This article is from the Food Preserving FAQ, by Eric Decker email@example.com with numerous contributions by others.
Given that a lot of people here might be working with fresh fruit, here's
an excellent way to keep the kitchen fruit fly population down. I learned
this from my brother, who works in a research lab where escaped fruit flies
are always a problem.
Materials: 1 glass jar; 1 piece of paper and a piece of tape, or a plastic
baggie and a rubber band; a little *cider* vinegar (not white vinegar), or
wine or beer; a couple of drops liquid soap or detergent.
Procedure: Tape the paper together to make a funnel shape that will rest
inside the mouth of the jar, but have a fairly broad opening. Or, tear a
hole in the corner of a baggie, put it in the jar as a funnel, and secure
it around the rim using a rubber band. Put cider vinegar (or wine or beer)
in the bottom of the jar (1/4 inch or 0.5 cm or so). Add a couple of drops
of detergent to the vinegar. Place the paper funnel on the jar. Set on the
kitchen counter near the fruit.
How it works: Flies are attracted to the cider vinegar, which they interpret
as decaying fruit. They go into the jar (the funnel makes entry easier than
exit) and either fall onto or land on the surface of the liquid. The
detergent decreases the normal surface tension, so they sink and drown. Easy
We tested this at our parents' house when the apple crop came in. A single
trap caught >100 flies in 2 days.
Acknowledgment: Thanks to firstname.lastname@example.org who suggested the baggie
method last time I posted this, and to others who suggested beer/wine.
[Little bits of overripe fruit and cheap sherry are also irresistible to