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The Duration of Residual Effect of DDT Sprays on Building Materials Used in Rural Venezuela¹

1. Blocks of five building materials commonly used in rural homes in Venezuela were sprayed with DDT suspension, emulsion, or solution at 1 gm. per sq. m. (approximately 100 mg. per sq. ft.). Tests of residual effect were made at intervals with wild-caught mosquitoes.

2. A test employing a 4-hour period of exposure was not sufficiently sensitive to demonstrate differences in effectiveness of the various surfaces and sprays. It showed, however, that with all types of spray the most consistent mosquito kills were obtained on encalado (a whitewashed surface), and that the residual effect of the DDT was more lasting on this material than on the other substances tested. Of the mosquitoes applied to an escalado surface 10 months after spraying, 100 per cent were killed. On the other surfaces good kills (72 to 100 per cent) were obtained 9 months after spraying.

3. Tests with a 15-minute period of exposure showed that suspension of DDT wettable powder gave far more effective residuals on the solid materials than were produced by emulsion or solution of DDT. Solution was also inferior in the case of the thatching materials; emulsion gave a better residual on these than on the solid surfaces, but was in general inferior to suspension.

4. Of the five types of surface on which effective residuals were produced by DDT suspension, encalado was the one on which the residuals gave good kills for the longest period of time (94 per cent mortality at 4.5 months). The two thatching materials showed good residual effect at 3 and 4 months respectively. On adobe and bahareque the residual effect was good at 1 month and poor at 3 months.

5. In view of the meager information relative to the length of exposure of mosquitoes to sprayed surfaces under natural conditions, it is suggested that in order to obtain a high percentage of kills of mosquitoes entering rural Vene zuelan houses, it would be necessary to spray with a suspension of DDT wettable powder (1 gm. per sq. m.) every 3 months during the season of malaria transmission.

¹ This work was carried out with the support and under the auspices of the Division of Malariology of the Ministry of Health and Social Welfare of Venezuela, the International Health Division of The Rockefeller Foundation, and the Institute of Inter-American Affairs, Dr. Arnoldo Gabaldon, Director of the Division of Malariology, aided with valuable advice and suggestions.