BUREAU OF ENTOMOLOGY AND PLANT QUARANTINE 29 
and black blow fly, 14 percent. More flies were trapped around farm 
buildings than in open areas. 
Radioactive DDT Used to Study Sites of Action of Insecticides 
Studies of the distribution of radioactive DDT in house flies showed 
that approximately TO percent of the insecticide was absorbed by the 
cuticle and the rest by the internal organs and tissues. More DDT 
is required to kill house flies when absorbed through the dorsal sur- 
face of the thorax from local applications than when absorbed through 
the feet from residual deposits. Absorption of DDT into the cuticle 
continues after flies have been killed with DDT. 
Mosquito larvae exposed to DDT suspensions in water showed 
higher kills at 70° than at 95° F. However, the amount of DDT 
absorbed by the larvae was about the same at each temperature. 
Bio-assay of acetone extracts from the two groups of second-instar 
larvae used as test insects showed that the insects exposed to DDT 
at 95° F. had decomposed a higher percentage of absorbed DDT. 
Clear Lake Gnats Controlled With TDE Larvicide 
No adults of the Clear Lake gnat were found during 1950 in Lake 
County, Calif., by local mosquito- and gnat-abatement officials. Sev- 
eral years of research by this Bureau, in cooperation with California 
State agencies, to develop control measures for the Clear Lake gnat 
culminated, in the fall of 1949, in the treatment of the 40,000-acre 
lake with TDE. Apparent complete control of the gnat larvae was 
reported last year. The absence of the Clear Lake gnat in the summer 
of 1950 confirmed the efficacy of the treatments. 
Cultivated Heliopsis Plants Yield Extract More Toxic to House Flies 
Than That From Wild Plants 
Petroleum-ether extracts prepared from the roots of Heliopsis 
scabra and II. parvi folia plants, grown in experimental plots at 
Sacaton, Ariz., and Beltsville, Md., by the Bureau of Plant Industry, 
Soils, and Agricultural Engineering, proved to be more toxic to house 
flies than extracts from roots of the parent wild plants. The extrac- 
tive from 6-month-old H. scabra plants grown at Sacaton was more 
toxic than that from the 11-month-old Beltsville plants, but the 
reverse was true of H. parvifolia. The extractive from the II. scabra 
root grown at Sacaton was more toxic to house flies than that from 
any sample of the plant previously obtained. 
A sample of Heliopsis scabra collected at Cloudcroft, N. Mex., on 
extraction yielded the highest percentage of scabrin (0.37 percent 
of dry root) thus far obtained from the species. 
Heliopsis helianthoides was found growing abundantly in a field 
near Circleville, W. Va. The extractive from a sample of this root 
was separated into three fractions, one of which appeared to consist 
mainly of scabrin. 
In tests against German roaches, scabrin was toxic in refined kero- 
sene solution, but not very effective when applied as a pyrophyllite 
dust. Refined kerosene solutions of scabrin were highly toxic to 
house flies when tested by both the turntable and Peet-( J rat I v methods. 
