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.

extension.colostate.edu

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.