Beers, Elizabeth H.

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3. Pesticide Resistancee. Pesticide Resistance1. Twospotted spider mite, apple, pearElizabeth H. BeersTree Fruit Research & Extension Center1100 N. Western Ave.Wenatchee, WA 98801Evidence for Reversion to Susceptibility to Vendex in Twospotted Spider Mite(Tetranychus urticae) populations from pear orchards in Washington. Resistance toorganotins (OTs) in mite populations from pome fruits in the Pacific northwest has been welldocumented. Much of the original research was on the OT cyhexatin, but cross resistance to theclosely related compound fenbutatin oxide (Vendex) was thought to occur. When cyhexatin wasvoluntarily withdrawn from the market by its manufacturer in 1987, growers were obliged toswitch to other acaricides. Those already experiencing poor control with cyhexatin quicklyfound that fenbutatin oxide provided no better control. As field failures became common, thefurther use of fenbutatin oxide was precluded.Mite control on apples since 1989 has generally been accomplished either with biologicalcontrol (ca. 90% of the acreage) or with propargite. Mite control on pears has been achievedthrough the use of avermectin for pear psylla control. Rates of avermectin used for pear psyllaare typically 16-20 fl oz/acre, much higher than the rate needed to control mites (ca. 5-10 floz/acre). Avermectin has been available for use on pears under an EmergencyExemption(Section 18ofFDFRA) fromthe 1988 through the 1994 growing seasons. Typically, it has beenapplied 1-2 times per season. Use of this material has been extremely widespread inWashington, with ca. 95% of pear acreage treated.Fenoxycarb was available for the first time in 1994under an Emergency Exemption forpre-bloom pear psylla control. The high cost ($200/acre)of this 2-spray programwas adisincentivefor growers to applyfurther controls for pear psylla unless absolutely necessary.Thus, mite populations became established in some pear orchards for the first time in sevenyears. This allowed a re-evaluation of susceptibility or resistance status to various miticides. Onapples, there appeared to be a significantly greater numberof reports of mite populations than inprevious years. Although reasons for this are unknown, the increased use of carbamates in thepest management programs for fruit thinning, western tentiform leafminer (Phyllonorycterelmaella Doganlar&Mutuura), andaphid (Aphis pomiDeGeer) controlmaybe contributory.T. urticae were taken from commercial orchards inWashington. Threewere from pearand one was from a mixed population of P. ulmi and T. urticae on apple (Beebe 16b). A fifthpopulation was from a laboratory colony reared on bean (Table 1). The populations fromcommercial orchards were reared on lima bean until sufficient numbers were obtained forbioassay. Bean leaf disks (2 cmdiam.) were floated bottom surfaceuppermost in a plasticportion cup filled with cotton and distilled water. Five adult females were transferred to thelower surfaceof each disk. The bioassay consisted of six concentrations with ten replicate disksper concentration (50 females per concentration). The concentrations of fenbutatin oxide weremade fromVendex 4L . Miteswere treated topically for 5 s (with a 5 s settlingtime) with aPotter Spray Tower calibrated to deliver 1.1 kg/cm2. The initial concentration series was2,000,1,000,500, 250, 125 and0 mg (AI)/liter. All concentrations but the checkkilled virtually 100%of the mites in the first two populations assayed. Based on these data, a new series (125, 62.5,31.25,15.625, 7.8125, and 0 mg [AI]/liter) was used for the succeeding bioassays.Allpopulations testedwere reared for ca. 52 days in colonyon bean, or approximately 5generations (Table 1). It is possible that some reversionmay have occurred while the mites wereheld incolony, butthe degree towhich this occurred is unknown. The estimated LC50s rangefrom 8.3 to 15.8 mg (AI)/liter (mean 12.6 mg[AI]/liter) (Table 2). These are admittedly poorestimates given the high %2 values, however, they are several orders ofmagnitude lower thankS50those found in 1989 (Knight et al. 1990). In the 1989 study, the72h LC50s for T. urticaepopulations from apple and pear ranged from 98 to 4,507 with a mean of 843 mg (AI)/liter; therewas an 8-fold difference between thelowest LC50 found in 1989 andtheaverage LC50 found inthis study. High mortality (>94-100%)was found in the three highest concentrations (Table 3).Substantial mortality (67-100%) occurred at the next highestconcentration (15.6mg (AI)/liter),and low to moderate mortahty occurred at the lowest concentration (7.8 mg (AI)/liter).Despite the questions raised by the intervening time in colony, these data provide the firstindication that mite populations in Washingtonmay have reverted to susceptibility to fenbutatinoxide. The LC50S observed in this study should correspond to good field control with rates aslow as 0.42 lb (AI)/acre (13.4 fl oz/acre).Table 1. Characteristics of T. urticae populations bioassayed with fenbutatin oxide, 1994Population County SourceDatedCollectedDateAssayedSmith - HillSmith - HomeLaboratoryBryantBeebeChelan pearChelan pearChelan vegetablesChelan pearDouglas apple22 July22 July26 July12 Sept.12 Sept.12 Sept.12 Sept.12 Sept.Table 2. Probit analysis <Df five populations of T. urticae bioassayed with fenbutatin oxide, 1994Fopulation X1 Heterogeneity LC50 (95% R L.) Slope ±SESmith - HiUSmith - HomeLaboratoryBryantBeebe30.9020.00c29.4113.254o.oocI 10.301 10.835) 0.000 12.6589.804 15.8771. 1.085 15.229) 0.000 8.340(10.804 -19.745)4.129 ±0.67711.117 ±8.79127.614 ±59316415.897 ±0.98629.997 ± 5443247Table 3. Mean percentage mortality of T. urticae adult females treated with variousconcentrations of fenbutatin oxide, 1994Uncorrected % mortalityConcentration(mg (AI)/liter) Beebe* Bryant2 Laboratory Smith/Hill Smith/Home125 100.00 a 100.00 a 100.00 a 100.00 a 100.00 a62.5 100.00 a 100.00 a 100.00 a 97.78 a 100.00 a31.25 100.00 a 94.17 a 100.00 a 100.00 a 100.00 a15.625 100.00 a 67.59 b 76.00 b 84.00 a 87.78 a7.8125 25.21 b 8.44 c 28.61c 30.67 b 11.67 b0 6.00 c 13.08 c 86.67 ab 12.21 c 10.00 bMeans within columns followed by the same letter are not significantly different (Waller-Duncanfc-ratio r-test, fc-ratio=100).zData transformed (arcsineVy) before analysis

Evidence for reversion to susceptibility to Vendex in twospotted spider mite (Tetranychus urticae) populations from pear orchards in Washington

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Evidence for reversion to susceptibility to Vendex in twospotted spider mite (Tetranychus urticae) populations from pear orchards in Washington

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