Summary of field trials in 1964-69 in Rangoon, Burma, of organophosphorus larvicides and oils against Culex pipiens fatigens larvae in polluted water.

In tests in Rangoon, Burma, of the larvicidal activity of various organophosphorus compounds against C. p. fatigans, it was found that the activity usually lasted much longer in septic tanks and pit latrines than in open drains. Dursban, Abate, fenthion and several other emulsifiable concentrates caused high larval mortality at a concentration of 0.05 ppm but 0.5 ppm was normally required to obtain a minimum of 1-2 weeks of complete larval control. For the desired residual activity, dosages about 40-400 times the laboratory LC(95) values were normally required, depending on the compound used.The most effective formulation was Dursban emulsifiable concentrate, which, at a concentration of 0.5 ppm, was effective for 3, 7 and 12 weeks in concrete drains, pit latrines and septic tanks, respectively. Petroleum oils applied at rates of 25-40 US gal/ac (approx. 237 l/ha-380 l/ha) and a pyrethrum derivative at a concentration of 1.0 ppm were toxic to larvae but not highly residual. Some emulsifiable-concentrate/oil mixtures appeared to be outstandingly effective, although inconsistent results also occurred. Granular formulations were normally less effective than the emulsifiable concentrates.