First we should ask
What's the target? Open-air jug traps draw whatever is attracted to a vinegar/ molasses mix, and that can include beneficial species. I'm not familiar with using rotten fruit ... but that sounds kind of after the fact if codling moth (for instance) is the goal. Soap breaks surface tension so more insects drown.
Spotted wing drosophila, the newest fruit fly on the block, is drawn to the smell of vinegar, which makes sense, since raspberries and other soft fruits subject to SWD larvae are going to ferment. See the
February 2014 newsletter for details on SWD traps. The cup trap "entrance" insures that only small insects like SWD will get into the trap.
It would be great if anyone who has ever tried an open-air jug trap reports what's to be found. Stink bugs? Moths? Mitch McConnell?
Lost Nation OrchardZone 4b in New Hampshire