Resistance was documented in 1995 to commonly used organophosphorus, carbamate, and pyrethroid insecticides in populations of western flower thrips, Frankliniella occidentalis (Pergande), from six commercial greenhouses in Ontario. Adult female thrips were placed in glass vials treated with technical-grade insecticides and mortality at 18 h was compared with a single discriminating concentration, the computed LC99 of a reference laboratory population. Baseline dose–response regressions for insecticides commonly used in Ontario greenhouses were obtained for the laboratory population of western flower thrips. The organophosphorus compounds chlorpyrifos and malathion and the carbamates methomyl and bendiocarb were the most toxic materials tested; whereas the pyrethroid deltamethrin and a phosphoroamidate acephate were the least toxic. The addition of piperonyl butoxide to solutions of deltamethrin was highly synergistic. The mixture of deltamethrin and endosulfan (1:1) was moderately synergistic. Populations of western flower thrips from commercial greenhouses were resistant to deltamethrin, but deltamethrin mixed with piperonyl butoxide or endosulfan was synergistic in all cases. None of the populations were resistant to all of the insecticides tested. Recommendations are presented for the development of a resistance-management strategy for western flower thrips.

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Bioassay-directed fractionation of extracts of chrysanthemum leaves using a choice test permitted isolation of a fraction that exhibited repellent activity against the western flower thrips (WFT). Analysis of this fraction from cultivars exhibiting varying degrees of host-plant resistance to WFT by high performance liquid chromatography revealed a distinctive peak, the height of which correlated with the degree of resistance of those cultivars to WFT. The peak was attributed to a novel unsaturated isobutylamide, N-isobutyl-(E, E, E, Z)-2,4,10,1 2-tetradecatetraen-8-ynamide.