Beers, Elizabeth H.

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3. Pesticide Resistancee. Pesticide Resistance1. Twospotted spider mite, European red mite, apple, pearElizabeth H. BeersTree Fruit Research & Extension Center1100 N. Western Ave.Wenatchee, WA 98801Evidence for Resistance to Agri-Mek in Twospotted Spider Mite (Tetranychus urticae)populations from pear orchards in Washington. Spider mites were taken from variouscommercial pome fruit orchards in Washington. In general, Tetranychus urticaewas mostcommon on pear, and Panonychus ulmi on apple. P. ulmipopulations were tested within 48 h ofcollection from the field, and stored at ca. 5°C during the interim. T. urticaepopulations wereeither tested immediately upon receipt from the field (within 48 h), or reared on lima bean untilsufficient numbers were obtained for bioassay.Bean leaf disks (2 cm diam.) were floated bottom surface uppermost in a plastic portioncup filled with cotton and distilled water. Ten adult females were transferred to the lowersurface of each disk. The bioassay consisted of six concentrations (0.01,0.005,0.0025,0.00125,0.000625 and 0 mg (AI)/liter), with highest concentration at the discriminating concentration forT. urticae as reported by Knight et al. (1990). The same concentration series was used for P.ulmi, although Knight et al. (1990) considered the data too variable to recommend adiscriminating concentration. The highest concentration had 10 replicates, the otherconcentrations five. The concentrations of avermectin were made from Agri-Mek 0.15EC.Mites were treated topically for 5 s (with a 5 s settling time) with a Potter Spray Towercalibrated to deliver 1.1 kg/cm2. They were held at24°C for 72 h, then evaluated byproddinggently with a camel hair brush. Data were analyzed initially with POLO-PC. Resistance ratioswere calculated as the LC50 colony tested/LC^Q baseline for that species. The baseline usedwas the mean LCcq for that species as reported oy Knight et al. (1990), viz., 0.0026 for P. ulmi,and 0.0011 mg AMiter for T. urticae. Overall, fit of the probit model was poor for the majorityof the populations (Table 1). Fiducial limits were generated for only 6 of the 17 populationstested. The difficulty in interpreting avermectin bioassays may have contributed to variability ofresults; some severely intoxicated mites were still capable of locomotion. Actual levels ofmortality were probably underestimated.Despite these caveats, there is evidence for resistance to Agri-Mek in all T. urticaepopulations from commercial orchards. The maximum allowable use in Washington during thisperiod (1988-1994) on pear would be 2 applications/season at 20 fl oz/acre/application; most ofthe orchards received 6-12 applications. Using the criteria of Flexner et al. (1988) (see footnote),none of the P. ulmipopulations tested were resistant. All T. urticae populations except thelaboratory colony were considered either in transition to resistance (6 populations) or resistant (3populations). Even the T. urticae population from apple, with no history of avermectin use,showed a transitional level of resistance.The likelihood of predicting field failure with a discriminating concentration bioassay isrelated tothe difference between the discriminating concentration, the LCop and the field rate.In this case, the discriminating concentration of 0.01 mg (AI)/liter (0.8 ml formulation/acre) isstill ca. 700-fold lower than the full field rate of 20 fl oz/acre (7.02 mg [AI]/liter). Although thisis an extremely large "safety margin", continued selection pressure could rapidly increase thelevel of resistance. Mite populations inWashington pear orchards are at a stage where thedevelopment and implementationof a resistancemanagement strategy, through rotation ofacaricidesor enhanced biological control, is a necessity if we are to prevent field failures ofAgri-Mek.klTable1.ProbitanalysisofpopulationsofP.ulmiandT.urticaebioassayedwithavermectin,1994MiteCropLC5090%FiducialLimitsSEofResistanceSpeciesSourceOrchardn(mgAl/liter)lowerupperslopeslopeX2Ratio2ERMAppleArnold,G.#11820.00290.00120.00413.9181.3571.9551.11ERMAppleArnold,G.#21660.00370.00200.00544.5230.8136.3171.41ERMAppleBeebe16b2960.0016~~6.9031.6068.3100.60ERMAppleMathison2030.0026----27.51431705950.4250.99ERMAppleMattawaOrganic1640.0039~—5.4891.25913.2471.48ERMAppleSoaringEagles1550.0032——5.2551.31118.8191.22ERMAppleVaughn2320.00180.00150.00213.1640.4052.0800.68TSMAppleCV-112890.00713.1311.0156.2566.41TSMBeanLabcolony2930.0025--~26.18422419494.2352.26TSMPearBryant2870.0076—~2.5290.5637.8476.93TSMPearGriggs,Marcus2490.0143--—2.3421.0493.58113.04TSMPearLane,Bruce1870.00540.00330.00734.4820.9624.0874.92TSMPearMcDivett2210.0109—--22.76717123172.3499.91TSMPearSmith,Fred(hill)2560.00560.00370.00724.2830.6017.1485.06TSMPearSmith,Fred(home)2870.0048~—20.1584.34TSMPearSoaringEagle2850.0155--~4.1153.4392.96814.08TSMPearTalley,Randy2910.00720.00460.00876.3841.1207.0356.52^ResistanceRatio=ObservedLC50/SusceptibleLC50.Baselinesforsusceptiblemitepopualationswere0.0026(ERM)and0.0011(TSM)(Knightetal.1990).ResistanceRatioClassification(Flexneretal.,1988:RR<3,notresistant;3>RR<7,intransition;RR>7,resistant)4?"OO

Evidence for resistance to Agri-Mek in twospotted spider mite (Tetranychus urticae) populations from pear orchards in Washington

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Evidence for resistance to Agri-Mek in twospotted spider mite (Tetranychus urticae) populations from pear orchards in Washington

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