They usually hatch from fresh fruit and multiply as long as they have fruit to eat, even in the trash. If you want to make a fruit fly trap take a chunk of ripe banana and put it in a fruit jar and leave it out on your kitchen counter. They will swarm to the banana and when you see them put a lid on the jar & take it to the sink and run some water in it and drown them. You can reuse the chunk of banana and keep doing it till you catch them all. Make sure all other fruit is put away in the refrigerator etc. and empty all of your trash cans before doing this. We had a problem in a 12 story dorm building one May after the students moved out. Some of the late movers put their trash in the trash cans in the basement laundromats on a weekend and no one was working to empty the trash on the weekend since nearly everyone had already gone home. Imagine trying to catch all those fruit flies (in the basement where lounges, bathrooms, offices, and lounges were). It took a couple of days and we got them all this way.
