Current Issue

Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Pumpkin eaters: Fruit flies, Drosophilidae


Grooming eyes, antennae, mouthparts, and legs seem very important to the fruit fly. 


Jack O’ Lanterns will soon be recycled by fruit flies.

With Halloween a quickly fading memory, I visited my Jack O’ Lanterns one last time and took them on their final journey to the compost heap. While lamenting the passing of my pumpkins, I was delighted to see dozens of tiny winged engineers already at work with the decomposition process. Flies are important recyclers of dead plants and animals. They provide a vital ecological service by unlocking nutrients tied up in complex molecules and returning them to food webs. In this episode we meet the fruit fly, a master of decomposing plant material. The common name fruit fly is often used to describe small ( ~ 3 mm) flies with bright red eyes in the family Drosophilidae (a.k.a. vinegar or pomace flies). Larger flies sporting spotted or banded wings in the family Tephritidae also go by the name fruit fly by virtue of their appetite for fruit and other parts of plants. Details of the former will be investigated today and strange dealings of the latter await another episode.

While adult fruit flies feed on the surface of my pumpkins, their offspring are busy dining inside.

In autumn I regularly receive questions about hordes of tiny fruit flies buzzing around fruit bowls, kitchen sinks, and counters tops. They seem to appear from nowhere and lend credence to Aristotle’s notion that living organisms like tiny flies can originate spontaneously from non-living or putrefying things. Now-famous experiments by Francesco Redi and Lazzaro Spallanzani pretty much disproved Aristotle’s theory of spontaneous generation, but the appearance of hordes of tiny flies remains vexing even for bug geeks.

To help untangle this mystery, consider the change of seasons.  Autumn in many parts of the country is characterized by damp cool weather by virtue of hurricanes, tropical, storms, and incessant weekly showers. These moist conditions are nearly ideal for decomposing tons of leaves, fruits, and other vegetable matter that accumulated during summer and early autumn. My compost pile is a writhing mass of invertebrates intent on converting vegetable protein into animal biomass as quickly as possible. A cloud of fruit flies hovers over my compost each day and some of these winged raiders undoubtedly infiltrate my home when the door opens.

Despite a lack of eyes and legs, fruit fly larvae have no trouble navigating and devouring nutrient rich vegetable matter.

Like many kitchens, mine is occupied by a bowl that occasionally contains a piece of fruit that has gone a little squidgy. Yeasty odors of acetic acid and ethanol emanating from an over ripe banana serve as powerful attractants for the flies. After arriving at the banana, the female fruit fly deposits eggs. Each gal lays on average roughly 500 eggs during the course of her life time. Small translucent larvae hatch from the eggs. They glide through the overripe fruit slurping-up nutritious fermenting fluids as they develop and grow. When ambient temperatures are warm, as they have been for the last several weeks, fruit flies can complete a generation in less than two weeks. With their capacity for reproduction, populations around the fruit bowl can explode seemingly overnight.

Fruit flies can also enter your home as stowaways when you purchase overripe fruits or vegetables from the market. These goods may arrive preloaded with a complement of eggs or tiny larvae. To reduce chances of bringing home an infestation, inspect your produce carefully and wash fruits and vegetables. If fruit is unrefrigerated and displayed in a bowl, check it out regularly and toss over-the-hill items before they generate flies. Fruit flies can also breed in sink or floor drains, garbage pails, or recycling containers in homes, restaurants, and offices where decomposing organic material accumulates. Inspect these areas regularly, clean up spills, and disinfect surfaces.

The nefarious spotted wing drosophila has caused millions of damage to fruit crops since its discovery in the United States less than a decade ago.

Fruit flies are more than just an indoor nuisance. Several species are important pests of agricultural crops. The spotted wing drosophila, Drosophila Suzuki, first detected in the US in 2008 in California, has now spread from coast to coast and border to border. It is a major pest of strawberries, blueberries, raspberries, black berries, and cherries and in 2008 crop losses in California, Washington, and Oregon were estimated to exceed $500,000,000.

Yeasty odors of fermenting fruit and wine vinegar lure fruit flies to their death.


For the cloud of fruit flies wafting around your home, consider building a vinegar trap to catch and kill these noisome rascals.  Traps can be purchased commercially and several trap designs are available on the internet. My vinegar trap consists of an 8 oz clear plastic tumbler filled with 4 oz of wine vinegar and a few drops of dish detergent. Within 24 hours of placing the trap on the counter, more than 100 fruit flies were lured to their death.  Stealing a line from Robert Armstrong, “Oh no, it wasn’t the banana that killed the beast, ‘twas the fragrant odor of yeast.”



We thank Liz and her buggy bananas for providing the inspiration for this episode. The interesting references “Trapping spotted wing drosophila, Drosophila suzukii (Matsumura) (Diptera: Drosophilidae), with combinations of vinegar and wine, and acetic acid and ethanol” by P. J. Landolt, T. Adams, and H. Rogg; “Spotted Wing Drosophila: Potential Economic Impact of a Newly Established Pest” by M. Bolda, R. Goodhue, and F.  Zalom; and “Flies, gnats, and midges” by W. A. Kolbe in “The Handbook of Pest Control”, were used in preparing this Bug of the Week.