If you want to get back at flies do it now. They are very slow at reacting this time of year. You don't have to even try that hard. If it lands on your knee just slap it they barely move and usually not in time.
Fire pit evening?
AI:
Yes, flies are slower in the fall because their body temperature is dictated by the environment, and the cooling temperatures slow down their metabolism and flight ability. As temperatures drop, flies become sluggish.