We are once again at the time of the year when the perennial annoyance of "gnats" in the house is driving people buggy. The difficulty is not the occasional fruit fly buzzing around fruit on the countertop, but rather large numbers of fruit flies appearing at windows and over sink drains every day.
The indoor gnat problem of the fall is usually the common fruit fly (Drosophila). This is a small, red-eyed, fly only 1/4 inch long with alternating light and dark stripes on the abdomen. Although commonly associated with overripe and fermenting fruits and vegetables, fruit need not be present to have fruit flies, as they can originate from other sources.
Fruit flies can reproduce anywhere there is fermenting organic matter that stays wet or moist. In the house, likely places for flies to breed include slow-moving or seldom-used sink, bathtub, shower, or floor drains in which a layer of slime (gelatinous film) has built up above the water line. Other moist accumulations of fermenting organic matter are possible such as wet areas under dripping pipes and refrigeration equipment, garbage containers, and discarded bottles and cans. Regardless of where the flies originate, they will be at windows throughout the house and at sinks because they are attracted to light and to moisture.
Controlling fruit flies requires locating and eliminating the source of the infestation. Fly breeding areas are occasionally very difficult to locate and perseverance and imagination will be required. Check first the possible breeding sites given above. One way to check individual drains is to temporarily cover the drain with a plastic film (e.g., Saran Wrap ) 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.
Cleaning slow moving drains with a stiff brush may be all that is required. 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. Other moisture problems should be corrected as practical.
Household insecticide sprays for flying insects 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. Eradication will require eliminating the source of the infestation. Fruit flies are usually a temporary annoyance as they eventually disappear on their own without a source ever being determined.
This article originally appeared in the October 14, 1994 issue, p. 148.