The effect of repeated spraying of insects on their resistance to insecticides iii. Conditioning by the administration of sublethal concentrations

The administration of carbon dioxide in sublethal concentrations or for sublethal periods of time gave rise to an increased resistance to its effect in a strain of Drosophila melanogaster susceptible to the toxic effects of this gas. The effect did not appear to be permanent. The successive administration by spraying of sublethal concentrations of DDT and BHC did not increase the resistance of a strain of D. melanogaster to their effects, either in the insects sprayed or their progeny. Only when the dosages were such as to give a high death-rate did a significant increase of resistance of the progeny of the treated insects take place; the tendency was if anything to increase susceptibility, although the indications were not in favour of DDT acting as a cumulative poison. In a preliminary series of tests the topical application of DDT in sublethal doses to the adult insects resulted in a reduction in the amount or in the rate of egg laying.