Insecticide resistance resulting from sequential selection of houseflies in the field by organophosphorus compounds.

Although cross-resistance in houseflies to the organophosphates has eliminated numerous potentially useful compounds from field use, the "subgroup" specificity of this phenomenon has permitted housefly control to be carried out for nearly a quarter of a century by changing from one toxicant to another within this class of insecticides. A question of considerable importance in insect control is whether the development of resistance to one subgroup of organophosphates will be at the expense of resistance to a subgroup applied previously. The development over several years of resistance in a field population selected sequentially by a number of organophosphates was studied. It was observed that the resistance spectrum expanded progressively to include, finally, organophosphates originally thought to belong to more than one subgroup-namely, malathion (resistance greater than 100 times), fenchlorphos (114 times), diazinon (163 times), coumaphos (greater than 100 times), Ciodrin (greater than 100 times), fenthion (18 times) and naled (9.3 times). Resistance to each compound continued to rise to levels considerably higher than those achieved at the time when the field use of the compound ended. The possible coexistence of subgroup cross-resistance in a population is discussed in the light of these results.