Susceptibility of Populations of Twospotted Spider Mites (Acari: Tetranychidae) from Florida, Holland, and the Canary Islands to Abamectin and Characterization of Abamectin Resistance

Populations of twospotted spider mite, Tetranychus urticae (Koch), from nurseries in California, Florida, the Canary Islands, and Holland that have been treated with abamectin for the control of Lyriomiza spp. and twospotted spider mites were evaluated for their susceptibility to abamectin using a leaf residual bioassay. Mite populations sampled during 1990-1992 varied greatly in their susceptibility to abamectin, with resistance ratios at the LC95 ranging from 0.5 to 175 in a I-d leaf residual assay. Resistance was correlated with the numbers of applications per year and years of use of abamectin. Based on correlation analyses, there may be differences in the development of resistance to abamectin in mites from California and mites from Holland. The data on number of applications per year show that resistance was not detected in California nurseries that had used the product <6 times per year or for 4 yr, whereas in Holland, resistance was detected in nurseries that had used the product 3 times per year or for 2 yr. Although functional resistance was detected in this laboratory bioassay, no field failures were observed. Characterization of abamectin resistance using residual and contact assays and by the use of the synergists piperonyl butoxide (PBO) and S, S, S,- tributyl phosphorotrithioate (DEF), indicated that differences may exist in the mechanisms used by different T. urticae populations to develop resistance.