Relative fitness of malathion-resistant phenotypes of Oryzaephilus surinamensis L. (Coleoptera: Silvanidae).
Two malathion-resistant strains of the stored-grain pest Oryzaephilus surinamensis (L.) were cultured in the laboratory in the UK in the absence of insecticide. Each successive generation was tested for malathion resistance using a discriminating-dose test. There was a decrease in the percentage of resistant beetles, both in cultures derived from field infestations and in those that had originally been selected for resistance in the laboratory. A least-squares estimate showed the fitness of the resistant beetles in unselected cultures to be 68% of that of the susceptible beetles in one strain and 77% of that of the susceptible beetles in the other. The corresponding values in selected cultures were 63 and 76%. The nature of this disadvantage was not known, but its discovery could lead to a method of controlling insecticide-resistance in this important pest.