SOME INSECTICIDAL PROPERTIES OF THE FATTY ACID 
SERIES 1 
By E. H. Siegler, Associate Entomologist, Fruit Insect Investigations, and C. H. 
Popenoe, Associate Entomologist, Truck Crop Insect Investigations, Bureau of 
Entomology, United States Department of Agriculture 
The use of soaps as contact insecti¬ 
cides is of long standing in economic 
entomological practice, although there 
has apparently been no serious investi¬ 
gation leading toward the determina¬ 
tion 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 investi¬ 
gators, this brief preliminary paper, 
including some of the theoretical as¬ 
pects and a promising practical appli¬ 
cation 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 odd- 
carbon 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 CnH 2N 0 2 were available: 
Acetic, propionic, butyric, valeric, ca- 
proic, oenanthylic, caprylic, pelargonic, 
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 chrysanthe¬ 
mum 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 to 
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 acids- 
It 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 in¬ 
serted 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, provid¬ 
ing ample stability for uniform experi¬ 
mental tests. Throughout the study 
parallel tests, using a commercial 40 per 
cent nicotine sulphate solution in com¬ 
bination with soap, were conducted for 
comparison, and ample check or un¬ 
treated material was kept under obser¬ 
vation. 
Practically all tests were made on a 
laboratory scale in an outdoor insec¬ 
tary. 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. 
1 Received for publication Sept. 12, 1924—issued January, 1925. 
Journal of Agricultural Research 
Washington, D. C. 
(259) 
Vol. XXIX, No. 5 
Sept. 1, 1924 
Key No. K-138 
