SELECTION FOR AND AGAINST RESISTANCE TO INSECTICIDES IN THE ABSENCE OF INSECTICIDE - A CASE-STUDY OF MALATHION RESISTANCE IN THE SAW-TOOTHED GRAIN BEETLE, ORYZAEPHILUS-SURINAMENSIS (COLEOPTERA, SILVANIDAE)

Ln the absence of insecticide there may be intense selection against a lleles conferring insecticide resistance, so that these alleles will d ecline in frequency. On the other hand, selection will act to favour a lleles at modifier loci which cause resistance alleles to become coada pted and increase in fitness even in the absence of insecticide. The r elative effectiveness of these two processes is of practical importanc e and theoretical interest. The fate of polymorphism for malathion res istance was studied in the saw-toothed grain beetle, Oryzaephilus suri namensis (Linnaeus). Five to six lines of each of four strains (8401, 8401sel, 8518B and L1040), each polymorphic for resistance phenotypes, were maintained in insecticide-free culture for up to nine generation s, over which period the response of individuals to a discriminating d ose of 65 mg m(-2) malathion was determined. There was significant rev ersion to susceptibility in the related strains 8401, 8401sel and 8518 B. Assuming resistance to be primarily controlled by a single major lo cus, resistant homozygote and heterozygote fitnesses in the absence of insecticide were estimated to be 0.92, 0.83 and 0.68, respectively. T he initial frequency of the resistant allele was inversely related to the time since last exposure to insecticide, and also to the fitness o f the resistant allele. This could be explained by the selection of mo difier alleles that reduce the fitness cost of resistance in the absen ce of insecticide. Ln strain L1040 the proportion of resistant individ uals actually increased over time (after the second generation) in the absence of insecticide. The fitness of the susceptible allele in this case was 0.83. In all strains there was significant variation among l ines in relative fitness and initial resistance allele frequency. Anal ysis of knockdown proportions revealed highly significant variation be tween generations that is not easily interpreted in genetic terms but is better explained by limitations of the knockdown bioassay.