StreetScience: Fruit Fly Season

By therealbrute, flickr.

By therealbrute, flickr.

En Español

If flies are swarming your fruit bowl, garbage or compost, have no fear — you’re not alone.

Fruit flies abound in Bay Area homes, multiplying and thriving right now as the weather turns warm and apple cores, peach pits and decaying fruit — relished by these pesky insects, also known as vinegar flies — are plentiful.

“They love garbage,” said Dr. Michael Eisen, associate professor in the department of molecular biology at UC Berkeley. “If you’re a species that likes to live in fermenting fruit, what would be better than humans? We’re going to the supermarket and collecting and concentrating food for them.”

It’s the yeast in rotting fruits and vegetables that attracts fruit flies. But they also happily lay eggs — loads of them — inside the beer left at the bottom of your Corona bottle or in the scum on the side of your sink or trash can.

Females drop as many as 500 eggs near the surface of fermenting food material. The eggs hatch into larvae 24 to 30 hours later, and in no time new adults are flying around your kitchen.

“If things get away there can be a lot of flies in a pretty short order,” said Bay Area pest controller Art Slater. “You got a couple potatoes gone bad and it can fill a kitchen with flies.”

Fruit gone bad fills composting bins, and San Franciscans are composting more than ever after a citywide mandate last year. The rotting fruit draws the vinegar flies like kids to ice cream on a hot day.

“Compost is a heap of fruit fly media. It’s just exactly what they eat,” Slater said.

Compost, full of fermenting fruit and organic material, is an ideal home for fruit flies.

If you don’t line or clean out your trash, he added, the flies can lay eggs right in the dried juices that form on the bins’ sides.

A compost pile is a fruit fly’s ideal home, Eisen said — warm, damp and laden with rotting fruit. But that doesn’t necessarily mean compost brings more fruit flies than other forms of garbage.

“I don’t know whether we’re increasing the population by composting,” Eisen said. “We already provide them with pretty nice digs — garbage dumps, fruit trees, fruit bowls in the kitchen. They are really amazingly abundant.”

A closer look at the 3 mm-size flies would show that most have red eyes and a tan-colored body. They begin life as eggs and develop into larva, then pupa, which feed on the fruit before emerging as fully-formed adults. They become fertile after about 10 hours of adulthood, and the cycle begins again.

This rapid life cycle and fast regeneration has some benefit for science and makes the fruit fly one of the most studied creatures.

They are as easy to find as setting some rotting bananas on your windowsill, Eisen said. They’re also an enormously popular lab specimen.

“It’s not an accident that the thing that shows up in the kitchen first is the thing that scientists are most drawn to study,” he said.

Scientists began studying fruit flies in the early 1900s because they were easy and inexpensive, and many of their genes are similar to humans’.

“They were very significant in the developing field of genetics,” said Karen Kalumuck, a biologist at the Exploratorium. “Even though it’s been 100-plus years, they’re still making discoveries that apply to humans.”

Kalumuck cited as a current example scientists at UCSF who are studying a mutant fruit fly species that does not carry a gene for detoxifying alcohol.

“The ones that have this mutation really do stagger around. They’re hoping it may help shed some light on alcohol intolerance or addiction.”

In terms of eradicating the flies from your kitchen, the best thing you can do is remove the source. Females hang around where they lay their eggs, so if they hover around the sink, clean it, or if they’re buzzing around the fruit, toss it out.

Apart from being annoying, fruit flies don’t cause much harm or health risk, Slater said.

“The important thing is washing your hands — a bath is much more important than fruit flies.”

4 Comments

  1. Andy

    Small cup with beer at the bottom + a little dish soap + plastic wrap cover with small holes = tons of dead fruit flies.

    They love beer so they get into the cup, but the shape of the holes make it hard to get out. Then they get drunk, but the soap changes the viscosity of the water so if they get wet, they’re toast.

  2. Hadley Robinson

    Here are some comments from Hilary Near, commercial zero waste consultant from the city’s Department of the Environment:
    There are a couple of other ways to discourage fruit flies from accompanying compostables collection. First, of course, empty the compost collection container frequently. Don’t leave food exposed; layer dry newspaper inside of the composting collection container. Some people even keep food scrap collection containers in the freezer! A container of apple cider vinegar adjacent to food scraps attracts the flies and drowns them.

  3. neighbor

    glad to see this article! i’ve been fighting these little buggers. i can keep my kitchen clean, but what can i do about the green compost container downstairs, which gets collected only once a week?! i have actually seen fruit flies go up the stairs from the garage into my unit!! is there something i can spray?

  4. annie

    clean wine glass with an inch of rice vinegar at the bottom and a few drops of dishwashing fluid in it to break the skim on the water. the flies are attracted to the vinegar, but drown in the vinegar. for maximum impact, be sure not to get any vinegar on the sides of the wine glass when pouring.

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