SUSCEPTIBILITY OF 2-SPOTTED SPIDER-MITE (ACARI, TETRANYCHIDAE) POPULATIONS IN CALIFORNIA TO ABAMECTIN

Twospotted spider mite, Tetranychus urticae Koch, populations from 10 ornamentals nurseries in California in which abamectin has been used for the control of Liriomyza sp. leafminers and twospotted spider mites were evaluated for their susceptibility to abamectin with a leaf residual assay. Mite populations sampled during 1991 varied greatly in their susceptibility, with ratios at the LC(95) ranging from 1 to 658 in a 1-d leaf residual assay. Resistance was correlated with the number of applications and the total time of abamectin use. Resistance 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 LC(95) in a 1-d leaf residual assay The resistance ratio of the selected population in a contact assay was only 16-fold (LC(95)) relative to a susceptible standard population. Although resistance to abamectin was detected in the laboratory, no field failures were observed at nurseries.