cold hardiness of horseradish flea beetle (phyllotreta armoraciae (koch))

ISSN 1392-3196 ŽEMDIRBYSTĖ=AGRICULTURE Vol.99,No.2(2012) 203

ISSN 1392-3196 Žemdirbystė=Agriculture,vol.99,No.2(2012),p.203‒208UDK635.1:581.15:577.21(043.3)

Cold hardiness of horseradish flea beetle (Phyllotreta armoraciae (Koch))

KülliHIIESAAR,RiinaKAASIK,IngridH.WILLIAMS,EhaŠVILPONIS,KatrinJÕGAR,LuuleMETSPALU,MarikaMÄND,AngelaPLOOMI,AnneLUIKInstituteofAgriculturalandEnvironmentalSciences,EstonianUniversityofLifeSciencesKreutzwaldi1,51014,Tartu,EstoniaE-mail:[email protected]

Abstract SupercoolingabilityandcoldhardinessinthehorseradishfleabeetlePhyllotreta armoracia(Koch)(Coleoptera:Chrysomelidae: Alticinae) was investigated. Mean supercooling points (SCP) varied from −10.1 to −23.9°Cdependingontheseason:SCPstartedtodecreaseinAugust,achievedtheirlowestvalueinmid-Januaryafterfourmonthsat5°C,increaseddrasticallyinFebruaryandstayedatthishighleveluntilMay.Incontrast,somebeetleshadadistinctlyhighSCPduringoverwinteringandsurvivedfreezingattheirSCP.ThebeetlesacquiredsufficientcoldhardinessalreadyinSeptember–Octoberwithlethaltimefor50%mortalityof448h;thetemperaturenecessarytokill50%ofbeetlesafter24hexposurewas−12.2°C.Thebeetleswereabletosurviveverylowtemperaturesonlyduringdiapauseperiod.TheircoldtolerancedecreasedonthefifthmonthofhibernationwithLtemp50above−8°C.Atthesametimethebeetlesretainedarelativelygoodabilitytowithstandthemoderatelylowtemperatureof−6°Cforlongerperiodsafterterminationofdiapauseasover60%ofthemsurvived312hexposureinFebruary;however,allthefeedingbeetlesinMayweredeadalreadyafter96hexposure.

Keywords:Phyllotreta armoraciae,coldhardiness,supercooling,mortality,seasonality.

Introduction ThehorseradishfleabeetlePhyllotreta armora-

cia (Koch) (Coleoptera:Chrysomelidae:Alticinae) isaunivoltinemonophagousspeciesfeedinginnatureonlyon horseradish, Armoracia rusticana G.M. and Sch.(Brassicaceae) (Nielsenetal.,1979). In the laboratory,thebeetles canbe rearedon severalglucosinolate-con-tainingcruciferousplants,suchasBrassica napusL.,Si-napis albaL.,Brasssica nigraKoch,andtheyshownopreferencebetweenB. nigraandA. rusticana(Vig,Ver-dyck,2001).However,inchoicetests,beetleshavepre-ferredfeedingontheirhostplant,horseradish(Hagerupetal.,1990).HorseradishiswidelycultivatedinEuropeasaspiceforpicklesorasasidedish.Italsotendstona-turalizeoutsidethecultivatedareas(onabandonedfields).The horseradish flea beetle is the most prevalent andmostdamaginginsectpestofthehorseradishplant.Thebeetlesareirregularlydistributed:insomehabitatsplantsare densely colonized by them and the leaves entirelyperforated,whereastheleavesremainintactthroughoutthegrowingseasoninotherlocations.Accordingtoourlong-term observations, the first beetles emerge fromtheiroverwinteringsitesonasunny,warmdayattheendofAprilandstarteatingimmediatelywhenthehorsera-dishhasbarelysprouted.Themajordamageiscausedbyadultsinspringtime.Femaleslaytheireggsonthepeti-olesofyoungleavesinMay,andthelarvaeburrowintothe petioles (Nielsen et al., 1979).ThenewgenerationbeetlesemergeattheendofJulyandinthebeginningofAugustandfeedonthesameplantforaboutonemonth.Inmid-September,theystartleavingtheplantsfortheir

overwinteringsites.Theadultsoverwinterindebrisandsoilcrevicesnotfarfromthehostplants(Vig,2002).Inoneofourobservationstations,inherbcollectiongardenwherehorseradishhasbeengrown formanyyears, theplantswerecontinuouslyheavilydamaged, irrespectiveof the severity ofwinter.As a dispersedweed in openfields,plantsweremuchlessdamaged.

So far, there are no reports about hibernationandcoldhardinessinthehorseradishfleabeetle. Gene-rally, insects survive low temperatures by using freezetolerance or freeze avoidance strategies (Zachariassen,1985),butsomespeciesareknowntoemployboth(Du-man,1984;Gehrkenetal.,1991).Itisnotknownwhichstrategyenables P. armoraciae tosurvive longandun-stablewinters;thereforethisstudyfocusedonfactorsin-fluencingsupercoolingabilityandcoldhardinessinthisspecies.

Ourstudyaimedtoassess:1)seasonalchangesinsupercoolingpoints(SCP)ofadults,2)seasonalchan-gesincoldtoleranceofbeetlesafterexposuretoconstantsubzerotemperaturefordifferentperiodsoftime,3)sea-sonalchangesincoldtoleranceofbeetlesafterexposureofbeetlestodifferentsubzerotemperaturesforaconstantperiodoftime.

Material and methodsBeetles.Theexperimentswereperformedfrom

2008to2009.Thehorseradishfleabeetleswerecollectedby amanual aspirator fromhorseradish in themedical

204 Cold hardiness of horseradish flea beetle (Phyllotreta armoraciae (Koch))

herb garden of Tartu University (Estonia) (58°18′ N,26°41′ E). For supercooling points measurements, thebeetleswerepickedfromthehostplantsinAugustandfromsoilcrevices1–3mfromthebedsorfromdifferentweedsonwhich theywere resting inSeptember,whenmosthadalreadyleftthefoodplant.Partofthebeetlescollected in September (a pre-winter group)were keptinafreezer(RF)withnoaccesstofood,at5±1ºCandabsolutedarknessin1-lglassjarswhichwerehalf-filledwithlightlymoistenedpeatuntiltheexperiments.Onlyafewbeetlesdugintothesoilwhereasmostofthemstayedonthewallsandcoversofthejars.

Supercooling points (SCP).TheSCPofthebeet-lesweremeasuredbyachromel-alumel thermocouple-thermometer“RS-232DataLoggerThermometer”(“TESElectrical Electronic”, Taiwan). The low temperatureswereattainedbydeep-freezeHF-103 (−30ºC).Prior tosupercooling, the beetles were slightly anaesthetizedwithethertorenderthemimmobileandthenfixedindi-viduallybyathinVaselinelayertothetopofthethermo-coupleandsealedinaplastictube.Tubeswereplacedinacotton-linedcontainerandtransferredintothefreezingchamber.Acoolingrateof1°Cmin-1wasused.Thetem-perature atwhich freezingproduceda releaseof latentheatwastakenastheSCPoftheindividual.Afterasud-denspikeinthemeasuredtemperaturethebeetleswereremovedfromthefreezerandleftatroomtemperaturetorecoverfor24hbeforetheassessmentoftheircondition.Thebeetleswereconsideredfit if theywereactiveandabletomoveinanormalmanner.Beetlesnotrespondingtotactilestimulationwereconsidereddead.

Supercoolingmeasurementsincludedthebeet-les of the same generation. We started with summerbeetles inAugust 2008 andfinishedwith the overwin-teredbeetles inMay2009.SCPweremeasuredonceamonth, except inNovember. InAugust andSeptemberonly,field-freshbeetleswereusedandbeforestartingtheexperimenttheywereheldfor24hatroomtemperature(23±1°C),withnoaccesstofoodtoallowthemtoemptytheirguts.Inallothermeasurementsbeetlesstoredat5±1°Cwereused.Thenumberofbeetles(N)ineachassess-mentvariedfrom16to20.AfterSCPmeasurementsthesexofbeetleswasdetermined;40%weremaleand60%female.AssexdidnotaffecttheirSCP,mixedsexbeetleswereusedinallexperiments.

Cold tolerance. The cold tolerance of beetlesfromvarious seasonalgroupswasassessed. In thefirstexperiment,theeffectoflong-termexposuretothemor-tality at constant temperatureof−6ºCwasdetermined.Batches of beetleswerewrapped into glass vials linedwithfilter-paperand transferred into the thermostat for24,48,72,96,120,144,or312h.Inthesecondexperi-ment,thebeetleswereexposedtovarioussub-zerotem-peratures: −6, −8, −10, −12, −14, −16, −18 or −20ºCforaconstantperiodof time,24h.Steadycoolingandwarming rateswereachievedusinga liquid thermostat“Ministat230w-2”(“Huber”,Germany).Eachvialwassuppliedwiththermocoupletoregisterthetemperature.Thetemperaturefluctuationinsidethevialsdidnotex-ceed 0.5ºC. The beetles were cooled at a rate of 1ºCmin-1downtotherequiredtemperatureandwarmedupto 20ºC at the same speed after the termination of theexposure.AftertransferringfromthermostatthebeetleswereplacedinPetridisheslinedwithlightlymoistenedfilter-paper and left at room temperature to recover for24–48h;afterwhichsurvivalwasrecorded.

The cold tolerance was estimated at differ-ent dates: 1) inSeptemberwhen thebeetleswere end-ing their pre-winter feeding (field-fresh beetles), 2) attheendofOctober,3)inDecemberand4)attheendofFebruary (non-feedingbeetles stored inRFat 5±1°Candabsolutedarkness),5)inMayafteremergencefromoverwinteringsiteswhenthefoodwasalreadyavailable(field-freshbeetles).Eachgroupofbeetleswasreplicatedthreetimes.ThetotalnumberofbeetlesisrepresentedinTables1and2.

Statistical analyses.Analysesofvariance(one-wayANOVA)wereusedtoassesstheeffectofseasonal-ityonSCPofbeetles.DifferencesinSCPvaluesbetweenmeasurementdateswereanalysedwithFisherLSDtest.Differences in mortality of beetles between seasonalgroupsdependingonexposuretimeortemperaturewerecomparedbytwo-wayANOVA.DifferencesinmortalitydependingonexposuretimeinFebruarywerecomparedbyone-wayANOVA.Lethaltimetoreach50%mortalityofbeetles(Ltime50)afterexposuretoconstant−6°C,andlower lethal temperature atwhich 50%of beetles died(Ltemp50)whenexposedforaconstant24hperiodwereestimated by Probit Analyses (SAS/STAT version 9.1,SASInstituteInc.,USA).

ResultsSeasonal changes in supercooling ability of

horseradish flea beetles.Figure1illustratestheseasonalchanges in SCP of horseradish flea beetles. ThemeanSCPvariedsignificantlyovertheperiodAugusttoMay(F8;144=38.16,p<0.001); itdecreasedgradually fromAugusttoJanuary,increaseddrasticallyinFebruaryandthenremainedatahighleveluntilMay.

Inmid-August, during the pre-winter feeding,themeanSCPof the beetleswas−14.2ºC. In Septem-ber,whenfeedinghadfinished,themeanSCPdecreased,althoughtherewerenostatisticallysignificantdifferen-cescomparedwiththepreviousperiod.TheSCPvaluestended to decrease further during October–December,whenthebeetlesexhibitedsignificantlylowerSCPthaninSeptember.ThelowestmeanSCP,−23.9ºCaswellasthelowestindividualSCP,−27.2ºCwasattainedinmid-January.

FromAugusttoFebruary,abroadrangeofSCPvalueswasobserved.IndividualswithSCPvaluesofover−10°CsurvivedtheirSCP(markedwithadottedcircleonFig.1);afterre-warming,thesebeetleswereabletomoveinanormalwayandgatheredonthewhitemus-tardleavesweofferedthem.ThebeetleswithSCPbelow−10°CdidnotsurvivetheSCPmeasurement.InFebru-ary,theSCPofmostbeetleshadincreasedandachievedameanvalueof−10.1ºC;bythis time,onlyonebeetlemaintained a very lowSCP (−20.1ºC).FromMarch toMay,themeanSCPdidnotchange,remainingconstantatabout−8ºCwithverysmallindividualvariability.AllthesebeetlessurvivedtheirSCP(Fig.1).

Effect of duration of exposure on mortality of horseradish beetles from various seasonal groups at constant −6ºC.Themeanmortality of beetles fromdifferentseasonalgroupsafterexposureto−6ºCfordif-ferentperiodsof timeisshowninFigure2; itchangedsignificantly over time.The probit analysis results, thetimerequiredtocause50%mortalityofbeetles(Ltime50)arepresentedinTable1.Theoverlappingof their95%fiduciallimitsindicatednosignificantdifferencesinmor-talitybetweenestimatingdates.


Note. Different letters above columns indicate significantdifferencesbetween themeans(Turkey’s test,P<0.001);N=20forbothgroupsinMayandAugust,N=17foreachgroup fromSeptember to January,N=16 foreachgroupfromFebruarytoApril.

Figure 1.Seasonalchangesofsupercoolingpoints(°C,mean±SEM)inadultPhyllotreta armoraciae

Notes.Bars(mean±SEM)withdifferentlowercaseletterswithinthesameexposuredurationaresignificantlydifferent(Fisher LSD test, P < 0.05). Different upper case lettersbelowthebarsindicatethesignificantdifferenceatvariousexposuredurationswithinthesameevaluationdate.

Figure 2.Meanmortality(%)ofPhyllotreta armoraciae beetlesfromdifferentseasonalgroupsafterexposureto−6.0°Cfordifferentperiodsoftime

Table 1.Timerequiredtocause50%(Ltime50)ofPhyl-lotreta armoraciae beetles at a constant temperature−6°C

Estimationtime N Ltime50 h

95%fiduciallimitsh

September 285 448a 337.5to748.2December 282 408a 321.7to599.3

May 236 50b 42.8to56.0Note. Ltime50 of different seasonal groups followed by sameletters are not significantly different based on the overlap oftheir95%fiduciallimits.

Theseason(F3;49=40.36,p<0.001)aswellasthedurationofexposure(F1;49=341.27,p<0.001)hadasignificanteffectonthemortalityofbeetles:thelongertheexposure,thehigherthemortalityinallgroups.Therewasalsoasignificantinteractionbetweenseasonanddu-rationofexposure(F3;49=79.03,p<0.001).

Thebeetlesacquiredhighcoldtolerancealreadyinautumn.Thehighestcoldtolerancewithlowestmor-talitywasobservedinOctober;whenmorethan60%ofbeetlessurvived312hexposureat−6°CandcalculatedLtime50was448h.InDecembercoldtoleranceofbeetlesdid not change substantially; therewas no statisticallysignificantdifferencefromthepreviousperiod(p >0.05),their 95% fiducial limits of calculated Ltime50 valuesoverlapped.InFebruary, thebeetlesstarted losing theircold tolerance and themortalityofbeetleswas signifi-cantlyhigherthaninOctoberorDecember(p<0.001),stillsignificantlylowerthaninMay(p<0.001).LTime50 valueinFebruaryisnotshown;thedatadidnotenablecalculationoftheirfiduciallimits,becausemortalityrateremainedalmostonaplateauatdifferentexposuredu-rations, although the percentage ofmortality increasedslowlyandsignificantlywithprolongedexposure(F4;10=3.50,p=0.049).MostsensitivetolowtemperaturesweretheoverwinteredfeedingbeetlesinMaywithnosurvi-ving96hexposureat−6ºC,Ltime50equalledto50h.

Effect of low temperatures on mortality of horseradish beetles from various seasonal groups after constant 24 h exposure.Mortalityofbeetlesfromvariousseasonalgroupsafter24hexposuretodifferentlowtem-peraturesispresentedinFigure3andTable2.Mortalityincreasedwiththedecreaseoftemperaturesinallgroupsirrespectiveoftheseason(F1;30=426.85,p<0.001).Theeffectofmeasuringdatewasnotsignificant(F1;30=1.98,p=0.12),howevertheinteractionbetweendateandtem-peraturewassignificant(F3;30=6.13,p<0.001).

Note.Differentlettersabovethedatapointsindicatestatisti-cally significant difference between the different seasonalgroupsatthesametemperature(FisherLSDtest,P<0.05).

Figure 3. Mortality ofPhyllotreta armoraciae beetlesfrom different seasonal groups after 24 h exposure tovarioussubzerotemperatures

ThebeetlesacquiredahighcapacitytosurviveverylowtemperaturesforashortperiodalreadyinSep-temberwhentheyhadstoppedfeeding.InOctober, thecoldtoleranceofbeetlesincreasedslightly;nevertheless


therewasnosignificantdifference inmortalityatmosttemperaturesincomparisonwiththepreviousperiod.Inthe frame of this experiment, themost tolerant to lowtemperatureswere thebeetles inOctoberwithLtemp50 =−13.8hwithover10%ofbeetlessurviving−18°C.InFebruary,theircoldhardinessdecreasednotably,assig-nificantlyfewerbeetlessurvivedatalltemperatures;thetemperaturenecessarytokill50%ofbeetleswas−7.7°C.The toleranceofbeetles remained lowuntil thespring.InMaytheLtemp50was−7.1°C.ThelowesttemperaturethebeetlescouldsurviveaswellinFebruaryasinMaywas−12.0°C.

Table 2. Temperature required to cause 50%mortalityPhyllotreta armoraciaebeetles(Ltemp50)after24hex-posure

Estimationtime N Ltemp50

°C95%fiduciallimits

°CSeptember 546 −12.2a −12.7to−11.7October 570 −13.8a −14.7to−12.9February 282 −7.7b −8.3to−7.2May 307 −7.1b −7.7to−6.4

Note.Ltemp50ofdifferentseasonalgroupsfollowedbydifferentlettersaresignificantlydifferentbasedonthelackofoverlapoftheir95%fiduciallimits.

Discussion Supercooling points (SCP) measurements in

combination with survival data indicated that the coldhardiness and supercooling ability of horseradish fleabeetleschangedseasonally.Thisphenomenonhasbeenreported by many authors for different insect species(Sømme, 1982;Koštal, Šimek, 1995;Vernon,Vannier,2002;Kochetal.,2004,Worland,2010).SCPaswellascoldtoleranceareprimarilyinfluencedbytwoprocesses:coldacclimationanddevelopmentofdiapauseandwiththephysiologicalchangesaccompanyingtheseprocessesliketheaccumulationofcryoprotectantsandantifreezes,dehydration of organism, inactivation of ice-nucleatingagentsetc.(Zachariassen,1985;Denlinger,1991;Block,2003;Hodkova,Hodek,2004;Koštal,2006).ThegradualloweringinSCPandincreaseincoldtoleranceinhorse-radishfleabeetlesstartedwiththeonsetofthediapauseinAugustwhenthetemperaturedecreasedanddaysshort-ened,deepenedastheautumnprogressedandthebeetlesstoppedfeeding(acclimationanddiapausedevelopment)and achieved themaximum inmid-January (deep dia-pause).InFebruarySCPofbeetlesincreaseddrasticallyandsimultaneously,whereascoldtolerancedecreased.

Horseradishfleabeetleshadhighsupercoolingabilityandwereabletosurviveashort(24h)exposuretorelativelylowtemperaturesorlongerexposurestoamoderatelylowconstanttemperature(−6°C)fromSep-tember to January during pre-diapausing and diapau-sing periods. Relatively low mortality of beetles wasobservedin thisperiod.Generally inmostof thebeet-lesthediapauseisterminatedbeforetheendofJanuary(Koštal,Šimek,1996).Basedonspecificationofinsectdiapause(Koštal,2006)horseradishfleabeetlesbelongtothespecieswhosediapausereacheditsendspontane-ously despite keeping them at constant above freezingtemperature and resumption of their development fol-

lowedimmediately.InFebruary,afterfifthmonthsat5°Cwhensomeoftheoverwinteringbeetlesweretransferredto room temperature and providedwithwhitemustardleaves they started eating and, in a littlewhilemating.By this time SCP of beetles increased drastically andremained unchanged untilMay; at the same time tem-perature atwhich 50% of beetleswould die (Ltemp50)increased.Still, probit analysesdatadidnot enable es-timationof thefiducial limits for lethal time (Ltime50),becausemortality didnot changemuchwith extensionof exposureduration atmoderately low temperature inFebruarywhenthedevelopmentwaspreventedonlybylowtemperature.Evidently,thecohortofbeetlestestedatdifferentdatesconsistedofindividualswithdifferentvia-bility.Oninspectionofthebeetlesoverwinteringat5°Cwe found that the lessviablebeetles (nearly25%)hadbeen deadbyFebruarybecauseofundeterminedreasons,definitelynotbecauseof the low temperatureandonlythestronger individualswithachanceof survivaluntilspringwereincludedinthetrial.Activefeedingbeetlescollected inMay prove to be the most cold sensitive,noneofwhichsurvived96hexposureto−6°C.

The overwintering insectswould die not onlybecauseofthefreezing.Severalphysiological,biochemi-calandmetabolicdysfunctionsinducedbycoldarede-scribedbyRenault(2011).AccordingtoDanks(2005),mortalityinmanyinsectsresultsfromtheeffectofcoldratherthanfreezing.Long-termexposuretakesweeksormonthsandleadstosignificantmortalityintheabsenceoffreezing(Rojas,Leopold,1996).Thetemperatureof−6ºCused forprolongedexposurewashigher than themeanSCPofhorseradishbeetlesinallseasonalgroups.Thatiswhywesupposethatmortalityinbeetlescouldre-sultfromthecumulativeeffectofchillinjuriesduringex-posuretimesnotfromthefreezing.Despitetheincreasedmortalityafterterminationofdiapauses,arelativelylargeproportionofthebeetlesmaintainedtheirabilitytowith-standmoderatelylowtemperatureforalongerperiodofexposure.This reflects the adaptation of beetles to thelongperiodtheymustspendinunfavourablecoldcondi-tionsbeforetheonsetofwarmweather.

Generally,insectssurvivelowtemperaturesbytheuseoffreezetoleranceorfreezeavoidancestrategies(Zachariassen,1985;Lee,1991).Freezeavoidanceusu-allyinvolvesarelativelylowSCP,whereasfreezetole-rant insects usually have a highSCP (Turnock,Fields,2005).HighSCPoffeedinginsectsisexplainedbyfoodparticlesintheiralimentarytractwhichinduceiceforma-tionatrelativelyhightemperatures(Zachariassen,1985)and with high water content in its organism (Block,2003).OverwinteringhorseradishfleabeetlesaremostlyfreezeavoidingwithlowSCPinthediapausingperiod,however,atthesametime,afewspecimenshadarela-tivelyhighSCPirrespectiveoftheseasonorfeedingrate(Fig. 1).IndividualswithSCPupto−10°CsurvivedtheirSCP even though their gut contents were not causingfreezingsincetheywereinnon-feedingstate.Soonafterre-warmingtheywereabletomoveinanormalway;in-dividualswithaSCPsubstantiallylowergotkilled.Thissuggests that this species may simultaneously possesstwo different strategies. In the literature, some speciesemployingbothstrategiesaredescribed(Gehrkenetal.,1991;Koštal,Havelka,2000;Sinclaireetal.,2003).For


example, freeze-tolerant sugar-beet maggot would be-comefreezeavoidantasaresultofmicrohabitatselection(Rinehartetal.,2009).AccordingtoBaleetal.(2001),theresponsetomultiplefreeze-thawcyclesinvolvesbet-hedging strategy with some individuals retaining theirinitial freeze tolerance, whereas others become freezeavoiding.Thehorseradishfleabeetleoverwintersinde-bris and soil crevices a fewcentimetresbelow the soilsurfacenotburrowingdeeplyintheground(Vig,2002).In snowless, unstable winters they are highly exposedto repeatedly fluctuating temperatures and freeze-thawcycles,sotwodifferentstrategieswouldenablethemtosurviveunpredictablewinters.

Winter survival of insects is greatly related tothechoiceofoverwinteringsitesandontheseverityofagivenwinter.Onereasonforanoccasionallyhighpopu-lationdensityinNordictemperateareascouldprobablybe the presence of suitable overwintering sites nearby;for instance in our experiment, horseradish bed had ahedgerow locatedwithin10mof it anda trenchfilledwith debris closely surrounding it. In large openfieldswith less suitablemicrohabitats theplantswere almostundamaged.

Conclusions1.Supercoolingabilityaswellascoldhardiness

ofhorseradishfleabeetleschangedseasonally.Thelow-estmeansupercoolingpoints(SCP)(−27ºC)weremea-suredinJanuary;therapidincreaseofSCPwereobservedinFebruary,whenthemeanSCProseto−11ºC.Thehigh-estcoldtolerancewithlowestmortalitywasobservedinOctoberwhenthetimerequiredtocause50%mortality(Ltime50)ataconstanttemperature−6°Cequalled448h;temperature required to cause50%mortality (Ltemp50)after24hexposureequalled −13.8°C.Thebeetlesaremostsusceptibletolowtemperaturesinspringtimeafterterminationofthediapause.

2. Horseradish flea beetles have two differentoverwinteringstrategies:beetleswith lowsupercoolingability(SCP−7…−12ºC)duringdiapauseperiodbelongtothefreezetolerantgroupandbeetleswithhighsuper-coolingability(−18…−24ºC)belongtothefreezeavoid-inggroup.

AcknowledgementsThisresearchwassupportedbyEstonianScien-

ceFoundationgrants9449,8895andtheEstonianMinis-tryofEducationandResearchtargetedfinancingprojectSF 0170057s09.

Received16112011Accepted16032012

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Krienų spragės (Phyllotreta armoracia (Koch)) atsparumas šalčiui

K.Hiiesaar,R.Kaasik,I.H.Williams,E.Švilponis,K.Jõgar,L.Metspalu,M.Mänd,A.Ploomi,A.LuikEstijosgyvybėsmokslųuniversitetoŽemėsūkioiraplinkosmokslųinstitutas

Santrauka Tirta krienų spragėsPhyllotreta armoracia (Koch) (Coleoptera:Chrysomelidae:Alticinae) geba ištverti žemątemperatūrą ir atsparumas šalčiui. Priklausomai nuo sezono, vidutinė žemiausia temperatūra, kurią toleravospragės,buvonuo−10,1iki−23,9°C.Žemiausiatoleruotinatemperatūraėmėmažėtirugpjūčiomėnesį,žemiausiasreikšmespasiekėsausiomėn.viduryje,poketuriųmėnesiųekspozicijosesant+5°Ctemperatūrai,staigiaipadidėjovasariomėnesį ir tame pačiame lygmenyje išliko iki gegužėsmėnesio.Atvirkščiai, kai kurie vabalų individaižiemojimometuturėjoaukštątoleruotinąžemiausiątemperatūrąirišgyvenojuosšaldantjųtoleruotinoježemojetemperatūroje. Krienų spragės pakankamą atsparumą šalčiui įgijo jau rugsėjo–spalio mėnesiais: 448 valandųekspozicija buvo letalinis laikas, lėmęs 50% spragių mirtingumą; temperatūra, būtina žūti 50% spragių po24valandųekspozicijos,buvo−12,2°C.Spragės labaižemoje temperatūroje sugebėjo išgyventi tikdiapauzėsmetu.Jųatsparumasšalčiuisumažėjopenktąžiemojimomėnesį,Ltemp50esantdaugiaunei−8°Cšalčio.Kartuspragėsišlaikėganagerągebėjimąištvertividutiniškaižemą−6°Ctemperatūrąilgesnįlaikąpasibaigusdiapauzei,nesdaugiaunei60%spragių išgyveno312valandųekspozicijąvasariomėnesį.Tačiaugegužėsmėnesįvisosbesimaitinančiosspragėsžuvojaupo96valandųekspozicijos.

Reikšminiaižodžiai:Phyllotreta armoraciae,atsparumasšalčiui,didelisatšaldymas,mirtingumas,sezoniškumas.

cold hardiness of horseradish flea beetle (phyllotreta armoraciae (koch))

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