House fly (Musca domestica L.) strains resistant to organophosphorus insecticides are significantly different in a number of ways from those resistant only to the chlorinated-hydrocarbon insecticides.

Chlorinated-hydrocarbon resistance is characterized by the development of uniformly high levels of resistance, by limitations due to the saturation of absorption systems, and by marked cross tolerances, especially to closely related compounds, but not to the organophosphorus insecticides. Further, it is relatively stable or decreases only very slowly after removal of selection pressure.

Organophosphorus resistance is characterized by biochemically-limited resistance levels peculiar to each particular insecticide, beyond which it has not been possible to increase resistance by further selection pressure. It is more specific and cross tolerances to other organophosphorus insecticides are less marked. However, house fly strains selected with organophosphorus insecticides routinely develop high levels of resistance to chlorinated-hydrocarbon insecticides even though resistance to the selecting organophosphorus compound may be minimal. Finally, resistance to the organophosphorus insecticides is relatively unstable, decreasing initially at a rapid rate on removal of selection pressure.

Studies on the mechanisms of resistance to the organophosphorus insecticides have shown that resistance is not primarily related to higher levels of cholinesterase activity, cholinesterase less susceptible to inhibition, or the efficiency of toxifying systems. Rather it is related to detoxication mechanisms and the rate of destruction of the active compound in relation to the rate of accumulation in critical concentration at sites of action.