A modified excised leaf disc method to estimate the toxicity of slow- and fast-acting reduced-risk acaricides to mites.

The "modified excised leaf disc method" is based on leaf discs that fit tightly the bottom halves of 50-mm petri dishes. The bottom half of each petri dish is covered with wet cotton wool to prolong leaf freshness. The side wall of each bottom half has a small hole to allow the petiole of the leaf disc to protrude outside the petri dish. The top half of each petri dish has a 28-mm (diameter) window. For phytophagous mites the window is covered with a 40-microm mesh Pecap polyester screen. Using this method it was possible to estimate the LC50 value of bifenazate to be 0.00413 g (AI)/liter and the LC50 value of spirodiclofen to be 0.40050 g (AI)/liter to the twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae). The method reduces losses due to escapees and allows observations to be made as long as 9 d after treatment.