INSECT CONTROL INVESTIGATIONS—KNIPLING 
mula in order toimprove its insecticidal 
action and to conserve pyrethrum, 
and this was done in cooperation with 
other researchers in the Bureau. It 
was soon shown that DDT had effec- 
tive killing power against mosquitoes 
and especially against flies but lacked 
the rapid knock-down properties that 
pyrethrum possessed. Many tests 
were conducted with various concen- 
trations of DDT and pyrethrum in 
order to obtain the proper ratio of 
these insecticides. Various solvents 
and other auxiliary materials were 
employed in testing different formu- 
lations. —The outcome was a lique- 
fied gas aerosol formula developed in 
cooperation with L. D. Goodhue, 
E. R. McGovran, and others of the 
Beltsville Research Center, which 
contained 3 percent of DDT, 0.4 per- 
cent of pyrethrins, 5 percent of motor 
oil, and 5 percent of cyclohexenone. 
The U.S. Public Health Service con- 
ducted toxicological tests with this 
formula and judged it safe for use. 
As the research on aerosols was in 
progress, efforts were also directed 
toward the development of highly 
concentrated sprays. It was shown 
that within wide limits the amount of 
insecticide dispersed governed the kill 
of flies or mosquitoes regardless of the 
amount of liquid carrier actually 
atomized. For example 1 cc. of a 
20-percent DDT solution finely atom- 
ized in a given space was as good as 
20 cc. of a 1-percent solution applied 
in the same manner. The same 
principle held with pyrethrins. As a 
result of these studies, sprayers smaller 
than a flashlight were developed, 
which contained the insecticide equiv- 
alent to about 2 gallons of ordinary 
household-type pyrethrum sprays. Al- 
though such devices and spray form- 
ulas were not utilized during the 
war, primarily because the aerosol 
bomb had been supplied and dis- 
tributed in adequate numbers, the 
investigations definitely established the 
potentialities of highly concentrated 
solutions for use as sprays. 
Residual _treatment—The _ greatest 
339 
single advance in the control of insects 
of medical importance was the de- 
velopment of the residual or surface 
treatment concept of mosquito and 
mosquito-borne disease control. Even 
before DDT became available, the 
principle of controlling insects by 
means of surface treatments utilizing 
pyrethrins and other toxicants was 
under investigation at the Orlando 
laboratory. ‘The high degree of toxic- 
ity and stability of DDT seemed to 
offer promise for this method of insect 
control; consequently when this in- 
secticide was available it was tried 
with preliminary indications of suc- 
cess. Studies were therefore intensi- 
fied on this problem. It was not 
known at the time that R. Weisman 
of Switzerland had already demon- 
strated the possibilities of DDT as a 
residual treatment in controlling in- 
sects through his highly significant 
work on flies. The Orlando labora- 
tory independently developed DDT 
for fly control and proved its value 
for controlling mosquitoes and cer- 
tain other insects. 
Since the residual or surface treat- 
ment method of controlling mosqui- 
toes as well as other insects was new, 
many aspects needed investigation. 
Various kinds of surfaces were treated 
with DDT using different amounts 
and different kinds of preparations. 
Mosquitoes were confined for speci- 
fied periods of time in specially con- 
structed cages treated with the DDT 
(pl. 4, fig.1). Tests demonstrated that 
the killing action lasted for weeks, 
then months. Finally, after about 
8 months the laboratory tests were 
discontinued even though the better 
treatments were still killing adult 
mosquitoes. 
While the laboratory tests were still 
in progress, the principle of residual 
treatments was tried in actual field 
tests (pl. 4, fig. 2). In 1943 several 
types of buildings were sprayed with 
DDT. By the end of the mosquito 
season, as long as 70 days after the 
first treatments were applied, Anoph- 
eles mosquitoes entering the build- 
