Unless you're super vigilant about kitchen cleanup (which we are not), you'll be invaded by a swarm fruit flies at some point. An easy, cheap and nontoxic solution involves balsamic vinegar, a sheet of paper, and a mason jar.
To build the dungeon of death, pour about a half-inch of balsamic vinegar into a jar. Reach into the recycling bin and pull out a freshly mis-printed piece of office paper, roll into a funnel and put into the mouth of a jar.
Simply put: the flies are drawn down the funnel by the sweetness of the vinegar, go inside the jar and can't figure out how to get back out. They eventually drop to their deaths in the vinegar.
Sigh...kinda sad...but not really. Fruit flies are pretty gross.