Short Communication

The stability of tebufenpyrad resistance in two-spottedspider mite (Acari: Tetranychidae) under laboratoryconditions

GRANT HERRON* and JEANETTE ROPHAILNSW Agriculture, EMAI, PMB 8, Camden NSW, 2570, Australia; *Author for correspondence

Received 12 November 2001; revised 19 February 2002; accepted in revised form 3 June 2002

Key words: METI-acaricides, Resistance-management, Resistance-reversion, Resistance-stability

Abstract. The successful use of chemical rotations to manage insecticide resistance requires reversionbetween alternate chemical applications. We tested a tebufenpyrad resistant population of Tetranychusurticae Koch after some 55 months laboratory culture without pesticide selection and found LC50 levelresistance had dropped from 63.29- to 2.41-fold. However, the population was still heterogeneous withLC99 level resistance at 38.03-fold. It is likely that a lack of reversion contributed directly to the initialtebufenpyrad control failure.

Introduction

Resistance to the mitochondrial electron transport inhibitor (METI) tebufenpyradwas first confirmed in Australian Tetranychus urticae Koch in 1997 (Herron andRophail 1998) and has now been reported in Japan (Kunimoto et al. 1998), Korea(Kim et al. 1999) and Europe (Devine et al. 2001). Tebufenpyrad failed in Austra-lia after five applications over four consecutive seasons. Resistance occurred eventhough the product was essentially used in accordance with the product-label strat-egy that restricted use to one application per season necessitating rotation withnon-METI products. A key assumption for an effective rotation strategy is that thefrequency of resistant individuals will decline during the application of an alternateinsecticide (Tabashnik 1990).

Such a decline can be caused by a number of factors including a fitness cost. Inthis study we retested a tebufenpyrad-resistant population of T. urticae after morethan 4 years laboratory culture without pesticide selection. We aimed to see if re-sistance levels had changed markedly.

Experimental and Applied Acarology 26: 253–256, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

Materials and Methods

The reference susceptible strain was collected from an unsprayed source in Sydneyduring 1987. It has previously been screened with commercial formulations of pes-ticide to confirm susceptibility (Herron et al. 1998). Population WA was collectedfrom an apple orchard in Western Australia on the 31 January 1997 at which timetebufenpyrad resistance was found to be 63.29–fold at the LC50 (Herron andRophail 1998). A subculture since called WA-R, was maintained on French bean(Phaseolus vulgaris L.) without chemical selection in an isolated rearing unit at28°C until retested in August 2001.

Tebufenpyrad (Pyranicar 200 g / L Wettable Powder, Novartis Crop ProtectionAustralasia Pty Ltd), was bioassayed using an adulticidal technique (Edge andJames 1982). Briefly, 20–25 young adult female mites were transferred to individual30 mm bean leaf discs. Mites on the leaf discs were then sprayed with aqueousproduct using a Potter spray tower producing a deposit of 1.6 mg cm−2. For eachbioassay mites were sprayed with a range of serial concentrations plus a water onlycontrol. Sprayed leaf discs (two per concentration) were maintained under constantlight on moistened cotton wool for 48 h at 28 ± 0.4 oC, 70% RH, after which mor-tality was assessed. Bioassays were replicated three times giving six treated leafdiscs per dose. If control mortality exceeded 15% the bioassay was rejected.

Data were analysed using a Probit program written in GENSTAT 5 statisticalsoftware (Barchia 2001). LC50 and LC99 values plus their 95% fiducial limits, werecalculated using the probit method outlined in Finney (1971) and included controlmortality correction (Abbott 1925). Resistance factors (RF) at the LC50 or LC99

level (RF50 and RF99) plus their associated 95% confidence intervals (CI) were cal-culated as outlined in Robertson and Priesler (1992).

Results

After some 55 months laboratory culture without pesticide selection LC50 leveltebufenpyrad resistance in population WA-R was 2.41–fold (Figure 1). However,the dose-response for population WA-R included a distinct inflection causing LC99

level resistance to be 38.03–fold.

Discussion

In the absence of tebufenpyrad, resistance has reverted from an original high of63.29–fold (Herron and Rophail 1998) to 2.41- fold. This contrasts to the study ofDevine et al. (2001) who noted METI-acaricide resistance to be generally stableboth in the laboratory and in the field. Interestingly, their TUK4 strain showed littlereversion but still tolerated 11 tebufenpyrad applications before failure.

254

In Australia, T. urticae resistant to tebufenpyrad clearly have the potential forreversion yet resistance still increased in the original field-population WA relativelyquickly (ie five applications over 4 seasons). The Australian population remainedquite heterogenous for tebufenpyrad resistance even after 55 months culture (indi-cated by the LC99 level resistance of 38.03x). Such heterogeneity and consequentpotential for rapid selection may quickly undermine practical resistance manage-ment based on rotation. Therefore lack of reversion may have contributed directlyto the initial Australian tebufenpyrad control failure against T. urticae.

Acknowledgements

We are grateful to Dr Idris Barchia for the statistical analysis.

References

Abbott W.S. 1925. A method for computing the effectiveness of an insecticide. J. Econ. Entomol. 18:265–267.

Barchia I. 2001. Probit analysis and fiducial limits in Genstat. In: Doogan V., Mayer D. and Swain T.(eds), Genstat 2001. Mecure Resort, Goldcoast, Australia, p. 3. 31st January–2nd February 2001.

Figure 1. Dose-response relationship for tebufenpyrad susceptible (Susc.), laboratory non-pressured re-sistant (WA-R) and original 1997 field-collected resistant population (WA) (for comparison) of Tetrany-chus urticae; horizontal bars at LC50 and LC99 represent 95% fiducial limits; resistance factors at theLC50 and LC99 level (RF50 and RF99) are given with their 95% confidence interval.

255

Devine G.J., Barber M. and Denholm I. 2001. Incidence and inheritance of resistance to METI-acari-cides in European strains of the two-spotted spider mite (Tetranychus urticae)(Acari: Tetranychidae).Pest Man. Sci. 57: 443–448.

Edge V.E. and James D.G. 1982. Detection of cyhexatin resistance in two-spotted mite, Tetranychusurticae Koch (Acari: Tetranychidae) in Australia. J. Austral. Entomol. Soc. 21: 198.

Finney D.J. 1971. Probit Analysis (Third Edition). Cambridge University Press, Cambridge.Herron G.A., Edge V.E., Wilson L.J. and Rophail J. 1998. Organophosphate resistance in spider mites

(Acari: Tetranychidae) from cotton in Australia. Exp. Appl. Acarol. 22: 17–30.Herron G.A. and Rophail J. 1998. Tebufenpyrad (Pyranicar®) resistance detected in two-spotted spider

mite Tetranychus urticae Koch (Acari: Tetranychidae) from apples in Western Australia. Exp. Appl.Acarol. 22: 633–641.

Kim Y.J., Lee H.S., Lee S.W., Kim G.H. and Ahn Y.J. 1999. Toxicity of tebufenpyrad to Tetranychusurticae (Acari: Tetranychidae) and Amblyseius womersleyi (Acari: Phytoseiidae) under laboratoryand field conditions. J. Econ. Entomol. 92: 187–192.

Kunimoto Y., Nishino S., Otuji J. and Inda K. 1998. Effect of uneven acaricide application on effectivecontrol of spider mites in chrysanthemum fields. Japan. J. Appl. Entomol. Zool. 42: 135–140.

Robertson J.L. and Priesler H.K. 1992. Pesticide bioassays with arthropods. CRC Press, Boca Raton.Tabashnik B.E. 1990. Modelling and evaluation of resistance management tactics. In: Roush R.T. and

Tabashnik B.E. (eds), Pesticide resistance in arthropods. Chapman and Hall, New York, pp. 153–182.

256