Certain adjuvants have been shown to enhance the activity of pyrethroid insecticides, in particular the synthetic pyrethroids. Typical of these pyrethroids are alpha-cypermethrin, lambdacyhalothin, betacyfluthrin and deltamethrin. Respectively these are the active insecticide ingredient in the emulsifiable concentrates, Fastac (Cyanamid), Karate (ICI), Bulldock EC (Bayer) and Decis Forte EC (AgrEvo) and the Fastac ultra low volume (ULV) . The preferred adjuvants are based on the alkyl esters of fatty acids with the higher levels of unsaturation which appear as more effective penetrants of the waxy protective surfaces of insects. The ethyl esters appear most effective. Another adjuvant is based on the alkyl esters of dibasic acids such as di-isooctylmaleate.