[almostnormal]

Has anyone tried CO2?

The idea: Deep trash can for the tasty banana peels. Flood the trash can with CO2. CO2 is heavier than air. If the trash can is deep enough maybe a sufficiently high concentration can be maintained without too frequent flooding.

The attractive smell should still be noticeble for the flies. And the environment around the trash of their desires is deadly.

[dekhn]

I have used CO2 to disable flies (they just go to sleep) so that I can put them in a venus flytrap. Although, in my case, I was doing this in a science lab so I had access to a specialized device (a porous marble stone with CO2 being diffused through it). Once they're disabled, you can put them in the trap, trigger the little hairs, and when the fly wakes up, the trap stays closed due to the fly wiggling around, and a month later they open up and a dessicated fly falls out.

[Turskarama]

I don't think it's _enough_ heavier than air to stay in there too long, it will diffuse out probably within only a couple of hours.