On a fall night, under an outdoor light, a bee stumbles in circles, far from its hive. It cannot fly. Soon, its tormentor reveals itself: a writhing maggot breaks through its neck—killing the bee.
The end began a week earlier, likely near a flower, when the bee met an adult Apocephalus borealis fly. This tiny parasitic fly grabbed onto the back of the bee, stabbing the unfortunate pollinator between the plates of its abdomen with a needle-like ovipositor to lay its eggs inside of the bee’s exoskeleton. Within days, the fly’s eggs hatched into hungry larvae, which ate the bee’s guts before gobbling up its flight muscles—leaving the bee grounded. After emerging, the larva formed a pupa and hatched into an adult fly within a week. Then, it set off to seek out mates and another colony-nesting insect to lay its eggs in. And that’s how it got its name: the “zombie fly.”
A. borealis is part of the family of phorid flies—flies that scuttle rather than fly to avoid getting eaten. A. borealis, in particular, belongs to a group known as ant-decapitating flies, which specialize in laying their eggs on colony-nesting insects such as ants, wasps, and bees, and help cycle energy and matter through local ecosystems in the process. Mostly, they flew under the radar (even as they burrow beneath the skin). But fifteen years ago, scientists have discovered A. borealis infecting a new host—one freighted with unparalleled economic and cultural importance.
It began early one morning in June 2010, as John Hafernik, an evolution and entomology professor at San Francisco State University, walked to campus. Puzzled by dying honeybees under a streetlight, he decided to collect them in a vial. After leaving them on his desk for a week or two, he saw pupae in the vial. A week or two after that, adult flies emerged from the pupae. He recognized them as phorid flies. Hafernik reached out to Brian Brown, an entomology curator and phorid fly expert at the Los Angeles County Museum of Natural History, who identified them as A. borealis. Brown realized Hafernik was the first to ever observe this parasitoid attacking honeybees.
Hafernik wondered whether A. borealis might be more widespread than people realized. Since 2006, bee colonies had mysteriously collapsed across the country. Even hives with plenty of stored resources saw the worker bees disappear, leaving behind the queen to die. Social insects like honeybees can disperse when in the presence of parasites or other pathogens: could A. borealis be causing colony collapse disorder? Hafernik suspected honeybees—brought by Europeans to North America in the last few centuries—were vulnerable to A. borealis, because they have had “little evolutionary time” to adapt to the new threat. “Honeybee queens are not bred for resistance to the flies,” he says.
Hafernik and a team of researchers created a community science project, ZomBee Watch, so that beekeepers and scientists could report honeybees that had been parasitized by A. borealis. Contributors discovered infected honeybees across the west coast of North America, from Vancouver Island to Santa Barbara. Clusters have also been observed in New England and Nova Scotia.
Since A. borealis was discovered in honeybees, colony collapse disorder has shined a spotlight on the fly’s behavior. It’s unusual for a parasitoid to target so many different hosts, says Brown. He suspects they might be attacking honeybees by accident. Stacey Philpott, an ecology professor at UC Santa Cruz, has studied how honeybees and yellow-faced bumblebees interact with A. borealis, and her work suggests that honeybees fall victim to the fly more often in specific environments, like urban gardens. A. borealis also seems to prefer bumblebees, by far. To her, that indicates A. borealis lands on honeybees mostly when bumblebees are harder to get. “The fact that they’re on honeybees is just an unfortunate quirk of our moving species around,” says Brown.
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After years of study, thanks in part to the citizen scientists and experts contributing to ZomBee Watch, scientists think this fly isn’t the main driver of colony collapse disorder. For one thing, it seems to thrive only in reasonably moist environments that don’t get too cold. In California, for example, flies have been reported only a few miles from the coast, according to Brown, whereas colonies have collapsed across North America. Scientists now believe there is no single cause for colony collapse disorder, and are generally more concerned about viruses spread by varroa destructor mites instead.
Now, we can appreciate phorid flies for the part they’ve always played in our ecosystems. Functionally, these flies help keep populations of colony-nesting insects from becoming too dominant, and thereby gives opportunities for the hundreds of pollinators that live a solo lifestyle. “Parasitoids are inherently so interesting,” says Philpott. “[And] having the phorids present in an ecosystem may be an indicator of ecosystem health.”
But Brown discourages seeing the fly only for its function. “They’re out there to gather enough energy to reproduce, it’s all any organism does,” he says. “They’re not there to fulfill a role that someone’s got for them.”
Though it’s unlucky for honeybees that this remarkable fly adapted to infect them, the attention A. borealis has now received gives us a chance to explore the weird world of parasitoids, and it reveals some of the mysteries that still exist in the animal world. There’s much we still don’t know about A. borealis: how it detects its hosts, exactly how it zombie-fies the bees—or where it might be found next.
Phorid flies breed in the fall and spring. Report zombee sightings here.
