In terms of the most frustrating pests to spot in your home, fruit flies are near the top of the list. They are incredibly persistent, they multiply at terrifying speeds, and they make your otherwise spotless kitchen feel immediately unsanitary. While there are DIY vinegar traps you can build once you have been invaded, the absolute best defense against a fruit fly swarm is cutting off their food supply at the source. You probably already threw out the overripe bananas and wiped down your sticky countertops. But if the flies are still hovering, there is one damp, dark spot that is practically rolling out the red carpet for them—and it is hiding in plain sight.