Six different bioassay methods were evaluated using propargite (Omite 30% wettable powder (WP) and fenbutatin oxide (Torque 50% (WP) and 55% suspension concentrate (SC)) with twospotted spider mite, Tetranychtt~ urticae Koch (TSM) and European red mite, Panonychus ulrni Koch (ERM) to document their utility and precision for estimating median lethal concentrations (LC). For each method, two post-treatment exposure periods and mortality criteria were used. Post-treatment exposure period and mortality criterion had a significant influence on the precision of LCso estimates for all tested miticides with all bioassays methods. Twenty four hour (h) post-treatment exposure was found to be the most suitable for the slide dip and Petri dish methods while 48h was the most appropriate for leaf disc methods. Scoring moribund mites as dead was the most satisfactory criterion for ensuring that biossays were as simple and precise as possible. The Petri dish residue-Potter tower method (PDR-PT) estimated the responses of TSM and ERM to propargite with high precision. The same method was not as precise for fenbutatin oxide formulations. Because significant mite run-off occurred with the leaf disc methods, their precision was not fully established. The slide dip method gave less precise estimates of LCs0 values for propargite (WP) and fenbutatin oxide (WP), while the same method gave more prccise LC~ estimates for fenbutatin oxide (SC) than the PDR-PT method. The toxicity of candidate miticides was found to be method-and species-dependent.
INTROD UCTION
A bioassay is a procedure used to determine the relationship between a physiologically active agent and its effect on a living organism. In entomological research, bioassays are important for the determination of pesticide toxicity and the detection of resistance within field and laboratory populations (Hoskins and Craig, 1962).