No less than 50 arthropod species of public health importance have developed resistance to the chlorinated-hydrocarbon or organo-phosphorus insecticides. The problem is most serious in the anopheline vectors of malaria, of which no less than 16 have developed resistance to dieldrin, and 4 to DDT and dieldrin; resistance now covers more than 5% of the world malaria cradication program. Resistance is of importance not only in the house fly (Musca domestica L.), but also in the body louse (Pedicutus humanus humanus L.), yellow fever mosquito (Aedes aegypti (L.)), Culex pipiens quinque-fascialus Say, human flea (Pulex irritans L.), and two species of bed bugs (Cimex lectularius L. and C. hemipterus Fabr.).

The World Health Organization has taken counter-measures in a planned program incepted in 1956. It has standardized test methods for detecting resistance in adult mosquitoes, mosquito larvae, and body lice, while tests for house flies, bed bugs, sandflies, blackflies, and other species will he standardized soon. It has acted as a clearing house of information on resistance among 400 workers in some 60 laboratories. It is stimulating basic research on the biochemistry and genetics of resistance, according to an overall plan. It promptly advises governments on substitute insecticides, and is promoting the development of new insecticides.