Target site and enzyme changes associated with selection of subcolonies of a multiresistant house fly strain with methyl parathion or permethrin

Abstract The effects of intensified selection pressure on subcolonies of a multiresistant house fly strain were determined with studies of different parameters associated with the responses of insects to insecticides. This study included three resistant strains (R, routinely selected by dual exposure to malathion and permethrin; Rmp, a substrain of R selected intensively with methyl parathion; Rper, a substrain of R selected intensively with permethrin); and S, an insecticide-susceptible strain. The Rmp and Rper strains were tested after selection of 20 consecutive generations. Topical toxicity tests indicated resistance of the Rmp strain to methyl parathion increased 17-fold compared with the parental R strain, but there was only a slight increase in tolerance to permethrin. Resistance to permethrin increased 2.6-fold in the Rper flies while tolerance to methyl parathion was unchanged. Compared with the S insects, there was a substantially decreased sensitivity of brain AChE to in vitro inhibition by methyl paraoxon in all three resistant strains. The order of AChE sensitivity to inhibition among strains was S > Rper = R > Rmp. Susceptible or resistant isomeric forms of AChE, respectively, were homogeneous in the S and Rmp strains, but were a heterogeneous mixture in R and Rper insects. Studies of the specific binding of [ 3 H]saxitoxin by brain membrane receptors demonstrated significant differences between susceptible and resistant strains in sodium channel density; the order of maximal binding capacity ( B max ) was S > R = Rmp = Rper. In vitro studies of key enzyme systems demonstrated that, compared with the parental R strain, there were substantial increases in titers of microsomal oxidases and glutathione- S -transferases in the Rmp and Rper flies. There were no major differences among the three resistant strains in cuticular penetration of trans -permethrin or in titers of DDT-dehydrochlorinase or permethrin esterase. All these enzyme systems were more active in the three resistant strains than in S insects.