Potential of Selective Insecticides for Managing Uraba lugens (Lepidoptera: Nolidae) on Eucalypts

FOREST ENTOMOLOGY

Potential of Selective Insecticides for Managing Uraba lugens(Lepidoptera: Nolidae) on Eucalypts

S. MANSFIELD, T. M. WITHERS, S. F. GOUS, K.J.B. POTTER, D. J. KRITICOS, M. C. WATSON,AND M. O. KIMBERLEY

Forest Biosecurity and Protection, Ensis, PB 3020, Rotorua, New Zealand

J. Econ. Entomol. 99(3): 780Ð789 (2006)

ABSTRACT The leaf skeletonizer Uraba lugens Walker (Lepidoptera: Nolidae), an Australianspecies, locally known as “gumleaf skeletonizer,” is well established in New Zealand. This insect hasthe potential to become a serious pest of forestry and amenity eucalypts (Eucalyptus spp.) and is thefocus of a long-term management program. The use of synthetic chemical or biological insecticidesis one possible control method within an integrated control program. A series of doseÐresponse trialswere conducted using laboratory bioassays to test the efÞcacy of several insecticides againstU. lugens:pyrethroids, spinosad, Bacillus thuringiensis kurstaki Berliner (Btk) and an insect growth regulator,Mimic. Pyrethroids and spinosad proved highly effective against U. lugens larvae, achieving 100%mortality after 3Ð6-d exposure. The performance of Btk was lower against gregarious skeletonizinglarvae compared with solitary chewing larvae. When good coverage of the target foliage is achieved,�90% mortality is possible with Btk. Mimic performed poorly against U. lugens compared with otherinsecticides tested (�60% mortality). The Eucalyptus species on which larvae were feeding signiÞ-cantly altered insecticide efÞcacy. Treatments applied toEucalyptus nitens (Deane & Maiden) Maidenhad reduced efÞcacy compared with E. cinerea F. Muell. ex Benth. or E. fastigata Deane & Maiden.Cooler temperatures also reduced insecticide efÞcacy, presumably by decreasing movement and foodconsumptionbyU. lugens.Recommendationson sprayapplications tocontrolU. lugens inNewZealandare given.

KEY WORDS Btk, Eucalyptus, Mimic, spinosad, Uraba lugens

The leaf skeletonizer Uraba lugensWalker (Lepidop-tera: Nolidae), locally known as “gumleaf skeleton-izer,” occurs in every state of Australia except theNorthern Territory (Elliott et al. 1998). This species isnow well established in Auckland, New Zealand,where it was Þrst recorded in 2001. However, theAuckland population was considered too widespread(�20,000 ha affected) for eradication to be feasible(Ross 2003). This Australian insect has the potential tobecome a serious pest of forestry and amenity euca-lypts (Eucalyptus spp.) in New Zealand because awide range ofEucalyptus spp. and closely related Myr-taceae, such as Angophora spp. and Lophostemon spp.,are suitable for larval development (Morgan and Cob-binah 1977, Farr 2002). Periodic outbreaks ofU. lugensoccur in Australia and can lead to extensive defoliationof eucalypt forests. Outbreaks sometimes cover�150,000 ha (Campbell 1962, Strelein 1988, Farr2002). The New Zealand infestation is currently con-Þned to metropolitan Auckland, whereU. lugensposesa risk to amenity trees and to public health. U. lugenslarvae have venomous spines that inject histamine intohuman skin upon contact with live caterpillars or shedskins. Initially, contact with larvae produces a stingingsensation; later, the affected skin may develop a per-

sistent, itching rash (Southcott 1978, Elliott et al.1998). Further spread of U. lugens in New Zealand isinevitable, and this pest poses a signiÞcant risk to bothhuman health andEucalyptusplantations in the future.

In Australia, U. lugens has one to two generationsper year during which it undergoes six to 14 instars,depending on climate (Allen and Keller 1991). TheAuckland infestation has two generations each year,with larvae present from approximately January toMarch (summer generation) and May to October(winter generation; Withers et al. 2003). More south-ern regions of New Zealand are predicted to supportonly one generation per year under present climaticconditions (Kriticos et al. 2005).U. lugens larvae have two different feeding modes.

Early instars (ÞrstÐÞfth) feed gregariously and skele-tonize foliage by chewing the softer portions of theleaves, leaving the veins untouched. Later instars(sixthÐ14th) are solitary feeders that chew entireleaves (Campbell 1962, Harris 1974). This change infeeding behavior is likely to affect the degree of in-secticide exposure experienced by larvae within thesame generation, particularly for products that actsolely by ingestion. Colder temperatures experiencedby the winter generation of U. lugens slow larval

0022-0493/06/0780Ð0789$04.00/0 � 2006 Entomological Society of America

growth, development, and food consumption com-pared with the faster developing summer generation.Seasonal differences are therefore likely in larval re-sponses to insecticide exposure.

Although broad-spectrum and selective insecticidesare available for the control of lepidopteran pests inNew Zealand, no insecticides are currently registeredfor use against U. lugens.Where U. lugens infestationsoccur in sensitive areas (e.g., urban zones or nearwaterways), the use of broad-spectrum insecticidessuch as pyrethroids will be inappropriate because oftheir environmental effects, human toxicity, and im-pacts on nontarget organisms (Kidd and James 1991).In New Zealand, resource consent is often requiredbefore aerial application of insecticides (Ministry forthe Environment 1991, Environment Waikato 2005). Amore attractive product for pest control within envi-ronmentally sensitive areas is the biopesticideBacillusthuringiensis kurstaki Berliner (Btk), which has beenused previously in New Zealand against other lepi-dopteran forestry pests in urban areas (Hosking et al.2003). However the insecticidal activity of this prod-uct can vary according to the host plant species towhich it is applied (Farrar et al. 1996). Other optionswith a degree of selectivity to the pest insect that arereadily available include insect growth regulators(IGRs) such as Mimic and the insecticide spinosad,which has been released recently in a new formulationwith organic certiÞcation (Entrust Naturalyte, DowAgrosciences 2005). These two compounds show re-duced toxicity against most beneÞcial insects com-pared with broader-spectrum insecticides (Gurr et al.1999, Ma et al. 2000, Williams et al. 2003) and have lowmammalian toxicity (Kidd and James 1991, Thompsonet al. 1999). Either spinosad, Mimic, or a biopesticidemay be far more suitable compared with broad-spec-truminsecticides for thecontrolof insectpests inareaswhere humans or livestock are at risk of exposure.U. lugens is now the focus of a long-term manage-

ment program in New Zealand (Ross 2003). Sprayingof synthetic chemical or biological insecticides may bea useful method to control U. lugens either alone or incombination with other methods such as biologicalcontrol. However the effectiveness of these insecti-cides needs to be quantiÞed before any recommen-dations can be made. This study presents a series ofdoseÐresponse trials that were conducted to test theefÞcacy of four chemical typesÑpyrethroids, spi-

nosad, Btk, and MimicÑagainst U. lugens on some ofthe more important eucalypts growing in New Zea-land. The impacts of larval feeding mode, tempera-ture, and host plant species on the efÞcacy of some ofthese products also were examined.

Materials and Methods

Spray Application. Eucalyptus nitens (Deane &Maiden) Maiden foliage came from the KapengaBlock (Atiamuri Highway, 38�14�23� S 176�13�22� E).Eucalyptus cinerea F. Muell. ex Benth. and Eucalyptusfastigata Deane & Maiden foliage came from trees atTe Papa Tipu Innovation Park, Rotorua (38�09�00� S176�15�56� E). Foliage was collected the day beforespray application and stored at 5�C overnight. Beforespray treatment, individual branch segments (200 mmin length) with sufÞcient foliage for two or threebioassays were cut and placed in conical ßasks con-taining water. A moving belt track sprayer, designedand built on-site, was used to apply the treatmentsolutions. Small droplets (150 �m volume median di-ameter) were produced using an ULVA 8 (MicronSprayers Ltd., Bromyard, United Kingdom) spinningdisc operated at 12,500 rpm, mounted 80 cm above thetrack sprayer conveyor belt. All insecticides exceptBtk were applied at 50 liter/ha total volume, withchanges in application rate achieved by a dilutionseries. For Btk treatments, changes in application rateswere achieved by altering belt speed and ßow rate.The product rates chosen were based on recom-mended insecticide rates for other lepidopteran pestsin New Zealand (Table 1). Actual insecticide dosagesachieved were calculated by adding Tartrazine fooddye (10 g/liter) to the 50 liter/ha treatments. Plasticcards (four replicates per treatment) were positionedon the track sprayer, interspersed between foliagesamples from each treatment. Total dye recovery wasdetermined by washing the card traps with 100 ml ofwater and measuring the UV absorbance of each sam-ple at 427 nm with a Shimadzu UV MINI 1240 UV-VISspectrophotometer. Btk dosages were similarly deter-mined from deposits on plastic cards with the UVabsorbance measured at 262 nm. DoseÐresponse re-sults are presented as actual rates applied as millilitersor grams per hectare of active ingredient.Bioassays. Treated foliage was divided between the

required numbers of test arenas to allow �100 mm2 of

Table 1. Nominal application rates for each insecticide used in the dose–response trials

Experiment 1 Experiment 2 Experiment 3

Decis Forte(liters/ha)

Dominex 100(liters/ha)

SuccessNaturalyte(liters/ha)

Foray 48B(liters/ha)

DelÞng/liter

Dipelg/liter

Foray 48Bliters/ha

SuccessNaturalyteliters/ha

EntrustNaturalyte

g/liter

Mimicg/liter

0.03 0.02 0.05 0.5 5 5 1.25 0.10 0.3 0.430.06 0.04 0.10 1.0 10 10 2.5 0.20 0.6 0.860.11 0.08 0.20 2.5 15 15 3.75 0.40 1.2 1.720.23 0.15 0.40 5.0 20 20 5 0.80 2.4 3.440.45 0.30 0.80 10.0

Treatments were applied at 50 liter/ha total spray volume except for Foray 48B in experiment 1, which was applied as undiluted productaccording to speciÞcations. Each experiment had its own set of controls (application rate, 0).

June 2006 MANSFIELD ET AL.: INSECTICIDES FOR MANAGING U. lugens 781

foliage on each stem. Foliage samples were then trans-ferred to the upper test chamber (395 ml) of eacharena, with stems inserted into water within the lowertest chamber (535 ml) and held in place by a foldedpaper wick. All arenas for a particular trial were pre-pared with the foliage, labeled, and transported tothe appropriate controlled temperature facility (22�Cunless stated otherwise) before larvae were intro-duced. U. lugens larvae used in these trials were col-lected from the Þeld in Auckland and reared to therequired feeding stage (skeletonizing versus chew-ing feeding mode) at 22�C, if necessary, before com-mencing each trial. The exact feeding stage used isdescribed below for each experiment. Larvae weretransferred from foliage on which they were feedingwithin rearing containers onto a leaf within each ex-perimental arena by gently lifting and then depositingwith a Þne camelÕs-hair brush. When early instarswere used, they were always introduced onto thesame leaf, because previous experience showed thatlarvae settled and initiated feeding more quickly ifthey could reestablish themselves into a gregariousgroup. A fresh brush, sterilized in 90% ethanol andallowed to dry, was used to transfer larvae onto control(unsprayed foliage) and between different spraytreatments. The effect of insecticide treatments andother experimental factors on mortality was examinedusing chi-square tests (experiment 1) or logistic anal-ysis (experiments 2 and 3, PROC CATMOD, SAS In-stitute 1999).Experiment 1: Broad-Spectrum, Selective, and Bio-logical Insecticides. In response to the then recentincursion by U. lugens, the Þrst experiment aimed torapidly test the efÞcacy of several commonly usedinsecticides against U. lugens. The chosen insecticidesranged from low to high selectivity toward lepidop-teran pests. Four insecticides (Decis Forte, Dominex100, Success Naturalyte, and Foray 48B) were testedat six rates including untreated controls (Table 1).Two of the insecticides used, Decis Forte andDominex 100, are pyrethroids (active ingredients aredeltamethrin and �-cypermethrin, respectively) thathave broad-spectrum activity against insects via con-tact and ingestion (Kidd and James 1991). The activeingredient of Success Naturalyte is spinosad, whichhas a degree of selectivity to Lepidoptera and also actsvia contact and ingestion (Thompson et al. 1999).Foray 48B is a formulation of Bacillus thuringiensiskurstaki (Btk), a toxin that acts solely by ingestion andis active primarily against Lepidoptera (Kidd andJames1991, Sampleet al. 1996).TheadjuvantDu-Wettwas added at 0.4% of total spray volume with theinsecticides Decis Forte, Dominex 100, and SuccessNaturalyte but not Foray 48B. Early instar gregari-ously feeding, skeletonizing larvae (n� 10 per arena)and later instar solitary, chewing larvae (n � 5 perarena) were evaluated with each treatment and larvalfeeding mode replicated Þve times. Many species ofeucalypt, including E. nitens, demonstrate hetero-blasty, i.e., a disparate foliage form (Brennan et al.2001). Larvae of U. lugens attack both juvenile andadult leaves of E. nitens. In this Þrst experiment, we

placed an opposing pair of fully expanded juvenileleaves of E. nitens into each arena. Foray 48B treat-ments were applied 16 May 2003, and these experi-ments took place within the Invertebrate Transitionaland Containment Facility (New Zealand Forest Re-search Ltd., Te Papa Tipu Innovation Park, Rotorua).The other three insecticide treatments were applied21 May 2003. Experiments with these more volatileinsecticides could not be conducted at Rotorua be-cause of the contamination risk to other insect cul-tures within the Containment Facility. Consequently,arenas containing the treated foliage were trans-ported to Auckland (kept at 12�C during transit), andthe experiments took place at the MAF TreatmentTechnology Centre at Auckland Airport. Mortalitywas recorded 2, 3, 4, and 6 d after larvae were trans-ferred to the treated foliage.Experiment 2: Host Plant Species and BiologicalInsecticide Efficacy.We were concerned that the po-tential efÞcacy of the selective insecticide Btk mayhave been masked in the previous experiment by in-adequate coverage on the waxy leaf surface of juvenileE. nitens(see Results, experiment 1). Furthermore, wedid not want to rely on the results of only one com-mercial formulation to draw conclusions about theefÞcacy of Btk. This second experiment was designedto increase spray coverage on the leaf surface and totest the efÞcacy of different Btk formulations by usingthe dominant foliage type of two host trees commonlyattacked byU. lugens in Auckland. Three formulationsof Btk (DelÞn, Dipel, and Foray 48B) were tested atÞve rates including controls treated with tartrazineand adjuvant only (Table 1). The adjuvant Pulse wasadded at 0.15% of the total spray volume for all threeformulations to increase droplet spreadability. Arenascontained either two to Þve leaves of mature E. nitensfoliage or four to eight leaves of E. cinerea foliage.Early instar gregarious, skeletonizing larvae (n � 10per arena) and later instar solitary, chewing larvae(n � 10 per arena) were again evaluated with eachcombination of host plant, insecticide treatment, andlarval feeding mode replicated Þve times. All treat-ments were applied 29 January 2004, and these exper-iments took place within the Invertebrate Transitionaland Containment Facility at Rotorua. Mortality wasrecorded 2, 4, 7, and 14 d after larvae were transferredto the treated foliage.Experiment 3: Host Plant Species, Temperature,and Selective Insecticide Efficacy. This experimentwas designed to test the efÞcacy of three selectiveinsecticides against U. lugens and to compare the ef-Þcacy of a new organic certiÞed formulation of spi-nosad with the conventional formulation. These in-secticides might be useful for controlling U. lugens incommercial plantations where the species E. nitensand E. fastigata dominate. From the stakeholder re-sponse to the U. lugens incursion into New Zealand ithad become apparent that insecticide applicationsmight be needed to control both summer and winterlarval generations. Therefore, in this experiment wetested insecticide efÞcacy againstU. lugensonE. nitensand E. fastigata at different temperatures. Two for-

782 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 99, no. 3

mulationsof spinosad(SuccessNaturalyteandEntrustNaturalyte) and Mimic (an IGR, active ingredienttebufenozide) were tested at Þve rates, includingcontrols treated with tartrazine and adjuvant only(Table 1). Mimic is a nonsteroidal ecdysone agonistthat stimulates premature molting in lepidopteran lar-vae, acting via contact and ingestion (Smagghe and DeGheele 1995). Arenas contained two to four matureleaves of either E. nitens or E. fastigata. For all threeinsecticides, the adjuvant Pulse was added at 0.1% ofthe total spray volume applied toE. nitens, and Spread-wett was added at 0.1% of the total spray volumeapplied to E. fastigata. Insecticide treatments wereapplied11May2004, andarenascontaining the treatedfoliage were then transported (at 12�C) to Hor-tResearch (Mt Albert, Auckland). At HortResearch,experiments were conducted in controlled tempera-ture rooms at either 15 or 20�C to simulate winter andsummer pest generations, respectively. Only early in-

star gregarious, skeletonizing larvae (n � 10 perarena) were evaluated, and each combination of hostplant species, temperature, and insecticide treatmentwas replicated Þve times. Mortality was recorded 1, 2,3, 5, and 7 d after larvae were transferred to the treatedfoliage.

Results

Experiment 1: Broad-Spectrum, Selective, and Bi-ological Insecticides. The pyrethroids (Decis Forteand Dominex 100) and spinosad (Success Naturalyte)were highly effective at all rates tested against U.lugens, particularly against early instar skeletonizinglarvae with 100% mortality after only 3 d of exposure.These insecticides took slightly longer (4Ð6 d of ex-posure) to achieve 100% mortality of later instar chew-ing larvae. In comparison, Foray 48B was effective atthe two highest rates, particularly for later instar

Fig. 1. Mean mortality (�95% CI) of skeletonizing (circles and solid line) and chewing (triangles and dashed line)U. lugens larvae on E. nitens after up to 7 d of exposure to six rates of Decis Forte, Dominex 100, Foray 48B, and SuccessNaturalyte.


chewing larvae (�90% mean larval mortality after 6 dof exposure) but less effective at lower rates (Fig. 1).Because mortality was essentially 100% for three of thefour insecticides at the lowest rate tested, statisticalanalysis amounted to a comparison of controls with alltreated organisms (Table 2).Experiment 2: Host Plant Species and BiologicalInsecticideEfficacy.After 7 d of exposure to Btk, therewas a signiÞcant trend of increased mortality at thehigher doses of the three formulations (DelÞn, Dipel,and Foray 48B), although the observed mortality wasnot as high as is desirable for effective pest control.After 14 d of exposure to the three Btk formulations,insecticide rate, larval instar, and eucalypt specieseach had a signiÞcant effect on mortality (logisticanalysis, Table 3). DelÞn and Dipel (Fig. 2, mean larvalmortality ranged from 90 to 98% onE. cinerea and from72 to 82% on E. nitens) tended to be more effectivethan Foray 48B (mean larval mortality at 14 d rangedfrom 82 to 96% on E. cinerea and from 56 to 88% onE. nitens). However, efÞcacy of all three Btk formu-lations was reduced, and there was greater variation inmortality when the larvae were feeding on E. nitenscomparedwithE. cinerea.Mortalitywasalso lowerandmore variable for early instar skeletonizing larvaecompared with later instar chewing larvae, particu-larly on E. nitens.With the extended monitoring pe-riod, some chewing larvae did pupate (on average, lessthan one larva per arena in each insecticide treat-ment). Pupae were counted as live larvae when quan-tifying larval mortality.Experiment 3: Host Plant Species, Temperature,and Selective Insecticide Efficacy. For each spinosadformulation (Success Naturalyte and Entrust Natur-alyte), insecticide rate, temperature, and eucalyptspecies had a signiÞcant effect on average mortality at7 d after exposure (logistic analysis, Table 4). OnE. fastigata, U. lugens mortality was �96% for the

three highest rates of both formulations at 15 and 20�C(Fig. 3). Mortality was generally lower for both for-mulations onE. nitens,particularly at 15�C, where onlythe highest rate applied caused �90% mortality. Bothformulations were generally effective on E. nitens at20�C. Up to 7 d after exposure, Mimic was not veryeffective against U. lugens on either E. fastigata orE. nitens, although insecticide rate and temperaturedid have statistically signiÞcant effects on mortality(Table 4). Mortality was somewhat greater at 20than 15�C but remained �60% at most rates applied(Fig. 3).

Discussion

The broad-spectrum pyrethroids and the spinosadformulations were highly effective against all stages ofU. lugens larvae tested, even at the lowest applicationrates. There was no apparent difference in perfor-mance between the conventional (Success Natur-alyte) and organic (Entrust Naturalyte) formulationsof spinosad. The Btk formulations tested did not resultin 100% mortality, although it was possible to achieve�90% mortality under some conditions (discussed be-low). In the trial comparing different formulations ofBtk, DelÞn and Dipel performed better than Foray48B, although it took �7 d for treatment effects tobecome apparent. The potency of the horticulturalproducts DelÞn and Dipel (measured as InternationalUnits of Btk per milligram; Novachem Services 2004)was �3 times greater than that of Foray 48B, which isintended for use in forestry (Bauce et al. 2004). Al-though the nominal rates applied in these experimentswere calculated to be equivalent between the differ-ent products, higher potency formulations are knownto provide better efÞcacy at lower droplet densitiesthan less potent formulations (van Frankenhuyzen1995, Bauce et al. 2004). This relationship betweenpotency and droplet density could explain the poorerperformance of Foray 48B relative to DelÞn and Dipel.Mimic also performed poorly against U. lugens com-pared with other insecticides tested. In laboratorytests, Mimic has been shown to be effective againstanother eucalypt pest,Mnesampela privata (Guenee)(Lepidoptera: Geometridae), but not until 14 d afterexposure (Elek et al. 2003). Logistical constraints pre-vented monitoring of the third experiment beyond 7 d.If this had been possible as in the second experiment,the treatment effects might have been greater.

The change from gregarious, skeletonizing feedingto solitary chewing that occurs in U. lugens approxi-mately halfway through larval development had the

Table 2. Significance tests of insecticide treatment on mortal-ity of skeletonizing and chewing U. lugens larvae after up to 6 dexposure compared with mortality of unexposed controls

Insecticide Larvae �2

Decis Forte Skeletonizing 270Chewing 112

Dominex 100 Skeletonizing 270Chewing 116

Foray 48B Skeletonizing 77Chewing 37

Success Naturalyte Skeletonizing 270Chewing 100

P � 0.01 and df � 1 for all tests.

Table 3. Tests for significance of host species, instar, and application rate on mortality of U. lugens larvae at 14 d after exposureto three Btk formulations (Delfin, Dipel, and Foray 48B)

Factor dfDelÞn Dipel Foray 48B

�2 P � �2 �2 P � �2 �2 P � �2

Host species 1 8.79 0.003 33.84 �0.0001 19.45 �0.0001Instar 1 44.29 �0.0001 37.99 �0.0001 19.98 �0.0001Rate 4 180.82 �0.0001 150.64 �0.0001 196.85 �0.0001


greatest impact on the mortality because of Btk. Goodcoverage of the target foliage with Btk is essential toensure maximum ingestion by the target organism,irrespective of product potency (van Frankenhuyzen1995). Chewing larvae not only consume larger

amounts of foliage but also eat the entire leaf, con-suming veins as well as both leaf laminar surfaces,whereas skeletonizing larvae feed on just one surfaceof a leaf for considerable periods and avoid consumingthe leaf veins and oil pits. If insecticide coverage onto

Fig. 2. Mean mortality (�95% CI) of skeletonizing (circles and solid line) and chewing (triangles and dashed line)U. lugens larvae after 14 d of exposure to Þve rates of three Btk formulations (DelÞn, Dipel, and Foray 48B) on two eucalyptspecies (E. cinerea and E. nitens).


the leaf underside is poor, then skeletonizing larvaefeeding on the underside may avoid insecticide expo-sure. The more patchy the coverage of Btk on the leaf,the higher the probability that gregarious, skeleton-izing larvae may inadvertently avoid feeding ontreated foliage. Any one or all of these factors may beresponsible for increasing the insecticide efÞcacyagainst chewing compared with skeletonizing U. lu-gens larvae.

The degree of spray coverage achieved in theseexperiments with the track sprayer may have beenaffected by the leaf morphology of the differenteucalypt species used (Forster et al. 2004). Manyeucalypt species are heteroblastic. Mature foliage ofE. nitens has long, nonglaucous, amphistomatal leavesthat hang vertically; thus, the foliage may have re-ceived poorer coverage from the track sprayer com-pared with the leaves of E. cinerea, which are glau-cous, hypostomatal, ovoid, and held horizontally(Brennan et al. 2001, James and Bell 2001). JuvenileE. nitens leaves are broader and more glaucous thanthe mature foliage and are also held horizontally to thestem. U. lugens suffered a similar degree of mortalitywhen exposed to Btk on juvenileE. nitens (experiment1) and E. cinerea (experiment 2), i.e., foliage that isheld horizontally. Under operational conditions, how-ever, leaf morphology may have less inßuence on Btkcoverage. Compared with the doseÐresponse trialsusing the track sprayer, skeletonizing and chewingU. lugens larvae suffered higher mortality with lessvariability among replicates on foliage of E. cinerea(horizontal orientation) and matureE. nitens (verticalorientation) after Btk was applied using a backpacksprayer (Gous et al. 2004). It is likely, therefore, thatthe leaf coverage achieved when Btk is sprayed ontofoliage will be the most signiÞcant factor inßuencingits ability to control the early instars of U. lugens.

Leaf morphology is not the only factor that differsbetween host plants. Physical and chemical charac-teristics such as the degree of leaf waxiness and thenature of leaf oils also vary among eucalypt species(Edwards and Wanjura 1990, Li et al. 1994). Larvalsurvival and developmental rates of U. lugens are bet-ter on some eucalypt species than others (Cobbinah1985). E. nitens and E. nicholii Maiden & Blakelywere the most suitable larval hosts for U. lugens of18 species tested, including E. cinerea (Potter andStephens 2005). In these doseÐresponse trials, thegreater suitability of E. nitens foliage for larval devel-opment may have enabled more U. lugens larvae tosurvive treatment with Btk compared with larvae thatfed on E. cinerea. Beveridge and Elek (2001) similarly

showed that Novodor (active ingredient Bacillus thu-ringiensis tenebrionis) was less effective against achrysomelid leaf beetle when it was applied to amore favorable host (E. nitens) compared with a lessfavorable host (Eucalyptus regnans F. Muell.). Alter-natively, the insecticidal activity of Btk may havebeen affected by the presence of secondary com-pounds in the leaves, as has occurred with other lep-idopteran pests (Hwang and Lindroth 1993, Appeland Schultz 1994). For example, higher tannin levelsin white spruce, Picea glauca (Moench) Voss, foliagewere associated with increased survival of spruce bud-worm,Choristoneura fumiferna(Clemens) larvae afterexposure to Btk (Carisey et al. 2004).

The efÞcacy of Btk against U. lugens also could beinßuenced by seasonal changes in plant chemistry. Wedid not speciÞcally test for seasonal effects in any ofthe experiments reported here. The high variability inefÞcacy both between and within the different for-mulations of Btk in experiment 2 was not inßuencedby seasonal effects because all the foliage used wascollected on the same date. Nonetheless, we cannotrule out that efÞcacy may vary depending on the timeof year applied, independent of temperature. In allspray trials, we used fully expanded leaves, not youngleaves from the distal portions of branches, to mini-mize interactions because of variations in leaf chem-istry (Hwang and Lindroth 1993, Appel and Schultz1994). Further research may be required to reduce thevariability in efÞcacy of Btk if it is to be the insecticideof choice for the management of U. lugens.

Physical characteristics of eucalypt leaves (such aswaxiness) also affect the wettability of the leaf surfacewhen treated with insecticides. The term wettabilityrefers to the degree of droplet adhesion to the leafsurface and is related to the microscopic roughness ofthe leaf (Wirth et al. 1991). One method of measuringthis attribute is to determine the contact angle formedby droplets of a 20% (vol:vol) solution of acetone inwater when applied to fresh leaves. Smaller contactangles (�60�) indicate an easier to wet surface,whereas larger contact angles (�100�) indicate a dif-Þcult to wet surface (Vollhardt 1992, Gaskin et al.2005). The current study found lower efÞcacy andgreater variation in bioassay response on Btk-treatedmature E. nitens foliage compared with E. cinerea fo-liage. EfÞcacy also was somewhat reduced and vari-ability greater on E. nitens foliage than E. fastigatafoliage when treated with spinosad or Mimic, partic-ularly at lower temperatures. The foliage ofE. fastigataand E. cinerea is easier to wet than E. nitens foliage,another factor that may have contributed to the ob-

Table 4. Tests for significance of host species, temperature, application rate on mortality of U. lugens larvae at 7 d after exposureto three insecticides (Entrust Naturalyte, Success Naturalyte, and Mimic)

Factor dfEntrust Naturalyte Success Naturalyte Mimic

�2 P � �2 �2 P � �2 �2 P � �2

Host species 1 16.57 0.0014 16.43 �0.0001 2.66 0.1027Temp 1 10.24 �0.0001 16.09 �0.0001 8.42 0.0037Rate 4 82.44 �0.0001 266.14 �0.0001 17.64 0.0014


served differences in efÞcacy between host plant spe-cies (Gous et al. 2004, Gaskin et al. 2005). Thesedifferences in wettability may still have inßuenced

results despite the addition of adjuvants to the spraymixtures. Organosilicone adjuvants such as Du-Wettare frequently used to improve spray deposition on

Fig. 3. Mean mortality (�95% CI) of skeletonizing U. lugens larvae after 7 d of exposure at 15�C (circles and solid line)and 20�C (triangles and dashed line) to Þve rates of three insecticides (Entrust Naturalyte, Success Naturalyte, and Mimic)on two eucalypt species (E. fastigata and E. nitens).


hard to wet foliage (Gaskin et al. 2000, NovachemServices 2004). However, a detailed investigation ofthe most suitable rates and types of adjuvant for in-secticide treatments applied to eucalypt foliage wasbeyond the scope of this project.

Temperature had a signiÞcant inßuence on the ef-fectiveness of insecticides for control of U. lugens.Cooler temperatures seemed to reduce efÞcacy, andone likely mechanism for this result was that coolertemperatures decreased mobility and food consump-tion rates by U. lugens. Developmental rates are sen-sitive to temperature in U. lugens (Allen and Keller1991, Kriticos et al. 2005). Similar examples of tem-perature inßuencing insecticide efÞcacy have beenrecorded in the literature. The effectiveness of spi-nosadand the IGRdißubenzuronagainst grasshoppersdeclined at cooler temperatures (Amarasekare andEdelson 2004). Similarly, greenhouse thrips reducedtheir ßight activity at cooler temperatures, resulting inlower mortality from endosulfan treatment (Shippand Zhang 1999). Although this study did not specif-ically test the effect of temperature on Btk formula-tions, the LT50 of spruce budworm exposed to Btkhalved from 16 to 28�C (van Frankenhuyzen and Nys-trom 1987). Insecticides whose mode of action isthrough ingestion alone, rather than both contact andingestion, are likely to show the greatest reduction inefÞcacy against U. lugens at cooler temperatures.

This study has provided important information tosupport the registration of insecticides for control orlocal eradication ofU. lugens populations in New Zea-land, if and when this is required. Lepidopteran-se-lective chemical and biological insecticides with theactive ingredients spinosad or Btk are suitable forcontrol of U. lugens in sensitive environments andshould achieve similar efÞcacy to broad-spectrumcompounds. Mimic did not seem to be effective fromthe results of this study, although a longer monitoringperiod after exposure may be necessary to fully eval-uate this compound against U. lugens. When broad-scale (aerial) pesticide applications are requiredagainst U. lugens, as is possible if future outbreaksoccur in valuable eucalypt plantations, Btk-containingproducts can now be used, despite not achieving thesame level of efÞcacy as other compounds tested inthis study. Products containing Btk are more compat-ible with biological control agents, particularly para-sitoids, because Btk is speciÞc to Lepidoptera (Sampleet al. 1996, Ma et al. 2000). In contrast, the moreeffective spinosad has been shown to have negativeeffects on beneÞcial Hymenoptera (Consoli et al.2001, Schneider et al. 2004). Good coverage of thetarget area will be most important when applying Btkproducts against the earlier instars exhibiting skele-tonizing feeding patterns and during cool tempera-tures, such as occurs during the early winter larvalstage of U. lugens. Higher than minimal applicationrates and increasing the wettability of the leaf throughthe use of adjuvants added to the spray mixture maybe required to compensate for inhibitory host planteffects on some eucalypts, and at some times of theyear.

Acknowledgments

Thanks to Kevin Steele, Benjamin Benharrosh, DianeJones, Judith Gardner, Robyn Gaskin, Sarah Skinner, SarahHailes, Arnaud Blouin, Chris Inglis, and Chris Scott for tech-nical support. Insecticides used in these trials were donatedby Gary Elliott (Elliott Chemicals, Pukekohe, New Zealand)and Brian Husband (Dow Agrosciences, New Plymouth,New Zealand). The assistance of staff at MAF TreatmentTechnology Centre and HortResearch and access to facilitieswas greatly appreciated. Research funding was provided byBiosecurity New Zealand.

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Received 27 November 2005; accepted 7 February 2006.


Potential of Selective Insecticides for Managing Uraba lugens (Lepidoptera: Nolidae) on Eucalypts

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