Hi pateco,
Yes, screens offer a very good protection against mosquitos. On South shore of Montréal, There is a lot, a LOT of mosquitos during end of June until end of July. When we go sailing on the Lake Champlain (New York State and Vermont), there is no more mosquitos... it depends of the period of the year.
Screens are very useful and allow us to stay in the cockpit all the night without getting crazy and appreciating our rhum! They make a nice airflow possible. Since we used them for last summer, I woud'nt get to the boat without! One of my project for next spring is to make a new central window in my dodger with screen to allow more airflow when anchored in a bay.
Sebastien