Experimental Evaluation of Resistance Management for Twospotted Spider Mite (Acari: Tetranychidae) on Southern Oregon Pear: 1987–1993

Abstract

Resistance in twospotted spider mite, Tetranychus urticae Koch, was measured 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 both within year, (4) between year rotations of both organotins and hexythiazox, 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 LC50s to the organotins in the consecutive organotin treatments after the 5th application; we also noted field failure after the 6th application (year 3). LC50s in the consecutive hexythiazox program increased after the 5th use (year 3), but field failure occurred only after the 11th application (year 6). In the within season alternation test, we detected a changed LC50 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 rotation, LC50s for organotins did not increase until after the 12th application (6th organotin application); for hexythiazox, LC50s did not increase until after the 11th (6th hexythiazox spray); field failure of hexythiazox occurred after the 12th application (year 6). In the combination half rate program, LC50s for both compounds increased after the 11th application (=5.5 full rate treatments of each); field failure occurred after the 13th application (year 7). Overall, use in the field was not extended by rotations or half rate combinations compared with consecutive uses, but benefits from these programs may occur because of slow registration of new acaricides. Alternate, consecutive uses 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 persistence of both resistances and implications of the study to pest management are discussed.