A Blog by Ed Yong

Parasitic fly spotted in honeybees, causes workers to abandon colonies

Throughout North America, honeybees are abandoning their hives. The workers are often found dead, some distance away. Meanwhile, the hives are like honeycombed Marie Celestes, with honey and pollen left uneaten, and larvae still trapped in their chambers.

There are many possible causes of this “colony collapse disorder” (CCD). These include various viruses, a single-celled parasite called Nosema apis, a dramatically named mite called Varroa destructor, exposure to pesticides, or a combination of all of the above. Any or all of these factors could explain why the bees die, but why do the workers abandon the hive?

Andrew Core from San Francisco State University has a possible answer, and a new suspect for CCD. He has shown that a parasitic fly, usually known for attacking bumblebees, also targets honeybees. The fly, Apocephalus borealis, lays up to a dozen eggs in bee workers. Its grubs eventually eat the bees from the inside-out. And the infected workers, for whatever reason, abandon their hives to die.

There are hundreds of species of Apocephalus flies, and they’re best known for decapitating ants from the inside. The larvae, laid within an ant, migrate to the head and devour the tissue inside. The brainless ant wanders aimlessly for weeks, before the larvae release an enzyme that dissolves the connection between the ant’s head and body. The head falls off, and adult flies emerge from it.

A. borealis has a similar modus operandi, but it targets bees not ants. Core discovered its penchant for honeybees by sampling workers that had been stranded in the lights of his faculty building, and other locations throughout the San Francisco Bay area. The fly was everywhere. It was parasitizing bees in three-quarters of the places that Core studied, and its DNA confirmed that the species that attacked honeybees was the same one that kills bumblebees.

When Core exposed honeybees to the flies in his lab, he saw the same events that befall unfortunate ants. The flies lay eggs in a bee’s body and weeks later, larvae burst out from behind the insect’s head. It’s no surprise that the infected bees, with up to 13 larvae feasting on their brains, seem a little disoriented. They walk round like zombies, pacing in circles and often unable to stand up.

They also abandon their hives. Core found that the dying insects literally head towards the light. Large numbers of them become stranded within bright lights. Many flying insects show a similar attraction, but the stranded bees were stock still rather than buzzing about. They would also head towards lights on cold, rainy nights when other insects seek shelter.

It’s not clear why the bees would leave. It could be the fly’s doing, since several parasites can change the behaviour of their hosts. One virus compels caterpillars to climb to high spots where their bodies liquefy, releasing virus particles that rain down on the foliage below. Some fungi steer infected ants towards places with the right conditions for its spore capsules to develop (which erupt from the ants’ heads). Given such manipulations, perhaps the fly changes a bee’s daily rhythms or its sensitivity to light.

Alternatively, the bees could flee their hives to avoid infecting their colony-mates. Such altruism is common among social insects – terminally ill ants, which have been infected with a fungus, will often walk off to die alone.

None of this means that the fly is the sole cause of CCD. The other viruses and parasites that have been linked to CCD are still important. The fly could even transmit some of them. Core found that many flies tested positive for the Nosema parasite and the “deformed wing virus” (DMV).

But certainly, Core’s evidence suggests that the flies are playing some role. Not only do the infected bees behaved in a way consistent with CCD, but the timing also fits. The flies attack the bees most heavily between October and January, and then once more in the late summer. Both peaks happen just before the times of year when CCD is most common in the Bay Area.

Core hopes that beekeepers will now help to measure and monitor the spread of the flies. The easiest way is to stick light traps near a hive, and to check the trapped bees for fly larvae. Core himself will try to find out where the flies are attacking the bees, to find ways of stopping them.

Similar flies attack honeybees in Central and South America, but North American bees have always seemed to be free of such parasites. That’s no longer the case. Core thinks that A.borealis may be a new threat. After all, honeybees have only recently started to gather around electric lights at night.

Honeybees are so economically important that they are very carefully studied. If the flies have been attacking them for decades, we’d probably know about it. Either it has recently expanded its choice of host or it always targeted honeybees at low levels and is now picking up its game.

Both scenarios are equally worrying. While bumblebees live in small colonies that die at the end of each year, honeybees live in massive hives that persist throughout the seasons and are often found in close quarters. If A.borealis has gained the ability to parasitise honeybees, its population could explode. That would spell problems not just for honeybees, but for bumblebees and the fly’s other victims. As Core writes, “The domestic honey bee is potentially A. borealis’ ticket to global invasion.”

Reference: Core, Runckel, Ivers, Quock, Siapno, DeNault, Brown, Derisi, Smith & Hafernik. 2011. A New Threat to Honey Bees, the Parasitic Phorid Fly Apocephalus borealis. PLoS ONE Citation TBC

10 thoughts on “Parasitic fly spotted in honeybees, causes workers to abandon colonies

  1. While this is interesting, the CCD link is overplayed, both here and in the original study. There is not even a demonstrated geographic correlation between fly distribution and CCD. Seems like every bee researcher is bending over backwards to link their stuff to CCD, but based on this paper I’m not inclined to view the phorid suggestion as any more plausible than the cell phone fiasco.

  2. I assume the flies would only be responsible for driving infected bees out of their nests if by doing so, the flies would increase their chances of happening upon new uninfected bees. From your description, it sounds like the bees are not taking their parasites any place of value to the flies. On the other hand, would a bee with 13 fly larvae eating its brains have the wherewithal to be altruistic?

    Either way, there’s nothing better than a creepy brain-eating larva story.

  3. If this fly has (relatively) recently started parasitizing honeybees, would there have been enough generations for evolution to select outcomes that benefit either? Maybe something the larva release just happens to trigger behavior related to the bee’s homing ability; heading to the light is happening until a more successful adaptation emerges? Interesting to think this is an opportunity to study the evolution of this kind of relationship between two unrelated species.

    And I couldn’t end without commenting on “The head falls off, and adult flies emerge from it.” I hope that something I don’t dream about tonight, if not I know who to thank.

  4. As a beekeeper and scientist, I find this very interesting. It would explain a lot about why so many hives have been found completely abandoned, without the dead piled up inside or outside, as so often happens with other causes of mass bee death.

    This discovery is certainly a good starting point for further study. It will be interesting to hear of the extent of A. borealis’ parasitization of honeybees, and see if it might be the elusive cause of CCD.

    It’s not as though the flies evolved with artificial light, so discovering the benefit of having their hosts to head toward natural light is more sensible. It seems that by controlling the bees to go out into the light, the emerging adult flies would be well-distributed for seeking new hosts for their offspring. Remaining in a dark hive full of rotting bee carcasses would probably be detrimental to the flies’ health. If the flies originally parasitized bumblebees, consider that a bumblebee nest might also not be the best place for the fly larvae to metamorphose.

    The cell phone theory had 15-minutes of fame, and really wasn’t taken as seriously in the beek and entomology community as among the media and general public. But the other major possible contributors to CCD _do_ exist, and _do_ plague honeybee hives world wide. CCD _is_ a big deal. We just don’t yet know enough about it to solve the problem. That’s why so many bee researchers are working on it.

  5. Some years ago we had a wasp infestation inside the walls of our home (some smaller relative of hornets by type). Rather than pay for extermination I just fitted a long extension to a shop vacuum and mounted it under their entrance outside and waited. One by one foragers fell into the trap, either coming or going- many hundreds, if not more. Once they stopped coming I attached the hose to the exhaust of my car and ran that for about 20 minutes. In the days after I noticed many small flies coming and going from the nest entrance. In a week the colony was kaput. It makes me think that nests in crisis are releasing some chemical signal that attracts the flies- didn’t see any coming or going before the vacuuming.

  6. I just found a number of dead bees around the outside of a house in Katy Texas missing their heads. They don’t appear to be honeybees or bumble bees. Very strange times we live in these days.

  7. I grow a large variety of peppers and flowering plants which you know attract bees & small colonies of wasps. For 17 years I’ve lived in the same house in south Florida and have several bee hives high in my oak trees.

    While I’m no beekeeper or scientist this theory very likely explains many of my recent observations over the last several months. After nightfall not one bee is attracted to our outdoor lights but some where around 4:00 AM, over 2 hours before sunrise I find 50 to 100 bees attracted to our lights. They seem to pill over each other either grooming or cleaning other bees. By the time I go outside around 7:00 AM I find numerous if not hundreds of dead bees around the lights on the ground or behind the bulbs.

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