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Factors Influencing the Outcome of Mark-Release-RecaptureStudies with Culex tarsalis (Diptera: Culicidae)Author(s): William K. Reisen, Hugh D. Lothrop, and Branka LothropSource: Journal of Medical Entomology, 40(6):820-829. 2003.Published By: Entomological Society of AmericaDOI: http://dx.doi.org/10.1603/0022-2585-40.6.820URL: http://www.bioone.org/doi/full/10.1603/0022-2585-40.6.820

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POPULATION AND COMMUNITY ECOLOGY

Factors Influencing the Outcome of Mark-Release-Recapture Studieswith Culex tarsalis (Diptera: Culicidae)

WILLIAM K. REISEN,1 HUGH D. LOTHROP, AND BRANKA LOTHROP2

Arbovirus Research Unit, Center for Vector borne Diseases, School of Veterinary Medicine,University of California, Davis, CA 95616

J. Med. Entomol. 40(6): 820Ð829 (2003)

ABSTRACT Three potentially important aspects of mark-release-recapture experimentation wereaddressed: 1) source of mosquitoes for release, 2) time of release, and 3) weather during recapture.Culex tarsalis Coquillett mosquitoes collected as adult host-seeking females from dry ice-baited traps(CO2 traps) operated within the study area (local) were recaptured more frequently than femalescollected from traps operated outside the study area (foreign) or reared from Þeld-collected imma-tures (reared).These results supportedpublished studies onAnopheles andOchlerotatus that indicatedmosquitoesmay “memorize” ßight paths within their environment. Releasing gravid females provideda potentially useful replacement for reared females, because these gravids oviposited at wetlands andthen dispersed to upland traps. Releasing local, foreign, or reared mosquitoes just after sunrise or justbefore sunset did not alter recapture success or thedistancedispersed. Elevatedwind speeds inhibiteddispersal from protected microhabitats with citrus orchards and resulted in most recaptures beingfound at the leeward portion of the orchard.

KEY WORDS mark-release-recapture, dispersal, mosquitoes, Culex tarsalis

MARK-RELEASE-RECAPTURE procedures are a useful toolin the study of mosquito population ecology and dis-persal (Service 1993). During release experiments de-signed to describe the movement of Culex tarsalisCoquillett adults from larval production sites into up-land adult congregation sites, we observed consider-able variability among the recapture rates of differentcohorts released concurrently. The current paper de-scribes subsequent experiments that investigated theeffects of cohort age, geographic origin, time of re-lease, and wind conditions on recapture success.Several investigators (Charlwood et al. 1988, Ren-

shaw et al. 1994, McCall et al. 2001) have suggestedthat some mosquitoes “memorize” their home range,thereby establishing ßight paths and reducing theirloss rates compared with foreign individuals forced tolearn new ßight paths after release within a foreignenvironment. Loss rates combined deletions attribut-able to mortality, emigration, and removal sampling.“Memorizing” a home range may impart a selectiveadvantage forOchlerotatus species such asOc. cantans(Meigen) that are univoltine, long-lived, and mustreturn to a limited number of semipermanent devel-opment sites for oviposition (Renshaw et al. 1994) or

for an Anopheles species such as An. farauti Laveranthat repeatedly must retrace its movements amongoviposition sites, bloodmeal hosts, and diurnal restingshelters on a 2-d cycle (Charlwood et al. 1988). Intu-itively, such “learned” behavior would seem less ad-vantageous for a dispersive colonizing species such asCx. tarsalis that inhabits unstable arid landscapes inwhich location of new aquatic habitats would seemessential and these resources can be exploited rapidlyby facultative autogeny (Reisen and Reeves 1990). Inthe current paper, we investigated the idea that adulthost-seekingCx. tarsalis females releasedback into thesame study area have a higher recapture rate thanfemales emerging from Þeld-collected immatures andnever previously exposed to upland landscapes or fe-males collected from outside the study area and ori-ented to a different landscape.Crowding females during the diurnal resting period

may increase their dispersive behavior after release.Temporary larval and teneral adult crowdingofOchle-rotatus taeniorhynchus (Weidemann) stimulated anenhanced dispersal response (Nayar and Sauerman1969), although this behaviorwas not universal amongFlorida mosquitoes, including Culex (Nayar andSauerman 1973). In our standard mark-release proto-cols, Þeld-collected immatures and emerging adults aswell as host-seeking females are held in cages in roomor insectary conditions until late afternoon to pre-clude diurnal mortality associated with hot, dry Þeldconditions. To test the idea that these holding condi-

Feeding of mosquitoes on chicks was done under Protocol 9607approved by the Animal Use and Care Administrative Advisory Com-mittee of the University of California, Davis.

1 Arbovirus Field Station, 4705 Allen Road, BakersÞeld, CA 93312.2 Coachella Valley Mosquito and Vector Control District, 43-420

Trader Place, Indio, CA 92201.

0022-2585/03/0820Ð0829$04.00/0 � 2003 Entomological Society of America

tions may increase ßight activity and dispersal, wecompared the recapture rates of host-seeking femalesand females emerging from Þeld-collected immaturesreleased in early morning just after collection withfemales released just before dusk.Increasedwind conditions during the spring of 2002

afforded the opportunity to describe the impact ofwind speed on mosquito dispersal behavior from asheltered release point. Previous rapid and long dis-tance movement by marked Cx. tarsalis females wasattributed to wind-assisted dispersal (Bailey et al.1965, Reisen and Lothrop 1995), but the current ser-endipitous observations indicated that dispersal wasreduced during elevated wind speeds, agreeing withpreviousobservationsonFloridamosquitoes (Bidling-mayer et al. 1985, 1995).

Materials and Methods

Location. All experiments were done at marshes,adjacent desert brush, and upland orchard and vineyardhabitat near the northwestern shore of the Salton Sea inRiverside County, CA, at some of the same sites used inour previous microhabitat comparison studies (Lothropand Reisen 2001, Lothrop et al. 2002). Wind and tem-perature records from weather station 136 (latitude33�19�N, longitude115�57�W,elevation�225 feet)of theCalifornia Irrigation Management Information Systemwere downloaded from the California Integrated PestManagement website (http://www.ipm.ucdavis.edu/).Rainfall did not occur during any experiment.

Mark-Release-Recapture Methods. Methodologyfollowed our previous mark-release-recapture studiesin Coachella Valley (Reisen and Lothrop 1995).Brießy, adult Cx. tarsalis were reared from Þeld-col-lected immatures under natural photoperiod (reared)or females were collected host seeking at dry ice-baitedCDC-style traps (CO2 traps) (Sudia andCham-

berlain 1962)operatedwithin the studyarea(local)oralong the eastern shore �13 km distance across theSaltonSea (foreign). Previous studieshave shown thatthere is considerable genetic exchange among Cx.tarsalis collected throughout the Coachella Valley(Gimnig et al. 1999), so these adults were consideredto be genetically similar. Reared femaleswere held for1Ð3 d after emergence and offered 10% sucrose; bothmales and females were released. In experiment 3,some host-seeking females were fed on a chick andthen held for 4 d until gravid. Adults were transferredto one or more screen-covered 5-gal (18.9 liters) plas-tic buckets depending on the numbers collected andexperimental design. Numbers in each bucket wereestimated from triplicate counts of females restingwithin 10% of the surface area delineated by verticallines (Dow et al. 1965). Females in each cohort weredustedwith a unique ßuorescent color in the Þeld justbefore release using an insuffalator.Recapture was attempted for three to four succes-

sive nights after release usingmultiple CO2 and newlydesigned suction traps that effectively sampled theßying populationwithout an attractant (Lothrop et al.2002). Because previous release-recapture studies in-dicated that most released mosquitoes were recap-tured during the Þrst 3Ð4 d after release (Reisen et al.1992, Reisen and Lothrop 1995), we limited the du-ration of our recapture effort, because our objectivewas not to estimate survivorship or maximum ßightrange, but rather movement from potential breedingsites into upland congregation sites. All capturedmos-quitoes were anesthetized with triethylamine, exam-ined under UV light to detect dust, identiÞed to spe-cies and sex, and then enumerated.

Experimental Design. Study area selection; mos-quitoes captured,marked, and released; and trap type,placement, and number varied among experimentsthat were described in Table 1 and below. Local host-

Table 1. Summary of mark-release-recapture experiments in Coachella Valley, 2001–2002

Experiment: purpose Mosquito sourceRelease

Released Recap % Chi2Site Time

1: InÞltration (24Ð26 Apr 01, 28.5C) Local CO2 trap Center PM 3,240 126 3.89 4.53a

Local CO2 trap Shore-1 PM 3,293 87 2.640.03NS

Local CO2 trap Shore-2 PM 3,080 89 2.89Reared Center PM 1,060 3 0.28Reared Shore-1 PM 967 1 0.10Reared Shore-2 PM 877 2 0.23

2: Source and time of release(23Ð25 May 01, 33.6C)

Local CO2 trap Center AM 1,230 80 6.50 1.08NSLocal CO2 trap Center PM 820 64 7.80Foreign CO2 trap Center AM 2,330 24 1.03 1.84NSForeign CO2 trap Center PM 1,660 26 1.57Reared Center AM 216 8 3.70 2.99NSReared Center PM 400 5 1.25

3: Source and feeding status (19Ð21Jun 01, 32.4C)

Local CO2 trap Margin PM 3,250 112 3.45 45.59a

Foreign CO2 trap Margin PM 890 79 8.88 0.98NSGravid Margin PM 280 6 2.14

4: Source (15Ð18 Apr 02, 20.5C) Local CO2 trap Center AM 1,794 296 16.50 1.65NSForeign CO2 trap Center AM 2,707 407 15.04

5: Source (30 Apr-3 May 02, 20.7C) Local CO2 trap Center AM 1,355 387 28.56 54.4a

Foreign CO2 trap Center AM 2,260 407 18.01Totals 31,709 2,209 6.97

a Chi-square signiÞcant (P � 0.001), NS (P � 0.05).

} }

}}}} }}}

November 2003 MARK-RELEASE-RECAPTURE OF Cx. tarsalis 821

seeking females that were collected by CO2 trapswithin each study area and released during late after-noon ineachexperiment served as a control groupandwere included in all experiments. Recapture ratesamong local and foreign host-seeking females werecompared on four occasions within differing land-scapes, whereas recapture rates between host-seekingand reared females were compared on two occasionswithin the current study and repeatedly in previousexperiments (Reisen and Lothrop 1995). Because re-lease sites, cohorts, and design varied among experi-ments, differences in recapture rates were comparedamonggroupswithineachexperimentbycontingency�2 (Hintze 1998).Experiment 1 compared recapture rates among ten-

eral females released at emergence sites and at anuplandcitrusorchard.Uniquelydusted reared femalesand males were released at two emergence sites nearthe Salton Sea and for comparison within a citrusorchard semiisolated by desert (Fig. 1). Uniquelydusted host-seeking females from combined local andforeign CO2 traps were released concurrently forcomparison. Recapture was attempted over threenights using 25 suction and 9 CO2 traps positioned asshown in Fig. 1.Experiment 2 compared the recapture rates of three

cohorts released within a managed freshwater marshadjacent to a dense row of Tamarisk (Fig. 2). Mos-

quitoeswere reared fromÞeld-collected immatures orwere local and foreign host-seeking females collectedat local or distant CO2 traps, respectively. Approxi-mately one-half of each cohort was released just aftersunrise (AM) or just before sunset (PM), resulting insix uniquely dusted groups of mosquitoes. Recapturewas attempted using 10 suction and 5 CO2 traps lo-cated near the release site and 12 suction and 9 CO2

traps positioned from 0.7 to 1.7 km distant from therelease site (Fig. 2).Experiment 3 compared the recapture of local and

foreign host-seeking females in vineyard and orchardhabitat. Females were released at an emergence sitealong the shore of the Salton Sea and recapturedupland using 19 CO2 traps operated for three nights(Fig. 3). In this experiment, reared females were re-placed by gravid females to test the possibility thatthese females would oviposit near the release pointand then disperse similarly to the unmarked parousfemale population.Experiment 4 compared the recapture rates of local

and foreign host-seeking females released at the sameorchard used in experiment 1. Recapture was at-tempted by 12 CO2 traps positioned along the marginof the orchard and by 6 CO2 traps positioned from 0.5to 1.5 km distant. Relatively cool temperatures andwinds from the N-NW measuring 25Ð40 kph during

Fig. 1. Map of study area used for experiment 1 showing the distribution of release sites in relation to suction and CO2

trap locations.

822 JOURNAL OF MEDICAL ENTOMOLOGY Vol. 40, no. 6

Fig. 2. Map of study area used for experiment 2 showing the distribution of suction and CO2 trap locations in relationto the single release point.

Fig. 3. Map of study area used for experiment 3 showing the distribution of CO2 trap locations in relation to the singlerelease point.

November 2003 MARK-RELEASE-RECAPTURE OF Cx. tarsalis 823

each night seemed to compromise dispersal from theorchard (Fig. 4).Experiment 5essentially repeatedexperiment 4, but

again dispersal seemed compromised by elevatedwind speeds during nights 1Ð3 after release that thensubsided on night 4.

Results

Overall, 31,709Cx. tarsalis femalesweremarkedandreleased during 2001 and 2002, of which 2,209 (6.97%)were recaptured during experiments 1Ð5 (Table 1).Recapture rates varied signiÞcantly among experi-ments and cohorts, ranging from 0.10Ð3.89% duringwarm, still conditions during experiment 1 to 18.0Ð28.6% during cool, windy conditions in experiment 5.In experiment 1, a total of 72,030 and 1,289 un-

marked female and male Cx. tarsalis was collectedduring the three nights of removal sampling, respec-tively. Examining the capture pattern and reproduc-tive status of these unmarked adults provided insightinto the distribution of the natural population. Themean number of host-seeking females collected atCO2 traps placed at the edge and within the orchard(2,068.9 females/trap night) was not signiÞcantly dif-ferent (F � 0.35; df � 1, 28; P � 0.56) from the meannumber collected at traps placed near the Salton Sea(2,356 female/trap night) when tested by an analysis

of variance (ANOVA) (Fig. 1). In contrast, signiÞ-cantly more females (F � 4.46; df � 2, 72; P � 0.015)were collected at suction traps at the edge of theorchard (93.7 females/trap night) than at traps withindesert (63.5 females/trap night) or shore vegetation(53.9 females/trapnight).At theorchard, signiÞcantlymore females per trap night (F � 13.9; df � 3, 32; P �0.001)were collected at suction traps positioned alongthe E (147.7) and N (137.9) margins facing the SaltonSea than at the W (61.6) and S (27.8) margins (leastsigniÞcant difference (LSD) test, P � 0.05), perhapsindicating the direction from which most of thesefemales originated (Fig. 1). SigniÞcantly more males(F � 15.2; df � 2, 72; P � 0.001) were collected atsuction traps along the shore (40.1 males/trap night)than in the desert brush (7.3 males/trap night) or theorchard (8.7 males/trap night), indicating limitedmale dispersal. Regardless of suction trap location,captured females were all inseminated and mostly atfollicular stages I-IIa indicative of host-seeking; only17% of females from suction traps near the shore weregravid and none were recently blood fed (Table 2).Parity rates of females from traps at the orchard andalong the shore were signiÞcantly greater (�2 � 6.3,df� 2,P� 0.05) thanparity rates of females from trapsin thedesert.Overall, only four femaleswerebiparous;the remaining females were uniparous and noneweresacculate, indicating recent oviposition.

Fig. 4. Map of study area used for experiments 4 and 5 showing the distribution of CO2 trap locations in relation to thesingle release point.

824 JOURNAL OF MEDICAL ENTOMOLOGY Vol. 40, no. 6

Overall, 3.14% of 9,613 females that were collectedin CO2 traps, marked, and released the followingevening were recaptured, signiÞcantly more (�2 �65.4, P � 0.001) than 0.21% of 2,904 females rearedfrom Þeld-collected immatures (Table 1). The recap-ture rate of marked CO2-trapped females in suctiontraps (0.37%) was statistically similar (P � 0.05) toreared females (0.14%), perhaps indicating that thesereared females began host seeking during the 3-d re-capture period, and therefore were available for col-lection in CO2-baited traps. In agreement, the tworeared females captured in CO2-baited traps werecollected on night 3. Reared females were 1Ð3 d old atrelease and should have mated and begun host-seek-ing activity on the evening after release, based on ourprevious release experiments (Milby et al. 1983,Reisen et al. 1992). Recaptures per CO2 trap nightaveraged 7.5, 2.1, and 1.3, and recaptures per suctiontrap night averaged 0.5, 0.2, and 0.1 on nights 1Ð3 afterrelease, respectively. Too few marked females werecollected at suction traps to compare their spatialdistributionwith thatof theunmarkedpopulation, andtoo few reared females were recaptured for furtheranalysis. There was no difference (P � 0.05) in therecapture rates among CO2-trapped females releasedat sites 1 and 2 near the Salton Sea, and collectivelythese females were collected less frequently than fe-males released within the orchard (�2 � 4.53, P �0.03), perhaps reßecting differences in female dis-persal behavior as well as sampling effort (5 CO2 and12 suction traps within the orchard, 3 CO2 and 7suction traps each near distant release sites 1 and 2[Fig. 1]).Experiment 2 compared recapture and dispersal

rates among females collected at local or distant CO2

traps with reared females released �2 h after dawn or2 h before dusk in duck club habitat (Fig. 2). The timeof release (AM versus PM) did not signiÞcantly affectthe recapture rate (Table 1) or the mean distancedispersed (data not shown) for the three cohorts, andthereforedatawerecombined in further analyses. Therecapture rates for foreign CO2-trapped (1.25%) andreared (2.11%) females were not signiÞcantly differ-ent (�2 � 2.31, P � 0.13), but both were recapturedsigniÞcantly less (�2 � 151.9, df � 2, P � 0.001) thanlocal CO2-trapped females (7.02%). Reared femalesdispersed a signiÞcantly shorter mean distance fromthe release site (mean �95% CL � 82 � 25 m) thanfemales originating from either foreign (mean �95%

CL� 375� 145 m) or local (mean�95%CL� 247�94m)CO2 traps, perhaps indicating that therewas notan immediate egress from the study area.Experiment 3 compared the recapture rates and

meandistances dispersed for local and foreign femalesfrom CO2 traps with females collected by CO2 traps,blood fed, and then released when gravid (Table 1).Although comparatively few gravid females were re-leased, the recapture rate of this cohort (2.14%) wasnot statistically different (P � 0.05) from females col-lected in local CO2 traps (3.45%). Most local (57%)and foreign (68%) host-seeking females were recap-tured on the night after release, whereas most (67%)gravid females were recaptured on night 2, after theyhad oviposited and resumed host seeking. In markedcontrast to experiment 2, the overall recapture rate offoreign females (8.88%) was signiÞcantly greater(�2�45.6%,P�0.001) than the recapture rateof localfemales (3.45%). There were no signiÞcant differ-ences in the mean distance dispersed at recaptureamong the three released cohorts: mean �95%CL forlocal� 1,337� 602m, foreign� 824� 386m, gravid�1,470� 940m. Although the number of gravid femalesreleasedwas small, recapture resultswerepromisingandindicated that gravid females oviposited soon after re-lease and then dispersed as parous host-seeking femalessimilar to those collected by CO2 traps.In experiment 4, there was no signiÞcant difference

between the recapture rates (P � 0.05) of local(16.5%) and foreign (15.0%) females collectedbyCO2

traps (Table 1; Fig. 4), and their distributions amongCO2 trapswas similar (r � 0.93, df� 16, P � 0.001; Fig.5). However, the distributions of the recaptured localand foreign mosquitoes within the orchard andthroughout the study area were not correlated (P �0.05) with abundance of the unmarked female popu-lation (Fig. 5A). Although differences over the entirestudy area might be expected because of immigrationfrom emergence sites near the Salton Sea, differenceswithin the orchard were unexpected. Elevated windspeeds and cool temperatures throughout experiment4 (Fig. 6a) appeared to preclude emigration from theorchard (Fig. 7a), and resulted in unexpectedly ele-vated recapture rates (Table 1). Most emigration ap-peared tooccurafterwind speedsdeclinedearly in theevening of day 2 after release, after which recapturesincreased markedly at traps outside of the orchard.Experiment 5 essentially repeated experiment 4 in

the same habitat (Fig. 4) and under similar conditionsof elevated wind speeds and cool temperature (Fig.6b). Although local and foreign females originatedfrom the same traps used in experiment 4 and werereleased at the same point at the same time of the day,signiÞcantly more local than foreign females wererecaptured during experiment 5 (Table 1). Similar toexperiment 4, the distribution of local and foreignfemales recaptured per trap night was correlatedamong trap sites (r � 0.95, df � 16, P � 0.001), butwere not correlated with the unmarked female pop-ulation (P � 0.05) (Fig. 5b). The pattern of recapturediffered temporally from experiment 4, because windspeeds did not decrease until days 3Ð4 after release

Table 2. Distribution and age structure of unmarked femalescollected in suction traps in Experiment 1

Mosquitoorigin

Mean per trapnight Dissected

(n)Gravid(%)

Parous(%)

Males Females

Orchard 8.7b 93.7a 30 0 76aBrush 7.3b 63.5b 25 0 48bShore 40.1a 53.9b 25 17 77a

Values within columns followed by the same letter are not signif-icantly different (P � 0.05).

November 2003 MARK-RELEASE-RECAPTURE OF Cx. tarsalis 825

(Fig. 6b), when most emigration to traps outside theorchard was detected (Fig. 7b).

Discussion

Our research addressed three factors that may in-ßuence the results of mark-release-recapture experi-

ments: 1) mosquito age and source, 2) time of release,and 3) wind. Origin of marked females signiÞcantlyaffected their recapture success in each experiment,except experiment 4 when there was no differencebetween local and foreign cohorts. In agreement withour previous studies in Coachella Valley (Reisen andLothrop 1995), reared females were recaptured less

Fig. 5. Distribution of marked and unmarked females collected per trap night among trap locations in experiments 4 and5.

Fig. 6. Wind speeds during experiments 4 and 5.

826 JOURNAL OF MEDICAL ENTOMOLOGY Vol. 40, no. 6

frequently than females collected host seeking at CO2

traps, regardless of site of origin or timeof release. Thefew reared females recaptured were taken at trapsnear the release point shortly after release, indicatingthat females were old enough to be recaptured andmay not have dispersed rapidly from the study area.Because Þeld-collected Cx. tarsalis usually matepoorlywithin cages (Reisen et al. 1985),most releasedfemales were presumed to be uninseminated. How-ever, males were abundant at each release site andwere released concurrently, somatingwas anticipatedto occur rapidly after release, as documented in ourprevious studies (Reisen et al. 1982).Low recapture rates of cohorts reared from Þeld-

collected immatures were not observed in our previ-ous release experiments at foothill (Nelson et al. 1978,Nelson andMilby 1980, Reisen et al. 1981) or riparian(Reisen et al. 1992) habitats in Kern County, CA. Onepotentially important factor not measured in thepresent experiment was autogeny, because the num-bers of reared females available for release were min-imal. Some Cx. tarsalis populations exhibit facultativeautogeny during the Þrst gonotrophic cycle (Eberleand Reisen 1986, Reisen and Reeves 1990), and highrates of autogeny within reared cohorts may havenegatively impacted recapture in CO2 traps by delay-ing the time of host seeking until after they ovipositedtheir initial egg raft�4d after emergence (Nelson andMilby 1982). Previous studies (Reisen et al. 1995)indicated that autogeny rates in Coachella Valley var-ied markedly among pupal collections. High parityrates among vernal host-seeking populations, such asin experiment 1, often were indicative of elevatedautogeny rates (Reisen and Reeves 1990).Previous environmental conditioning and orienta-

tion may have altered recapture success among ex-

periments. Local mosquitoes collected for releasefrom CO2 traps positioned within the recapture gridwere of mixed ages and therefore of previous orien-tation experience, but were recaptured signiÞcantlymore frequently than foreign mosquitoes in two ofthree experiments; in experiment 4, there was no sta-tistical differencebetween these cohorts.Results fromexperiment 3, in which foreign mosquitoes were col-lected more frequently than local mosquitoes, mayhave been biased somewhat by elevated recapturerates from trap 12 positioned �30 m from the releasepoint, in which 40 local females (36% of total recap-tures) and 45 foreign females (57% of recaptures)were recaptured. If these recaptures were deleted,recapture of local and foreign females decreased from3.5 to 2.3% and from 8.8 to 3.8%, respectively, becom-ing more similar, but still different statistically (�2 �5.4, P � 0.02). Cx. tarsalis is a dispersive colonizingspecies that preferentially oviposits in newly createdsurface pools (Beehler andMulla 1995). With this lifestrategy, “memorizing” a home range would seem tohave less of a Þtness advantage than for a mosquitospecies such as An. farauti that has a relatively shortgonotrophic cycle and inhabits comparatively stableperidomestic environments (Charlwood et al. 1988),or Oc. cantans that must return to the same groundpools for oviposition (Renshaw et al. 1994). In addi-tion, the detection of previous orientation is moredifÞcult for a multivoltine population of mixed agessuch as Cx. tarsalis than for a univoltine species suchas Oc. cantans. Cx. tarsalis do not exhibit ovipositionsite Þdelity similar to domestic Culex (McCall andKelly 2002), because few females seem to return tonewly created ponds for oviposition (Beehler andMulla 1993).The time of release did not signiÞcantly affect re-

capture success. This was unexpected, because mos-quitoes released in early morning were forced to rap-idly Þnd suitable diurnal resting sites and then spendthe day under hot, desiccating conditions beforeegressing the following evening to begin dispersal andhost-seeking activities. By comparison,mosquitoes re-leased during late afternoon spent the interveningtime period within the air-conditioned laboratory,where they were humidiÞed with moist toweling.However, females released during morning wouldhave more time for acclimatization and preparationfor evening activity than females released during lateafternoon that initiated ßight within 1Ð2 h after re-lease. Both cohorts were starved during the diurnalperiod, so energy for ßight and dispersal should havebeen similar. Comparable recapture rates and dis-persal distances also indicated that the length of timeduring the day that Cx. tarsalis were crowded withinholding cages didnot inßuencedispersal upon release.Long distance dispersal by Cx. tarsalis frequently

has been related to wind direction and speed (Baileyet al. 1965). In experiments 4 and 5, wind speedsaveraging �10Ð15 mph (17Ð25 kph) inhibited egressfrom the orchard, resulting in elevated recapture ratescompared with experiment 1, in which the wind was�3 mph at sunset and marked females readily dis-

Fig. 7. Meannumberofmarked local and foreign femalescollected at CO2 traps positioned within and outside theorchard in experiments 4 and 5.

November 2003 MARK-RELEASE-RECAPTURE OF Cx. tarsalis 827

persed throughout the study area. Mosquitoes withinthe orchard moved to the perimeter and were recap-tured in CO2 traps, but did not egress readily to CO2

traps outside of the orchard until after wind speedsdecreased to �10 mph. Overall, recaptures per trapnight generally were greatest at leeward traps, indi-cating that females avoided elevated wind speeds atthe windward side of the orchard. Similar behaviorwas shown previously for Florida mosquitoes (Bid-lingmayer et al. 1985, 1995).Our studies indicated that recapture success was

generally greatest when released females originatedfrom CO2 traps operated from within the plannedrecapture grid. The time of release did not seem crit-ical, and therefore cohorts could be released duringeither morning or evening. However, we did not re-solve difÞculties encountered recapturing reared fe-males. Future releases of teneral females will includeconcurrent estimates of autogeny rates.

Acknowledgments

We especially thank Marc Kensington, University of Cal-ifornia, and Arturo Guitierrez, Coachella Valley Mosquitoand Vector Control District (MVCD), for technical assis-tance, and theCoachella ValleyMVCD for logistical support.John Edman, University of California, kindly read the manu-script and commented on our data interpretation. This re-search was funded by Grant 00-16 from the University ofCalifornia Mosquito Research Program, and funds allocatedannually by the Coachella Valley MVCD.

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Beehler, J. W., and M. S. Mulla. 1995. Effects of organicenrichment on temporal distribution and abundance ofculicine egg rafts. J. Am. Mosq. Control Assoc. 11: 167Ð171.

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Received for publication 10 October 2002; accepted 6 August2003.

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