简介：Interactionsbetweenecologicalcommunitiesofherbivoresandmicrobesarecommonlymediatedbyasharedplant.Atripartiteinteractionbetweenapathogenicfungus-hostplant-herbivorousinsectisanexampleofsuchmutualinfluences.Insuchasystemafungalpathogencommonlyhasanegativeinfluenceonthemorphologyandbiochemistryofthehostplant,withconsequencesforinsectherbivoreperformance.HerewestudiedwhetherthebiotrophicfbngusPodosphaeraferruginea,attackingthegreatburnetSanguisorbaofficinalis,affectscaterpillarperformanceoftheendangeredscarcelargebluebutterflyPhengaristeleius.Ourresultsshowedthatthepathogenicftmgusaffectedthenumberandsizeofinflorescencesproducedbyfood-plantsand,moreimportantly,hadindirect,plant-mediatedeffectsontheabundance,bodymassandimmuneresponseofcaterpillars.Specifically,wefoundtherelationshipbetweencaterpillarabundanceandvariabilityininflorescencesizeonaplanttobepositiveamonghealthyfood-plants,andnegativeamonginfectedfood-plants.Caterpillarsthatfedonhealthyfood-plantsweresmallerthanthosethatfedoninfectedfood-plantsinonestudiedseason,whiletherewasnosuchdifferenceintheotherseason.Weobservedtherelationshipbetweencaterpillarimmuneresponseandtheproportionofinfectedgreatburnetswithinahabitatpatchtobepositivewhencaterpillarsfedonhealthyfood-plants,andnegativewhencaterpillarsfedoninfectedfood-plants.OurresultssuggestthatthisbiotrophicfungalinfectionofthegreatburnetmayimposeasignificantindirectinfluenceonP.teleiuscaterpillarperformancewithpotentialconsequencesforthepopulationdynamicsandstructureofthisendangeredbutterfly.