Indeed they do. And quickly. If you find a fruit fly in your glass, fish it out or dump the wine immediately. Aside from the unpleasant prospect of swallowing a bug, the alcohol in the wine softens the fly’s body and it releases a nasty-smelling enzyme into the wine.
If you have never recognized the smell of drosophila melanogaster, just pour 2 oz. of wine into a glass, set it out, and wait for the little buggers to fall in. While they are drowning, pour another 2 oz. into a fresh glass. Smell and taste the fresh glass. Then smell the drosophila glass. Once you learn to detect that aroma, you will know it anywhere. I can smell one fly in a bottle of wine, in the dark.
During harvest, wineries are heavily populated with fruit flies. Thousands of them will congregate around fermentors, tanks, and equipment. According to LiveScience, they can find wine or fermenting juice from half a mile away. This is one of many reasons that constant hygiene is important in a winery. The CO2 rising off fermenting must provides a blanket of gas that deters fruit flies from setting foot on fermenting fruit, but few winemakers trust that to last long—as the wine approaches dryness, the fermentation slows, and the blanket of gas decreases.
When visiting a winery during harvest, there a couple of things to watch for—a complete lack of fruit flies in a tasting room that is near the cellar or crushpad probably means the winery is employing chemical bombs at night.
If you have a chance to tour the winery cellar or crushpad during harvest, look at the equipment and notice if it is being cleaned regularly, and certainly after every use. The floor and pad should be hosed and swept at the end of each day, and the drain thoroughly rinsed. Pressed must, stems, etc. should be removed from the area as quickly as possible.
Even with attentive action, fruit flies are a common population around harvest, along with yellowjackets, bats, and tarantulas. Your best bet is to accept small pours, keep an eye on your wine, fish the little buggers out with your finger or ask for a new pour, and be patient. The lifespan of drosophila is very short—just a few hours. As nights grow colder they eventually cannot survive and disappear. Until next year …