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44

Residual life of chemicals in experiments II and III

Tables 3 and 4 Rive the maximum per cent mortality obtained from both series of experiments II and III, respectively. In all experiments, the second series showed a decided decrease in effectiveness due to the longer period of time elapsed from the chemical treatments to the time the insects contacted the chemical.

In experiment II (table 3), the light dosage showed dieldrin to have the longest residual life. Dieldrin was followed by heptachlor, aldrin and chlordane, which had the shortest life. In the same experiment with the medium dosage, dieldrin again demonstrated the highest residual action. With this dosage, however, dieldrin was followed by heptachlor, with chlordane outlasting aldrin. At the heavy concentration, dieldrin gave the best results, followed by aldrin, chlordane and heptachlor in decreasing order of residual life.

In experiment III (table 4), the picture differed greatly from the previous two experiments. There was more uniformity with respect to residual life among the chemicals in the light and heavy concentrations. With the light dosage, heptachlor showed the shortest residual life. Next in order came aldrin, chlordane and dieldrin. Aldrin possessed the longest residual life of all chemicals with the medium dosage; and it was followed by chlordane, heptachlor and dieldrin. The latter did not show any residual action whatsoever. In the heavy dosage, dieldrin demonstrated the longest residual activity. Aldrin, chlordane and heptachlor followed in order of decreasing residual life.

It is interesting to note that the order of residua11ives of chemicals in experiment II is for the most part exactly opposite to that in experiment III.