The tiny fruit flies that invade homes in the fall are scientifically known as Drosophila melanogaster. It's different from the other fly species called "fruit flies" that are serious pests of fruits while still ripening on the tree. The concentration of fruit farming in northwest Michigan is often blamed for the swarms of Drosophila that appear in the fall, but there is actually little connection between the two. The small fruit flies are often just as numerous and annoying in areas as far away as Gaylord and Grayling, where there are no orchards to blame.
The adult flies are able to detect minute traces of odors. They are strongly attracted to the odors of ripe fruits, vegetables, yeasts, fresh breads, wine, beer, vinegar, and certain soaps, shampoos or cleaning products. The slight residue of beverages in empty cans or bottles is enough to attract the adults.
It seems that the flies aren't around until late summer, but they're actually present in our area throughout the year. A relatively small number of them survive over the winter in protected areas that provide some insulation from the cold. In the spring, adults lay eggs in decaying plant material and other areas of high moisture where decay organisms like bacteria and fungi thrive. Each female may lay 400 eggs, and under favorable conditions the maggots that hatch can mature to adult flies and start the next generation in as little as seven days. Like compound interest, the numbers of fruit flies can increase very rapidly in a short time.
Warm and moist environmental conditions are most favorable to their survival and reproduction. Years with frequent significant rainfall, especially in the spring and mid-summer, seem to lead to the highest numbers of fruit flies in the fall. All sorts of plants can contribute to the fruit-fly banquet -- they don't have to be fruits. Decaying leaves, lawn clipping piles, compost bins with too much moisture and garbage cans can all be excellent breeding areas for Drosophila fruit flies.
It's virtually impossible to keep these tiny flies from entering homes. We can't eliminate all of the odor sources that attract them into homes, or we would have nothing to eat and no products to clean our homes or ourselves.
Here are some recommendations for the fruit-fly fight.
Indoors:
-- Securely cover or wrap food in the home, or keep it in the refrigerator. Check stored potatoes for signs of decay or softness, as these are very attractive to the adults.
-- Frequently clean kitchen surfaces to eliminate food residues. Use unscented cleaning agents if possible.
-- Keep windows closed during food preparation and meal time. Quickly remove food scraps from the house; don't keep scraps in a compost bucket or garbage can inside for any length of time. Rinse empty cans and bottles and store them outside if possible.
-- If you have a few "point sources" of attractive odors that you can't remove, a small fan aimed right at the source will disperse the odor and keep the flies from concentrating in such annoying numbers at that spot.
-- Prepare traps for the flies utilizing vinegar (red wine vinegar is the best) as bait. The simplest form of these is just an open jar containing about 1/2 inch of vinegar with a drop or two of dish soap added to break the surface tension.
Outdoors:
-- Collect and dispose of any fallen fruits from fruit trees and other ornamental plants. Don't water parts of the landscape that dry out very slowly. Mulch lawn clippings back into the turf, or if you collect them, be sure to spread them out thinly where they are being dumped.
-- Securely cover garbage cans. Turn compost bins regularly to disperse moist intact layers of fresh food scraps.
Dr. Duke Elsner is a Michigan State University extension agricultural educator in Grand Traverse County. You can contact him at elsner@msu.edu.
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