Susceptibility of Twospotted Spider Mite (Acari: Tetranychidae) Populations in California to Ahamectin

Twospotted spider mite, Tetranychus urticae Koch, populations from 10 or- namentals nurseries in California in which abamectin has been used for the control of Lirio-myza sp. leafminers and twospotted spider mites were evaluated for their susceptibility to abameetin With a leaf residual assay. Mite populations sampled during 1991 varied greatly in their susceptibility, with ratios at the LC95 ranging from 1 to 658 in a I-d leaf residual assay. Resistance Was correlated with the number of applications and the total time of abamectin use. Resistauce was not detected in nurseries that had used the product <6 times per year and for a total of <30 applications over a 6-yr period. In these nurseries, spider mite populations had up to 77% survivorship at a discriminating concentration of 3 ppm of abamectin. Twospotted spider mites collected from a rose nursery not included in this analysis were subjected to further selection With abamectin in the laboratory. After 38 cycles of selection Over 9 mo, resistance in the selected population (relative to a susceptible standard population) increased from 13- to 1,597-fold at LC95 in a I-d leaf residual assay. The resistance ratio of till' selected population in a contact assay Was only 16-fold (LC95) relative to a susceptible standard population. Although resistance to abamectin was detected in the laboratory, no field failures were observed at nurseries.