Biting insects, including the 2,500 different species of mosquitoes, are not usually active (although they may survive) at temperatures below 10 degrees. Even in the north of Siberia, Verkhoyansk - one of the coldest places in the world - has temperatures above 10 degrees in summer, often above 20 degrees. Similarly midges breed in Scotland from April to October, and "Autan" is a good repellent. How long repellents last depends, among other factors, on the concentration bought (DEET varies from 20% to 95%). Temperature is important, but malaria is endemic in tropical or subtropical regions - and not elsewhere - for other reasons. There are a million deaths every year from malaria, 90% in Sub-Saharan Africa, shameful considering there is effective prevention and treatment at a cost, if only there were the resources available.