Toxicity of some newer insecticides to larvae and pupae of Culex fatigans Weidemann.

The toxicity of DDT, dieldrin, endrin, malathion, parathion, Systox and P4 to larvae and pupae of Culex fatigans has been studied. Two colonies of mosquitos were maintained for the purpose-one at 88 degrees +/-2 degrees F (31.1 degrees +/-1.1 degrees C) and 70%-80% relative humidity, and the other at 72 degrees +/-3 degrees F (22.2 degrees +/-1.7 degrees C) and 45%-55% relative humidity.For test purposes 25 larvae or pupae were exposed to acetone solutions of insecticides in 25 ml of water, and mortality counts were taken at the end of 24 and 48 hours. The percent mortality was plotted on a logarithmic dosage-probit paper and the LC(50) (the concentration required to kill 50%) was read from the slopes obtained. The confidence limits for LC(50) and the slope functions were worked out statistically.Of the insecticides tested, parathion was found to be the most toxic to larvae. The others in their order of toxicity were endrin, dieldrin, DDT, malathion, P4 and Systox. The LC(50) at 24 hours ranged from 0.003 parts per million (p.p.m.) for parathion to 0.18 for Systox. The third-instar larvae were more susceptible to insecticides than those in the fourth-instar stage.Pupae were more resistant to insecticides than larvae. Parathion and endrin gave LC(50)'s of 0.034 and 0.04 respectively. P4 and malathion gave very poor results. Dieldrin and Systox acted slowly on pupae, and the 48-hour mortality is given for these. DDT showed extremely low toxicity to pupae.DDT, dieldrin, endrin and parathion were more toxic to larvae reared and tested at the lower temperature than to those reared and tested at the higher. The temperature after exposure to DDT altered toxicity to a greater extent than the temperature at which the larvae had been reared.