A laboratory method for the evaluation of residual effectiveness, based on 15- and 30-minute exposures of 3-day-old adult flies to DDT residues, is described, and the following results were obtained:
(1)Although house flies were more susceptible than mosquitoes to fresh applications of DDT, the residual effectiveness of the deposits was lost more rapidly in tests using flies than in comparable tests with mosquitoes. This phenomenon was coupled with the presence of the insects themselves, but mechanical removal of the DDT deposits by the insects was apparently not a significant factor. The adult male house flies were more susceptible than the adult females to DDT residual toxicity.
(2)Laboratory evaluations showed deposits of 50 mg. DDT per sq. ft. to be markedly less effective than deposits of 100 and 200 mg. of DDT per sq. ft. Further field studies on the application rates of 100 and 200 mg. DDT per sq. ft. are being made to determine the most practical dosage for operational use.
(3)Experimental evidence indicated that in applications of DDT emulsions containing 2½, 5, and 10 per cent DDT to obtain theoretically equal deposits on paper, plywood, and rough wood surfaces, the type of surface treated was a more important factor than the concentration of the DDT in the emulsion spray in influencing residual effectiveness.
(4)Residues from 2½ per cent DDT as water-wettable suspensions showed more marked loss of residual effectiveness than residues from 2½ per cent DDT as an emulsion in a series of comparable tests particularly under conditions of rain exposure.
Sanitarian (R), United States Public Health Service.
Biologist, United States Public Health Service.
Senior Scientist, United States Public Health Service.