EXPERIMENTAL EVALUATION OF RESISTANCE MANAGEMENT FOR 2-SPOTTED SPIDER-MITE (ACARI, TETRANYCHIDAE) ON SOUTHERN OREGON PEAR - 1981-1993

Resistance in twospotted spider mite, Tetranychus urticae Koch, was me asured for 7 yr in the following 5 treatments of organotin (cyhexatin- -1987, fenbutatin oxide-1988 to 1993) and hexythiazox: (1) consecutive organotin use, (2) consecutive hexythiazox use, (3) alternation of bo th within year, (4) between year rotations of both organotins and hexy thiazox, and (5) a combination at half rates of both types of compound . In replicated field plots, acaricides were applied twice per season. We detected significant increases in LC(50)s to the organotins in the consecutive organotin treatments after the 5th application; we also n oted field failure after the 6th application (year 3). LC(50)S in the consecutive hexythiazox program increased after the 5th use (year 3), but field failure occurred only after the 11th application (year 6). I n the within season alternation test, we detected a changed LC(50) Of organotin after the 10th application (5th organotin application); for hexythiazox the change occurred after the 11th (6th hexythiazox spray) . Field failure of hexythiazox occurred in year 7. In the annual rotat ion, LC(50)S for organotins did not increase until after the 12th appl ication (6th organotin application); for hexythiazox, LC(50)s did not increase until after the 11th (6th hexythiazox spray); field failure o f hexythiazox occurred after the 12th application (year 6). In the com bination half rate program, LC(50)S for both compounds increased after the 11th application (=5.5 full rate treatments of each); field failu re occurred after the 13th application (year 7). Overall, use in the f ield was not extended by rotations or half rate combinations compared with consecutive uses, but benefits from these programs may occur beca use of slow registration of new acaricides. Alternate, consecutive use s may give 8 years of control (if hexythiazox is used first) compared with 5-6 years of control for other programs. Resistance to organotins conferred cross-resistance to hexythiazox. The fitness and persistenc e of both resistances and implications of the study to pest management are discussed.