Fruit Flies Are Picky About Where They Lay Their Eggs, Choosing Citrus Fruits Over Other Spots

By Ajit Jha on December 5, 2013 12:44 PM EST

Fruit fly on an orange.
On your kitchen counter, it might seem as though fruit flies will show up for just about any type of fruit you leave around for them. But when given a choice about where to lay their eggs, those flies will go for citrus most of the time, new work shows. (Photo: Shutterstock)

While fruit flies will hover over just about any fruit, they are highly choosey when it comes to laying the eggs. They prefer citrus fruits to lay their eggs on, according to a new studyThe complex food preference is surprisingly simple, according to this study published in Current Biology; according to a press release, "it boils down to a single odorant receptor found on a single class of sensory neurons,"

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The study calls for a revision of our earlier understanding on fruit flies. Typically, we imagine fruit flies to be pretty indiscriminate in their day-to-day activities. "Fruit flies are especially attracted to ripened fruits and vegetables in the kitchen," said Michael F. Potter, extension entomologist, University of Kentucky College of Agriculture. "But they also will breed in drains, garbage disposals, empty bottles and cans, trash containers, mops and cleaning rags,"

However, according to Marcus Stensmy, one of the study's authors, fruit flies "are more picky than previously thought" in context of their egg-laying habits. "They are not indiscriminately using any old fruit encountered, but show a clear preference towards citrus," he said in a press release. The reason for this preference is pretty logical: it's a defense mechanism against parasitism. Parasitoid wasps prefer feeding their young with fruit fly larvae, but have a natural aversion to citrus. This aversion was demonstrated in the work of Stensmyr and his colleague Bill Hansson, from the Max Planck Institute for Chemical Ecology.   

The study also explains how fruit flies identify citrus: by its characteristic smell. This smell comes off a handful of chemical ingredients, including limonene in particular. Driven by a single receptor, Or19a, flies are able to zero-in on citrus, while the flies that lack this one super receptor won't be able to identify an apple from an orange. The discovery highlights how simple mechanisms can lead to complex behaviors in animals. "Complex behaviors can often be broken down into multiple subroutines," Stensmyr said. While the end behavior is complex, "each of the subroutines that together feed into the decision process can have a simple genetic basis," according to Stensmyr. 

One question that the study team has no clear answer to, however, is why wasps will avoid citrus, especially if there are fruit fly larvae to be found. Stensmyr and Hansson admitted that they had no clear answer, but speculated, nonetheless, that there could possibly be other aspects of citrus fruit such as their thick skin making them unsuitable for wasps. There's one important practical implication of the study: you may want to watch out for aging limes and lemons at the bottom of your fruit bowl, as that's basically paradise for fruit flies.   

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