Pest Management Science Pest Manag Sci 63:555–558 (2007)

Rapid ReportReport of resistance to the neonicotinoidinsecticide imidacloprid in Trialeurodesvaporariorum (Hemiptera: Aleyrodidae)Kevin Gorman,1∗ Gregor Devine,1 Jude Bennison,2 Peter Coussons,3

Neville Punchard3 and Ian Denholm1

1Rothamstead Research, Harpenden, Hertfordshire AL5 2JQ, UK2ADAS Boxworth, Boxworth, Cambridge CB3 8NN, UK3University of Luton, Park Square, Luton, Bedfordshire LU1 3JU, UK

Abstract: Susceptibilities of UK and mainland European samples of Trialeurodes vaporariorum (Westwood) tothe neonicotinoid insecticide imidacloprid were investigated over a 7 year period. All 24 strains collected between1997 and 2003 showed similar baseline levels of susceptibility to that of a known susceptible laboratory strain whenexposed to a diagnostic concentration (128 mg L−1) of formulated imidacloprid. Two samples collected during 2004,one from the UK and one from The Netherlands, demonstrated reduced susceptibility at this concentration. Usingdose–response assays, the presence of resistant individuals was disclosed in both these strains; some individualswere unaffected at doses high enough to induce phytotoxic effects. This report represents the first confirmedcases of neonicotinoid resistance inducing control failures in T. vaporariorum, and highlights a need for carefulvigilance to sustain the effectiveness of imidacloprid and related neonicotinoid insecticides. 2007 Society of Chemical Industry

Keywords: Trialeurodes vaporariorum; whitefly; glasshouse; greenhouse; bemisia; imidacloprid

1 INTRODUCTIONThe glasshouse or greenhouse whitefly, Trialeurodesvaporariorum (Westwood), inhabits the world’s tem-perate regions and is frequently found in protectedenvironments as a major pest of fruit, vegetable andornamental crops. All life stages apart from pupaecause crop damage through direct feeding; as abyproduct, honeydew is excreted, and that constitutesa secondary source of damage since it encouragesinfestations of moulds and fungi.1 The third andpotentially most damaging characteristic is the abil-ity of adults to transmit several ‘clostero’-like plantviruses.2,3 Owing to the success of biological con-trol agents against T. vaporariorum, the majority ofrecent scientific literature has focused on relation-ships with their commercially available parasitoids andpredators. Consequently, there is little published infor-mation regarding the status, mechanisms or impact ofinsecticide resistance in contemporary T. vaporariorumpopulations.

As a target of many insecticides, T. vaporario-rum is known to have developed resistance to sev-eral chemical classes. Pyrethroid and organophos-phate resistance was well documented in UK pop-ulations during the 1970s and 1980s,4,5 and stillreduces efficacy of these conventional classes today.6

Resistance in UK populations also affects the per-formance of some insect growth regulators, such asbuprofezin and teflubenzuron.6 There are no doc-umented cases of resistance to the neonicotinoidclass of insecticides in T. vaporariorum. This con-trasts with strong resistance to neonicotinoids con-firmed in populations of the cotton whitefly, Bemisiatabaci (Gennadius), collected from several coun-tries including Spain, Italy, Germany, Israel andGuatemala.7,8

Neonicotinoids are becoming increasingly impor-tant for controlling a range of insect pests and,although resistance has been comparatively slowto develop, it is now becoming widespread ina number of species.7 Aside from B. tabaci,resistance to the forerunner of the class, imida-cloprid, has been reported in agricultural insectpests including Colorado potato beetle, Leptinotarsadecemlineata (Say),9 western flower thrips, Franklin-iella occidentalis (Pergande),10 and brown planthop-per, Nilaparvata lugens (Stal).11 In spite of thelack of up-to-date resistance monitoring data forT. vaporariorum, there have been a limited numberof publications re-establishing baselines for a rangeof compounds that include imidacloprid and othercurrent products.6,12,13

∗ Correspondence to: Kevin Gorman, Plant and Invertebrate Ecology Division, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UKE-mail: kevin.gorman@bbsrc.ac.uk(Received 20 July 2006; revised version received 13 November 2006; accepted 1 December 2006)Published online 16 April 2007; DOI: 10.1002/ps.1364

2007 Society of Chemical Industry. Pest Manag Sci 1526–498X/2007/$30.00

K Gorman et al.

2 MATERIALS AND METHODS2.1 Whitefly strainsThe strains of T. vaporariorum included an insecticide-susceptible laboratory reference strain and 24 fieldstrains (22 from the UK, one from The Netherlands,one from Spain) collected from commercial plant pro-duction glasshouses with varied treatment histories.A number of the sites (UK-3, UK-13, UK-14, UK-21, UK-22, SPAIN-1 and NED-1) were known tohave previously used imidacloprid to control whiteflyoutbreaks. All colonies were reared on French beanplants, Phaseolus vulgaris (L.), cv. ‘Canadian Wonder’,under a 16 h photoperiod at 24 ◦C, and maintainedwithout exposure to insecticides.

2.2 InsecticidesImidacloprid 200 g L−1 SL (‘Confidor’; Bayer Crop-Science, Leverkusen, Germany) was used throughout.

2.3 BioassaysResponses to imidacloprid were determined usinga systemic uptake assay.6 French bean leaveswere allowed 40 h to uptake either the requiredconcentrations of imidacloprid diluted in distilledwater, or water only for controls. Leaf discs werecut and stored on a 1% agar bed held within a plasticpetri dish. Adult whiteflies were added and confinedusing a close-fitting ventilated lid. Bioassays consistedof a minimum of three replicates per concentration,each consisting of 20–30 females aged between 0 and7 days. All bioassays were maintained at 24 ◦C, withadult mortality recorded after 72 h.

2.4 Data analysisWhen appropriate, dose–response data (correctedfor control mortality) were subjected to probit

analysis using the PoloPlus computer program (LeOraSoftware, California). For discriminating dose assays,confidence limits were calculated using binomialdistributions generated by GenStat.

3 RESULTS3.1 Discriminating concentration assaysOf the 24 field strains assessed using a diagnosticconcentration of imidacloprid (128 mg L−1), 22possessed levels of susceptibility (>80% mortality)similar to that of the LAB-S reference strain (Fig. 1).Two strains collected during 2004 (UK-21 and NED-1) demonstrated increased survival at this dose andwere subjected to full dose–response assays alongsideLAB-S.

3.2 Dose–response assaysDose–response assays (limited by phytotoxicity atsystemic concentrations >1000 mg L−1) disclosedreduced levels of susceptibility to imidacloprid in UK-21 and NED-1 (Fig. 2). Although not significantlydifferent at LC50 (Table 1) owing to the presenceof susceptible individuals, dose–response data forUK-21 and NED-1 at higher concentrations (512and 1024 mg L−1) demonstrated plateaus in mortalitybetween 50 and 70%. Consequently, response slopeswere significantly lower than that obtained for LAB-S(which gave 100% mortality at 512 mg L−1), and thepresence of resistant individuals likely to compromisethe field efficacy of this compound was confirmed.

4 DISCUSSIONThe data presented have served to chart the develop-ment of imidacloprid resistance in T. vaporariorum and

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Figure 1. Responses of all T. vaporariorum strains to a discriminating concentration of 128 mg L−1 imidacloprid. 95% confidence limits for LAB-S,UK-21 and NED-1 were 72.6–92.7, 52.9–79.7 and 10.4–31.4 respectively.

Table 1. LC10, LC50 and LC90 values (mg L−1) with slopes for LAB-S, UK-21 and NED-1 against imidacloprid

Strain n LC10 95% CL LC50 95% CL LC90 95% CL Slope (±SE)

LAB-S 446 0.475 0.12–1.13 8.12 4.32–14.10 138 68.9–398 1.04(±0.081)UK-21 314 0.001 0.000–0.061 3.96 0.03–26.5 21 959 NCa 0.342(±0.076)NED-1 696 0.054 0.001–0.383 29.7 9.66–96.3 16 364 NCa 0.467(±0.055)

a NC = not calculable.

556 Pest Manag Sci 63:555–558 (2007)DOI: 10.1002/ps

Resistance to imidacloprid in T. vaporariorum

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Figure 2. Dose–response relationships for (a) LAB-S, (b) UK-21 and(c) NED-1 against imidacloprid; fewer than three data points perconcentration are visible in some instances owing to overlappingpoints.

warn that in some areas current selection pressures aresufficient to favour resistant genotypes. At present itis not known whether the resistance observed in UK-21 and NED-1 was selected in situ at their respectivecollection sites or whether it was imported from otherlocations. As is often the case with field samples, com-plete treatment histories for most of the collection sitesare not available. However, UK-21 and NED-1 wereamong the sites known to have used imidacloprid forwhitefly control.

Results presented for high doses (up to 1024 mgL−1) demonstrate a potent resistance phenotype capa-ble of further selection and spread. Plateaus in mortal-ity between 50 and 70% could conceivably reflectintrinsic limitations in imidacloprid uptake. How-ever, in this case the plateaus observed occurredfrom 10 mg L−1 upwards, within the range inwhich LAB-S showed a linear dose–response rela-tionship. In addition, the fact that concentrationshigher than those used for bioassays (>1024 mg L−1)induced phytotoxicity demonstrates that imidaclopridlevels within plant tissue had not reached a maxi-mum.

Three untreated generations between the discrimi-nating dose and dose–response assays are a potentialexplanation for a discrepancy between the observedmortalities with NED-1, which may be an indica-tion that, without selection pressure, this resistance isto some extent unstable. In other species, enhancedoxidative detoxification has been implicated as an imi-dacloprid resistance mechanism,11,14,15 and addition-ally a field-collected strain of N. lugens has been shownto contain a target-site mutation conferring reducedsensitivity to imidacloprid.16 The relative importanceof enhanced detoxification and target-site modificationin imidacloprid-resistant populations of N. lugens isunclear at present. However, it is notable that the strainpossessing target-site insensitivity exhibited fitnesscosts when compared with a susceptible population.17

Both types of mechanism appear to confer broad-spectrum protection against a range of commerciallyavailable neonicotinoid insecticides.14,15,18,19

When coupled with an extensive monitoring pro-gramme, resistance management tactics that includerestricting application frequencies and alternatingmodes of insecticidal action can be effective toolsagainst the development and spread of imidaclopridresistance.20,21 Compared with the levels and distri-bution in B. tabaci,14,22–24 neonicotinoid resistance inT. vaporariorum still appears to be at an early stage.However, its confirmation is of considerable concern,as growers have very few available insecticides that areeffective against this pest. Imidacloprid is widely usedfor persistent whitefly control on protected container-grown ornamental plants in the UK,25 particularly onsusceptible hardy nursery stock (HNS) species (fromwhich UK-21 was collected). Biological control of T.vaporariorum on these HNS plants can be difficultowing to, for example, low early-season temperaturesor high susceptibility of the host plants to the pest.Operational tactics for managing resistance,7 in addi-tion to further work documenting its distribution andunderlying mechanism(s), are consequently a highpriority.

ACKNOWLEDGEMENTSThis work was funded in part by the PesticidesSafety Directorate of the UK Department of Environ-ment, Food and Rural Affairs (Defra). The authors

Pest Manag Sci 63:555–558 (2007) 557DOI: 10.1002/ps

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thank numerous colleagues at Rothamsted and withinthe horticultural industry for valuable discussion andfor providing insect samples. Rothamsted Researchreceives grant-aided support from the Biotechnologyand Biosciences Research Council of the United King-dom.

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