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Sunday, January 08, 2012

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Apocephalus borealis, bee killer fly

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.”

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