INSECTICIDE RESISTANCE AND RESISTANCE MANAGEMENT Fitness Consequences of Malathion-Specific Resistance in Red Flour Beetle (Coleoptera: Tenebrionidae) and Selection for Resistance in the Absence of Malathion

Malathion resistance in the red ßour beetle, Tribolium castaneum (Herbst), is a worldwide problem and is very stable once it becomes widespread in natural populations. In the absence of insecticide the proportion of resistant phenotypes may rapidly decline but the development of resistance does not always involve reduced Þtness. Malathion-speciÞc resistance in T. castaneum seemsnot to involve any loss ofÞtness in laboratoryorÞeldconditions. Susceptiblebeetles were in competition with resistant beetles at different initial frequencies and modiÞcations of susceptible gene frequency were estimated in these laboratory populations over 10 generations. A signiÞcant decrease in susceptible gene frequency was observed in Tribolium populations over time. The selection coefÞcient of the susceptible allele was estimated and the Þtness of susceptible alleles in all tests was observed to range from 0.89 to 0.93 compared with the Þtness of resistant genotypes, which was assumed to be 1. Data provided evidence that the resistant strains exhibited Þtness advantages in the absence of malathion. We also compared the biotic potential (fecundity and developmental time) of the susceptible strain, the homozygous malathion-speciÞc resistant strain, and their hybrids. Malathion-speciÞc resistant strains showed an 8Ð23% increase in biotic potential relative to the susceptible strain. These Þndings are consistent with those of malathion-speciÞc resistance in T. castaneum; the Þtness of the insects seems independent of the genetic background and the Þtness of the resistant insects is not affected by this resistance mechanism.