Why do vinegar flies love onions?

By |  December 20, 2017

Photo: Mark Sheperdigian, BCE

In restaurants, grocery stores and even homes, you should never look at an onion without thinking about vinegar flies (Drosophila spp.) or small fruit flies, as they are also called. The association has to do with the way onions are shipped and sold in open mesh bags. When stacked up in large crates, the onions on the bottom get crushed and begin to rot around their crushed parts. Within a week or so, the yeast that grows as a result readily attracts and feeds Drosophila.

Whenever you see bulk onions in a produce section or in a storeroom, walk by and gently jostle them. If the flies are there, they will take flight immediately. Look under counters to check for onions that have fallen and rolled out of sight. They will continue to breed vinegar flies until someone has the good sense to get on the floor and look underneath — or, more likely, until the flies finish the onion.

Because vinegar flies are common in onions in grocery stores, they also are common in homes that have onions. If a client is complaining about “little gnats” or small fruit flies in the kitchen, the three most likely sources are the onions, the fruit bowl or the garbage disposal. We’ll talk about garbage disposals in a future post.

You can reach Mark Sheperdigian, BCE, vice president of technical services, Rose Pest Solutions, Troy, Mich., at shep@rosepest.com.

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