Running through the plains, I always assume a deathsquito is stalking me. 80% of the time I am right.
Hear the buzz, block attack, then swing. Works every time--assuming you are geared for it.
When blocked, they pause for a second and it is a perfect time to counter. Don't need to parry. Just hold up the shield and they bump into it and wait a second for you to hit them.
Save their skeeter tails, tho. Your archer will thank you.