BUREAU OF ENTOMOLOGY AND PLANT QUARANTINE 87 
country for insecticidal purposes. Several plant products were shown to be 
highly toxic to insects and indicate that sonic of these plants may have value 
for the commercial production of insecticides. 
In a search for insecticides that will he effective againsl insects hut will not 
be harmful to the plants or render them dangerous for human consumption, :i 
large number of organic compounds have been tested. In these tests mosquito 
larvae are used as test insects and those compounds that do not show toxicity 
to this insect arc eliminated. The compounds that arc toxic to mosquito 
larvae in concentrations of i^i) parts or less per million will he tested further 
on other insects. During the year some 250 compounds have been tested in this 
way. about 50 of which have proved to he toxic at concentrations of 20 parts or 
less per million. About 20 of these compounds were azo compounds, and this 
group apparently contains a number of compounds promising as insecticides. 
INSECTICIDE INVESTIGATIONS 
The chemical investigations on insecticides for the year followed closely the 
lines pursued for several years past. The only essential change in the organiza- 
tion was the reopening of the tield laboratory at Whittier, Calif., t<> resume the 
study of fumigation of the California red scale. 
Forty-three puhlications dealing with the many studies of the Division of 
Insecticide Investigations were issued, mostly in journals outside the Depart- 
ment. Numerous other articles were prepared and are awaiting review or 
printing. Three public-service patents were issued to members of the Division. 
CHEMICAL INVESTIGATIONS ON INSECTICIDAL PLANTS (TOBACCO, DERRIS, 
PYRETHRUM, ETC.) AND THEIR CONSTITUENTS 
The study of pyrethrum powder and its active constituents, the two pyreth- 
rins, was continued, but no satisfactory method was found for the isolation 
of the pure compounds in quantity sufficient to permit of a comprehensive com- 
parison by entomologists of their relative toxicity. It was discovered, however, 
that the formulas heretofore accepted for these materials are incorrect, and 
the new formulas established by this work explain satisfactorily all of the 
recorded reactions of the compounds, including some that could not he recon- 
ciled with the older formulas. The new formulas identify one of the derivatives 
of the pyrethrins, namely, tetrahydropyrethrolone with dihydrojasmone, the 
odorous principle of the jasmine flower, indicating the possibility of synthesis 
of related compounds. It was further demonstrated for the lirst time that the 
pyrethrins are present in fresh pyrethrum flowers, and are not formed during 
the drying of the flowers. The flowers of the common daisy (Chrysantliemum 
leuoanthemum) were examined by the new methods developed for the study of 
pyrethrum, and although both methoxyl-containing and acid-reacting materials 
were found in appreciable quantities, no pyrethrin semicarbazones could he 
isolated; hence the pyrethrins are absent from flowers of the daisy, and this is 
in agreement with the known worthlessness of daisy flowers as insecticides. 
Recent reports by other investigators have indicated that nicotine occurs in 
the tobacco plant as a glucoside which is more poisonous to dogs and guinea 
pigs than is nicotine itself. Investigations by the Division show thai this is 
not the case in the one sample of Texas Cuban tobacco examined, but that ir 
may ho partly true of Maryland tobacco, and the subject is being more fully 
investigated. The fixed-nicotine preparation known as nicotine peat, originally 
developed in the Division, was made on a semicommercial scale, and several 
hundred pounds were obtained for large-scale held tests, a method of making 
nicotine dusts from which the nicotine is given off very rapidly was developed 
ami should prove useful in the control of pea aphids and similar in-. :ts where 
it appears necessary to establish a high concentration quickly. New micro- 
analytical methods for the determination of small quantities of nicotine were 
developed as an aid in the study of fumigation with this compound. 
Work was continued with rotenone-hearing plants, mainly derris and cube. 
This dealt largely with improve incuts in the methods of analysis and Led to a 
much better understanding of the conditions necessary for t lie complete recovery 
of the rotenone content. A method was also proposed for the calculation of the 
approximate toxic value of a sample from a consideration of both the rotenone 
and total extractive contents, since it i- recognized that it is not satisfactory 
to rely on the rotenone figure only. The unreliability of optical rotation as a 
means of estimating the rotenone content of derris or cube was demonstrated. 
and a study was begun of the water-soluble constituents of these plant.-. 
