Like jno,I never came across any mosquitoes in Seville.Although it is on the Guadalquivir River it is in a sufficiently dry area not to have these pests.
May is one of the best months to visit Seville,as it's warm without being as hot as July,August and Sept.
Seville is one of the largest untouched historic Spanish cities,the "real" Spain.
It is also not that big a city to cover on foot,as the most historic centre is quite compact,and most of the best sites are quite close together.
One thing,Spaniards don't start eating till about 9pm(at the earliest) but you CAN get a meal at usual(UK) times,and the restaurants shouldn't be too busy.Much of Seville(like most of non tourist Spain)used to have a siesta (with most places closed) from about 3pm to 6pm,but this is fast changing now that Spain is in the EU.
These sites have good information on Sevile.
http://en.wikipedia.org/wiki/Seville
http://www.andalucia.com/cities/sevilla.htm