The western flower thrips, Frankliniella occidentalis (Pergande) is a serious pest on a wide range of crops throughout the world. In Denmark F. occidentalis is a pest in greenhouses. F. occidentalis is difficult to control with insecticides because of its thigmokinetic behaviour and resistance to insecticides. Since F. occidentulis spread to become a worldwide pest in 1980’es, resistance to a number of different insecticides has been shown in many populations of F. occidentalis. This flower thrips has the potential of fast development of resistance owing to the short generation time, high fecundity, and a haplodiploid breeding system. Although resistance in F. occidentalis is a common problem, the underlying mechanisms conferring resistance have only been studied on a few populations. A purpose of this study was to gain more knowledge about possible resistance mechanisms in F. occidentulis and, furthermore, to evaluate fast and sensitive biochemical assays for their potential to detect resistance in field populations of F. occidentulis. Resistance to the carbamate methiocarb was studied, as this compound is the central insecticide for control of F. occidentalis in Denmark. The study was carried out on laboratory populations and populations recently collected in the field from commercial greenhouses in Denmark. Bioassays showed that the populations differed in their susceptibility to methiocarb by 30-fold. The biochemical mechanisms of methiocarb resistance in F. occidentalis were studied with synergists in bioassays (piperonyl butoxide, a cytochrome P450- monooxygenase inhibitor, and S,S,S-tributylphosphorotrithioate, an esterase inhibitor), assays in vitro of activity of detoxification enzymes (general esterases and glutathione S-transferases) toward model substrates, and assays in vitro of insensitivity and activity of acetylcholinesterase, the target site enzyme for methiocarb. The results from bioassays with synergists included indicated involvement of cytochrome P450- monooxygenases and esterases in methiocarb resistance in the most resistant populations. Selection with methiocarb on one of the populations to increase the level of resistance resulted in increased activity of acetylcholinesterase and glutathione S-transferases. Hence, altered acetylcholinesterase and glutathione S-transferases may also contribute to methiocarb resistance. Assays of acetylcholinesterase sensitivity to inhibition by methiocarb, dichlorvos and eserine suggested insensitive acetylcholinesterase in some of the resistant populations. Thus, two alterations of acetylcholinesterase may be possible to confer resistance in F. occidentulis: insensitive acetylcholinesterase and increased acetylcholinesterase activity. The results of the study suggest that both metabolic and target-site resistance mechanisms are involved in conferring methiocarb resistance to F. occidentalis. The potential of the enzymatic assays for detecting resistance in F. occidentulis was studied by testing if enzyme activities or inhibition of acetylcholinesterase in different resistant populations were significantly and consistently different from those measured in the susceptible population. None of the enzymatic assays showed strong and consistent correlation to the level of methiocarb resistance in the different populations. Therefore, the enzymatic assays used in this study appeared to have modest value for detecting resistance to methiocarb in field populations of F. occidentalis. The particular host plant of a polyphagous insect population may affect activity of detoxification enzymes and tolerance to insecticides. Another part of this study investigated the possible effects of host plant shifts on activity of general esterases, glutathione S-transferases, and acetylcholinesterase as well as tolerance to methiocarb in F. occidenfulis. Thrips from a population adapted to growth on bean plants were transferred to new host plants, sweet pepper and chrysanthemum, to establish two new populations. The size of the adult thrips was decreased on the new host plants, indicating a poorer performance due to the host plant shift. The specific acetylcholinesterase activity differed between the populations, but because the populations also differed in total protein per thrips, no difference was found when acetylcholinesterase activity was expressed as total activity per individual. Specific activity of esterases and glutathione Stransferases were largely unaffected by the host plant shifts; the only significant effect observed was a slightly lower level of glutathione S-transferase activity in the population cultured on pepper plants. Tolerance to methiocarb was not affected by culturing the thrips on the new host plants.