Biochemical mechanisms of resistance in strains of Oryzaephilus surinamensis (Coleoptera : Silvanidae) resistant to malathion and chlorpyrifos-methyl

The acetylcholinesterase, carboxylesterase, and cytochrome p350 monooxygena se activities of three strains of Oryzaephilus surinamensis (L.) were exami ned to better understand biochemical mechanisms of resistance. The three st rains were VOS49 and VOSCM, selected for resistance to malathion and chlorp yrifos-methyl, respectively, and VOS48, a standard susceptible strain. Cros s-resistance to malathion and chlorpyrifos-methyl was confirmed in VOS49 an d VOSCM. Acetylcholinesterase activity was not correlated to resistance amo ng these strains. VOS-49 and VOSCM showed elevated levels of carboxylestera se activity based on p-nitrophenylacetate, alpha -naphthyl acetate, or beta -naphthyl acetate substrates. PAGE zymograms showed major differences in c aboxylesterase isozyme banding among strains. VOSCM had one strongly staini ng isozyme band. A band having the same Rf-value was very faint in VOS48. T he VOS49 carboxylesterase banding pattern was different from both VOSCM and VOS48. Cytochrome P450 monooxygenase activity was based on cytochrome P450 content, aldrin epoxidase activity, and oxidation of organophosphate insec ticides, all elevated in resistant strains. The monooxygenase activity vari ed with insecticide substrate and resistant strain, suggesting specific cyt ochromes P450 may exist for different insecticides. The monooxygenase activ ity of tile VOS49 strain was much higher with malathion than chlorpyrifos-m ethyl as substrates, whereas VOSCM monooxygenase activity was higher with m alathion than chlorpyrifos-methyl as substrates. Results are discussed in t ile context of resistance mechanisms to organophosphate insecticides in O. surinamensis.