The laboratory technique of biological assay of arsenical dust as anopheline larvicides is described. From a preliminary survey of a wide variety of arsenites, copper arsenite and calcium arsenite were selected for more intensive study as larvicidal dust. Since copper arsenite demonstrated a definite superiority to paris green as an anopheline larvicide, a pilot plant was constructed for the manufacture of a sufficient quantity of this preparation for field testing. In the field it has been demonstrated that copper arsenite is at least the equivalent of paris green as a larvicide despite the fact that quantitative sampling indicated that only a small fraction actually reaches the water surface. Laboratory studies on the influence of particle size, utilizing samples which have been fractionated by the use of the Federal air classifier, indicate that fractions which average 5 or 15 microns in diameter are more efficient larvicides than fractions averaging 25 microns in diameter. Since the copper arsenite, as prepared in the pilot plant, has been of uniformly fine particle size, it is believed that its superiority over paris green may be in part attributed to this physical property. Studies on the influence of dilution of larvicides, indicate that paris green or copper arsenite may be diluted with soapstone as much as 1:19 without reducing the larvicidal efficiency. Attempts to determine the minimum lethal dose of paris green for fourth instar larvae indicate that this is less than .0000004 grams. An entirely satisfactory determination of this has not been possible. Copper arsenite has exhibited a marked larvicidal superiority over paris green for first instar anopheline larvae.
From Laboratory Service, Health and Safety Department, Tennessee Valley Authority, Wilson Dam, Alabama. Read before the 37th annual meeting of the American Society of Tropical Medicine, St. Louis, Missouri, November 10–13, 1941.