Dispersal of Adult Females of Culex Annuliros?Ris in Griffith, New South Wales, Australia

JUNE 1992 DTSPERSALor Cx. arvrur,rgosrn8

DISPERSAL OF ADULT FEMALES OF CULEX ANNULIROS?RIS IN GRIFFITH, NEW SOUTH WALES, AUSTRALIA

M. S. O'DONNELL,Ir G. BERRy,l T. CARVAN, lNo J. H. BRyAN'r

. {Bp-TEAPI The dispersal of Culex annulirostris, a major arbovirus vector in Australia, was studied in Griffith, N.S.W. using a- mark-release-recapturetechnique. From an empirical model of aU"trri, fitted^to da,ta_onrecaptured.adults, the averagi distance dispersedwas 6.8 im (98% c.t. +.f-aO.b [mi, Vn!.5.0-%of the lopulation dispersed4.8 km or more. Maximum recorded dispersal was 8.T tm, a"J j individuals traveled more than 5 km in 1 day. The relevance of the findings to control strategy is discussed.

INTRODUCTION and constant movement of the mosquitoeslim- ited the accuracyof this estimate. In Australia, Culex annulirostrls Skuse is the Mosquitoes were marked with powdered flu- most important vector of Murray Valley enceph- orescent pigments with a particle size range of alitis virus and a major vector of Ross River "Radglo"), 4-6 p.m (Ciba-Geigy blown into the virus. It is also a seriousbiting nuisance,and in container from a large-bulbpipette. Preliminary irrigated areas and during floods, may occur in tests had demonstratedthat this technique pro- very high densities (see Lee et al. 198g for re- duced durable labels which were readily view). During an epidemic of Ross River virus detect- able and the resulting at Griffith, New South Wales in 1984 (Hawkes colors were distinguisha- ble under long wavelength ultra-violet et al. 1985), control measures were directed illumi- nation. against the larvae of Cx. annulirostris in the With the exception of the first (Feb- urban area and a 5 km zone surrounding it. An release ruary 5) when approximately initial decline in adult density was not sustained, 50% of the adults had been caught the previous and invasion from untreated areas was sus- night and held for 24 h prior to pected. Therefore, dispersal of this specieswas labeling, marked mosquitoeswere investigatedto provide information for planning released on the night of capture, at approxi- effective control measures. mately midnight. Disturbance was minimized as far as possible by allowing the majority of individuals to leave the container "voluntarilv." MATERIALS AND METHODS Mosquitoes were released,on several occasions at each of 4 locations (Fig. 1), with a different Mosquito capture, marking, releaseand recop- color label (green,red, yellow and orange)being ture: The study was undertaken from February used for each releasepoint. 4 to 23,1985at Griffith (34" l7' S, 146' 02, E) Releasepoint 1 was approximately 4 km E S and its environs, N.S.W., Australia. Adults for E of the town center, and mosquitoes released marking were captured between 1700 and 2100 at this site were labeled green. The second (red h (Eastern Standard Time), from locations label), third (yellow label) and fourth (orange known to have large Cx. annulirostris popula- label) release points were 2.5, 5 and 9.b km, tions, in I CDC light traps (Sudia and Cham- respectively,from releasepoint 1 (Fig. l). berlain 1962) baitcd with dry ice and 4 EVS The study area contained a wide variety traps (Rohe and Fall 1979). Captured mosqui- of environments including urban areas, irrigated toes were taken to the laboratory, lightly anaes- rice paddies, orchards, vineyards and grazing thetized with ether or chloroform and trans- land. Initially, recapture traps were arranged ferred to plastic containers, 20 cm diam x 16 cm within 5 km of the first release point, at the deep,with fine muslin mesh at both ends.When intersection of 2 roads. Recapture stations were they had recovered,the number alive and active positioned on each road at 0.25, was estimated; delayedrecovery, large numbers 0.5, t, 2, B, 4 and 5 km from this release point. The areas betweenthe main eres were sampledat stations l 2.5 and 5 km from the center, on each School of Public Health and Tropical Medicine of 8 '(now -Building additional radial Department of Public Health), A 22, arms. One EVS trap was op- University of Sydney,N.S.W. 2006,Australia. erated nightly, at each of these 44 recapture '-\q* South Wales, Department of Health, Albury, stations, from February 7-8 to 18-19. N.S.W. 2640,Australia. From February 19-20 3 to 2l-22, the experi- Present address: Tropical Health program and mental area was extended 4.5 km to the SW to Department of Entomologl, Medical School, Herston include release point 4, and 13 stations were Rd., Herston, 4006,Australia. a Queensland, added to the recapture grid (Fig. 1). Only Present address:c/o J. H. Bryan. 28 traps were operated on the final night of trap- 160 JouRNer,oF THEArranmcer Mosqurto CoNtnol Assocrntlott VoL. 8, No. 2 ping, February 22-23, concentratedalong a SW- Analysis of results: Taylor (1978, 1980) has NE axis. The use of the 3 peripheral release suggestedthat the dispersalof many insects can points allowed the detection of movements up be describedby the generalequation: to 14.4km. N: exp(a + b X) (1) Collections from recapture traps were examined for labeled individuals with an ultra-violet where N is the density at distance X from the lamp operating at 366 nm; the presenceof mark- dispersal center (releasepoint) and o, b and c ing powder and its color were confirmed by are parameters.Apart from the parameter c, this microscopic examination (20-40x) of fluoresc- is a generalized linear model (McCullagh and ing individuals. Nelder 1983) and was fitted to the recapture The effect of marking on survival was moni- data by maximum likelihood methods,using the tored in the laboratory. Ten samples each of computer program Genstat V. approximately 20 labeled females were main- For each trap, for each set of releases,the tained in a darkened, humid environment, at recapture rate was consideredas: ambient temperature (approximately 20-30' C) for 6 wk on a diet of207o sucrosesolution. Ten Number of mosquitoesrecaptured batches of control females were taken from the Number of trapping nights samenight's catch before anaesthesia and label- which allows for variations in trapping intensity. ing. Survival was recorded at intervals of t-2 The number of recaptured mosquitoes was as- days, and the 2 populations were compared by sumed to be proportional to the density of la- determination of their Product Limit Survivor- beled mosquitoes (N) and to the number of ship Estimators (Kaplan and Meier 1958) and trapping nights. application of the logrank (Peto test et al. 1977). The effects of releasepoint and direction of The 4 releasepoints and 67 trapping stations movementon the recapturerate were examined, were mapped at a scale of 1:25,000.The dis- including extra parameters in the model for tances betweeneach releasepoint and eachtrap these factors and assessingtheir statistical sig- were measured to the nearest 25 m, together nificance by an analysis of deviance. The trap with their angle relative to north. Although an- effect was assumedto be randomly distributed gles were measuredto the nearest degree,they and was included as a component of the residual were grouped into quadrants, 1-90", 91-180", deviance. l8l-270" and 271-360" for statistical analvsis. The best fit of the model was determined by varying the value of the exponent c and selecting that value which minimized the deviance. The 95% confidencelimits of c were derived as those \ values which gave a deviance that was greater -tI than the minimum devianceby the critical chi- squarevalue (1 d.f.) of 3.84. Hawkes (1972) discussesthe calculation of the averagedistance dispersedby the population under study for the specific caseof c : 0.5. This r\ approach may be generalizedfor any value of c \ . \ -t . /-., a a a f J* | as: J. I Town of c ,/ 1 . Average distance dispersed \ criffttF f- o ol lo t f (3d) a lt-!o : a a (-bFr ('2d) rz) ' a o I 2J+ a aa where I representsthe gamma function and d oaa" : l/c. o The specificforms of equation (2) appropriate Oa o to the best fit of c and its 95% confidencelimits ooa were determined and the corresponding esti- .,o^ooo mates of averagedistance dispersedcalculated. The relative numbers of mosquitoesat different distancesfrom releasepoints were calculatedby multiplying the density, derived from equation Fig. 1 The location of releasepoints (*) and trap- (1), by the circumferenceof the circle with that ping stations at Griffith, N.S.W. Trapping stations distance as its radius. The maximum distance marked (o) were addedto the grid on 19-20 February. dispersedby specified proportions of the popu- JUNE 1992 Drspnnser- or Cx. nNttutIRosrRIS 161

Iation was obtained by numerical integration of Table 1. Labeling details and numbers of fernale the relative numbers. Culex annulirosfris releasedat Griffith, N.S.W., An alternative, more conservative approach during February 1985. was also adopted; the determination of mean Numbers distance traveled, as described by Lillie et al. Release Color of Release released (1981). Distances up to that of the observed point label dates (approx.) maximum flight were divided into a series of 1 Green Feb.5-7 25,000 annuli, at intervals of 100 m, up to 500 m from a Red Feb.8-11* 18,000 the releasepoint and thereafter, at 500 m inter- 3 Yellow Feb.12-15 20,000 vals. The mid-point of each annulus (z) was 4 Orange Feb.16-18 12,000 taken as the representative value of distance Total 75,000 from releasepoint and the corresponding den- * Mosquitoeswere not releasedon February 9 sity (N,) was calculated by means of the fitted model. The density was multiplied by the area of the annulus (A ) to give a measure of the number of (labeled)mosquitoes in eachannulus. The mean distancetraveled was calculatedusing significant (P < 0.001), releasepoint was taken the formula: into account in subsequentanalyses. Direction was found to exert a significant ) N'' A' z Meand.istance travek' ro = -jT" r.t, effect (P < 0.001), but there was also a signifr- .T- cant interaction between releasepoint and direction (P : 0.01). The effects were mainly due where is the summation of annuli. I to lower recaptures in quadrant 2 (SE) from The terms "average distance dispersed" and releasepoints 2 and 3. As there was no indica- "mean distancetraveled" refer to the samepop' tion of a consistent direction effect of the type ulation parameter, but are retained throughout that could, for example be due to a prevailing this paper to distinguish betweenthe 2 methods wind, the effect of direction was not considered of calculation. further. Since the main presentation of results Date-specific labels were not used in this on the parameter b in equation (1) and study, but the successiveuse of 4 colors made it depends and the interaction term possibleto obtain estimatesof the rates of move- inclusion of direction of b by less than 2Vo,the ment. The minimum, maximum and median changedthe estimate affected by this deci- times in the field and the correspondingrates of results are not materially dispersal were determined for each recaptured sion. (1978,1980) general individual, and the mean values of these param- The best fit of Taylor's recaptures eters calculated for the whole population. More dispersal equation to the observed : giving a precise estimates could be derived for 10 indi- was obtained with a value of c 0.55, (222 recaptured on either the same night as minimum devianceof 267.7 d.t.). The 95% viduals, : : releaseor the following night. confidencelimits were c 0.26 and c 0.89. The specific forms of equation (2) for these values of c, the corresponding values of b and RESULTS the calculated average distances dispersed by female Cx. annulirostris are shown Approximately 75,000 female Cx. annuliros- the labeled with the distance Iimits for fris were releasedbetween February 5-18, 1985 in Table 3, together proportions whole labeled pop- as shown in Table 1 with a period of 17 days specified of the labeled mos- betweenthe first releaseand the final trapping. ulation. The relative numbers of Labeling did not adverselyaffect survival over a quitoes by distance, derived from the fitted comparableperiod (x2 : O.277at 1 d.f., P > 0.5; regressionsare shown in Fig. 3. The average Fig. 2). The median longevity of the Iabeled distancedispersed was calculatedas 6.8 km, with : population was 17 days and the maximum was 95% confidence Iimits of 4.1 km (s 0.89) and 42 days. 40.9km (c :0.26). A total of 2I5 labeled Cx. annulirosfrus was Using the methodof Lillie et al. (1981)(equa- recovered (Table 2). Of these, 95 were labeled tion 3) the mean distance traveled was 3.8 km green,90 red, 26 yellow and 4 orange. Nineteen (c : 0.55)with95% confidencelimits of 3.5 km were captured at distancesof 5 km or more from (c : 0.89) and,4.2km (c : 0.26). their release point. The maximum recorded The estimated rates of movement derived flight was 8.7 km. from all recapturedCx. annulirostris(n:215) The recapture rates for release points 1-4, were: minimum, 0.7 km/day; median, 1.1 km/ respectively,were in the proportions1.0, 1.8, 1.3 day; maximum, 2.1 km/day. The rates of the 10 and 0.7. As these differences were statistically individuals caught within 1 day ofreleaseranged 162 JounNlr, oF THEAupnrcln Moseurro CoNrnor, Assocreuot VoL.8,No. 2

Percent survivol

17 20 30 No. of doys ofter lobelling

Fig. 2. The survival of labeledand unlabeled femaleCul.ex annulirostiis in the laboratorv.

Table 2. Distances from releasepoint at which ally distinct. Migratory flights are primarily lo- labeled Culcx annulirostris femaleswere recaptured comotory with vegetativebehavior, such as feed- and recapture rates for each distance category. ing and reproduction, suppressed.Trivial move- Number of ments are the incidental result of responsesto Total recaptures vegetative stimuli. Distance from number Total per trap In mosquitoes, migratory behavior, if ex- releasepoint recap- number night (:re- pressed, generally occurs shortly after emer- (km) tures nights capture rate) genceand before the femalesare reproductively 0.125-0.375 25 58 0.431 active (Service1976). Provost (1952,1957) for 0.376-0.750 50 99 0.505 example, identified a clear post-teneral migra- 0.75r-1.250 13 n1 0.183 tory phase (non-appetential) in the salt-marsh L.257-r.750 18 56 0.321 speciesAedes taeniorhynchtn (Wied.), followed 1.75r-2.250 19 tt2 0.170 by a period of shorter flights (appetential) as- 2.25r-2.750 22 274 0.103 2.75t-3.250 22 199 0.111 sociatedwith feeding and reproduction. He con- 3.25t-3.750 3 66 0.046 sidersthis pattern to be generalfor mosquitoes. 3.75t-4.250 11 115 0.096 Southwood(1962) and Johnson (1969)equated 4.25t-4.750 q 159 0.057 Provost's division of mosquito flights into non- 4.75t-5.250 7 265 0.026 appetential and appetential with their division 5.251-5.750 q r42 0.063 into migratory and trivial movements. 5.75r-6.250 0 38 0 It is probable that the study described here 6.25L-6.750 1 58 0.017 was concernedprimarily or solely with the meas- 6.75r-7.250 1 62 0.016 urement of 7.251-7.750 , 91 0.044 dispersal by trivial movements be- 7.75r-8.250 0 31 0 causetraps baited with dry ice, as used in the 8.251-8.750 1 4l 0.024 recapture grid, selectively attract those females 8.75r-74.750 0 247 0 which are seeking a blood meal. Following the hypothesis proposedby Johnson, Kennedy and Southwood,such femaleswould have completed from 0.6 to 7.5 km/day. Two individuals ex- any migratory phase. As trivial movements are ceeded5 km in 1 day, traveling 6.5 and 7.5 km. typically shorter than migrations (the main exception being Southwood's (1962) category of "vagrants") part DISCUSSION and occupyonly a ofthe adult's life, it is very likely that the flight ranges ob- Movements which result in dispersal may be servedin this study underestimatethe dispersal classifiedas either migratory or trivial (Johnson of Cx. annulirostris. 1960, 1966, 1969; Kennedy 1961; Southwood In the absenceof transovarial transmission, 1962),the 2 types of movement being behavior- infection with, and transmission of, arboviruses Cx. tyNuunosrnts loti

Table 3. Values of averagedistance dispersedand distance limits for specifiedproportions of the labeled Culex annulirostris female population for the three regressionequations corresponding to the best fit of c and its 95Voconfidence limits. Distancelimits (km) Form of Averagedistance equation(2) dispersed(km) 50% 90% 95V 4783.6 o.26 -0.5725 40.9 18.5 r52.7 b3.846 12.558 0.55 -0.0313 6.8 4.8 ID.U 19.9 b1.818 2.5548 0.89 -0.00140 4.1 3.4 8.2 10.2 bl.l24 Note: In the regressionsand equations (2) above, distance (X) is expressedin meters. Derived distance parametershave been subsequentlyconverted to km.

ve No. of mos quitoes

c:0.26

c:0.55

c:0.89

10 20 30 +0 50 60 Distoncefrom reieosepoint (km)

Fig. 3. The relative numbers of labeled mosquitoes by distance from release point as estimated by Taylor's (1978, 1980) model; the best fit (c : 0.55) and its g5% confidence limits (c : 0.26, c: 0.89) are shown. will only occur in this post-migration phaseand from the models. Taylor's (1978, 1980) general the estimates of dispersal presented here are, dispersalequation (1) gavea good fit to the data therefore, relevant to the consideration of Cr. with c : 0.55.From the correspondingequation annulirostris as a vector. (2), the averagedistance dispersedwas 6.8 km The data were evaluated by direct examina- with98% confidencelimits of 4.1 km (c : 0.89) tion and 2 methods of modeling. From direct and 40.9km (c : 0.26). examination of the data, the maximum flight As the spatial distribution of the dispersing was 8.7 km. Of the 215 recapturedspecimens, mosquitoeswas markedly skewed(Fig. 3), the 19 had traveled 5 km or further and would have averagedistance dispersedhas limited meaning. traversed the 5 km barrier zone established at This qualification doesnot apply to the distance Griffith during 1984.The proportion of the mos- limits, within which specifiedproportions of the quito population reachingor passingthis bound- population dispersed,which are, therefore, more ary cannot be assessedby direct examination of readily comprehensible and a more complete the data, as no account is taken of variation in description of the dispersal process.Using c : sampling intensity with distance. Considering 0.55, 50% of the population dispersed4.8 km or the low recapturerate (

valuesofdensity for all distancesare determined curred. If his data are analyzed by the model and subsequent calculations, e.g., the average applied in the present study, the average dis- distance dispersed, derived from a summation tance dispersedwas 7.1 km, in close agreement of the correspondingnumbers of individuals. It with the results of the present study. As Russell is, therefore, the more powerful method for an also used EVS traps, migrants were probably analysis of dispersal but assumesthat the rela- also underestimatedin his studv. tionship based upon the observed recaptures retains the same form over all distances.If this assumption is valid, the calculated values of CONCLUSIONS average distance dispersed and distance limits Culex annulirostris is a highly mobile species; demonstrate that Cx. annulirostris is capableof extrapolation from the results suggeststhat it is dispersingwell beyond the upper limit observed capable of flights up to and beyond 10 km. If in this study and that approximately half of the this hypothesis is correct, then in areas with population could cross a 5 km barrier zone. high mosquito population densities, such as The upper confidencelimit, correspondingto Griffith, substantial numbers will cross a 5 km c : 0.26, yielded a very high estimate of the barrier zone. If the concept of a barrier zone is averagedistance dispersed,40.9 km and corre- to be included in future control strategies, our spondingly large distance limits. In the absence findings suggestthat it should be increased to of supporting observations, caution is necessary at least 10 km. However, such an increasewould in interpreting these values, which are largely entail a substantial increase in the area requir- due to the Iow precision with which the densities ing control measuresand a far greater commit- labeled of mosquitoes were measured at the ment of resources.For example,if it is assumed greater distances from the release point. The that the Griffith town area can be encompassed relationship between the parameters in the within a circle of 2 km radius, a circular area of model is such that even relatively small varia- 7 km radius would have been treated in 1984. tions in b, at low values of c, can have marked To increase the barrier zone to 10 km, a circle effects on the derived values of averagedistance with a 12 km radius would have to be treated, dispersedand distance limits. As Taylor (1980) increasing the area by a factor of 2.94. Further discusses,the simpler form of the model, as used studies are required so that appropriate control here, is not a full description of the dispersal strategiescan be implemented. process and the development and inclusion of additional parameters is desirable. The modelingmethod of Lillie et al. (1981)to ACKNOWLEDGMENTS determine mean distance traveled, as adapted joint for use in this study, is a compromise between This project was carried out with funding the purely descriptive approach and the extrap- from the N.S.W. Department of Health and the olations implicit in Taylor's model. It allows the Commonwealth Department of Health. behavior of the whole population to be exam- We thank: P. Currie, M. Eick, R. Franken- ined, but avoids the difficulties of extrapolation berg, M. Geary, M. Kanjo, D. Murray and R. by limiting the analysis to observed dispersal Turner for technical assistance,including long distances. It is, however, conservative and and unsocial hours, during fieldwork; Drs. J. Baker and for many valuable dis- underestimates dispersal becauseof the prob- R. C. Russell cussions;Ms. T. Guilfoyle for assistancein the lems in adequately sampling the very low den- preparation of figures; the staff of Griffith Shire sities in areasdistant from releasepoints. Thus, Council, Griffith Base Hospital and the people in this study, the best fitted model gave an of Griffrth for their co-operationand Ciba-Geig:y estimateof 3.8 km (95Voc.I. 3.5-4.2km) for the (Australia) Ltd. for the donation of fluorescent mean distance traveled compared with 6.8 km powders. for the averagedistance dispersed. The data on daily rates of dispersalare limited and, at best, approximations; but the estimates REFERENCES CITED obtained are compatible with the hypothesis that Cx. annulirostris is capableof considerable Hawkes,C. 1972.The estimationof the dispersalrate mobility. of the adult cabbage root fly (Erioi.schia brossicae (Bouche)) presence Only one other study of dispersal in this spe- in the ofa brassica crop. J. Appl. (1986) Ecol. 9:617-632. cies is known to the authors. Russell Hawkes, R. A., C. R. Boughton, H. M. Naim and N. recorded flights of up to 7 km (the limit of his D. Stallman. 1985. A major outbreak of epidemic trapping grid) from a larval habitat into the polyarthritis in New South Wales during the sum- surrounding country, where no larval sites oc- mer of 1983/1984. Med. J. Aust. 143:330-333. JUNE 1992 Dlspnnslr, oF Cx. ANNULIR0STRTS

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