© Stuart M Bennett 2009
Drosophila funebris
(Fruit or Vinegar Fly)

If you have been seeing small flies or gnats in your kitchen or supermarket, especially around the fruit and vegetable areas, they're probably fruit flies. Fruit flies are a problem all the year round, especially now that we have super markets and centrally heated homes both of which allow the fly to breed at full stretch

Tomatoes, melons, squash, grapes and other perishable items brought in from the garden are often the cause of an infestation developing indoors. Fruit flies are also attracted to rotting bananas, potatoes, onions and other unrefrigerated produce purchased at the grocery store.

Biology and Behavior:

Fruit flies are common in homes, restaurants, supermarkets and wherever else food is allowed to rot and ferment. Adults are about 1/8 inch long and usually have red eyes. The front portion of the body is tan and the rear portion is black.

Once the female has been located by smell, sound or sight, she may turn out to be coy and despite all the males efforts she will need to be wooed. This is not because she is fussy, the courtship ritual ensures that the male belongs to the right species and the right sex. Paradoxically, some of the most complex rituals seem to have been evolved by the least charismatic of insects. The fruit fly female will only make up her mind once she and her prospective mate have gone through a preliminary dance. It begins with the male approaching her and drumming on her head with his front feet. This then releases the next stage of the dance where they shuffle from side to side whilst facing one another. Finally, the male feels confident enough to spread out his wings and twist their leading edge downwards, a simple flourish that seems to be full of meaning to her because she now allows him to mount her.

Fruit flies lay their eggs near the surface of fermenting foods or other moist, organic materials. Upon emerging, the tiny larvae continue to feed near the surface of the fermenting mass. This surface-feeding characteristic of the larvae is significant in that damaged or over-ripened portions of fruits and vegetables can be cut away without having to discard the remainder for fear of retaining any developing larvae. The reproductive potential of fruit flies is enormous; given the opportunity, they will lay about 500 eggs. The entire lifecycle from egg to adult can be completed in about a week.

Fruit flies are especially attracted to ripened fruits and vegetables in the kitchen. But they also will breed in drains, garbage disposals, empty bottles and cans, trash containers, mops and cleaning rags. All that is needed for development is a moist film of fermenting material. Infestations can originate from over-ripened fruits or vegetables that were previously infested and brought into the home. The adults can also fly in from outside through inadequately screened windows and doors.

Fruit flies are primarily nuisance pests. However, they also have the potential to contaminate food with bacteria and other disease-producing organisms.

Prevention:

The best way to avoid problems with fruit flies is to eliminate sources of attraction. Produce which has ripened should be eaten, discarded or refrigerated. Damaged portions of fruits and vegetables should be cut away and discarded in the event that eggs or larvae are present in the wounded area. A single rotting potato or tomatoe forgotten at the back of a display, or fruit juice spillage under a refrigerator can breed thousands of fruit flies. So can a recycling bin stored in the basement which is never emptied or cleaned. So we can see straight away that the best cure, as always, of treating problems such as this housekeeping on a regular basis. Housekeeping in these sorts of areas needs to be accountable, hence, some sort of checklist should be made available for the person who is assigned the job to fill out and to sign,then there is no excuse.

People who can their own fruits and vegetables, or make wine, cider or beer should ensure that the containers are well sealed; otherwise, fruit flies will lay their eggs under the lid and the tiny larvae will enter the container upon hatching. Windows and doors should be equipped with tight-fitting (16 mesh) screens to help prevent adult fruit flies from entering from outdoors.

Eradication:

Once a structure is infested with fruit flies, all potential breeding areas must be located and eliminated. Unless the breeding sites are removed or cleaned, the problem will continue no matter how often insecticides are applied to control the adults. Finding the source(s) of attraction and breeding can be very challenging and often will require much thought and persistence. Potential breeding sites which are inaccessible (e.g., rubbish disposals and drains) can be inspected by taping a clear plastic food storage bag over the opening overnight. If flies are breeding in these areas, the adults will emerge and be caught in the bag.

After the source of attraction and breeding is eliminated, a pyrethrum-based, aerosol insecticide may be used to kill any remaining adult flies in the area. Also these days there are bait stations which are quite inocuous and can actually be placed in say a display of produce even in a supermarket

Interest Note:

What is it and why bother about it? Drosophila melanogaster is a fruit fly, a little insect about 3mm long, of the kind that accumulates around spoiled fruit. It is also one of the most valuable of organisms in biological research, particularly in genetics and developmental biology. Drosophila has been used as a model organism for research for almost a century, and today, several thousand scientists are working on many different aspects of the fruit fly. Its importance for human health was recognised by the award of the Nobel prize in medicine/physiology to Ed Lewis, Christiane Nusslein-Volhard and Eric Wieschaus in 1995.

Why work with Drosophila? Part of the reason people work on it is historical - so much is already known about it that it is easy to handle and well-understood - and part of it is practical: it's a small animal, with a short life cycle of just two weeks, and is cheap and easy to keep large numbers. Mutant flies, with defects in any of several thousand genes are available, and a project is now underway to sequence the entire Drosophila genome.

Life cycle of Drosophila: The drosophila egg is about half a millimeter long. It takes about one day after fertilisation for the embryo to develop and hatch into a worm-like larva. The larva eats and grows continuously, moulting one day, two days, and four days after hatching (first, second and third instars). After two days as a third instar larva, it moults one more time to form an immobile pupa. Over the next four days, the body is completely remodelled to give the adult winged form, which then hatches from the pupal case and is fertile after another day. (timing is for 25C; at 18, development takes twice as long).

Research on Drosophila: Drosophila is so popular, it would be almost impossible to list the number of things that are being done with it. Originally, it was mostly used in genetics, for instance to discover that genes were related to proteins and to study the rules of genetic inheritance. More recently, it is used mostly in developmental biology, looking to see how a complex organism arises from a relatively simple fertilised egg. Embryonic development is where most of the attention is concentrated, but there is also a great deal of interest in how various adult structures develop in the pupa, mostly focused on the development of the compound eye, but also on the wings, legs and other organs.

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