Testing blackfly larvicides in the laboratory and in streams.

The early discovery that DDT is extremely effective in controlling blackfly larvae led to its widespread use in control programmes. Recent evidence that DDT accumulates in the food chain has made it desirable to investigate the effectiveness of other, less persistent, insecticides in reducing populations of blackfly larvae.A method of testing larvicides in troughs was developed and tests were carried out with a number of insecticides both in the troughs and in streams in New York State. Fourfold or fivefold differences in the susceptibility of larvae to different formulations of the same chemical were noted. In the laboratory, emulsions were less effective than oil solutions or wettable-powder suspensions. The effectiveness of emulsions under field conditions, noted in the course of their widespread use in Africa, may be due to the ease with which they become uniformly distributed throughout the water even when no special effort is made to ensure even distribution. When aircraft are used to apply larvicides in oil solution the insecticide is similarly distributed in fine droplets resulting in control at unusually low dosages.In trough tests and in streams, methoxychlor and DDT in oil solutions were about equally effective; carbaryl in wettable-powder suspension was highly effective in the laboratory but relatively greater concentrations were required in streams. Abate in oil solution was effective at low dosages in the laboratory and, in a single test, in a stream.