BIOCHEMICAL-MECHANISMS OF ORGANOPHOSPHATE RESISTANCE IN RHYZOPERTHA-DOMINICA (COLEOPTERA, BOSTRICHIDAE) POPULATIONS FROM THE UNITED-STATES AND BRAZIL

Four possible biochemical mechanisms of organophosphate resistance wer e examined in populations of Rhyzopertha dominica (Fabricius) collecte d from eight locations in the states of Minas Gerais and Sao Paulo in Brazil and from seven locations in northeast Kansas, United States. Re sults from insecticide bioassays with synergists (triphenyl phosphate, diethylmaleate, and piperonyl butoxide) suggested that metabolic deto xification mechanisms might be involved in organophosphate resistance in some of the populations of R. dominica. However, overall low synerg ism, non-synergism or even antagonism presented by these synergists in all 15 resistant populations suggested that carboxylesterases, cytoch rome P450 monooxygenases and glutathione S-transferases were not likel y to play a major role conferring organophosphate resistance in R. dom inica. This hypothesis was strongly supported by our biochemical studi es showing no significant difference in these enzyme activities, as we ll as cytochrome b(s), P420, and P450 levels between the susceptible a nd resistant populations. In contrast, all resistant populations of R. dominica showed higher acetylcholinesterase activity than the suscept ible population both in the presence and absence of malaoxon. Furtherm ore, acetylcholinesterase from the resistant populations was less sens itive to inhibition by malaoxon than that from the susceptible. These results suggested that quantitative and qualitative changes in acetylc holinesterase might contribute to organophosphate resistance in these populations of R. dominica.