Studies on Contact Insecticides

Summary. The toxicities to Aphis rumicis L. of the fatty acids from formic to stearic and of the sodium and ammonium salts and methyl esters, applied as spray fluids, have been quantitatively determined. Two un‐saturated acids, undecenoic and oleic, are included. There is a rise in toxicity of the acids with increase of molecular weight as the series is ascended from acetic to undecylic acid. Formic acid is exceptional. Beyond undecylic acid, there is a fall in toxicity, and acids higher in the series than tridecylic show only slight toxic action. The sodium salts of the fatty acids are in most cases much less toxic than the corresponding acids, though the difference is less marked with the higher acids. Oleic acid and sodium oleate are of the same order of toxicity. The ammonium salts are also generally less toxic than the corresponding acids, but the differences are much less than in the case of the sodium salts. With some of the higher acids, e.g . myristic and oleic, neutralisation with ammonia increases toxicity. The relatively high toxicity of the ammonium salts may be due, at least partly, to liberation, by hydrolysis, of free fatty acid in a very finely divided state. Methylation of the fatty acids reduces toxicity; all the methyl esters are less toxic than the acids or ammonium salts. Both the ammonium salts and the methyl esters show, like the acids themselves, increase of toxicity with increase of molecular weight up to a certain point. The formates are exceptional. The fatty acids do not show marked toxicity to the eggs of Belenia tetralunaria Hüfn. at concentrations below 2 per cent. Possible relationships between certain physical properties (physical state. volatility, dissociation constants, partition coefficients and surface tension) of the fatty acids and their insecticidal action are discussed. Determination of partition coefficients as between olive oil and water and comparison of the figures with the relative toxicities show a steady rise in toxicity with a decrease in the partition coefficients (water/oil) from acetic to capric acid. Formic acid is again exceptional. With lauric and oleic aoids there is a break in the correlation. The bearing of the solubility relationships of the acids on these results is considered.