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8,No.
1USEOFDEGREE-DAYSTOTIMEAERIALSPRAYINGINWEST-CENTRALCOLORADOGARYLORENMCCALLISTERBiologyDepartment,MesaStateColbge,P.
O.
Box2642,GrandJunctinnCOgIS02ABSTRACT.
SeasonalfluctuationsinCulcxandAedeswerefollowedfrom1984to198usingdegree-days('Days).
Theannualpopulationmaximaofa"mosquitoseason"canbepredicted,.
Citex"firstbecomesaproblematabout1,550'Daysanddisappearsati,Arc"Days.
Aedesarepresentatb25'Days,experienceamidsummerdeclineat900'Days,thenresurgeat2,400'Daystomosilydisappeatat2,i00"Days.
_ThistimingappearstoworkinthearidwesternvalleyofGrandJunction,Colorado,duetothedependenc-ebythemosquitoonbreedingsitesrelatedtolocalirrigationsystems,whicharedryinthewinterandrehydratedatthesametimeeachyear.
INTRODUCTIONTherehasbeengreatinterestbybotanistsinusingtemperaturetopredictharvesttimeforcropsbecausetherelationshipbetweentemper-atureandplantgrowthiscloselyrelated.
Duetothisdirectcommercialbenefit,botanistsmadeearlyattemptstofrndmethodsofmeasuringthedailyaccrualofheat.
Candolle(1855)wasthefirsttocomputedailyheatunitsbysummingthemeantemperatureforeachday.
Theheatunitheusedwascalled"day-degrees.
"Thisgen-eralconcepthasbeenmodifiedfrequentlybybotanistsbuthasnotbeenextensivelyutilizedtopredictinvertebratedevelopment.
Growthanddevelopmentofinvertebratesalsodependsonambienttemperature,whichcanbeusedtopredicttimeofemergenceofinvertebratepests.
Nelson(unpublisheddata,1982)hasuti-lizeddegree-daystopredictcodlingmoth,Car-pocapsapomonella,emergenceinorchards.
Theemergenceoftheapplemaggot,Rhagolntispo-monelln,waspredictedbyadegree-daymodel(LaingandHeraty1984).
Whitfield(1984)ex-amineddegree-dayaccumulationandthedevel-opmentofsugarbeetrustmaggots,Leucaniaunipuncta.
However,itdoesnotappearthatdegree-dayshavebeenusedtopredictmosquitopopulationdynamics.
InthispaperwehaveexaminedseasonalCul.
exand.
Aedespopulationgrowthusingdegree-daysandhaveappliedthemtoaerialsprayprograms.
Aerialsprayingisnowtimedonbothdegree-daysandfieldmonitoringbasedonthefindingsofthispaper.
MATERIALSANDMETHODSInthisstudyweusedthefollowingformulafordegree-days(Nelson,unpublisheddata,1982):minimumtemperaturesintheFahrenheitscale;'Daysarethedailyaccumulatedheatunitsindegree-days.
Thecomputerprogramusedtogen-eratethedegree-daytablewasprogrammedinFahrenheitbecausetheweatherbureaureportstemperaturesinFahrenheitanditwastedioustoconstantlyconverttometric.
Thelowerthresholdfordevelopmentwassetat[$[sinceitseemedconsistentwithdiapauseterminationintheliteratureformosquitoesandotherin-sects.
Theupperthresholdwas88'F.
Degree-dayswerethendeterminedusingthedatainTable1beginningonMarch1ofeachyear.
MosquitotrappingbeginsaroundthefirstofMayduetoseasonalavailabilityofemployees.
Degree-daysfortheyears1983and1984weretakenfromtablesgeneratedinalocalcodlingmothstudy(Nelson,unpublisheddata,1982).
Degree-daysfor1985-87werecalculatedusingdailyweatherdatafromtheNationalAtmos-phericandOceanicAdministrationWeatherServiceatWalkerFieldAirportinGrandJunc-tion,Colorado.
TheRedlandsMosquitoControlDistrictislocatedinalargevalleyinwest-centralColo-rado.
Twomajorriverdrainages,theColoradoandtheGunnisonrivers,joinatthesoutheast-ernedgeofthevalleyandproceedwestastheColoradoRiver.
Alongthesouthedgeoftherivernumeroushousingsubdivisionshavebeenbuiltonthebluffsabovetheriver.
Atonetimetheareawasanagriculturalcenter,butmanyofthefieldsandorchardsarenowsmallhobbyfarmswithpoorlymanagedpasturesandgar-dens.
Thereisalargefloodplainalongtheriver,andnumerouscanyonsdminirrigationwaterfromthepasturesandlawns.
Thesandysoilalsoallowsthewaterfromlawnstopercolatedowntobedrockandseepoutatthebaseofthebluffs.
Althoughtheareaisatypicalsouthwestdesert,theriverandirrigationcreatenumerousbreed-inghabitatsformosquitoes.
MosquitoesweretrappedonenightaweekusingCDC-4lighttrapsbaitedwithdryice.
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t-l||_-t-'I[,::tt--iil:[::3_4|234|234|234r_2__J__WEEKOFMAYJUNEJUI,YAUCUSTSEMEMBERFig.
1.
Numberofadult,femaleCubxpertrapin4differentyears.
Thedegree-daysarealsoshown(dottedline).
DEGREEDAYS3000200033571000Seventrapswereusedin1987;previousyears'datawereobtainedbyusing4to6traps.
Mostofthesetraps(3-6)weresetaroundtheperim-eterofamosquitocontroldistrictand2othersweresetinareaslackingmosquitocontrol.
Onlyadultfemalemosquitoeswerecountedandiden-tifiedtospecies.
ThetotalnumberofCulexandAedeswercdividedbythenumberoftrapssetuponeachdate.
Thevalueobtainedwasroundedtothenearestwholenumber.
Itwasdecidedtousegenericdatainsteadofspeciesforseveralreasons.
Earlydatawerecol-Iectedbyuntrainedstudenthelpandidentifi-cationwasoftenapproximate.
IdentificationoftheCulexcomplexisespeciallydifficultinthisareasincethedifferentiationbetweenCx.
pi-piensLinn.
andCr.
quinquefasciatusSayisdif-ficultbecauseofhybridizationbetweenthespe-cies.
Also,thereare5speciesofAedesand.
4speciesofCulexfoundinthestudyarea,andonanygiventrapnightonespeciesoranothermaynotbepresent.
Finally,wewerenotattemptingtostudyspeciescharacteristicsbuttodevelopatoolfortimingoursprayingprogram.
RESULTSFourgeneraofmosquitoesareroutinelycol-lectedinlighttrapsinthisarea.
ThemostprominentgenusisCulex,whichaccountsforabout63%ofthetotal.
Aedesisalsocommon,makingupabout35%ofthetrapcounts.
Theother2generaarcAnophelesandCuliseta.
CulextarsalisCoq.
andCx.
pipiensarepresentinaboutequalproportions.
TherearefivespeciesofAedespresentinthearea,butAe.
dorsalrs(Mei-gen)andAe.
uercans(Meigen)arethemostabun-dant.
Aedesnigromaculis(Ludlow),Ae.
idahoen-srsandAe.
triuittatus(Coq.
)makeupthebal-anceoftheAedesspecies.
CulermosquitoeswerepresentbythethirdweekinMayofeachyearexcept1987.
NumbersofCul.
exdonotattainmaximumnumbersuntillateJulythroughthethirdweekofAugust.
CulexpopulationsdeclinequicklyinSeptember.
Fig-ure1showsCulcxdatainabutterflygraphwithaplotofdegree-daysincludedforeachyear.
ThenumberofCulextrappedbydateisindicatedbythewidthofthebox.
ThereisanobviousJuly/Augustpopulationpeak.
Cul.
expopulationsbe-gintoincreasebetween1,000and1,550'Days.
"IheCulexseasonusuallydeclinesaft,er2,675"Days.
Thedateofaeriallarvicidingisshownbyarrows.
Thenumberofadult,femaleAedestrappedeachweekdeclineoverthe4years.
Aedeswercalwayspresentbymid-May.
Populationsfluc-MARCH1992DpoRpn-DeYsnNoAnnrelSpRlvruc|=rolrosourroesI=AERIALSPRAYINGSDEGREE]'OTALI]AYSItL----r||--1I34t234[::[::'i::::[::'WEEROFrt_3_J|234JULYAUCU$t___34SEN'EMBDR2,675"Dayswerefirstaccumulated.
Thoseval-uesrepresenttheaveragevalueof:1)onsetand2)terminationoftheactivebreedingseasonforCulexspp.
,respectively.
Table3showssimilardataforAedesspeciesfrom1985and1986.
Fig.
2.
Numberofadult,femaleAedespertrapin4differentyears.
Thedegree-daysarealsoshown(dottedline).
Table2.
CalendardatesforthebeginningandendofadultCubxbreedingseasons.
1550'Days2675'DaysYear(begin)(end)198419851986r987July12July8July8July15Aug.
24Aug.
19Aug.
19Sept.
1tuatebutseemtobehighinearlysummerandthenagaininAugustorSeptember.
Figure2showsthisdatainbutterflygraphform.
Theearlyandlatepopulationsareeasilyseenin1984and1986.
Aedesgenerallyonlybecomeabun-dantaftertheaverageaccumulationof525"Days.
Thesepopulationsdeclinemarkedlyafter900"Daysandpeakagainatabout2,400"Days.
Aedesdisappearfromtrapsquicklyafter1,700'Days.
Thedatesofaerialsprayingareshownbyarrows.
Degree-daysweremeasuredeachyearfromMarchItoOctober1.
Thecoolestyear,intotalnumberofdegree-days,was1987with2,892'Daysaccumulated.
Thesecondcoolestyearwas1986with3,203'Days,followedby1985with3,269"Days.
Thewarmestyearwas1984with3,417"Days.
Table2showsthecalendardatesineachoftheseyearsonwhich1,550"DaysandDISCUSSIONMosquitopopulationsinwest-centralColo-radoareseasonalandgenerallyincreasefromMaythroughlatesummerandthenrapidlyde-clineintoOctober.
ThisisduetoseverewintertemperaturesandlackofbreedingsitespriortoMarchofeachyearwhenlocalcanalsarefilledforagriculturalirrigation.
Growthisseldomsteadybutratheroccursinaseriesofpeaksandsmallerdeclines.
Whenplottedonagraph,thiscreatesajaggedandskewedsigmoidcurve.
Thispatternwasdocumentedinwest-centralColo-radoasearlyas1958byOgdenandMarsden(1961).
1tOgden,L.
J.
andE.
Marsden.
1961.
AnappraisaloftheMosquitoProblemsinMesaCounty,Colorado.
USDHEW,PublicHealthService,CDC,incoopera-tionwithColoradoStateDepartmentofHealthandMesaCountyDepartmentofPublicHealth.
JounNer,orrspAunnrcANMoseurroCoNrRolAssocrltronVoL.
8,No.
1Table3.
CalendardatesforthebeginningandendofadultAedesbreedingseasons,1985-87.
*YearSeasonbegins525'DaysSeasonends900'Days2ndseasonbeginsSeasonends2400'Days2700'Days198519861987May26May26June5June13June13June.
20Aug.
8Aug.
9Aug.
20Aug.
20Aug.
20Sept.
2*Fourdatesareshownsincethepopulationappearstopeakatleasttwiceeachsummer.
CulcxandAedesarcmultivoltinewithover-Iappinggenerations.
Eachgenerationhasitsownseparatedegree-daytallyandisindepend-entofallothergenerations.
AIso,mosquitoesareusuallydependentuponrainsorothersourcesofflooding,andtheirappearanceduringacertaintimeofyearismostofteninfluencedbythetimingofsuchevents.
However,inwest-centralColoradomosquitoesaredependentuponbreedingareasthatdevelopduetospillsandseepagefromthelocalsystemofcanalsandditchesdevelopedforagriculture.
Thesearelefbemptyinthewintermonthsformaintenanceandbecauseofsoilseepageandsalinityprob-lems.
Consequently,breedingareasarere-strictedtothelocalriverbottomuntilMarchofeachyearwhenthecanalsarefilled.
Thisresultsintheuniqueconditionthatallmosquitopopu-lationsbegintodevelopataboutthesametimeeachyear.
Furthermore,hightemperatures,Iowhumidityandagradualwithdrawalofwaterfromlocal,maturecropsresultinthedeclineofbreedingareasataboutthesametimeeachfall.
Thisledustobelievethatlocalpopulationsmightbeabletobetrackedusingdegree-days.
Bymonitoringmosquitopopulationandde-gree-dayssince1982,wehavebeenabletocom-parepeaksofgrowthwithcalendardateanddegree-daysoverseveralyears.
Whilepopula-tionshavebeenobservedsince1982,onlydatafrom1984to1987arepresentedinthispaper.
Earlierdatawereusedforbaselineinformationandservedtogeneratetheideasforthisstudy.
ConsistentmosquitocontroleffortshavehadanobviouseffectonCulexpopulationsoverthepastseveralyears.
ThisisillustratedinFigs.
1and2bylesseningofthewidthofeachpopulationareafromyeartoyear,andalsobythedelayinpopulationpeaksfrommid-Julytoearlyormid-August.
The4-yraveragedegree-daysfortheonsetoffvstAedesmosquitoesis525"Days.
Thismaybedueasmuchtowateravailabilityastotem-perature.
Twoprominentriversintersectinthevalley,theColoradoandGunnisonrivers,andtheadjacentfloodplainisofbeninundatedinearlyMay,duetospringsnowmeltinnearbymountains.
Also,thefertiledesertvalleyhasanextensivesystemofirrigationwhichisdryinwinterbutisflooded,alongwiththefields,ataboutthissametime.
Aedesisafloodplainmosquito,depositingeggsandoverwinteringindampsoil.
Hence,largepopulationsemergeearlyinthesummer.
Thefour-yearaveragedegree-daysfromtheonsetofthefirstCulexpopulationpeakis1,550'Days.
ThishasfallenasearlyasJuly8oraslateasJuly15duringtheyearsstudied.
Culerpopulationsdeclinerapidlyafter2,675'Days.
ThisgenerallyoccurredbetweenAugust19andSeptember1,1983.
Culexfemaleswerecollectedafterthesedatesbutincreasesinpopulationoccurwithintheselimits.
Culexoverwinterasadults,andthereforethepopulationsmustbuildgraduallyoverthesummermonths.
Thisseemstobedependentontemperature.
AnencephalitisoutbreakoccurredinMesaCountyin1985,oneofthewarmestyearsofthefour,buttherewasalsoanearlyonsetdateofmosquitoes.
However,1984and1986alsohadearlycalendardatesforreachingL,550'Days.
Neitherofthesecharacteristicsappearsusefulinpredictingencephalitisrisks.
Degree-daysaccumulatedmorequicklyin1985thaninotheryears(seeFigs.
1and2).
Whileitappearsthat1984hadamorerapidrise,3,000'DaysdidnotoccuruntilwellintoSeptember.
In1985,1,500"Dayshadaccumu-IatedbyearlyJulyand3,000'Dayshadaccu-mulatedbylateAugust.
Ifvirusreplicationisalsodependentontemperature,thenearlywarmtemperaturewouldsuggestencephalitisrisk.
Bymonitoringdegree-daysonadailybasis,wecankeepindirecttrackofthedevelopmentofthemosquito.
Bycalculatingthemeandegree-daysaccumulatedonadailybasis,predictingthetimingofthefirstmosquitohatchinareasofseasonaloccurrencecanberelativelyaccu-rate.
Combinedwithfieldobservations,itcanaidinmakingmanagementdecisionssuchasthetimingofaerialspraying.
Noticethattherewere8aerialsprayingsin1984,5in1985,and4inboth1987and1988.
TheRedlandsMosquitoControlDistrictre-liesexclusivelyonlarviciding.
ThisisbecausethemosquitoproblemisseasonalandbecauseMAROH1992DpcRpn-DavslNoAnnrar,SpRevrxc23sprayinginsecticidesoverhousetopsandtheColoradoRiverisnotenvironmentallysound.
Forlarvicidingtobesuccessful,timingisessen-tialsincesprayingbeforeegghatchorafterpupationwastestimeandmoney.
Degree-dayshaveproventobeausefulmanagementtoolinthesecircumstances.
Thisisevidentfromthegeneralreductioninthenumberofaerialspray-ingsrequiredtoachievecontrol.
Sprayingtodayisbaseduponcombinedsurveillanceanddegree-davaccumulations.
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de.
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Geographiebotaniqueraisonee.
Masson,Paris.
Laing,J.
E.
andJ.
M.
Heraty.
1984.
Theuseofdegree-daystopredictemergenceoftheapplemaggot,Rhagoletispomonelln(Dipten:Tephritidae),inOn-tario.
Can.
Entomol.
116:1123-1129.
Whitfreld,G.
H.
1984.
Temperaturethresholdanddegree-dayaccumulationrequiredfordevelopmentofpostdiapausesugarbeetrootmaggots(Diptera:Otitidae).
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13:1431-1435.