Similar to the fly trap shown in an earlier Instructable, but (a) very simple to build and (b) specifically designed for those little buggers that love the produce that is so expensive or hard to grow.
Step 1: Materials
Sheet of heavy paper (I use cheap cardstock)
Small plastic jar, preferably clear (I use peanut butter jars)
Hot glue gun
Step 2: Doorway for the Little Creeps
Cut a circle of cardstock about 8" diameter. Cut a slit to the center of this circle. Precision is not important.
Step 3: Roll Into a Cone
Roll the circle into a cone. Again, precision isn't important but the cone top must be significantly larger than the jar mouth. You can use hot glue to seal. I used a strip of tape.
Step 4: Cut an Entryway
Snip off the tip of the cone just a bit. You want a hole about 1/8" diameter. Too small, and the fruit flies can't get in. Too large and they can find their way out.
Step 5: Assemble
Pour a half-inch of vinegar into the jar.
Run a bead of hot glue on the jar opening.
Glue the cone into the opening.
If there are any gaps between cone and jar, seal with hot glue.
Place the finished trap near your fresh fruit.
Mine caught eight flies in less than an hour.
After a few days, take it outside, immerse in soapy water to kill the flies that aren't already dead, and make a new trap.