A Laboratory Method for testing Residual Insecticides against Anopheline Mosquitos

Abstract

Three interconnecting cages, the largest of which was about 2 feet square, were used in experiments with Anopheles vagus bred from wild-caught larvae.Variations in the arrangement of these cages showed that:1. The mosquitos are quiescent by day, if not disturbed, whether kept in the light or in darkness.2. If they are in complete darkness by night, as in a photographic darkroom, they fly vertically upwards. They will readily escape from a cage about one foot square, with a hold half an inch in diameter in the top, in their continued attempts to fly upwards.3. They are attracted by a dim light at night and will fly sideways or to a lesser extent slightly downwards, in opposition to the tendency to vertical upward flight. In this way they can pass through openings which they would miss if in complete darkness. Light will not cause them to fly vertically downwards.4. Anopheles maculatus is less responsive to light under these conditions, than A. vagus.Using A. vagus, tests were made of the toxicity and residual effect of DDT, BHC and Chlordane as wettable powders on plywood panels. The order of toxicity when fresh was found to be BHC, Chlordane, DDT; the order of the residual effectiveness was DDT, BHC, Chlordane.A considerable mortality occurred amongst mosquitos surviving the overnight tests with DDT and BHC, if they were kept for a further 48 housrs.A larger proportion of females was killed than of males, especially by DDT and Chlordane.BHC was shown to have a marked repellent effect, comparable with that of citronella, but whereas citronella repelled without killing, BHC repelled, but also caused a high subsequent mortality.A. maculatus was tested once against DDT, and once against BHC in the main series of tests, and was much more readily killed than A. vagus, presumably because of its weaker response to light causing it to remain longer in the main cage in contact with the treated panels.