Either by the finding of larvae or by the flooding of dry material, natural rock holes in Aguadilla, Puerto Rico, were shown to be breeding Aedes aegypti on four occasions during a period of 5 months, 17 July to 15 December 1959. The rock holes were photographed and some of their characteristics recorded. Since the area had resisted control measures involving treatment with DDT and dieldrin, larvae from the rock holes were used to establish colonies which were subjected to toxicological tests in the F3 generation. The larvae were found to be highly resistant to DDT, dieldrin, and lindane. At 2.5 ppm the 24 hour percentage mortality of two replicates, counting dead-plus-moribund larvae, ranged from 76 to 92, 4 to 4, and 72 to 80 respectively; and counting dead only, the percentage mortalities ranged from 64 to 84, 0 to 4, and 56 to 68 respectively. Control failure in the area was ascribed to the abundance of artificial containers, the hidden, inaccessible rock hole breeding places, and the resistance of the strain to DDT and dieldrin. Results were compared with those from tests on larvae from five other localities. It was concluded that aegypti along the whole north coast of Puerto Rico is resistant to both DDT and dieldrin, or will shortly become so, so that continued use of these insecticides is believed to be of little value. The abandonment of the current idea that aegypti populations do not show resistance to both DDT and dieldrin is also suggested.
This investigation was supported by the Puerto Rico Department of Health and by Research Grant E-1225 from the National Institute of Allergy and Infectious Diseases, U. S. Public Health Service.
Present address: Department of Entomology, University of Florida, Gainesville.