Some Insecticidal Properties of the Fatty Acid Series

The use of soaps as contact insecticides is of long standing in economic entomological practice, although there has apparently been no serious investigation leading toward the determination of the active principle chemically responsible for their value as such. References available to the writers attribute the toxicity of soaps to their alkaline ingredients, expressing the belief that their other constituents, the fatty acids, are practically inert in their action toward insects. In the belief that the results obtained through an investigation of this problem during the past year justify their presentation for the consideration of other investigators, this brief preliminary paper, including some of the theoretical aspects and a promising practical application to insecticide entomology, is herewith presented. Soaps have been defined as the alkali salts of the fatty acids. Since those most commonly in use are formed from the fatty acids containing an even number of carbon atoms united in a straight chain, or the normal, saturated monocarboxylic fatty acids found in nature, this series was chosen as the subject of the tests first conducted, as herein outlined. A complete series of the even-carbon acids through stearic , acid was tested, several of the oddcarbon acids being likewise compared. When it was possible to obtain them, purified acids were used; although the technical grades were employed when necessary. The following homologues of the series CNH2N02 were available: Acetic, propionic, butyric, valeric, caproic, oenanthylic, caprylic, pelargonio, capric, lauric, myristic, palmitic, and stearic acids. The results obtained on the available species of aphids were striking. While no great toxicity has been encountered with the lower homologues, the caproic acid showed marked toxicity, and tests of the next in the series, caprylic acid, were gratifying. Kills of more than 90 per cent of the black chrysanthemum aphid were attained at 1 to 500 dilutions, with only slightly decreased results when the water proportion was increased to 1 to 1,000. Capric acid, the next even-chain homologue, killed more than 99 per cent of the green apple aphid at a dilution of 1 part ta 1,200 of water. Lauric acid at the same dilution was fatal to 92 per cent of the same aphid, while myristic acid at 1 to 1,200 killed 78 per cent of the same species. Paralysis is complete and practically immediate when aphids are subjected to toxic strengths of the fatty acidsIt was noted in tests on the black chrysanthemum aphid in comparison with nicotine sulphate (commercial 40 per cent) that in the case of the latter poison the mortality was indicated by the large percentage of dead aphids which dropped from the sprayed plants ► In the case of the fatty acids, the dead insects remained attached by their inserted beaks, affording no gauge of toxicity, such as was shown by the nicotine, until the actual counts were made. The fatty acids tested were applied in the free form, and as acid, neutral, and alkaline soaps, using potassium, sodium, and ammonium bases. In the free form, they were emulsified by means of various stabilizers, and when used with distilled water alone, by means of a colloid mill. In the later part of the investigation a solution of glue was used as a standard emulsifier, providing ample stability for uniform experimental tests. Throughout the study parallel tests, using a commercial 40 per cent nicotine sulphate solution in combination with soap, were conducted for comparison, and ample check or untreated material was kept under observation. Practically all tests were made on a laboratory scale in an outdoor insectary. The insects were thoroughly wetted by means of a small atomizer drawing from the bottom of the chamber. It is hoped that at a somewhat later date detailed information as to the results of the season's work may be presented.