Insecticide bioassays for western flower thrips (Frankliniella occidentalis) (Thysanoptera:Thripidae) and greenhouse whitefly (Trialeurodes vaporariorum) (Hemiptera:Aleyrodidae)

Bioassays were tested for their suitability to determine the resistance of western flower thrips (Frankliniella occidentalis) and greenhouse whitefly (Trialeurodes vaporariorum) to insecticides. Adult female greenhouse and lupin strains of western flower thrips were exposed to bean leaf discs treated with insecticide solutions for 24 h at 25 degrees C. The susceptibility of greenhouse strain western flower thrips was further assessed following exposure for 48 h at 25 degrees C to treated bean leaf discs and plastic Petri dishes, the inside surfaces of which had been sprayed with insecticide. The susceptibility of adult greenhouse whitefly was compared when exposed to leaf discs sprayed with insecticide, leaf discs dipped in insecticide solutions, and insecticide-sprayed Petri dishes. Whitefly mortality on leaf discs was assessed after 48 h at 25 degrees C and in plastic dishes after 24 h at 15 degrees C in the dark. A further series of bioassays compared the effect of insecticide on whitefly nymphs after 7 days at 25 degrees C when exposed to various orientations of buprofezin-sprayed leaf discs, and whole leaves dipped into buprofezin solutions. The lupin strain of western flower thrips was more susceptible than the greenhouse strain to fipronol, maldison, methiocarb, and methamidophos with resistance factors of 14, 19, 26, and 45, respectively. The greenhouse strain of western flower thrips was more susceptible to dichlorvos, lambdacyfluthrin, maldison, and methamidophos in the Petri dish bioassay than in the leaf disc bioassay, but the thrips were less susceptible to maldison in 1996 bioassays than in 1993 and 1999. Adult greenhouse whitefly were similarly susceptible to endosulfan in the three bioassays, but were more susceptible to methomyl in the Petri dish tests than in the two leaf disc bioassays. Dipped leaf discs gave lower LC(50)s of whitefly nymphs than other bioassays, but there was high variation between runs. The Petri dish bioassay was successfully adapted for adult western flower thrips and greenhouse whitefly, but its use is limited by the mode of action of different insecticides, and it is not suitable for insecticides that only affect juvenile whitefly.