Resistance to abamectin and reversion to susceptibility to fenbutatin oxide in spider mite (Acari: Tetranychidae) populations in the Pacific Northwest

Tetranychid mite populations from pome fruit orchards in Washington and Oregon were surveyed in 1994-1995 for resistance status to 3 acaricides, abamectin, fenbutatin oxide, and hexythiazox Lethal concentrations (LC) and lethal concentration ratios (LCR) were calculated from probit dose-response bioassays for various populations and compared with data from the same type of bioassay performed in 1989. After 7-8 yr of widespread commercial use of abamectin in pear orchards, all Tetranychus urticae Koch populations collected from pear showed low to moderate levels of resistance (LCR-LC(50)s of 5-27, depending on the reference population used). Higher levels of resistance were found in T. mcdanieli McGregor populations from pear in Oregon (LCR-LC(50)s from 27 to 81). One population of T. urticae from apple (where abamectin was neither registered nor used) also showed a low level of resistance. Conversely, Panonychus ulmi (Koch) populations, which were collected only from apple, showed no changes in susceptibility to abamectin relative to 1989 baseline responses. Fenbutatin oxide LC(50)s were several orders of magnitude lower for both T. urticae and P. ulmi populations in comparison to the highly resistant populations found in 1989. Fenbutatin oxide was replaced largely by abamectin on pear and propargite on apple in the intervening years. Reversion was most complete in populations of T. urticae and P.ulmi (Washington) and less marked in T. mcdanieli(Oregon). The ovicidal acaricide hexythiazox has been little used on pear, and it was not registered on apple from 1989 to 1995. There were no consistent changes found in the LC,s for any of the 3 mite species to hexythiazox in comparison to the baselines. The implications for resistance management schemes on apple and pear are discussed.