ALTERED ACETYLCHOLINESTERASE ASSOCIATED WITH ORGANOPHOSPHATE RESISTANCE IN RHYZOPERTHA-DOMINICA (F) (COL, BOSTRICHIDAE) POPULATIONS FROM BRAZIL AND THE UNITED-STATES

Microplate assay format was used to examine the susceptibility of acet ylcholinesterase (AChE) to inhibition by malaoxon and chlorpyrifos-met hyl oxon in individual adult insects of an organophosphate susceptible and two resistant populations (Marshall and Uberlandia) of Rh? Rhyzop ertha dominica (F.). AChE from these resistant populations was signifi cantly less sensitive to inhibition by malaoxon, but not by chlorpyrif os-methyl oxon, suggesting that other resistance mechanisms might also contribute to the overall resistance, at least to chlorpyrifos-methyl . The remaining AChE activities in the presence of 10(-4) M malaoxon w ere 22.8 and 20.3% in the Marshall and Uberlandia populations, respect ively, whereas the remaining activity in the susceptible population wa s 18.9%. Both resistant populations showed about 27% of remaining acti vity in the presence of 10(-6) M chlorpyrifos-methyl oxon, whereas the remaining activity in the susceptible population was 26.1%. One cycle of selection of the resistant populations with chlorpyrifos-methyl de creased the sensitivity to chlorpyrifos-methyl oxon (approximate to 3% ), but increased their sensitivity to malaoxon (3-7%), suggesting that selection with chlorpyrifos-methyl might result in deselection for re sistance to malaoxon in these populations. Such effects on the sensiti vity of AChE by different organophosphate selections suggest that diff erent modifications in AChE structure might have occurred in these res istant populations of R. dominica.