of the Xerces Society. We regret the error.