Do Coffee Grounds Really Keep Flies Away?

Unless you live in an underwater city (coming soon, according to science fiction), you're likely having seasonal battles with flies. Since these are reliably disgusting scavengers and disease carriers, it's normal to get excited when hearing about a hack to eliminate them. A video circulating on YouTube offers an enticing solution: burning coffee grounds. Supposedly, this produces a smell humans enjoy while promoting flies and other insects to leave you alone.

Are you curious about who in the insect world likes coffee and who doesn't? Circumstantial evidence suggests flies don't hate the aroma. Houseflies certainly aren't shy about hanging out in coffee shops, often causing these businesses to use pest control methods such as fly zappers. Also, as many coffee lovers know, some bugs (such as fruit flies) seem to enjoy hovering around an attractively damp coffee maker.

Pest control expert Guy Halvorsen shared with Homes & Gardens that the pungent smell of coffee covers up many of the scents bugs are drawn to — but as a pest deterrent, it's most effective against mosquitoes, cockroaches, and ants. Even he admits, "We have seen varying degrees of effectiveness in using burnt coffee grounds." While one of the unexpected ways to use coffee grounds in your home is as an insect repellant, the evidence is inconclusive that burning coffee grounds is effective at repelling flies.

Flies might be on the fence about coffee, but they're definitely not fans of smoke. They hate smoke's guts and consider it a sign of impending danger. Since this coffee-burning hack produces a fair amount of smoke, it seems likely that it would have a positive impact on your home's fly population. The YouTuber's directions are to pack coffee grounds into a small glass or cup, place a wick in the coffee, and dot the top with cloves. We gave this a shot, and following these directions, the wick wouldn't stay lit as soon as it burned close to the coffee. We attempted this because a horde of black flies had entered our home, but there was no smoke, fire, or fly repelling in this case.

Assuming it's possible to burn the grounds successfully, this could be a helpful trick. But burning a citronella candle is probably a better idea to smoke out flies (as well as mosquitos and other pests). You may need more than one or two candles for it to work, but the merits of citronella as a fly repellent are much more well-documented than burning coffee grounds. That said, regardless of which method you use, never leave an open flame unattended.