Frontiers in Zoology This Provisional PDF corresponds to the article as it appeared upon acceptance. Fully formatted PDF and full text (HTML) versions will be made available soon. A cuckoo in wolves' clothing? Chemical mimicry in a specialized cuckoo wasp of the European beewolf (Hymenoptera, Chrysididae and Crabronidae) Frontiers in Zoology 2008, 5:2 doi:10.1186/1742-9994-5-2 Erhard Strohm ([email protected]) Johannes Kroiss ([email protected]) Gudrun Herzner ([email protected]) Claudia Laurien-Kehnen ([email protected]) Wilhelm Boland ([email protected]) Peter Schreier ([email protected]) Thomas Schmitt ([email protected]) ISSN 1742-9994 Article type Research Submission date 30 May 2007 Acceptance date 11 January 2008 Publication date 11 January 2008 Article URL http://www.frontiersinzoology.com/content/5/1/2 This peer-reviewed article was published immediately upon acceptance. It can be downloaded, printed and distributed freely for any purposes (see copyright notice below). 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Acuckooinwolves’clothing?Chemicalmimicryina specializedcuckoowaspoftheEuropeanbeewolf (Hymenoptera,ChrysididaeandCrabronidae) ErhardStrohm1,2,JohannesKroiss1,2,§,GudrunHerzner1,2,ClaudiaLaurien-Kehnen3,4, WilhelmBoland5,PeterSchreier6,ThomasSchmitt2,7 1UniversityofRegensburg,InstituteofZoology,D-93040Regensburg,Germany 2UniversityofWürzburg,DepartmentofAnimalEcologyandTropicalBiology,Am Hubland,97074Würzburg,Germany 3UniversityofBonn,ZoologicalInstitute,PoppelsdorferSchloss,53115Bonn,Germany 4presentaddress:UniversityofBielefeld,BehaviouralBiology,33501Bielefeld,Germany 5MaxPlanckInstituteforChemicalEcology,DepartmentforBioorganicChemistry,07745 Jena,Germany 6UniversityofWürzburg,DepartmentofFoodChemistry,AmHubland,97074Würzburg, Germany 7UniversityofFreiburg,InstituteofBiologyI(Zoology),Hauptstr.1,79104Freiburgi.Br., Germany §Correspondingauthor Emailaddresses: ES:[email protected] 1 JK:[email protected] GH:[email protected] CLK:[email protected] WB:[email protected] PS:[email protected] TS:[email protected] 2 Abstract Background Host-parasiteinteractionsareamongthemostimportantbioticrelationships.Hostspecies shouldevolvemechanismstodetecttheirenemiesandemployappropriatecounterstrategies. Parasites,inturn,shouldevolvemechanismstoevadedetectionandthusmaximizetheir success.FemalesoftheEuropeanbeewolf(Philanthustriangulum,Hymenoptera, Crabronidae)huntexclusivelyhoneybeeworkersasfoodfortheirprogeny.Thebroodcells containingtheparalyzedbeesareseverelythreatenedbyahighlyspecializedcuckoowasp (Hedychrumrutilans,Hymenoptera,Chrysididae).Femalecuckoowaspsenterbeewolfnests toovipositonparalyzedbeesthataretemporarilycouchedinthenestburrow.Thecuckoo wasplarvakillsthebeewolflarvaandfeedsonitandthebees.Here,weinvestigatedwhether H.rutilansevadesdetectionbyitshost.Sincechemicalsensesaremostimportantinthedark nest,wehypothesizedthatthecuckoowaspmightemploychemicalcamouflage. Results Fieldobservationssuggestthatcuckoowaspsareattackedbybeewolvesinfrontoftheirnest, mostprobablyafterbeingrecognizedvisually.Incontrast,beewolvesseemnottodetectsigns ofthepresenceoftheseparasitoidsneitherwhenthesehadvisitedthenestnorwhendirectly encounteredinthedarknestburrow. Inarecognitionbioassayinobservationcages,beewolffemalesrespondedsignificantlyless frequentlytofilterpaperdiscstreatedwithacuticularextractfromH.rutilansfemales,than tofilterpaperdiscstreatedwithanextractfromanothercuckoowaspspecies(Chrysis viridula).ThebehaviortopaperdiscstreatedwithacuticularextractfromH.rutilansfemales 3 didnotdiffersignificantlyfromthebehaviortowardsfilterpaperdiscstreatedwiththe solventonly. Wehypothesizedthatcuckoowaspseithermimicthechemistryoftheirbeewolfhostortheir host'sprey.WetestedthishypothesisusingGC-MSanalysesofthecuticlesofmaleand femalebeewolves,cuckoowasps,andhoneybeeworkers.CuticleextractsofHedychrum nobile(Hymenoptera:Chrysididae)andCercerisarenaria(Hymenoptera:Crabronidae)were usedasoutgroups.Therewaslittlecongruencewithregardtocuticularcompoundsbetween H.rutilansfemalesandhoneybeesaswellasfemalesofC.arenariaandH.nobile.However, therewasaconsiderablesimilaritybetweenbeewolffemalesandH.rutilansfemales. Beewolffemalesshowastrikingdimorphismregardingtheircuticularhydrocarbonswithone morphhaving(Z)-9-C25:1andtheothermorphhaving(Z)-9-C27:1asthemajorcomponent. H.rutilansfemalesweremoresimilartothemorphhaving(Z)-9-C27:1asthemain component. Conclusions WeconcludethatH.rutilansfemalescloselymimicthecompositionofcuticularcompounds oftheirhostspeciesP.triangulum.Theoccurrenceofisomericformsofcertaincompounds onthecuticlesofthecuckoowaspsbuttheirabsenceonbeewolffemalessuggeststhat cuckoowaspssynthesizethecuticularcompoundsratherthansequesterthemfromtheirhost. Thus,thebehavioraldataandthechemicalanalysisprovideevidencethataspecialized cuckoowaspexhibitschemicalmimicryoftheodorofitshost.Thisprobablyallowsthe cuckoowasptoenterthenestwithareducedriskofbeingdetectedbyolfactionandwithout leavingtraitorouschemicaltraces. 4 Background Theinteractionbetweenhostsandparasitesorparasitoidsisoneofthemostimportantforces drivingevolutionaryandecologicalprocesses(e.g.[1]).Inordertoreducetheimpactof parasitoids,hostspeciesmayevolvemechanismstodetecttheirenemiesandemployadequate counterstrategies(e.g.[2-5]).Parasitoids,inturn,areselectedtoevolvemechanismsthat reducetheprobabilityofbeingdetectedbytheirhoststocircumventsuchcountermeasures. Thissetsthestageforrepeatedcyclesofadaptationsandcounteradaptations(“evolutionary armsrace”;seee.g.[6-10])betweenhostsandparasites,especiallyiftheparasitoidishighly specializedonasinglehostspeciesandhasalargeimpactonhostfitness[2,11,12]. Progenyofbroodcaringbeesandwaspsareparticularlysusceptibletoparasitism[13-15]. Femalesofthesespeciesstorelargeamountsofvaluablenutrientsaslarvalprovisionsin broodcells.Thesevaluableresourcesattractavarietyofbroodparasites,either cleptoparasitesthatreducetheamountofresourcesavailabletothehost’sprogenyor parasitoidsthatobligatorilykillthehostlarvae.Mostly,femalesoftheseparasiticspecies havetoenterthenestorthebroodcelltodepositeggsorlarvae.Thus,thetracesthatareleft byfemalebroodparasitesmightbedetectedbythehost.Asaresult,thehostsmightabandon thenestsorremoveordestroyeggsofbroodparasites[5,16].Broodparasitesmightalsobe encounteredinthenestbythehostandmightbedrivenaway,injured,orevenkilled(E. Strohm,unpubl.observations).Sinceinsectsheavilyrelyontheirchemicalsensesforany kindofrecognitionorlocalizationprocess[1,17,18],concealmentofabroodparasite's actualorpreviouspresencewillrequirechemicalcamouflage(compoundssequesteredfrom thehostorthehost’snest)orchemicalmimicry(compoundssynthesizedbythemimic, definitionssensu[19]).Inthisstudy,weinvestigatedtheinteractionbetweenahuntingwasp, 5 theEuropeanbeewolf,Philanthustriangulum(Hymenoptera,Crabronidae)anditshighly specializedbroodparasitoidHedychrumrutilans(Hymenoptera,Chrysididae).We investigatedthefollowingquestions:Arecuckoowasps(H.rutilans)detectedandrecognized bybeewolffemalesatall?Isthereadifferenceinhostresponsetowardsthecuckoosoutside andinsidethenestanddothecuticularhydrocarbonsplayaroleforthedetectionofthe cuckoowaspsinsidethenest?Isthechemicalcompositionofthecuticularhydrocarbonsof H.rutilansfemalessimilartotheirhostortotheirhost’sprey? FemalesoftheEuropeanbeewolfhunthoneybeeworkers(Hymenoptera,Apidae)asfoodfor theirprogeny.Severalparalyzedbeesaretemporarilycouchedinthemainburrow(upto1m long)oftheundergroundnest(see[20]fordetailsonnestarchitecture).Eventually,the femaleclosesthenestentrance,excavatesasideburrowandaterminalbroodcell,bringsin onetosixparalyzedbees,andovipositsononeofthebees[21].Thereupon,shecarefully closesthesideburrowandsubsequentlyhasnocontacttoherprogeny. Thecuckoowasp,H.rutilans,isaspecializedbroodparasitoidofthegenusPhilanthus[16, 22].However,sinceinCentralEuropeonlyonememberofthegenus,P.triangulum,isfairly abundant,H.rutilansiseffectivelymonospecificinthisregion.Thisconsiderabledegreeof specializationisexpressedbytheuniqueovipositionstrategyofH.rutilans.Mostchrysidid waspsovipositintothebroodcelloftheirhostsatadefinedstageoftheprovisioningcycleor afterthebroodcellhasbeenfinallyclosed[22].Inbeewolves,however,thebroodcellis excavatedonlyafterthefemalehasbroughtinthebeesandthenestentrancehasbeenclosed. Thus,thenestisblockedupandthefemaleisattendantuntilthebroodcellisfinallyclosed. Thisleaveslittleopportunityforacuckoowasptodepositanegginthebroodcell.Asa consequence,H.rutilansfemalespursuetwoalternativestrategies.Eithertheyrapidlypounce 6 andovipositonaparalyzedbeewhenthefemalealightswithitspreyandenterstheburrow ([23],E.Strohmunpubl.observation),orH.rutilansfemaleswaitinfrontofthenestuntilthe hostfemaleleavestoforageandthenentertheburrowandovipositontheparalyzedbeesthat