Malathion resistance in the red flour 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 fitness. Malathion-specific resistance in T. castaneum seems not to involve any loss of fitness in laboratory or field conditions. Susceptible beetles were in competition with resistant beetles at different initial frequencies and modifications of susceptible gene frequency were estimated in these laboratory populations over 10 generations. A significant decrease in susceptible gene frequency was observed in Tribolium populations over time. The selection coefficient of the susceptible allele was estimated and the fitness of susceptible alleles in all tests was observed to range from 0.89 to 0.93 compared with the fitness of resistant genotypes, which was assumed to be 1. Data provided evidence that the resistant strains exhibited fitness advantages in the absence of malathion. We also compared the biotic potential (fecundity and developmental time) of the susceptible strain, the homozygous malathion-specific resistant strain, and their hybrids. Malathion-specific resistant strains showed an 8-23% increase in biotic potential relative to the susceptible strain. These findings are consistent with those of malathion-specific resistance in T. castaneum; the fitness of the insects seems independent of the genetic background and the fitness of the resistant insects is not affected by this resistance mechanism.KEYWORDS : Tribolium castaneum, Coleoptera, insecticide, malathion resistance, fitness, selection Because of the intensive use of malathion in grain storage and on stored grain since the late 1950s, mal-athionresistance in red flour beetle, Tribolium castaneum (Herbst), is widespread. The first case of resistance was reported in 1961 (Parkin et al. 1962), and by 1974 this phenomenon was regarded as a common attribute of this species (Champ and Dyte 1976). Although malathion use has declined or even been abandoned in some countries from the early 1970s because of the widespread occurrence of malathion resistance in red flour beetle populations (Champ 1984), malathion-specific resistance is very stable in T. castaneum wild populations. Moreover, the malathion-specific resistant phenotype in T. castaneum populations has almost completely replaced the susceptible one throughout most of the world (Beeman and Nanis 1986). To explain this stability, it was assumed that there were no pleiotropic effects of the mutation and no or few reproductive disadvantages between malathion-specific resistant and susceptible strains.Because resistant insects were not present at high frequency before the use of insecticides, it is not surprising that resistant and susceptible strains should differ in properties other than their adaptation to insecticides, such as developmental time, fecundity, and fertility. D...
