Effects of sublethal doses of DDT on the reproduction and susceptibility of Culex pipiens L.

The effect of non-selecting doses of DDT on the induction of resistance was investigated in the house mosquito, Culex pipiens. Sublethal doses of DDT were applied to the adults in each generation, at levels which just fell short of causing mortality in these adults. A susceptible strain, a DDT-resistant strain, and a slightly DDT-tolerant strain were so treated for 6-7 generations.It was found that the treatment initially caused approximately 25% of the ovaries to degenerate, and reduced the proportion of females that fed and oviposited. This reduction in biotic potential became aggravated in successive generations of the DDT-resistant and DDT-tolerant strains, which failed to show any material increase in resistance level. In the susceptible strain, however, the biotic potential became enhanced, and considerable resistance was developed. It was concluded that the increase of resistance in this strain was due to hidden selection, of eggs in the ovary, and of females which failed to oviposit.