elm leaf beetles - extension · elm leaf beetle larvae after egg hatch and associated leaf injury....
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Fact Sheet No. 5.521 Insect Ser ies|Trees and Shrubs
Quick Facts•Adultelmleafbeetleschewholesinelmleaves.Thelarvaefeedontheleafsurface,producingskeletonizinginjuriesthatmakeleaveslooklacy.
•Adultelmleafbeetlesoftenspendwinterinandaroundbuildingsandmaybecommonnuisanceinvadersfoundwithinhomesduringthisperiod.
•Someinsecticidesthatmovesystemicallywithinplantscanbeappliedtothesoiltocontrolelmleafbeetles.
Elmleafbeetles(Xanthogaleruca luteola)arecommoninsectsthatchewleavesofelmtrees(Figure1).Thedarkgrub-likelarvaechewontheundersideofleavesbutavoidthelargerleafveins,producingatypeofinjurypatternknownasskeletonizing(Figure2).Leavesdamagedbyelmleafbeetlelarvaelooklacy,turnbrownandmayprematurelydropfromthetrees(Figure3).Adultbeetleschewirregularlyroundholesinthecenterofleaves.SiberianandEnglishelmsareparticularlyfavoredbythisinsect.Elmleafbeetlesalsocanbe
importantasanuisancepestinhomes,becausetheyoftenenterbuildingsinautumnwhenseekingwintershelter.Beetlesthatdoworktheirwaybehindwallsandotherareasofbuildingsmaythenbefoundindoorsuntilspring,becomingmostactiveduringwarmperiods.Fortunatelyelmleafbeetlesarestrictlyanuisanceinvadertypeofinsectthatdoesnotfeedonnordamageanythingwithinahome,althoughtheirverypresenceinahomeisacommoncauseofconcern.Historicallyelmleafbeetleshave
beenamajorinsectinColoradocommunitieswhereeverelmtreesarecommon.However,sincethemid-1990s,incidenceofproblemshasdeclinedinmostareasofColorado.InrecentyearsproblemshavemostoftenoccurredintownsalongtheArkansasRiverValley,lesscommonlyoccurringasspotoutbreakselsewhereineasternColorado.Beginningaround2006anadditional
beetlearrivedinColoradothatfeedsonelm,theEuropeanelmfleaweevil(Orchestes alni).Europeanelmflea
by W.S. Cranshaw*
*W.S. Cranshaw, Colorado State University Extension entomologist and professor, bioagricultural sciences and pest management. 1/2018
weevilproducessomeleafinjuries,suchasshotholesinleaves,whicharesimilartothoseproducedbyadultelmleafbeetles. This new insect of elms is occurs throughout the state and is discussed at the end of this sheet.
Life History and HabitsElmleafbeetlesoverwinterinthe
adult(beetle)stage.Inlatesummerandearlyautumntheyseekoutprotectedsitessuchaswoodpiles,loosemulch,andpiledeavestoshelterthroughwinter.Oftencracksandotheropeningsthatallowthemtogetbehindbuildingwallsserveaswintershelters.During
©ColoradoStateUniversityExtension.11/96.Revised1/18.
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Elm Leaf Beetles
Figure 1. Elm leaf beetle adults and damage.
Figure 2. Skeletonizing injury to elm leaves produced by elm leaf beetle larvae.
thisperiodthebeetlesareinasemi-dormantstate(diapause)andareakhaki-greencolor(Figure4).Whileinthisdormantstatetheydonotfeednorreproducebutmaybecomeactiveduringwarmdaysinlatewinterandspring.Inspringthebeetlesthatsurvived
winterbecomeincreasinglyactiveandmovetoelmtreeswhenleavesemerge.Asthebeetlesfeedtheircolorshiftstomoreyellow-greenandwithinafewweeksfemalebeetlesbeginlayingeggs(Figure5).Theeggsareyellow,typicallylaidontheundersideofleavesinmassesofoneortwodozen(Figure6).
Tiny,blacklarvaehatchfromtheseeggsandoriginallytheywillchewsmallpitsnearwheretheeggshadbeenlaid(Figure7).Astheygetolderandlarger,elmleafbeetlelarvaescatterthroughthetree.Theyfeedontheundersideoftheleaf,avoidingthelargerleafveins,“skeletonizing”theleaves(Figure8).Within3to4weeksaftereggsare
laid,thelarvaearefullgrownandtheythenwandertofindshelteredspotswheretheycantransformtotheirnextstage,thepupa.Mostcrawldownthetrunkandpupateatthebaseofthetree,sometimesinlargegroups(Figure9).Othersmaysettleintosomeofthelargerfissuresofthebark.Withinabouttwoweeks,newadults
emergefromthepupae.Theythenflybacktotheleaves,mate,andproduceasecondgeneration.Therefollowsasecondcycleoflarvaechewingleavesfrommid-JulyintoSeptember.Adultbeetlesthatareproducedattheendofthissecondgenerationwillfeedforabriefwhileonleavesbutdonotlayeggs.Theythenmovetooverwinteringsheltersiteswheretheyremainuntilthenextspring.
Natural ControlsFewnaturalenemiesfeedonelm
leafbeetles.Insectpredators,suchaspredaceousstinkbugsandplantbugs,mayfeedonvariousstagesoftheelm
leafbeetle.Therearesmallparasiticwaspsthatattacklarvaeandpupae,althoughincidenceoftheseappearstobeverylowinColorado.SomepupaemaybekilledbyBeauveriabassiana,afungusthatproducesdiseaseinmanyinsects,butinfectionswiththisdiseaserequireconditionsofhighhumidityandoutbreaksareuncommoninaridsites.Weatherprobablyisthemost
importantfactorthataffectselmleafbeetlepopulations.Longwintersoralatespringfreezemaykilllargenumbersofoverwinteringbeetles.Smalllarvaearesusceptibletobeingblownorrubbedofftreesduringwindstorms.Thismaybeparticularlycommonwhenlarvaeareforcedtofeedonolder,toughleaves.Thisincreasestheirwanderingandmanydieintheprocess.
Chemical ControlChemicalcontrolscaneffectively
controlelmleafbeetles.Thesearebestusedinareaswhereoutbreaksregularlyoccurorwhenlargenumbersofbeetlesareobservedlayingeggs,anindicationofapotentialoutbreak.Insecticidescanbeemployedinthreewaystomanageelmleafbeetle:1)useofsoildrenches/soilinjectionsofsystemicinsecticidesthatmovetokillinsectsonfoliage;2)useofsprayscoveringleavestodirectlycontactandkilllarvaeandadults;and3)trunk
Figure 3. Trees showing leaf injury by elm leaf beetle.
Figure 4. Elm leaf beetle adult, with the green coloration of the overwintering form. Photograph courtesy of Joseph Berger, Bugwood.org
Table 1. Insecticides that can be used to control elm leaf beetles. Products are listed by Active Ingredient (in bold). Some trade names/formulations of these insecticides are listed in parentheses.
Soil applied systemic insecticidesImidacloprid (BayerAdvancedTree&ShrubProtect&FeedConcentrateII,BonideAnnualTree&ShrubControl,ferti-lomeTree&ShrubSystemicInsectDrench;OrthoBugBGonYear-LongTree&ShrubInsectControl,Merit,Mallet,Zenith,Criterion,others)Chlothianidin (BayerAdvancedTree&ShrubProtect&FeedConcentrateII,Arena)
Insecticides applied as sprays to leavesAzadirachtin(Azatin,AzaGuard,Azasol,BioNeem,others)Bifenthrin(Talstar,Onyx,OrthoBugBGonInsectKillerforLawns&Gardens)Carbaryl(Sevin,Carbaryl)Cyfluthrin (Tempo,BayerAdvancedVegetable&GardenInsectSpray)Cyhalothrin(Scimitar,SpectracideTriazicideInsectKillerforLawns&Landscapes,CutterBackyardBugControlSprayConcentratePermethrin (Astro,Permethrin,Hi-YieldTurf,TermiteandOrnamental38Plus,BonideEightInsectControlYard&Garden,others)Spinosad (Conserve,BonideCaptainJack’sDeadBugBrew,NaturalGuardSpinosadLandscape&GardenInsecticide,ferti-lomeBorer,Bagworm,Leafminer&TentCaterpillarSpray)
Figure 5. Elm leaf beetle, with the yellow coloration of the form found during the growing season.
bandingwithinsecticidestokilllarvae astheymovetothebaseofthetreetopupate.
Soil Applied Systemic Insecticides.
Afewinsecticidesaresystemicinplants,capableofmovingthroughtheplantprovidingsothattheymaycontrolinsectssomedistancefromwheretheinsecticideisapplied.Mostusefulforcontrolofelmleafbeetleareinsecticidesthatcanbeappliedtothesoilandthatthencanmovetotheleaveswhereelmleafbeetlesfeed.Imidaclopridisthemost
commonlyavailableinsecticidethatmovessystemicallyinplantsandcanbeusedinelmleafbeetlecontrol.Lesscommonlyavailableischlothianidin,achemicallyrelatedmaterialalsointheneonicotinoidclassofinsecticides(Table1).Theseareusedasasoilapplicationappliedtoareaswhererootsabsorbtheinsecticidethenmovetheinsecticideintoandthroughtheplant.Theseinsecticidescanbeappliedaseithersoildrencharoundthebaseofthetreeorasaseriessoilinjectionsspreadtoareasunderthecrown.Ratesofinsecticideareindicatedinlabelinstructionsandareusuallyindicatedasbeingrelatedtoeitherthediameter(commercialformulations)orcircumference(over-the-counter/retailformulations)ofthetree.Afterapplicationtherewillbea
lagperiodbeforetheinsecticidemovesfromthesoilintotheleavesinenoughconcentrationtocontrolelmleafbeetles.Sincebothimidaclopridandchlothianidinmovefairlyslowlyinsoilthistypicallytakesabouttwotofourweeks.Mostimportantinhowquicklytheseinsecticidesmoveintotheleavesisthehowmoististhesoilwheretheinsecticideswereapplied.Foreffectivecontrolofelmleafbeetleitisveryimportantthatthesoiliskeptmoistintheareawheretheinsecticidewasapplied,whichunderColoradoconditionsusuallyrequiressomesupplementalirrigation.
Sprayed Treatments.
Severalinsecticides(Table1)canbeusedtocontrolelmleafbeetlesfeedingonleaves.Thesetreatmentsarebestappliedaftermosteggsarelaidbytheoverwinteringfemalesbutbeforethelarvaestarttocausesignificantinjurytotheleaves.ForthefirstgenerationofelmleafbeetleinsoutheastColoradothistypicallyoccursinlateMayorearlyJune.Mostoftheinsecticidesapplied
assprayswillhaveseveraldaysofpersistenceinabilitytokillelmleafbeetles.Thiscanbeausefulfeatureifmanyeggsremainunhatchedwhenspraysaremade.Otherinsecticides,suchastheneem-derivedproducts(azadirachtin)andbiologicalinsecticides(spinosad)haveshortpersistenceandshouldbetimedtoperiodsafteregghatch.
Trunk Banding
Somecontroloflateseasonelmleafbeetlefeedinginjuryispossiblebysprayingabandofinsecticideontrunkstokilllarvaethatcrawltothebaseofthetreewhentheyhavefinishthefirstcycleoffeedinginearlysummer.Thebandsshouldbeatleastonefootwideandplacedonthetrunkjustbelowwherethelowestmajorbranchesjointhetrunk.Anyinsecticideregisteredforuseonelmorshadetreescanbeusedfortrunkbanding.However,ratesofdilutionandusemustnotexceedtheamountsindicatedonthepesticidelabel.Pyrethroidinsecticides,suchasbifenthrin,permethrin,andcyfluthrin,haveshowneffectivepersistencewhenusedastrunkbands.
Elm Leaf Beetles in a Home
Inlatesummerelmleafbeetlesseekingwintersheltermaymoveintonearbyhomes,becomingaseasonalnuisanceinvader.Toavoidthispreventivestepscanbetakentoexcludetheinsects.Priortoperiodswhenbeetlesmovetohomes,sealallcracksthatallowentry.Caulkareasaroundwindowmoldingorundersiding.Screensshouldbeinplace.Typically,adultbeetlesstarttomigratetooverwinteringshelterinmid-August,butmovementsgreatlyincreasebyearlySeptember.
Figure 6. Egg mass of an elm leaf beetle.
Figure 7. Elm leaf beetle larvae after egg hatch and associated leaf injury.
Figure 8. Late stage elm leaf beetle larvae.
Figure 9. Elm leaf beetle pupae and last stage larvae, settled at the base of a tree.
ColoradoStateUniversity,U.S.DepartmentofAgricultureandColoradocountiescooperating.CSUExtensionprogramsareavailabletoallwithoutdiscrimination.Noendorsementofproductsmentionedisintendednoriscriticismimpliedofproductsnotmentioned.
Figure 10. European elm flea weevil adults feeding on new growth on spring.
Figure 11. Siberian elm leaf showing leaf mines and shot hole wounds produced by the larvae and adults of elm leaf beetles, respectively.
Figure 12. Shothole wounds produced by adults of European elm flea weevil.
Someincreaseincontrolispossibleifsealingisaccompaniedbyspotinsecticidetreatmentofthebuildingexterioratpointswherebeetlesmayfindentryintrobuildings,suchasaroundwindows.Severalformulationsofpyrethroidinsecticides(activeingredientsincludepermethrin,bifenthrin,cyhalothrin,cyfluthrin)areavailableforthisuse.Regularvacuumingismosteffective
forbeetlesthatarefoundwithinahome.Thisisbestdoneduringwarmperiodswhenmostofthebeetlesareactiveandmassonwindowsorwalls.Duringcoolerperiods,theoverwinteredbeetlesoftenreturntoshelteredareas.Elmleafbeetlesdonotreproduce
inhomes.Althoughsomemayemergeandmoveaboutduringwarmsperiods,thebeetlesthatspendwinterinbuildingsareharmlessandtheydonotfeednordamagehouseholdfoodsandfurnishings.Thesenuisanceproblemsendbymid-latespring,afterallbeetleshaveeithermovedoutdoorsordied.
European elm flea weevil – A new insect pest of elms
AnotherbeetlethatfeedsonelmandisnowpresentinColoradoistheEuropeanelmfleaweevil(Figure10).ThisinsectwasaccidentallyintroducedintoNorthAmericaandfounditswaytoColoradofairlyrecently,beingfirstidentifiedin2006.Inaveryshortperiodfollowingthisinitialfindingitwasfoundtobepresentstatewide.Europeanelmfleaweevilsare
muchsmallerthanelmleafbeetlesandarecapableofjumping.Theydonothavethehabitofusingbuildingsasshelteredsites,butremainoutdoorsthroughwinter.Theadultsthenmovetoelmtreesasbudsbreakandfeedontheemergingleaves,producingsmallshotholewoundsintheleaves.Femalesthenbegintolayeggs.
Eggsarelaidindividually,notasamass,andareinsertedintotheleaf.Uponhatch,thelarvaethenfeedwithintheleaf,betweentheupperandlowersurfaces,asatypeofleafminer(Figure11).Thepatternoftheleafminetheyproduceoriginallymeanders,gettingwiderastheinsectgrows,andterminatesattheedgeoftheleaf,
producingablotchymine.Thelarvaecompletetheirdevelopmentintheleafmineandthenpupate.AdultsemergeinlateJuneand
earlyJuly.Theythengothroughanothercycleofleaffeeding,producingmoreshotholefeedingwoundsonleaves(Figure12).Later
insummertheyabandontheplantsandseekareasforwintershelter,inbarkcracks,underfallenleavesandinsimilarprotectedareas.Onegenerationisproducedeachyear.Europeanelmfleaweevilswere
quiteabundantandproducedveryconspicuousleafdamageinthefirstfewyearsaftertheyarrivedinColorado.However,morerecentlynaturalenemiesoftheinsecthavebecomeestablished,notablyatypeofparasiticwasp,andthesehavegreatlyreducednumbersofEuropeanelmfleaweevilanditsassociatedinjury.InsecticidecontrolsarenotrecommendedforEuropeanelmfleaweevil,althoughtheymaybekilledbysomeofthetreatmentsusedforelmleafbeetle.