Comparative toxicity of selected organophosphate insecticides against resistant and susceptible clones of the greenbug, Schizaphis graminum (Homoptera : Aphididae)

Comparative toxicity of selected organophosphate (OP) insecticides against resistant and susceptible clones of the greenbug, Schizaphis graminum, were studied both in vitro and in vivo. Two resistant (OR-l and OR-2) clones of the greenbug showed marginal to high levels of resistance to all seven OPs tested, ranging from 11- to 327-fold greater than those of a susceptible ( OSS) clone. The OR-1 clone showed lower levels of resistance to phenyl (par athion and parathion-methyl) and heterocyclic (chlorpyrifos) OPs than to al iphatic OPs (dimethoate, omethoate, disulfoton, and demeton-S-methyl), wher eas the OR-2 clone showed a rather broad spectrum of resistance to nearly a ll OP insecticides examined. In vitro inhibition of acetylcholinesterase (A ChE) using six selected OP oxon analogues showed that alterations of AChE w ere involved in resistance to all OP compounds examined in both the OR-1 an d OR-2 clones. Although the levels of insensitivity of AChE to these OPs we re relatively low, ranging from 1.1- to 3.8-fold, the insensitivity spectru m of AChE to different OPs was rather broad. The general esterase activity in the OR-1 and OR-2 clones was 1.3-8.4-fold higher than that in the OSS cl one, depending on the substrates used. The AChE activity in both the OR-1 a nd OR-2 clones was 1.8-fold higher than that in the OSS clone. High resista nce levels of the OR-2 clone to phenyl and heterocyclic OPs appeared to be associated with the ability of the esterases to hydrolyze beta -naphthyl ac etate and more hydrophobic substrates.