Characterization of Resistance to Dicofol in Spider Mites (Acari: Tetranychidae) from New York Apple Orchards

Abstract

Susceptibility to dicofol of populations of Panonychus ulmi (Koch) and Tetranychus urticae Koch from New York apple orchards was estimated in 1985-1986 with a whole-plant residual bioassay. Nine of 14 P. ulmi populations collected from throughout the state in 1985 had <80% mortality in 1,000 μg/g (discriminating concentration) bioassays and were designated as resistant. T. urticae populations evaluated in 1985 were generally more susceptible to dicofol than were P. ulmi populations. Seven 10 T. urticae populations tested had no survivors in any 1,000 μg/g bioassays. In 1986, 12 of 13 P. ulmi populations evaluated from the Hudson Valley were susceptible to dicofol. Pressuring with dicofol of two isolated populations of T. urticae collected from orchards located more than 100 km apart illustrated the change in population response that occurs as a result of dicofol treatments. After one year of dicofol selection, both cultures were equally and highly resistant to dicofol. Probit analysis of the response of the two pressured cultures and two susceptible cultures indicated > 1,000-fold resistance to dicofol residues. Whereas susceptible mites were approximately equally affected by residual or topical exposure to dicofol, resistant mites (possessing a > 1,000-fold resistance to residual exposure) were < 15-fold resistant to topical contact with dicofol. Field trials conducted at orchard locations with P. ulmi resistant or susceptible to dicofol (as shown by bioassay results) illustrated that the resistance resulted in reduced efficacy of dicofol treatments. Advantages of the dicofol-spider mite system for evaluating resistance dynamics and resistance management strategies are described.