In view of the possible importance of Synosternus pallidus Taschenberg as a vector of plague to humans, field experiments are described which tested the residual action of DDT upon adults of this flea species in native huts of the Dakar region of French West Africa.
Data are also given on the immediate effect of DDT in relation to the biting activity of S. pallidus.
A 5% DDT-kerosene solution, when paint sprayed directly upon the floor or air saturated, gave an approximate 100% initial kill of adult S. pallidus and a residual kill which extended to 64 days or more.
A 10% DDT-talc dust, when applied to the floors of native huts, gave initial and residual kill of S. pallidus adults comparable to that of a 5% DDT-kerosene spray. The DDT dust seemed to produce a slightly lower initial kill and a slightly longer residual action than the spray, although the difference was not significant.
Dosages in excess of 100 mg. of DDT per square foot showed no appreciable advantage both as regards initial knock-down and residual action against S. pallidus.
The air saturation of native huts with a pure kerosene spray gave a poor initial kill and no residual effect of any significance on S. pallidus.
The biting activity (host seeking) of adult S. pallidus was inhibited approximately 10 minutes after the floor of a native hut was sprayed with a 5% DDT-kerosene solution at the rate of about 100 mg. of DDT per square foot.
Adult S. pallidus exhibited typical DDT-toxicity in a minimum of 10 minutes after floors were treated. These fleas died in from 3 to 5 hours after contact with DDT.
The initial and residual kill by DDT of S. pallidus, on a large scale under practical field conditions, suggests that this may be an effective addition to established procedure in the prophylaxis and control of flea-borne disease. This would obtain especially in conditions where the flea species is highly domestic, feeds on humans, and is known to be a vector of plague or other diseases from man to man.
Second Lieutenant, Sanitary Corps, Army of the United States.