>>Within the last two weeks or so, fruitflies (I assume) have appeared in
>>our house, mostly in the kitchen but no room is unaffected, and they seem
>>to be multiplying. I have removed all obvious sources of food - bowls of
>>fruit, etc. This doesn't seem to have helped.
Fruit flies are attracted to a number of odors, particularly fermenting
things. Beer, bread products, and vinegar (cider especially) are
particularly good (if you don't have any rotting fruit handy).
Traps commonly used in labs consist of a layer of attractant in the bottom
of a bottle, with a funnel inserted in the top. Flies go in but generally
can't get out because they *usually* don't want to fly much in an enclosed
space (or they just can't find the relatively small hole in the end of the
funnel) and so they tend to crawl upwards, which means they go up on the
bottle walls, but won't climb down the funnel to get to the exit. Not very
bright, but that's true of some people I know too, so I won't throw
stones. Be sure that there is no gap between the funnel and bottle lip, or
(obviously) the flies will walk right back out again: try sealing with
Saran wrap or something (we use parafilm in the lab).
The only problem is that beer and vinegar have high enough surface tension
that the flies won't sink readily, and therefore don't drown until they're
very weak and/or old. If you're into experimenting, you might try adding a
little dish detergent to the liquid to try and reduce the surface tension,
or just "uncork" the bottle outside periodically.
Chris Jones (jonesc at cshl.org)