Fruit Flies Hide Their Eggs in Oranges to Protect from Parasitic Wasps

Researchers have learned that tiny fruit flies will actively lay their eggs on citrus fruits because parasitic wasps that feed on fruit fly larva are repelled by the smell.

To draw their conclusions, the researchers analyzed the fruit flies' brains to see which areas were stimulated by different odors, pinpointing which odor receptors in the fly brains responded to citrus.

To make sure that the fruit flies were indeed actively laying their eggs on citrus, the researchers disabled the flies' citrus receptors. Fruit flies whose citrus receptors were silenced were unable to distinguish oranges from other fruits, the researchers learned.

"It is fascinating that a complex behavior, such as choosing an egg-laying site, can be broken down into multiple sub-routines that have such a simple genetic basis," said Marcus Stensmyr, a researcher at the Max Planck Institute for Chemical Ecology in Jena, Germany. "We were quite surprised that by silencing just this single odorant receptor, flies could no longer localize their preferred egg-laying substrate."

Stensmyr and his colleagues chose the fruit fly Drosophila melanogaster for their research. For these insects, where their mother chooses to lay her eggs is crucial to their survival. Once she lays the eggs, her maternal contract ends and the fly larvae are at the mercy of the environment.

In the process of isolating the citrus odor receptors in the flies, the researchers tested the insects' response to 450 odors, finding an overwhelming preference for oranges. Lemons, the researchers found, the flies did not perfer because of their higher acidity.

The parasitic wasps that lay their own eggs inside fruit fly larva are repelled by the orange scent, and do not appear to even use the scent to lead them to eggs otherwise ripe for infesting. It is unclear why the wasps avoid the citrus, though the researchers are certain that the Drosophila fruit fly actively seeks citrus out to ensure the best chance of survival for its eggs.

A paper detailing the research is published in the journal Current Biology.