The microimmersion bioassay: a novel method for measuring acaricidal activity and for characterising pesticide resistance in spider mites

A novel and versatile method is described for measuring the contact activity of acaricides against spider mites. The microimmersion (MI) bioassay involves drawing batches of 25 mites into small pipette tips under vacuum, immersing them for 30 s in 35 mul of a test solution, and then confining the treated subjects on clean foliage in holding cells. Evaluations of amitraz, bifenthrin, chlorpyrifos and dicofol against susceptible strains of Tetranychus urticae Koch showed the MI bioassay to be equally applicable to formulated and technical acaricides (the latter dissolved in acetone + distilled water; 20 + 80 by volume), and to give LC50 values that corresponded well with those from a conventional residual bioassay. The most important difference between bioassay methods was the consistently greater slope of probit lines from MI tests. Results were not highly sensitive to varying the immersion period between 15 and 60 s, and low control mortality was observed with ethanol or acetone concentrations of up to 500 ml litre-1, and surfactant concentrations of up to 10 g litre-1. The potential of this method for assessing the relative toxicity of both fast- and slow-acting acaricides, screening small quantities of experimental compounds for acaricidal activity, and improving the laboratory procedure for selection and characterization of acaricide resistance is discussed.