SUSCEPTIBILITY OF POPULATIONS OF 2-SPOTTED 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), fro m nurseries in California, Florida, the Canary Islands, and Holland th at 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 du ring 1990-1992 varied greatly in their susceptibility to abamectin, wi th resistance ratios at the LC(95) ranging from 0.5 to 175 in a 1-d le af residual assay. Resistance was correlated with the numbers of appli cations per year and years of use of abamectin. Based on correlation a nalyses, 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 o r for 4 yr, whereas in Holland, resistance was detected in nurseries t hat had used the product 3 times per year or for 2 yr. Although functi onal resistance was detected in this laboratory bioassay, no field fai lures were observed. Characterization of abamectin resistance using re sidual and contact assays and by the use of the synergists piperonyl b utoxide (PBO) and S,S,S,-tributyl phosphorotrithioate (DEF), indicated that differences may exist in the mechanisms used by different T. urt icae populations to develop resistance.