Yeah YD1, the flies do seem to reduce down during colder months, but you don't need many to get the meat blown. You don't need a lot of flies to hang around, just enough over a day or 2 (or 3) to let them do their job (compared to probably 1 day in hotter weather). You don't want too many blowing the meat as there may not be enough meat/food for them whilst growing.
I haven't actual tried to breed gents during those times, mainly because I am only targeting Salmon.
But whilst I'm filleting the Salmon, flies do still turn up - unless it's bitterly cold, windy and wet day, then pro