My glass of wine was buzzed by a fruit fly last week. A few hours later, it drowned in the dregs left in the glass. From early summer into fall, fruit flies are common in homes. There are also several species that live in warm winter areas of North America, and spread north each summer, rather like butterflies with northward migrations, but a lot less attractive. Fruit flies also overwinter in buildings, and live outside year round in warmer climates, hence the fly in the wine in November. Despite rigorous sanitation measures, it can be impossible to keep them out of your home by late summer. Flies spread around neighborhoods, with successive generations making their way indoors, following the scent of ripening fruits. Once indoors, they can live, and overwinter, hibernating until warmer weather returns.
The common fruit fly, Drosophila melanogaster, takes up residence in our homes each summer. Only three millimeters long, this fly can go from egg to adult in seven to ten days, and usually lives no more than thirty days. It is widely used in genetic studies, due to its short lifecycle and ease of maintenance. Females can lay one hundred eggs per day, and perhaps two thousand over one lifetime, so if you do nothing to stop them, your house will fill quickly with flies! Native to the tropics in Asia and Africa, this fly lives on all continents except Antarctica. It survives cold seasons by moving indoors, and then expands its range each summer. It also comes north in summer, riding along with containers of fruit. Here along the coast, this fly probably does not survive outside, as our winters are usually too cold. It overwinters outside in southern California and across the southern tier of states, as do other species of fruit flies.
Flies hang around ripening fruit, including berries, squashes, tomatoes, apples, bananas and stone fruits. They eat juices, and lay eggs in the fruit, and will also use vegetables. Actions that help reduce them indoors include: wipe surfaces to clean up after food preparation; do not store fruit or vegetables on countertops for days to weeks at a time; clean fresh fruits and vegetables as they come into your home, keep your garbage can covered, and the sink drains and disposal clean.
In a warm dry summer with ripening food outside as well as inside, you will inevitably have problems by August or September as the summer population explosion takes hold. If you have a compost pile outside, locate it well away from your door, so that flies cannot quickly and easily fly into your home. Placing a hummingbird feeder near compost piles helps too, as hummingbirds eat fruit flies and other small insects.
Commercial fly traps emit carbon dioxide, ethers and other yeasty scents. You can make your own traps using red wine, or a fermenting mixture of fruit juice with sugar and yeast, or apple cider vinegar with rotting fruit. Wine seems to disorient them the most, making it more likely that they will stay in the trap and drown in the wine. I have not tried a milk plus sugar and ground black pepper formula, but some people say this makes the best trap of all.
The trap requires a container. I use glass jars for traps; clear jars help you keep watch on how many flies have been caught, and whether the cover is keeping them inside. If they can walk out, then they will dip into the liquid for a drink––like a smoothie for humans––and then climb right back out and resume patrolling your kitchen, your house and even your face for food. They are tracking the scent of CO2 and fermentation juices when they fly around your face, not that this is appreciated by us!
Covers to keep the flies inside the trap include cones of paper with narrow openings at the bottom, taped down to jars, so that they cannot fly out. Long cones (more than 2 inches long) in tall jars (more than 6 inches tall) seem to work better than short ones; some flies figure out that they can walk around short cones to their freedom. Another cover is plastic wrap over the top of a jar with a few small holes in it, about one millimeter wide or slightly larger, pricked with a toothpick. They wriggle in via the holes, feed on the liquid inside, and then can't get out. Or use a plastic baggie with one corner snipped open (very small opening—just large enough to let the fly get in, 1-2 millimeters), placed over the top of the jar or glass. Smooth the baggie down inside to make a cone with the hole near the wine, vinegar or yeasty fruit juice inside the glass, and anchor it with a rubber band on the outside.
To dispose, you can dump the contents into soapy water for ten minutes, then wash down the drain, or dump outside, well away from any doors. Then you can clean and fill your trap for the next round. Cleaning and refreshing the trap every week seems to help get the last flies each fall. Mid to late fall is a good time to get the flies out of your house, and reduce the odds that they will overwinter indoors.