Bee colonies are dying off because stressed bees start foraging when they are too young to do the risky job, new research suggests.
The findings are published today in the Proceedings of the National Academy of Sciences.
Collapse of bee colonies is a major international problem, especially in the United States, but also threatens pollination of Australian crops.
"Like the rest of the world our colony death rate has gone up," says entomologist Dr Andrew Barron from Macquarie University.
Scientists believe this is caused when too many environmental stressors -- such as pesticides, pathogens and lack of nutrition -- build up to a 'tipping point'.
Barron and colleagues set out to investigate how stress on individual bees causes the loss of their whole society.
Young adult bees normally spend two to three weeks in the hive house cleaning and brood rearing before they head out of the hive and forage.
"Foraging is by far the riskiest job and the hardest job," says Barron. "So it makes sense for the society if bees only go out foraging once they've made other contributions to the society."
Previous studies had shown environmental stress can cause bees to go out foraging at a younger age than normal in an attempt to help the colony build up reserves.
Barron and colleagues set up an experiment to look at how well-equipped younger adults were to forage, and what impact this shift in colony resources had.
First they tagged thousands of bees in three experimental hives and three control hives with a small radio tracking device.
Then the researchers removed all the older bees from the experimental hive, forcing younger bees to go out searching for food.
Radio tracking enabled Barron and colleagues to record when each individual bee started foraging, how many foraging trips they made, how long the foraging trips were, and when the bee disappeared from the colony, presumed dead.
The researchers found that the young foragers didn't do a very good job and died faster than their older counterparts.
"We found the bees that started foraging when they were younger, survived a fewer number of days, completed far fewer successful foraging trips and they also took longer on each foraging trip," says Barron.
He says younger bees may be worse at foraging because they are less experienced at navigating than older bees, or because their flight muscle is not properly developed.
After collecting their field data, the researchers then used a mathematical model to predict the effect of foraging by younger bees on the rest of the colony.
They found that younger foragers were not able to support the colony "As a forager they are not able to bring back enough resources to support the colony ... they are spending so little time raising the brood. So everything breaks down," says Barron.
"If our model is correct, this effect of stress on individual bees would cause a colony to go through one of these tipping points."
Barron says in the future it may be possible to detect colonies that are at risk of failure by identifying when younger bees are out foraging, and where there are fewer bees looking after brood in the hive.