Imagine a hive, silent except for the queen, her only companions larvae maturing in capped honeycombs. Almost all the 50,000 to 80,000 worker bees that once served her have left en masse to die.
The hive is afflicted by Colony Collapse Disorder, or C.C.D., a phenomenon affecting bees globally that seems to have no single, obvious cause. Scientists have so far implicated pesticides (neonicotinoids, which farmers spray on corn, soy and other crops), habitat loss, parasites, and even diesel fumes.
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The factors have little in common, except that they all stress the bees. A single factor in a low dose is not enough to kill them, but the stress alone is enough to make the bees exhibit erratic behavior - such as orientation, mobility, defense and other skills - as though the they are slightly boozed.
The stress experienced by individual bees over a prolonged period of time begins to affect the entire colony. And at a critical level of stress, a tipping point, the colony heads toward failure, suggests a study published today in Ecology Letters.
John Bryden, a biologist at the Royal Holloway University of London and co-author of the report, compared the situation to a person carrying a heavy shoulder bag. We can all manage a certain weight, but add a bit too much and we accept defeat and hail a cab.
"Similarly, bee colonies can keep growing when bees aren't too stressed, but if stress levels get too high the colony will eventually fail," said Bryden, in a press release.
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Bee workers are redundant and dead individuals are quickly replaced by the colony. Impaired bees are a different matter, however, and they are allowed to work even as they hinder the colony's success. At some point, the hive becomes so dysfunctional that more bees die (of natural causes) than are born.
That's dangerous to the highly organized, stratified bee society. Bees begin life as larvae in honeycomb compartments. They mature into "hive" bees and for 21 days they maintain the temperature of the hive; they build and clean, ventilate and cool. They then mature into the "forager" bee, heading out to collect nectar and pollen. After another 21 days, they die. Hive bees are recruited to replace them.
When afflicted by C.C.D., the hive has too few workers left to forage and care for the larvae and the queen. The very social organization that makes bees so successful spells the colony's demise.
Bryden and his colleagues experimented with bumblebees in the study. They exposed the colonies to sub-lethal levels of neonicotinoid pesticide for 42 days and kept count of the numbers of dead and living.
The scientists then built a computer model that mimics a hypothetical bee colony containing both normal and impaired bees. When they input the results of their real-world experiments into the computer model, they found an excellent fit.
Although the scientists used a pesticide in their experiments, any stressor that affects bees (such as disease, weather, habitat loss, pollution) can trigger colony collapse, the scientists found.
Accumulation of multiple different stressors can push the colonies over the critical threshold of collapse, the study finds.
"This can explain why finding the link between colony failures and a single specific stress factor has so far proved elusive," the authors state.
IMAGE: The bumblebee species Bombus hortorum, seen here flying over yellow buttercup flowers, has the longest tongue among British bees. (Robert Pickett/Corbis)