An insecticide formulation comprising i) a naturally occurring pyrethrin (e.g. pyrethrins I and II, jasmolin I and II and cinerin I and II) and ii) an unsaturated fatty acid or fatty acid mixture containing an average of from 16 to 24 carbon atoms (C16-C24) and/or their alkali metal salts wherein the unsaturated fatty acids are derived from natural sources and wherein the ratio of the weight of unsaturated fatty acid and/or alkali metal salt thereof to the weight of natural pyrethrin is from 3:1 to 20:1. Preferably, the unsaturated acid is a mixture of unsaturated acids. Preferably, the formulation has a pH below 7, most preferably, between 3.5 and 7. Preferably, the ratio of fatty acid(s) and/or specified derivatives to pyrethrin is from 3:1 to 10:1. Preferably, the unsaturated fatty acids are unsaturated linear acids. Most preferably, the fatty acid composition comprises oleic acid and linoleic acid.
