For me it's always been a plate with strawberries placed inside the greenhouse and the fruit flies flying free and the plants catching them all naturally. Yes, it's a certain fermentation smell but nothing too bad. I just move the plate around and give all the plants possibility to get some fresh meat. It's just my lazy way of feeding dews and it will not work indoors or if you're sensible to fermentation smells. Works for me since i don't have time to grow them anyway!