Synthetic pyrethroids are synthetic derivatives of natural occurring pyrethrins and are used by the District to control yellowjackets and adult mosquitoes. First generation synthetic pyrethroids (e.g. allethrin) are more active than natural pyrethrin but are unstable in sunlight, breaking down very quickly. Second generation synthetic pyrethroids are more resistant to degradation by light and air and therefore can last longer than first generation synthetic pyrethroids which typically persist less than one day.