MALATHION-SPECIFIC RESISTANCE IN A STRAIN OF THE RUST RED GRAIN BEETLE CRYPTOLESTES-FERRUGINEUS (COLEOPTERA, CUCUJIDAE)

A strain of Cryptolestes ferrugineus (Stephens) bred for malathion-spe cific resistance was found to be 650 fold resistant at LD50 when compa red with a susceptible strain bred from the same stock. Resistance was more than 98% synergized by triphenyl phosphate and S,S,S-tributyl ph osphorotrithioate, but unaffected by piperonyl butoxide. AChE inhibiti on by malaoxon varied slightly between the strains. Non-specific ester ase activity as measured by the hydrolysis of alpha-naphthyl acetate w as slightly reduced in the resistant strain whereas there were no inte r-strain differences in the hydrolysis of beta-naphthyl acetate. Produ cts of in vitro metabolism of malathion were identified by thin-layer chromatography and gas chromatography-mass spectrometry as alpha- and beta-malathion mono-acids. It was therefore concluded that resistance was due to the hydrolytic breakdown of malathion by a malathion-specif ic carboxylesterase. The rate of in vitro malathion hydrolysis was fou nd to be 31 times greater in the resistant strain, bz vitro inhibition studies indicated that resistance is attributable to a carboxylestera se unique to the resistant strain. The implications of these results a re discussed in relation to work recently carried out on malathion-spe cific resistance in dipterous species.