Cut off the food sources. Fruit flies eat primarily fruit, so start by banishing all fresh fruit to the refrigerator or other airtight storage until the fruit fly infestation is eliminated. Other common sources of food for fruit flies include residues in garbage disposals and beer, wine and soft drink containers. Scrub sinks thoroughly, flush the garbage disposal, remove the recyclables from the house, and wash out recycling bins and trash cans with mild detergent.
Step2
Cut a semicircle from paper. The semicircle should be about twice the diameter of the glass. For a 3 inch diameter glass, cut a semicircle whose flat side is about 6 inches long.
Step3
Bring the two corners of the semicircle together, overlapping slightly, to bend the semicircle into a cone shape. Tape the ends together. Trim so the cone sits on the glass, pointed side down, without leaving gaps around the edge, with no more than 1/4 inch overhang above the lip of the glass.
Step4
Snip a small piece off the point of the cone, leaving a hole about the size of a pencil eraser.
Step5
Pour a quarter inch of vinegar into the glass and place the paper cone in the top of the glass.
Step6
Place the trap near where the largest concentration of fruit flies is observed. Attracted by the smell of the vinegar, the flies will fly down through the paper funnel and into the glass, but cannot get out again.
Step7
Empty the trap at least once a day by taking it outside, well away from any doors, and removing the paper cone. Most of the fruit flies will fly right off. Blow into the glass to coax any stragglers out of the trap. Reset and keep emptying until the fruit fly infestation is cleared up. Refill as needed to keep at least a quarter inch of vinegar in the glass.
Step8
When the flies are gone, simply wash out the cup and throw away the paper cone.
Tips & Warnings
Full elimination of a fruit fly infestation may take several days
Some fruit flies will fall into the vinegar and drown. While this does not affect the trap, you may wish to dump the vinegar and dead fruit flies out and put fresh vinegar bait in if more than half a dozen or so flies are floating in it.