真菌学教学课件：chapter 8 Mycopesticides1

WhenImovedin,Isawthatmysmallhouse,whichrestedupontimbers,hadsoftspotsinthefloorwherefungiwereemergingthroughthecarpets.Uponinvestigating,Iwasbothfascinatedanddisturbedtofindhugewavesofbrownandwhitemyceliumcoursingthroughthefloorsandwallsofmyhouse.

Unfortunately,thespreadofwood-digestingfungisuchastheartistconkiswellknowntoprecedeorcoincidewithtermiteandcarpenterantinvasions;thefungisoftenandmoistenthewood,settingthetableforthesemunchinginsects.

AndsoIdiscoveredthattheslumpinghousewasunderattackbycarpenterants(Camponotusmodoc).

Eachday,Iawoketofindpilesofsawdust

onthefloornearthewalls;overnightmoreofmyhousehadbeenmilledintosawdustbyants.Visitorsshoweddispleasureatthesawdustrainingdownuponthem,despitemybestattemptstomakelightofthesituation.

IsearchedtheInternettofindinformationaboutfungithatnaturallyparasitizeinsects,andIbegantoreadaboutMetarhiziummolds,especiallyMetarhiziumanisopliae,whichhadbeenusedtokilltermites.

Idecidedtogiveitatryandorderedastrainfromacommercialtissue-culturelibrary.

TheEmergenceofMycopesticidesSwarmingovertheplanetarelegionsofinsectswhosediversitybogglestheimagination.Currentestimatesofinsectdiversityrangefrom4to6millionspecies(Kirby2002),whilefungihoverbetween1and2millionspecies(Rossman1994).

Pesticideswereinventedtofightdestructiveinsectsandprotectcropsandstructures.However,manyofthechemicalsusedinpesticides,especiallytheorganophosphates,harmnontargetedorganisms,pollutewater,andimpairhumanhealth.

Theneedforalternative,nontoxic

pesticidesiscritical,sincethemedicalandecologicalimpactoftoxicpesticidesposesacascadinghealthhazardandaglobalthreattoourbiosphere.

Findingareplacementforenvironmentallydangerouspesticideswillbringusonestepclosertopreservingbiodiversityandsavingtheecosystem.

Thesearchforecologicallyrationalmethodstocontrolinsectshasfocusedonbiopesticides—

nature-basedremediesthatcauseonlynegligiblecollateraldamage,ornoneatall,tootherorganisms.

Biopesticides,especiallyselectfungi,donotposeapersistentthreattotheenvironmentafteruse,incontrasttomanyconventionalchemicaltreatmentsthatcauselong-termdamage.Theinterrelationshipsbetweenfungiandinsectsarecomplexandareonlynowbeingunderstood.Practicallyallinsectsengage,consume,orsuccumbtofungi.

Weknowthatthousandsofspeciesoffungi(calledentomopathogenicfungi)attack

thousandsofinsectspecies.Thisdancebetweenpredatorandpreyhasledtotheevolutionofsurvivalstrategiesforboth,someofwhichborderonthebizarre.Byexploringtheserelationships,wemaybeabletoadaptnewtechniquesforcontrollinginsectswithoutharmingtheenvironment.

In1834,AgustinoBassinoticedthatsporesofthefungusBeauveriabassianawerecausingthediseasemuscardine,aplaguethatimperiledtheinternationalsilktrade.Heiscreditedwithconceiving“germtheory,”amajortenetofmodernmedicine,wellbeforeLouisPasteurdiscoveredtheroleofmicrobesin1858.

Sincethe1990s,severalpatentshavebeenawardedexploitingthesemoldfungi,raisingexpectationsfortreatmentsintheemergingfieldofentomopathogenicmycology.

Mostofthesemycopesticidalpatentstargetaninsectspeciesusingafungusstrain,withorwithoutaneffectivedeliverysystem.Wheninsectscomeintocontactwithsporesoftheseentomopathogenicfungi,thesporesattachtotheinsectsandgerminate,boringhyphalpegsthroughtheinsects’exoskeletonsusingchitin-dissolvingenzymes.Otherportalsofentryincludetherespiratorytract,anus,andmouth.

Onceinside,themyceliumforksandrunsthroughtheinternalorgans,interfering

withthecreature’smetabolismandcausingmalaise,necrosis,anddeathinafewdays.

Theinsects,lookingmummifiedwithfuzzymycelium,thenbecomealaunchingplatformforfurthersporulation.

OnespeciesofCordyceps(Cordycepslloydii),whenitinfectsacarpenterant,compelstheinsecttoclimbtothecanopyoftheCostaRicancloudforestwhereitresides.Onceithasascended,theantlocksitsmandibleintoaleafanddies.ACordycepsmushroomeruptsoutofitshead

Byreleasingas-yet-unidentifiedchemicalcompoundsandcompellingtheinsecttoascendtothetopoftheforestcanopy,themushroomcanreleasesporesthatflyfarandwide,ensuringwiderdistributionthanwouldbepossiblenearertheground.Manysuchfungiuseinsectsasvehiclesfordistributionand,later,theircarcassesascentersforsporeproduction.

Worldwide,termitespeciesnumberatleast2,500,ofwhichabout150speciescanattackbuildings,butwithinthose150species,onlyafewcausewidespreadeconomiclosses.

Thedamage,primarilytobuildings,causedbynativetermitesintheUnitedStatesaloneexceeds$10billionperyear,whilethedamagefromtheimportedFormosantermiteexceeds$1billionannually.

Onanindividuallevel,thethreatoflosingone’shouse—formostpeopletheirlargestsingleinvestment—candriveahomeownertospendwhateverisnecessarytopreventdamagebyinsects.

Thisdrivetoprotectone’shomehasbeenamajoreconomicstimulusforthepesticideindustry,andaglobalmarketforinsect-defensemeasuresandcurescontinuestospurtheinventionofnewpesticides.Thesetypesoftechniquessucceedtemporarilyat.Plus,chemicaltreatmentscanbetoxictohumans,otheranimals,andtheenvironment.

Theuseofentomopathogenicfungiasabiologicalweaponagainstinsectshasmanyadvantagesbutalsofacesobstacles.

Severalcompanieshaveexpendedagreatdealofmoneytodevelopsporedeliverysystems,especiallyusingtheentomopathogenicgreenmoldMetarhiziumanisopliaeandthewhitemoldBeauveriabassiana.However,sprayinganentirebuildingwithsporesisimpracticalandunlikelytoassassinatethequeenofthecolony.

Althoughcaptiveinsectsareeasilyinfectedwithsporesinthelaboratory,insectcommunitiesinnaturehavedevelopedsophisticatedstrategiesfordefendingthemselvesagainstcontamination.Ingroupsofsocialinsects,spore-carryingworkersarerefusedentrytothenest,withseveraltiersofguardspreventinganydisease-bearinginsectsfromentering.

Throughoutthelate1980sand1990s,pesticideresearcherstriedtoplacesporesin.Thiswayofkillinginsectslookedpromisingatfirst,sinceentomopathogenicsporesquicklypenetratetheexoskeletonsoftermitesorantsuponcontact.Butthecolony’saversiontopathogenicsporesandtheinsectscarryingthemstymiedcommercializationoftheseinsectcontrolstations.Althoughincidentalcontactwithfree-flyingorsoil-bornesporesmaysucceedtoinfectindividualinsects,naturalselectionhasgivenadecidedadvantagetoinsectsthatcandetectentomopathogenicsporesandavoidthem.

Afungusthatwouldalwaysalertitsdesiredtargetinsectwithsmellysporeswouldbeselectedagainstunlesstherewasanalternativestrategyfortrickingtheinsectstoengagethefungus.Isurmisedthatsincetheinsectscandetectinfectiousspores,essentially“knowing”amoldplaguewhentheysense,orpossiblysmell,one.IrealizedthatIcouldcreateatemporarysporelessstateinMetarhiziumorBeauveriathatnotonlylackstherepellentsmellbutalsoactuallycontainsanattractant.WhenIfirstobtainedculturesofthegreenmoldMetarhiziumanisopliaeformyexperimentswithcarpenterants,Iselectedwhite-wedgesectorsofmyceliumthatlackedorhaddelayedsporeformation,andImadedescendantcultures.

Aftersuccessivetransfersoftheselectedpresporulatingmycelium,itswhitishformsoondominated.Inculturingthesemoldfungi,andthroughfurthergenerationsoftransfers,Iisolatedanonsporulatingmycelialphenotype.Ipursuedthesepresporulatinggrowthformsinvitroandsoondiscoveredthatthesporelessmycelium,stilllethaltoinsects,emitsattractants.

Sinceinsectsavoidknownlethalspores,attractingthemtopresporulatinginsecticidalfungihasobviousadvantages,includingthattheinsectsthemselvescouldbecomeagentsfordispersingthisinsecticidalfungus,whichcouldla