Fruit flies in the home: how to identify and eradicate them

Two types of fruit flies (three to four millimetres long) are common in households: the light brown fruit fly, with bright red eyes, and the dark brown fruit fly, with dark eyes (not red).

Fruit flies, despite their name, do not eat fruit at all. They eat yeast, the microbe that consumes fruit and emits alcohol. That’s why fruit flies are found around overripe fruits (and vegetables), especially when they begin to ferment. However, fruit need not be present to have fruit flies, as they can originate from other sources.

Fruit flies breed in fermenting organic material that stays consistently wet or moist. The most likely of such sites in the home is a slow-moving or seldom-used bathtub, sink, or floor drain in which a layer of slime has built up above the water line. Other source sites may include wet areas under dripping pipes and refrigeration equipment, garbage containers, indoor composters, and recycling bins. Adult flies are seen at windows and at sinks, as they are attracted to light and to moisture. (The life cycle of a fly is described in my article on cluster flies.)

How to control fruit flies

The best and only way to get rid of fruit flies is to get rid of what they like. Some useful tips:

• Overly ripe produce should be eaten, refrigerated, or discarded. Any damaged part of a fruit should be cut off and discarded since that’s where fruit flies often breed first. Wash bottles and cans during recycling, and clean out your recycling bin regularly.

• Check and clean your drains. One way to check individual drains is to cover the drain with a clear plastic bag taped to the floor or fixture, and leave it for a day or two. If the flies are breeding in that drain, the adults will emerge and be caught in the bag. Clean slow moving drains with a stiff brush. Drains that cannot be scrubbed can be “sterilized” by slowly pouring boiling water down along the sides of the drain. Bacterial drain cleaners that biodegrade the organic matter can also be used. Never pour insecticides down the drain.

• Some of the flies may emerge from open drains or standing water that has organic debris in it, like a mop pail. Eliminate standing water, if possible. If not possible, pour one tablespoon of vegetable oil down the open drain to create a thin film of oil on the top of the water.

• Several types of homemade traps effectively attract and kill adult fruit flies. A trap, for example, can be constructed by using a small amount of apple cider vinegar and soap in a jar.

• For current chemical control options, consult your local pest control representative. Follow closely the directions and cautions on the label.

Note: The fruit flies found in homes are the quasi-fruit flies (also known as vinegar or pomace flies) and belong to the family Drosophilidae; they do not eat fruit. The real fruit flies belong to the family Tephritidae and are serious pests of a number of fruit crops.

Rana Sarfraz is an entomologist and ecologist currently working at the University of British Columbia.