Pesky Little Flies

Fruit flies have stout bodies and red eyes
Fruit flies have stout bodies and red eyes.

By Laura Jesse
Iowa State University Extension

It is never fun to walk into the kitchen and notice little flies resting on the walls and cupboards. Where did these flies come from?  And most importantly, how do I get rid of them?

There are a handful of common flies that we find in homes. Two of the common small flies are fruit flies and fungus gnats. Fruit flies have stout bodies and red eyes. Fungus gnats look like small mosquitoes. Both flies are less than one-fourth of an inch in size.   

We consider these flies a nuisance pest because they don’t damage anything. Like all flies they have a complete life with an egg stage, larval stage, pupal stage and adult stage. The adult flies are the annoyance, but it is the larval flies that we need to locate and control. 

These flies get into homes from the outdoors or occasionally are brought in on rotting fruit. Household insecticide sprays labeled for fly control can be used to eliminate the adult flies that are present at the time of treatment but this will be only a temporary relief at best. If you want to get rid of the flies you need to focus on locating where the larvae are feeding. Fly breeding areas are occasionally very difficult to locate and perseverance and imagination will be required. Regardless of where the flies originate, the adults will be seen at windows and sinks, as they are attracted to light and to moisture.

Fruit flies can reproduce anywhere there is fermenting organic matter that stays consistently wet or moist. Rotting fruit is one possibility, but it takes almost two weeks for larvae to develop into adult flies. If you are a better housekeeper than I am, it is likely you will have thrown away any rotten fruit or vegetables before the flies have time to complete their life cycle. 

The most likely of such sites in the home is a slow-moving or seldom-used sink, bathtub, shower or floor drain in which a layer of slime (gelatinous film) has built up above the water line.One way to check individual drains is to cover the drain with a plastic film taped to the floor or fixture. If the flies are breeding in that drain, the adults will accumulate underneath the film within a day or two. Other moist accumulations of fermenting organic matter are possible and should be considered. These include wet areas under dripping pipes and refrigeration equipment, garbage containers and discarded bottles and cans.

The most effective method to eliminate fruit flies developing in drains is to clean the inside of the drain pipe to eliminate the organic matter. Clean slow-moving drains with a stiff brush or other tool. Drains that cannot be scrubbed can be rinsed with water under high pressure or "sterilized" by slowly pouring boiling water down along the sides of the drain pipe. Another possibility is the use of a bacterial drain treatment that biodegrades the organic matter. Follow label directions carefully for best results. There is no benefit to treating drains with bleach or ammonia.

Fungus gnats are frequently quite plentiful outdoors in fungi, damp soil and decayed vegetable matter. Though fungus gnats occasionally wander in from outdoors, a persistent problem with this nuisance in the house indicates an indoor breeding site. The immature stage of the fungus gnat is a small white maggot that lives in very moist areas high in decaying organic matter. This habitat may occur indoors with houseplants or in slow-running drains, moisture-accumulating cracks and crevices, refrigerator drain pans and other places where fungi and slime accumulate.

When houseplants are infested, it is often because they are overwatered. Fungus gnats cannot survive in houseplants if the soil is permitted to dry out almost to the leaf-wilting point between waterings.Otherwise, houseplant insecticide spray can be applied to the surface of the soil and around the edges of the pot.

Laura Jesse, Entomology, (515) 294-7400,
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,

There is one photo available for this week's column (this photo is 300 dpi, but only about one-half inch wide):
Fly123005.jpg [48 KB]