When I first saw a bumblebee fall from a jewelweed flower and plummet five feet to the ground, I thought I had witnessed an unusually clumsy bee. When another fell only a minute later, I started paying closer attention. When the third bee hit the ground, I discovered the culprit.
Some sort of crazy dive-bombing insect was systematically kicking out every bumblebee that entered the jewelweed patch by ramming itself into the bees’ abdomens. It was shaped like a fighter jet with narrow black wings that formed a V when resting on a leaf. The insect looked like it weighed about one-tenth of what a bumblebee weighs and yet it could shake a bee from a flower with one swift push. Not once did I see it actually pollinate a flower. What was this thing?!!
The answer, offered by my fellow UVA graduate student and go-to bumblebee expert, Rosemary Malfi, is far more gruesome than I imagined. This insect was a conopid fly (Physocephala tibialis). It was not defending the patch from other pollinators. It was waiting for bumblebees to pump them full of its own eggs.
The conopid fly is a parasitoid, a parasite that not only feeds on a host, but ultimately kills it.
Here’s how it works. An adult female conopid fly waits in a patch of flowers for a bumblebee to come along. Then, when a bee flies within range, she jams her ovipositor into its backside. “The ovipositor is like a can opener that opens the bee’s abdominal segments and shoots an egg inside,” says Malfi.
The bee might be a bit stunned by this attack at first, but after a couple of seconds it flies off and continues foraging, all while harboring a time bomb in its gut. After two days, the egg hatches into a hungry larva that begins feeding on hemolymph, the bumblebee’s blood. As the larva grows, it then begins munching on the bumblebee’s gut tissue. “The larva eats it from the inside,” says Malfi. “The bumblebee is alive during this whole process.”
Sometimes, the larvae seem to encourage bumblebees to dig their own graves. Just before death, about ten days after the egg was inserted, a bee may bury itself in the ground. In the cozy protection of the bumblebee’s buried body, the conopid fly larva develops into a pupa. It then emerges as an adult the following spring.
The conopid fly makes foraging for nectar and pollen a risky business for the bumblebee. But just how risky? Malfi wondered what the probability of being parasitized was as bees made longer and longer foraging trips away from the hive. This past summer, she was determined to find out.
Together with her graduate advisor T’ai Roulston, her husband, Lewis Bauer, and an undergraduate researcher, Clara Stuligross, Malfi designed an experiment that tracked the movement of over one hundred individual bumblebees at once.
The team purchased two bumblebee hives (Bombus impatiens) and placed them in clear boxes for easy viewing at the University of Virginia’s Blandy Experimental Farm in northern Virginia. In June they used crazy glue to attach tiny computer chips to the backs of over one hundred bumblebees. “The chips are smaller than the width of a chocolate chip,” says Malfi.
They placed a sensor that could read the chips at the opening of each hive so that every time a bee entered or exited the hive, the bee’s movement was recorded. They tracked the movement of bees for eight days and then repeated the same experiment with a different set of hives and bumblebees in July.
Their chip system allowed Malfi and her team to know the exact amount of time each bee spent away from the hive over the eight-day period. Back in their lab at the University of Virginia, the team dissected each of the computer-chipped bees in search of conopid larvae. They could then ask whether longer flight times resulted in a higher probability of conopid fly attack.
They found that the risk of encountering a deadly conopid fly was far greater than anyone expected. In June, bumblebees reached a 50% chance of infection when they spent about 30 hours away from the hive. In July, as conopid flies became more common, bumblebees reached a 50% chance of infection with only 20 hours away from the hive, and a nearly 100% chance of infection with 40 hours in flight. In both months close to 60% of foraging bees were infected with the deadly parasitoid.
When Malfi first saw these results, “I was a little shocked,” she says. “I think we hadn’t quite pieced together at that time how high the parasitism rate was. We didn’t know that going into the experiment.”
Malfi’s findings suggest that foraging for nectar and pollen is indeed a risky endeavor. But bumblebees need to forage to feed their own larvae back in the hive. Malfi estimates that each worker bumblebee collects enough food over its lifespan to support about nine to eleven larvae. If bumblebees must forage to keep their hives healthy, and foraging frequently results in conopid fly attack, then it would seem that conopid flies seriously threaten the health of bumblebee populations.
Malfi isn’t so sure. It’s possible that if infections tend to occur at the end of a bumblebee’s lifespan anyway, then the bee would still be able to contribute its share of resources to the colony before death. Malfi is now planning to use computer modeling to test how infection at different ages may affect the long-term health of bumblebee populations.
When I first noticed the conopid fly, it looked like a bully. Now that I know more, it looks like a creature right out of a horror movie. But Malfi is quick to remind people that the relationship between the bumblebee and the conopid fly is completely natural. These species have long coexisted. “No one has linked conopid flies with bumblebee population declines,” says Malfi.
Ultimately, Malfi would like to understand how other factors, like habitat loss and introduced parasites, may damage bumblebee populations. “But for us to better understand bumblebee declines with respect to these threats,” she says, “we have to understand the natural factors that regulate bumblebee populations.” And that includes natural enemies like the conopid fly.