Not so much a problem here in the UK but if your riding abroad they can be a nuisence.
Make sure you rub insect repellant over any exposed areas. Don't just do it once and think you'll be protected all day. Apply it at regular intervals during the day.
The other thing you can do it to dink Tonic water. Tonic water contains quinine which mosquitoes don't like. The other thing is to eat food containing hot chilli's it also wards them off.
