167

TRANSACTIONS OF THE ROYAL SOCIETY OF TROPICAL MEDICINE AND HYGIENE, VOL. 72, No. 2, 1978

A mark-release-recapture experiment with Culex tritaeniorhynchus Giles

W. K. REISEN, Y. ASLAM*, T. F. SIDDIQUI AND A. Q. KHANS

Pakistan Medical Research Center, (i Birdwood Road, Lahore, Pakistan

Summary A mark-release-recapture experiment with the

progeny of wild collected Culex tritaeniorhynchus was conducted at the village of Khokhar near Lahore, Pakistan, during June-July 1976, the onset of the monsoon. Recapture rates for each of three releases ranged from a low of 0*19”,, for males to a high of 0.58”,, for females. Male and female recaptures were well correlated with the dispersion patterns of the unmarked adults which suggested that the released adults selected similar resting habitats as the wild population. The direction of dispersal appeared random and marked, unfed females were captured biting buffaloes at the most distant collection site, 1.22 km from the release noint. The initial aonotronhic cvcle of the released iemales was completed six to- eight days after emergence. The last marked female and male specimens were collected 13 and 15 days after the day of release, respectively. Released virgin females were all inseminated by the second night. The modified Lincoln index and Jackson’s positive method (based on an eight-day sampling sequence) provided relatively similar estimates of population size which were considered realistic. Jackson’s negative method, based on a three-day recapture chain, provided a substantial underestimate of survivorship and thus an underestimate of popula- tion size.

Introduction Mark-release-recapture studies have recently

been initiated near Lahore, Pakistan, with C&x tritaeniorhynchus Giles, an important Asian arbo- virus vector, to provide ecological information necessary for planned genetic control experiments. Previous releases of marked females of this species were performed in Japan and Korea (JOLIVET et al., 1975) and Thailand (BAILEY & GOULD, 1975). While these studies provided valuable information of epidemiological interest, experimental design (only wild-caught females of unknown age were marked and released) precluded observations on survival, reproduction behaviour and estimates of population size. The present paper reports observa- tions on these phenomena for both males and females ascertained by the release of the laboratory- reared progeny of wild-caught females.

Description of the Study Area Geographical

The-area of the release and recapture centred around the villarce of Khokhar (nonulation annroxi- mately l,lOO), l&ated at the uorthern outskIrts of Lahore on the flood plains of the River Ravi. The immediate area surrounding the village consisted of

heterogeneous farmland with arum, melon, maize and sugar-cane fields interspersed with numerous cattle sheds. The study was contained within an area delimited to the north by the River Ravi (about 3/4 km wide during the study), east by the road between the villages of Bhamma and Jhugian, south by the Bund Road (a flood control dike) and west by a forested area (Fig. 1).

Weather and Climate During the study period (29th June to 15th July

1976), just before the onset of the monsoon, the Regional Meteorological Centre in Lahore reported a mean (f standard error of mean) maximum and minimum daily temperature of 38.6 & 0.9”C, and 27.3 i 0 + 7”C, respectively, with a relative humidity at 17.00 hours of 42.0 & 5*3’$. The mean rainfall in five rainy days was 11.2 mm. In maize fields at Khokhar at about 17.00 hours the mean temperature was 32.1 * 0~4°C with a relative humidity of 84.1 5 2 *4”&. The most frequent wind direction during late afternoon was north to south at 5 km hour, gusting to 7. At 21.00 hours (after the buffalo biting- collections) the mean temperature was 29.4 5 0*6”C, relative humidity was 75.3 i 3.00/, and the most frequent wind direction was west to east at less than 3 km/hour. On the evenings of the three releases at 21.00 hours, the wind was from west to east at velocities of 3 to 16, 3 to 11, and less than 3 km/hour, the temperature remained at 32’C and the relative humidity ranged from 54 to 74”,. Dust storms occurred on the evenings of Ist, 3rd and 5th July with about 45 km winds blowing first from NE to SW and then reversing to SW to NE.

Methods and materials Mosquito Production

16 days before the release, about 8,000 females were collected from buffaloes in the Khokhar area, of which about 17% oviposited (Subsamnle of 50 egg rafts: 2 * S.E.s =-112.4 +‘5*8 eggs/raft, 94.5 & 2.20,5 hatch, 107 ~2 $ 6.1 larvae/raft). Larvae from five egg rafts were added to each pan (density = 0.6 larvae/cm’ surface area) and fed a daily ration of finely sifted liver powder. Pupae were collected on three successive days by icing and were placed into plastic containers fitted with a 1 .l litre paper carton with a screen top to collect emerging adults. The next day, adults were lightly etherized, counted and marked with a fluorescent dust (different colour each day) using an apparatus

* Present address: Model Town, Lahore t Present address: Zoology Department, Forman

Christian College, Lahore

168 MARK-RELEASE-RECAPTURE WITH C. tritaeniorhynchus

Fig. 1. Map of the Khokhar study area with buffalo-biting and light-trap recapture sites indi- cated by numbers and letters, respectively (See Table III for collection results). R -z: release point.

similar to the one described by YASUN~ et al. (1972). After dusting, adults were returned to the same paper carton (bottom closed with a plastic petri dish bottom), offered a 3”,, sugar solution, and held for release that evening.

Release Three groups of marked mosquitoes were released

on three successive nights beginning 29th June 1976 (Table II). Each group was approximately 24 hours old and unmated as male C. tritaenio- rhynchus are not capable of inseminating females until 28 hours old. Adults were released at one of the primary C. tritaeniorhynchus breeding sites in the Khokhar area (“R” in Field 4, Figs. 1 and 2), having a mean abundance of 34.8 larvae/dip. The wild population resting in Field 4 prior to sunset on 30th June was found to consist of 60”,, newly emerged (unfed, uninseminated, nulliparous, CHRISTOPHERS’ (1911) ovariole Stage I) and 40”,, gravid females, and males whose terminalia were at varying degrees of rotation. Thus, our release of virgin males and females at this location may have approximated the natural situation.

Recapture Beginning the day after the first release, three

methods of recapture were used: (i) Szueeper collections: Each afternoon mosquitoes

were collected resting in the fields indicated in Fig. 2 using battery-powered sweepers similar to those described by DAVIS & GOULD (1973). All

fields in the study area were not swept daily or uniformly. Sweeper collecting effort (time) for each field depended mostly on field size and ‘or the number and dispersion of the mosquitoes. Initially, fields proximal to the release site were sampled more intensely; later additional more distal fields were sampled to evaluate dispersal. Selected fields consistently yielding a high number of marked and unmarked mosquitoes were sampled thoroughly on each recapture interval.

(ii) Buf/alo bitiqg collectiom : Each evening, starting 20 minutes after sunset, mosquitoes biting buffaloes were mouth-aspirated for 1 hour at the cattle sheds indicated by number in Fig. 1. In Pakistan, female C. tritaelliori7~lizrlf(s are highly attracted to buffaloes at this time (REISEN & BORE- HAM, 1976; REISEN ec al., 1976). Again collection effort was greatest at the more productive recapture stations.

(iii) Light trap collections: Every evening from dusk to dawn one New Jersey and a varying number of CDC traps powered by batteries were placed at cattle sheds where buffalo-biting collections were not being made (Fig. 1). At some sheds, traps and buffalo-biting collection methods were inter- changed on different nights.

All collections except the light traps were com- pleted before the release of the next batch of marked mosquitoes, and no marked mosquito was recaptured on the night of its release. Each daily collection was immediately frozen upon return to the laboratory. The following day specimens were examined under

W. K. REISEN et d. 169

Table I-Total numbers of mosquitoes ($2~-$) i n sweeper, buffalo biting and light trap collections at Khokhar from 30 June to 16 July 1976.

Species

Aedes Zineatopennis (Ludlow) Ae. pulverulentus Edwards Ae. w-albus (Theobald)

Anopheles annularis Van der Wulp An. culicifacies Giles An. nigerrimus Giles An. pulcherrimus Theobald An. stephensi Liston

Culex bitaeniorhynchus Giles Cx. pipiens fatigans Wiedemann Cx. fuscocephalus Theobald Cx. pseudovishnui Colless Cx. tritaeniorhynchus Giles Cx. univittatus Theobald Cx. vishnui Theobald

Mansonia uniformis (Theobald)

* gynandromorph

Buffalo Sweeper Biting

:z o-o o-o

O-O O-O

;,I; 3-o 2-O

40-36 206-O 17-6 973-o o-o 13-o

2-3 1-O 2-O O-O 7-9 24-O

108-283 1864-l” 32816-38794 73080-o

2-o 1-o O-O 1-o

7-l 97-o

Light Trap Total

O-O O-O :=z 1-o 1-o

2-O 6-2 O-O 2-O 6-O 252-36

17-l 1007-7 1-o 14-o

Ei 3-3

11-l 1-O 32-9

18-O 1990-284 7942-187 113838-38981

3-o ;I; 1-O

4-l 108-2

Table II-Summary of releases and recaptures of C. tritaeniorhynchus at Khokbar

Recaptured

No. Mosq. Released

Date 9 $

8 8

Dust Sweeper Biting Light Trap Colour No. % No. % No. % Tot y0 Tot y0

29 June 5,15510,901 Orange 14 0.27 11 0.21 0 0.00 25 0.48 33 0.30 30 June 20,339-24,145 Green 37 0.18 60 O-29 9 0.04 106 0.52 45 0.19

1 July 13,242-10,458 Pink 46 0.35 31 0.23 2 0.02 79 0.60 59 0.56

ultra-violet light for fluorescence and then separated according to species and sex. Marked females were dissected and examined for insemination, trophic and ovariole condition, and parity.

Calculations Because collection effort (time and numbers of

collectors) was variable, all data were standardized to numbers of mosquitoes collected per man-hour (MH) or light-trap night. Population size was estimated using three methods:

(i) Lincoln indices, modified to account for survivorship and low recapture rates (BAILEY, 1951), were calculated with population loss rates estimated by regression (GILLIES, 1961) and the addition rates estimated from the daily change in population size (YAWN0 & RAJAGOPALAN, 1973). Population size was estimated for resting males and females using total daily recaptures.

(ii) Jackson’s positive method (JACKSON, 1939)

was used to estimate the population size for males and females for each release.

(iii) Jackson’s negative method (JACKSON, 1939) was applied to estimate population size for resting males and females with

yz + ys r- =

y, + yz since marked mosquitoes were released only on three occasions.

For each method, the number of marked speci- mens recaptured during a recapture interval was not added to the total specimens caught, since the marked individuals were laboratory-reared and actually not part of the wild population (YAS~O & RAJAGOPALAN, 1972). In the present experiment this bias was considered minimal, since the re- capture rate was low and the total number of released specimens considerably less than the density of the estimated wild population.

170 MARK-RELEASE-RECAPTU WITH C. tritaeniorhynchus

Mean dispersal within the study area for the entire experiment was estimated from the mean recapture rate among stations grouped into 50 m intervals, where

a=: rx&I: Fzc with a = x=1 x=1

mean distance dispersed, fX = mean recapture rate in number of marked mosquitoes per man-hour within distance interval X, d, = outer radius of distance interval x, and n = most distant interval.

Results and discussion Fauna of Study Area

A total of 156,594 mosquitoes (four genera and 16 different species) were collected during re- capture attempts (Table I). C. tritaeniorhynchus was the most prevalent species comprising over 97% of the total. Most species, with the exception of C. bitaeniorhynchus, C. univittatus and Aedes

w-albus, were readily collected while biting buffaloes. Light trapping was markedly disappointing, while sweeper collection success was erratic, attesting to the clumped distribution of mosquitoes based on the availability of favourable microhabitats within the fields.

Recovery Rate A total of 38,736 female and 45,504 male C.

tritaeniorhynchus were released over a three-day period of which 210 females and 137 males were recaptured (Table II). The higher number of females recaptured was due to increased collecting effort as males were recovered mostly from sweeper collections, whereas females were taken in sweeper, buffalo-biting and light-trap collections (Table II). Our recapture rates, although less than l%, exceeded those of previous capture-mark-and- release studies with C. tritaeniorhynchus using CDC light traps (BAILEY & GOULD, 1975) or black light and CO, traps (WADA et al., 1969). However, the recapture sites in these two investi-

Table III-Summary of C. tritaeniorhynchus buffalo-biting (?/man hour) and light-trap (Q- J/trap night) capture rates. See Fig. 1 for location and distances from the release site

Map Ref. No.

: 3

t

76 8

1;

11

Abundance Collection Effort

(MH or trap night) Unmarked Marked

Buffalo Biting*

14.42 7.17 829-O 704.9 0.28 O-62** 20.49 1319.3 3.03

16.00 0.75 632.8 24.0 0.63 0.00

11.50 5.33 982.9 563.0 0.43 1.50 1.75 229.7 o-00 0.75 5.50 326.7 669.1 0.91 o-00

6.25 601.0 0.16

Total : 89.91 812.8 l-13

Light Trapt ? 6

: 23.5 15.8 Opoo 0.13 - - s

- - 2.8 1.9 o-00 o-00

E 2 23.0 4.0 - - 2.0 1.3 0.00 0.00 - - 0.00 0.00

E is 37.9 - 5.5 o-00 - 0.13

:++ 5 H 23

3.0 1.4 - - o-0 l-6 0.00 o-00 - - 0.00 0.00 18.5 - 1.5 0.00 - o-00

: 1 t

1.0 . BO:.:

- - 1.0 - ;:;

o-00 o-00 - - o-00 o-00 K 1.11 - 0.00

Total : 51 155.7 3.7 0.22 0.02

* Number of females per man hour of collecting effort ** plus 1 8 taken t per trap night

tt NJ. light trap

w. K. REISEN et al. 171

Table IV-Summary of C. tritaeniorhynchus sweeper collections; see Fig. 2 for locations and distances from release site

Abundance (No./MH)

Field No. Total Man-Hrs

1 7.15 2 4.16 2 4.83 1.75

5 4.90

7” 3.24 1.58 8 11.49

1: 2.25 l-50

11 2.08 12 0.50 13 0.25 14 5.66 15 4.66 16 0.58 17 0.33

:“9 7.53 0.25

Total : 64.69 Sex Ratio ( G/total)

Unmarked Marked

9 d ? 6 910.1 740.0 2.24 2.24 374.5 356.0 0*50 0.00

1305.7 886.3 3702 938.6 -3 4.29 4.57 4.51 7.43

288.4 352.7 0.82 3.06

313.2 676.0 430.9 991.5 0.31 l-00 2.00 0.00 624.5 719.8 1.91 3.13

1049.3 474.2 820.0 505.8 2.22 4.70 4.70 3.10

274.0 276.4 0.00 0.00 212.0 178.0 0.00 0.00 52.0 60.0 o-00 0.00

192.9 208.8 0.88 I.59 149.4 193.1 0.64 0.21 489-7 737.9 1.72 1.72 206.1 915.2 o-00 0.00

252.9 32.0 281.5 44.0 0.13 0.00 0.53 o-00

507.28 599.69 1.50 2.07 0.541 0.580

gations were more widely dispersed than those in the present study.

(6) of female mosquitoes among biting recapture stations (Fig. 1) was equal to 542.6 m.

Dispersal Females dispersed in all directions and were

recaptured at the farthest points sampled (Figs. 1 and 2, Tables III and IV). Females were collected biting buffaloes at Site 1, l-13 km west of the release point, after four days, and at Site 11,1*22 km northeast of the release point, after seven days (Fig. 1). Four females were recaptured biting buffaloes at Site 10, 0.65 km northeast of the release site, and seven females were taken at Site K, O-61 km northeast of the release site, the evening following the night of the release. These data suggested that non-blood-fed C. tritueniorhynchus were capable of dispersing at least 0.65 km within 48 hours of emergence. Some of the specimens recaptured at Site 10 while biting buffaloes were taken within 30 to 45 minutes after sunset, suggest- ing that at least a portion of this distance was probably traversed the evening of the release in search of suitable resting sites. These dispersal estimates were low compared to WADA et aZ. (1969), who recaptured females of unknown age 5.1 km from the release site 24 hours after release. The capture rate of marked and unmarked females (No./MH) collected while biting buffaloes at the Sites 1 to 11 were well correlated, suggesting that released females responded to the same host stimuli and were well mixed with the wild population (r = 0.808, P < 0.01). Estimated mean dispersal

Females and males were recaptured by sweepers farther than O-4 km from the release site (Fig. 2, Table IV). Marked adults dispersed in all directions and were well mixed with the wild population by the day following their release as indexed by the dis- persion of the recaptures (Fig. 2, Table V). The highest recapture rates occurred in tall, partially cut maize fields (Fields 3 and 6) and in a marshy area (Field 4) near the release site (Table IV). The capture rates of marked and unmarked (No./MH) males and females in Fields 1 to 19 were highly correlated suggesting that released adults rested in the same microhabitats as the wild population (males, r = O-745, females, r = 0.902, P < 0.01) (Table IV). Sweeper collections were hampered by agricultural activities such as the harvesting (e.g., Fields 12, 13) and irrigating (e.g., Fields 5, 11) of some maize fields which altered the vegetation patterns and thus the resting places of the mos- quitoes. Sweeper collections in the arum and sugar- cane fields yielded extremely low numbers of specimens which were not included in the popula- tion estimation. Estimated mean dispersal distance (c?) among fields sampled by sweeper collection (Fig. 2) was 159 -5 m for males and 165 *8 m for females.

Longevity Survival of the released adults was estimated from

the recapture rates for each collection method

172 MARK-RELEASE-RECAPTURE WITH C. tritaeniorhynchus

ARUM m

PLOWED

CUT MAIZE m

BUILDINGS p

POND

Fig. 2. Map of the Khokhar study area showing the juxtaposition of the various agricultural fields used for sweeper recapture attempts (See Table IV for collection results). R = release point.

pooled over the three releases (Fig. 3). In the biting collections (Fig. 3a), the number of recap- tured females dropped precipitously on the second night after release and increased slightly on Day 8 after the completion of the initial gonotrophic cycle, although parous females were first recovered on Day 5 (Table VS). The estimate of the length of the gonotrophic cycle in nature agreed well with laboratory life table experiments, where maximum age-specific fertility (rn3 was attained between six and seven days after emergence (RBISEN et al., 1977a).

Female and male sweeper recaptures also declined sharply the second day after the release (Fig. 3b and cl. Male longevity was much longer than expected; the last specimen was an orange-dusted male recaptured 15 days after release. The last dusted females were collected biting buffaloes and in sweeper collections 13 days after release. More marked males than females were recaptured in sweener collections. and the sex ratio of the re-

captures was slightly greater than that of the wild population and the released cohort (Table IV). This disparity could not be attributed solely to the greater vagility of the released females, since more males than females were colIected in Fields 14, 16 and 18 (Fig. 2). Recapture attempts were termin- ated 17 days after the first release-two days after the last marked specimen was recaptured. Further recaptures were attempted four days later without success (2 95 MH sweeper collection effort in five productive fields; 2 MH buffalo-biting at sunset at four productive locations). Unmarked specimens collected on this occasion were not included in the results.

Physiological Status of Recaptured Females The first day after the release, IO of 23 marked

females collected by sweepers m Fields 4 and 5 were unfertilized and all were at ovariole State I, while only 5/25 recovered in the remaining fields were unfertilized, and 3/25 of these had ovarioles

W. K. REISEN et al. 173

A. 8lflNG !? 8 2

Q 4 8 I2 I6

i” 4 8 I2 I6

2

0 0 4 8 I2 16

TI ME r0AY.S)

Fig. 3. Recapture rates (no./man hour of collection effort) of marked mosquitoes collected per day by sweeper (resting) and buffalo-biting transformed to loge ($ + 1):. Regression lines -of recapture seauences Davs 1 to 8 (o----o ) and Davs 3 to N (x- x) for resting females and males- included with extrapolations represented by dashed lines (See Table VII for regression coefficients).

matured to Stage III. These observations agreed closely with LEA & EDMAN (1972) who found a higher insemination rate among C. nigropalpus females dispersing more than 250 feet from the release point. Our data also indicated that some females had mated and even fed during the evening of the release. These findings agreed with laboratory observations in which some females were capable of being inseminated eight hours after emergence. As

some males were also capable of inseminating females at this time (after 28 hours from emergence), it was not ascertained whether females were in- seminated by wild or released males. By the second day after the release, all males and females had probably moved away from the release site as no marked adults were again recaptured in Fields 4 and 5. Despite repeated searching, mating swarms or pairings of C. tritaeniorhynchus have rarely been observed in Pakistan (REISEN et al., 1977b), although this behaviour has been observed frequently in Japan (KAWAI et al., 1967). The present observa- tions suggest that mating must occur either in the vegetation at the breeding site, during emigration to preferred resting sites, or among the vegetation at the preferred resting sites (in this case, maize fields).

Population Size Estimates Five primary assumptions underlie the use of the

mark-release-recapture method of estimating popula tion numbers: (i) marked individuals should not be affected by being marked, (ii) marked individuals must become completely mixed in the population, (iii) sampling is random with respect to mark status, (iv) samples are taken at discrete time intervals, small in relation to total time, and (v) population gains or losses are negligible or can be estimated (SOUTHWOOD, 1966). In the present study, adults were not considered to be adversely affected by the marking procedure fulfilling Assumption 1. The over-all mortality of 25 marked females and 25 marked males which were etherized. dusted and then held in the insectary for 16 da& at a temperature of 28 to 30°C was -32 and 68%, respectively, while the over-all mortality of 25 males and 25 females which were not etherized or dusted, but were also held in the insectary for 16 days, was 24 and 720/6, respectively. There were no significant differences (I’> 0 * 05) between the slopes of the fitted regression of survivorship (number alive on each day/total at start) on time in days for dusted and control females and males applying analyses of covariance (SOKAL & ROHLF, 1969).

As the capture rates of marked mosquitoes (no./MH) were well correlated with those of the unmarked individuals, released mosquitoes were considered well mixed in the population soon after release (Table V) fulfilling Assumption 2. Sampling was selective for sites containing more mosouitoes (marked and unmarked), but random with respect to mark status fulfilling Assumption 3. Samples were taken at daily intervals over the 17-day period fulfilling Assumption 4.

Birth-and immigration, and death and emigration rates were not considered negligible (Assumption 5). For the modified Lincoln Index estimates, curvi- linear regressions of the form In (y + 1) = a - bx were fitted for the decline in the recapture rates shown in Fig. 3. Initially the three releases were used as replicates to test for significant departures from linearity using a replicated, regression analysis of variance on the number of recaptures over days one to eight (SOKAL & ROHLF, 1969). Using an In (y + 1) transformation of the replicated re- capture rates (no./MH), there was a significant

Tabl

e V-

Dai

ly

capt

ures

of

mar

ked

(M)

and

unm

arke

d (U

M)

rest

ing

mos

quito

es

($J-

& g

roup

ed

by d

ista

nce

from

th

e re

leas

e po

int

(als

o se

e Fi

g. 2

)

Dat

e

Dis

tanc

e fro

m

rele

ase

(met

ers)

0 to

1oo

a 10

0 to

2oO

b >

2OO

c To

tals

MI-I

* M

e U

M

MH

M

U

M

MH

M

U

M

MI-I

M

U

M

30-V

I 4-

8 1.

49(4

) -

576-

597

I-VII

3.33

(4)

1%

3928

-291

2

2-V

II 2.

50(7

) :-:

14

-12

1397

-165

6

3-V

II 2.

50(7

) k;

47

19

40-2

091

4-V

II 2-

3 1.

83(5

) - 42

1913

-198

0

5-V

II 1.

50(4

) z l-l

545-

625

6-V

II 1.

83(6

) 1

7641

049

7-V

II O

-l 1.

99(6

) - 1-l

937-

837

nc

nc

1.59

(2)

z2

1.41

(3)

E 6-7

1.17

(3)

2 24

1.83

(3)

:; 2-8

0.67

(2)

‘. l-5

0.83

(2)

2-l

1-O

0.75

(l)

z

nc

llC

tlC

nc

1.49

1513

-208

4 nc

Sl

C

llC

4.92

984-

1247

1.

00(2

) 1 2-7

240-

248

4.91

671-

1681

0.

50(2

) 1

135-

120

4.17

1938

-517

1 0.

50(l)

I l-o

20

7-25

2 4.

16

533-

984

o-1

1.08

(4)

- O-l

570-

906

3.25

529-

532

0.67

(2)

1 1-O

22

4277

317-

244

o-1

1.41

(5)

- l-1

217-

382

3.33

4.15

4-8

3-3

25-3

3

o-5

4-3

22-2

6

l-2

O-2

6-

11

;I;

7-10

O-l

2-7

2-1

2-O

O-2

2-2

576-

597

5441

-499

6

2621

-315

1

2746

-389

2

4058

-740

3

1648

-251

5

1517

-185

8

1471

-146

3

Tota

ls

: 16

.97

lo-1

6 12

000-

1174

7 8.

25

l-11

6485

-119

43

5.16

O

-2

1593

-218

5 30

.38

11-2

9 20

078-

2587

7 18

-23

14-1

7 51

9 31

40

2423

12

-24

41-5

6

a Fi

elds

1,

2,4,

5,7,

9,

10,

11;

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elds

3,

6,8,

13

; C

Fie

lds

14,

19,

15,1

2,

16,

18,

17;

d M

H

= m

an

hour

s of

oa

rent

hesi

s)

; e T

OD

=

oran

ge d

uste

d co

hort.

M

iddl

e =

pree

n. a

nd b

otto

m

= Di

Dk

fTah

le

TTY

colle

ctin

g ef

fort

(num

ber

of f

ield

s sa

mpl

ed

in

W. K. REISEN et al. 175

Table VI-Dissections of recaptured females, all releases combined

Days after release

Sweeper Collections Biting Collections Ovariole Stage Parity* Parity

I II to v Nr P NP P

1 2 3 4 5 6 7 8 9

10 11 12 13

3 8 7 4 3 3

- 1 3 2 3 2

-

14 0 3 1 :

- -

- - - - - 1 - 1 - - - -

68 4 1

A -

*determined by the coiling of the ovarian tracheoles for females at ovariole Stage I; some specimens too dried for dissection

Table VII-Regression estimate of survivorship based on different recapture sequences

Collection Days

After Release Regression

Coefficient (b) Coefficient oft Survivorship No. Remaining“

Determination (r2) (eb) (Day 16)

Biting 99

Resting 90

:I,: l-14

l- 8 3-14 l-14

-0 - 155a(ns) -0~04Ob(ns) -0*088b(*)

-0*275a(**) --0.069”( **) -O.l2i’b(**)

Resting d$ l- 8 3-16 l-16

-0.291a(**) 0.781 0.748 591.7 -O.l06b(**) 0.698 O-899 9,275 * 0 -0.149(**) 0.721 0.865 5,176-g

0.261 0.856 5,328 * 6 0.303 0.961 23,357 * 6 0.351 0.916 12,595.6

0.744 0.759 671.6 0.545 0.933 15,970.3 0.608 0.881 7,654 * 2

a Tested for significant linear regression by analysis of variance, each release used as a replicate; NS (P ~0. lo),

b Tested for significant departure from 0, all 3 releases combined; NS (P > 0 *05), * (P < 0 *OS), ** (P < O-01). c Proportion of variation explained by regression (SOKAL & ROHLF, 1969). d Estimated number of marked mosquitoes remaining within the study area by Day 16.

(P < 0.01) linear fit for both the resting females and males; however, there was significant deviation

The pooled recaptures were then used to compare

from linear regression and the linear regression survivorship estimates calculated by fitting re-

mean square was non-significant (P>O*O5) for gression lines to three recapture sequences: (i) Days

the recapture rates of the biting females. For l-8, the length of the gonotrophic cycle; (ii) Days 3-N, where N is the day after the last marked

biting females, recapture was dependent upon the gonotrophic rhythm of the released cohort, and

specimen was recaptured as originally proposed by

thus would not be expected to provide a sigmficant GILLIES (1961). This approach negates the high

linear fit for days one to eight. Covariance analyses initial loss rate due to crowding-induced dispersal

(SOKAL & ROHLF, 1969) showed that there were no or handling mortality; and (iii) Days I-N, all

significant differences (P> 0.05) among the slopes recapture intervals (Fig. 3). Recapture sequence 1

of the three replicate releases for both resting males provided the most realistic estimate of survivorship, i.e. by Day 16 the fewest marked specimens were

and females. Untransformed resting recaptures were then pooled, again transformed to In (y + l),

estimated within the study area (Table VII).

and used to calculate the final regression expression Sequence 2 and 3 provided higher estimates of

to estimate survivorship for females and males survivorship which likewise would provide spur-

(Table VI). iously high estimates of population size, i.e. by Day 16, > 5000 marked individuals were estimated

176 MARK-RELEASE-RECAPTURE WITH c. tritaeniorhynchus

to remain within the study area (Table VII). We felt that more than 5,000 marked specimens within the study area would not go undetected using our recapture methods. In addition, recapture sequence 1 had the highest r2 values, while sequence 2 had the lowest; i.e. provided the poorest fit for the data.

The loss rates (emigration + deaths) for resting adults were thus considered constant (1 -survivor- ship) and were used to estimate the number of marked individuals remaining at each recapture interval. Therefore, population size could be estimated daily for each of eight recapture intervals using the modified Lincoln Index (BAILEY, 1951) and the addition rate (immigrating + emerging adults) calculated by the formula At = Nt + r-P&, (YASUNO & RAJAGOPALAN, 1973), where A = number of individuals added at time t, N = population size estimated at intervals t and t + 1, and P = the probability of surviving to the next interval (Table VIII). The population size and addition rate estimates presented for the Lincoln Index in Table VIII represent the means of eight daily estimates for resting males and females. We felt that the modified Lincoln Index provided the best estimates of population size as well as of loss and addition rates.

For comparison, population size and addition rates were also calculated using the Lincoln Index modified to account for the higher survivorship estimated by recapture sequence 2 (Fig. 3, Table VII). As expected, population estimates were considerably higher and more variable, i.e. the confidence intervals were broader (Table VIII). Estimates of loss and addition rates were less, suggesting a more stable population (Table VIII). We thought that these estimates of population size

were too high and that the wild population was much more dynamic than indicated by estimates based on GILLIES (1961) method for calculating survivorship (sequence 2). The variable nature of the wild population was suggested by fluctuations in the numbers of unmarked mosquitoes collected during daily recapture attempts (Table V).

Jackson’s positive method, based on an eight-day recapture chain, also provided realistic estimates of population size which fell within the fiducial limits of the modified Lincoln Index (Table VIII). There was considerable variation among the estimates for each of the three releases resulting in the very broad 95% confidence intervals for this method (Table VIII). The population addition rates estimated by Jackson’s positive method were considerably higher than those estimated by the modified Lincoln Index.

Survivorship estimates based on Jackson’s nega- tive method, which utilized only the first recapture intervals, were exceedingly low due to the high loss rate of marked individuals on the day after release (Fig. 3). These survivorship estimates were con- sidered unrealistically low and resulted in estimates of less than one marked individual remaining by Days 7 and 9 for resting females and males, res- pectively. This over-estimation of the loss rate (or under-estimation of survivorship) resulted in the lower estimate of population size (Table VIII).

When estimated by regression and the modified Lincoln Index, the loss rates were less than the addition rates. If population size were to remain constant, the loss and addition rates should be approximately equal; however, during the present eight-day period, population size was apparently increasing due to immigration and/or emergence.

;&le VIII-Population size estimates of resting females and males at Khokhar during June-July

Population size (x 106)

95% Confidence interval

(x 106) Loss rate Addition

rate -~~

MODIFIED LINCOLN INDEX (1)a ?? 3.515 1.961- 5.071 0.241 0.348 68 3.565 1*710- 5.419 0.252 0.320

MODIFIED LINCOLN INDEX (2)” ?? 8.162 3.970-12.356 O-067 0.261 cc3 7.903 2.339-13.466 o-101 0.204

JACKSON’S POSITIVE METHOD=

3% 1.559 1.726 -1.443-4.561 -1.852-5.304 - - 0.485 0.437

JACKSON’S NEGATIVE METHODd 99 O-165 - 0.894 - 36 0.342 - 0.777 -

a Based on 8 recapture intervals. b Based on recapture intervals 3 to 14 for females and 3 to 16 for males. c Mean and confidence intervals of 3 replicate releases. d Based on 3 release and recapture intervals.

W. K. REISEN et al. 177

Acknowledgements We would especially like to thank Messrs. J.

Ahmad, A. Aziz, S. H. Chishti, L. A. Chowdhry, I. M. Chaughtai, S. A. M. Gilani, N. Hussain, M. Saghir, A. A. Shahid and I. H. Zafar for their technical assistance in the field and laboratory. Drs. R. H. Baker and R. K. Sakai kindly provided the cohorts of mosauitoes for release, assisted in their recapture and offered helpful. suggestions during the preparation of the manuscript. Dr. F. J. Sonleitner, University of Oklahoma, Norman, Oklahoma; Drs. J. H. Frank and M. W. Provost, Florida Medical- Entomology Laboratory, Vero Beach. Florida : and Dr. W. W. Macdonald. Londdn School of Hygiene and Tropical Medicine; London, kindly reviewed the manuscript and provided valuable advice and comments. This research was supported by Grant No. AI-10049 from the National Institute of Allergy and In- fectious Diseases, N.I.H., U.S.A. The adminis- trative support of the United States Agency for International Development in Pakistan is gratefully acknowledged.

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Accepted for publication 26th November, 1977.