Insecticide Resistance Comes of Age

A. W. A. BROWN It is now 21 years since DDT -resistant house flies were first discovered in northern Sweden, and since the report of Wilson and Gahan that a DDT -resistant strain could be produced by laboratory selection at Orlando. Although the resistance of the San Jose scale to lime-sulfur was detected by Melander in Washington state nearly 60 years ago, the resistance problem as we know it came on the scene with the organic residual insecticides in 1946. Now that this year it has come of age, we might well examine how the economic entomologist has fared with this cuckoo in the nest; is it a clear case of defeat, or is it peaceful coexistence. At least we know the nature of the enemy. It is produced by selection, not habituation. You can expose flies to non-killing doses of DDT for 150 generations, as Liiers has done with DrosoPhila, and you don't get resistance. But as soon as the dose begins to kill a segment of the population, as Gaines Eddy and coworkers have shown with body lice, the resistance to DDT devel0JlS. As the susceptible individuals are selected out the more I"t'sislant genotypcs are left to producc the suc~eediug generations. The insecticide cannot produce postadaplalions; it can only favour the proportion of the preadaptations for resistance already just barely present in the !)opulation.