I live in a 1st floor apartment in the CBD. The yearly temperature range is 10-30 degrees Celsius. I've lived here 20 years and never treated my apartment. I can leave food on the bench all day in the middle of summer but when I get home I can count 3 or 4 flies in the whole apartment. When I go to work or a friend's house there are flies everywhere. When I get 6 flies in my apartment I start freaking out because I'm not used to them. This question has stumped me for 20 years.