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.
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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.
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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.
The U.S. Geological Survey is posting photos of insects on its
, offering a macro look at this hidden world. First up, this Festive Tiger Beetle (
Cicindela scutellaris
) was found on top of a butte in Badlands National Park that had ancient windblown sand at its crest. Here, this sand specialist can build its long burrows.
This Agapostemon bee species is one of the most common native bees in the eastern United States. In almost any field there can be hundreds, if not thousands, of these bees visiting a wide variety of blooming plants. One of the largest of the sweat bees, it still goes undetected if you don't get down on your knees, face close, among the flowers. This one was collected at Colorado National Monument, Mesa County, Colo.
This wild bee (
Hoplitis fulgida
), a female from Grand Tetons National Park, was collected as part of a study of climate change. Most species in this genus are black , but a few, like this one, are as the Latin in name implies, glittering jewels.
This unknown wasp was collected in Cecil County, Md.
This is an unknown species of Robber Fly from Charles County, Md. Robber flies, a very large and widespread type of fly, feed on many different kinds of insects, making them a key player in maintaining the insect balance in different environments.
One species of the rarely seen leafcutting bee, this is
Megachile integrella
from the sandhills of North Carolina. Leafcutter bees are so called because they cut plant leaves to create the cells in their nests. The bees tend to build their homes in rotted wood or in the strong stems of plants.
Phidippus clarus
is a type of jumping spider. This one was found in Beltsville, Md., but
Phidippus clarus
lives in fields and prairies across North America. It feeds on seasonal plants.
Eggplant Tortoise Beetles like eggplants (go figure), eating holes in the plants' leaves. From the underside, the insects look quite queenly, with their ruffled collars. This one was gathered at the Patuxent Wildlife Research Center, in Anne Arundel County, Md.
The Karner blue butterfly,
Lycaeides melissa samuelis
, is endangered. Karner blue butterflies feed on nectar from many different types of flowers, but their larvae can survive on the leaves of only one specific plant, which has been decimated by habitat loss or change.
Deer flies like this one, despite their groovy eyes, deliver a ferocious bite. And no wonder: when the female bites (males don't bite), she lacerates the skin and when the blood flows, sponges it up with her mouth. There are over 110 species of deer fly.
Centris bees, like this one, make their homes in holes, either in trees or in the ground.
The biggest visual difference between damselflies and dragonflies are their wing positions when resting. Dragonflies hold their wings open, while damselflies close them above their backs. This Ebony Jewelwing Damselfly (
Calopteryx maculata
) was found on a Beltsville, Md. stream.