Masses of tiny flies. You probably call them fruit flies, but true fruit flies are a different species and infest fruit in orchards. Entomologists call them vinegar flies because they're attracted to any souring, fermenting, or liquefying vegetable matter.
Where they live
They are found everywhere in North America.
Their life cycle
Vinegar flies become sexually active only two days after emergence, but hey, they only live eight to ten days.
Plants they attack
Some species are small enough to fit through window screen mesh—they are attracted to light—but mostly they hitch a ride into your home on produce. Besides fruit, other places vinegar flies breed and eat are in potatoes, onions, cracked tomatoes (usually infested while still on the vine), and foods containing yeast; in beer cans, wet mops, sink drains, recycling or garbage bins, compost buckets, and worm beds.
Why they're a problem
In theory, vinegar flies can transmit disease, because they live around food and alight on both clean and contaminated surfaces. But unlike houseflies, there's no evidence that they do. They are primarily a nuisance.
Organic damage control
Cleanliness is next to flylessness. The best way to eliminate vinegar flies, according to Steven B. Jacobs, senior extension associate at Penn State University, is to use good sanitary practices. Don't allow overripe fruits or vegetables to lie around; either dispose of them in a lidded container or keep them in the refrigerator. Rinse beer cans and other containers before they go into the recycling bin or garbage can. Rinse mops with hot water and allow them to dry out. Pour boiling water down sink drains (bleach and ammonia don't work). Empty and rinse compost buckets daily. Cover food scraps in the worm bin with bedding, and remove any scraps the worms don't appear to be eating.