Parasitic fly helps determine a bee's future as queen or worker

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Washington, July 29 : Researchers from the University of Washington and the Smithsonian Tropical Research Institute say that a tiny parasitic fly seems to be affecting the social behaviour of the nocturnal bee, and helps determine which individuals would become queens and which workers.

The researchers claim that their finding is the first documented example of a parasite having a positive affect on the social behaviour of its host.

They say that this task is accomplished by cleptoparasitism, fly larvae that steal food from the developing immature bees.

During the study, the researchers found that smaller bees emerging in a nest were dominated by their mothers, and that such bees were more likely to stay and act as helping workers.

Larger bees, on the other hand, were more likely to depart and start new nests as egg-laying queens, the researchers say.

Bees that emerge from cells, or brood chambers, that also house flies are smaller than their nest mates from fly-free cells. The flies may encourage worker behaviour in some bees.

"We often think of parasitism in terms of it affecting an animal's fitness, its survival or its ability to reproduce. Here the parasite is not living inside another animal, but is still stealing resources from the host," said Sean O'Donnell, a UW associate professor of psychology and co-author of the paper appearing in the current issue of the Journal of Insect Behavior.

"We think these fly parasites are not affecting the lifespan of the bees, and the bees' mothers benefit by having a helper, or worker, stay around to protect the nest, increasing survivability," he added.

O'Donnell and his colleagues studied two closely related tropical social bees, Megalopta genalis and Megalopta ecuadoria, and a family of very small parasitic flies called Chloropidae.

Upon conducting behavioural observations, the researchers found that non-reproductive foragers and guards were significantly smaller than the queen bee in a nest, though the relative size of individual bees varied from nest to nest.

As to how the flies are affecting the bees' behaviour, the researchers say that the bees nest in hollowed twigs and sticks hanging in the tropical understory, and the flies flick their eggs into the entrance to the bee nests.

The researchers say that some of the eggs randomly fall into chambers prepared by the bees, each to hold a larva and pollen that the larva eats.

The cells are then sealed, so if a cell does contain fly eggs the young flies are competing with the bee larva for a limited amount of food.

"There is a natural size variation in bees and this is based in part on the amount of food available in the cell. A fly or flies in a cell reducing the amount of food could be a potentially important factor. It seems that the more flies in a cell the smaller the bee is. The key here is relative body size compared to nest mates. The larger individuals become queens because they are not dominated," said O'Donnell.

He and his colleagues were able to culture the bees and flies from individual cells, and counted as many as 15 of the tiny flies in a single cell. Some cells did not contain flies.

"This study is a counterintuitive take on parasitic infection. It encourages us to look for complicated ecological relationships between different species. Parasitism may encourage sociality in some situations. Here it is promoting social behaviour," O'Donnell said.

ANI

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