Fruit flies are tiny little gnat-like flies that are usually apparent in kitchens, particularly in late summer when fruit is ripened. They are happy to lay their eggs in soil outside, but they also lay their eggs on any moist surface of fermenting stuff. Drains, waste disposal systems, empty bottles and cans, rubbish bags, cloths and mops are all perfect sources, as well as fruit and vegetables. They sneak in through open windows and doors or on the surface of fruit you have purchased
Wash any fruit you bring into the house and if you can, refrigerate fruit and vegetables to slow down the fermentation process. Empty rubbish bins regularly, or bag food items before disposal and keep sinks and drainage systems clean.