We have been in SP the end of October the last two years, and will be there the 29th this year. Take a good repellant (2 cans) and you will be fine. Mostly a problem in some of the passageways by the rooms after dark. The breeze usually keeps the little buggers under control.
**********Little Known Useless Info*********
The official San Pedro "Skeeter Season" started this year on August 16th at 4:45 pm. A terrific wave of rain swept in from the reef at 4:30 and lasted 15 minutes (zero visibilty), at 5 pm the sun returned and 4 billion mosquitos woke up and visited with us for the next 8 days. They are still there, we are not. Nothing serious, just a minor irritation. Don't try the Skin So Soft stuff...you need higher % DEET to be happy.