Differences in Performance of Aedes Aegypti Larvae Raised at Different Densities In

7/28/2019 Differences in Performance of Aedes Aegypti Larvae Raised at Different Densities In

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Vol. 31, no. 2 Journal of Vector Ecology 371

Differences in performance ofAedes aegyptilarvae raised at different densities in

tires and ovitraps under feld conditions in Argentina

Arnaldo Maci1

Instituto de Limnologa Dr. Raul A. Ringuelet, ILPLA, C. C. 712, 1900 La Plata, Buenos Aires, Argentina

Received 21 February 2006; Accepted 3 August 2006

ABSTRACT: Alteration of tness components was assessed in the yellow fever mosquito,Aedes aegypti, in automobiletires and vases (ovitraps) under eld conditions. Larval numbers were manipulated in both kinds of containers to comparelow, high, and control (natural) densities. Densities were set from a census of a wild population, then doubling and reducingto half the mean crowding, m*. Articially altered densities were not high or low enough to produce differences amongtreatments. Tires generated more vigorous larval populations and females with higher fecundity than did small containersalthough the mortality was more intense.Journal of Vector Ecology 31 (2): 371-377. 2006.

Keyword Index: Mosquito larvae, intraspecic competition,Aedes aegypti, articial containers, Argentina.

INTRODUCTION

Water-lled articial containers are distinctive habitatsfor many aquatic insects, and numerous individuals sharinglimited space and food can be found in a single container.Mosquitoes are common container-breeding dwellers andoften undergo inter- and intraspecic interactions that cangenerate phenotypical variations among individuals ofone population. Such variability, in addition to a geneticcomponent, is a result of the history during preimaginaldevelopment (Fish 1985), which is an important forceaffecting population dynamics. Intraspecic competition

during the larval stage alters adult tness (Christophers1960, Bradshaw and Holzapfel l986). In immature stages, ashortage of resources generates longer larval developmentas well as lower success and smaller size at metamorphosis.In adults, body size, survival, fecundity, mating success, andight capacity are all reduced (Steinwascher 1982, Reisenet al. 1984, Fisher et al. 1990, Bradshaw and Holzapfel1992, Bradshaw et al. 1993, Hard and Bradshaw 1993).All these demographic parameters are tness components,and they ultimately impact on population growth. Severalstudies have demonstrated the importance of using indexesof performance that estimate population growth, such asR

0

(Fisher et al. 1990), r(Livdahl 1984, Livdahl and Sugihara

1984), (Pianka 1988) and (Leonard and Juliano 1995,Grill and Juliano 1996), instead of quantifying tnesscomponents. However, such indexes require the knowledgeof specic attributes such as survival under naturalconditions, preoviposition period, and the relationship

between fecundity and adult weight. These attributes havebeen recorded in the yellow fever mosquito,Aedes aegyptiL.from the U.S.A. (Grill and Juliano 1996, Juliano 1998), butthey differ in some extent according to the population understudy.There is a lack of knowledge about such attributesfor strains ofAe. aegypti from Argentina. Although thereare papers about phenology (Campos and Maci 1996,

Micieli and Campos 2003, Vezzani et al. 2004, Carbajoet al. 2004), habitat characteristics (Vezzani et al. 2001)and biological control (Mart et al. 2004) ofAe. aegyptfrom Argentina, there is a lack of experimental studies onwild populations. The species was considered eradicatedfrom Argentina after a countrywide campaign in 1963

but reinfestation was detected in the north of the countryby 1986 (OPS 1990). Aedes aegypti spread as far south asBuenos Aires Province (Campos 1993), and the mosquitodistribution includes northern and central Argentina above35 S (Curto et al. 2002). The species is an effective vectoof dengue, the risk of which has increased in recent years in

Argentina (Avils et al. 1999), mainly due to travelers fromneighboring, epidemic countries (Seijo et al. 2000). In factthere were reports of dengue outbreaks in Argentina (Avilset al. 2000), including several cases in Buenos Aires (Seijoet al. 2000).

This paper focuses on the effects of either increasingor reducing the larval density ofAe. aegypti in a eld

population in Argentina. Two kinds of articial container(tires and vases) were compared because they are common

breeding habitats forAe. aegypti in urban areas and areusually used in outdoor tests. In this experiment, a naturasubstrate was used as a source of nutrients for the larvaethat allows a more realistic approach than using articial

food (such as aquarium sh food, liver powder, yeast orground dog chow), commonly employed in other studiesThe objectives were to determine (1) to what extent tnesscomponents (pupal weight, total biomass, survivorshipdevelopment time, and female fecundity) are affected iflarval density is articially increased or diminished, and (2)if these parameters differ between two common breeding

places forAe. aegypti.

1Research assistant, Comisin de InvestigacionesCientcas, CIC.


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372 Journal of Vector Ecology December 2006

MATERIALS AND METHODSStudy area

This work was conducted at Florencio Varela, 24 kmsouth of Buenos Aires City (34 50 S, 58 06 W), Argentina.The study area was located at Instituto de Limnologa(ILPLA, University of La Plata), in an 11-hectare campussurrounded by a dense urban area at least 100 m away fromthe nearest houses, with scattered large buildings and treeclusters. The articial containers used in this study wereset under a 30 m-long line of cypresses (Cupressus sp.)separated from each other by approximately 0.5 m. The lineof cypresses separated a large building from a grassy openland measuring 60 x 40 m. Weather was temperate to hot(ranging 15-32 C) and humid during spring and summer,with year-round rains.

Artifcial containers

As breeding habitats, 46 small containers (ovitraps) and46 automobile tires were used. Ovitraps were black plasticvases (13 cm high x 6 cm diameter) lined with lter paper,

numbered and xed with wire to the trees at 80 cm height,and separated from each other by 1 m. Automobile tireswere cut in half, numbered, and placed with the opening up,leaning on the ground, and separated from each other by 0.5m. Ovitraps and tires were set in October 2000 and partiallylled with rainwater (100 ml in the ovitraps and 1 liter inthe tires) to facilitate natural colonization by mosquitoes.Fluctuations of water level in both types of traps relied onweather conditions, but water was added when the volumewas about 50 ml (ovitraps) or 500 ml (tires). Containerswere checked daily with a ashlight to detect the presenceof mosquitoes and to measure water levels. To impedeinterspecic competition, all Culex egg rafts were discarded

using a teaspoon. Mosquito eggs, larvae, and pupae were notcounted to avoid excessive disturbance due to handling.

Mosquito colony

Larvae were obtained from a colony maintained atILPLA for more than ten generations under a L:D 16:8

photoperiod, 80% relative humidity and 26 2C. Adultswere maintained in a cage with free access to a sugarsolution. Females were fed with human blood and suppliedwith two 500 ml black jars lled with dechlorinatedwater to a depth of 1 cm and lined with paper towels.Eggs were collected, air-dried, and stored in plastic bagsuntil experiments started. Submerging eggs in water with

powdered yeast stimulated articial hatching. Resultinglarvae were rinsed once in dechlorinated water before beingused in any treatment. Mosquito larvae used at the start ofeach experiment were less than one day old.

Estimating larval crowding in nature

In order to estimate an index of larval density undernatural conditions, a destructive census of containers wascarried out and Lloyds mean crowding, m*, was estimatedduring February 2000. After discarding containers withoutlarvae, ve tires and ve ovitraps were selected using a tableof random numbers, recording water volume, number, and

stage of larvae and pupae. None of these containers wereused again. Lloyds mean crowding for tires and ovitrapswas calculated (Lloyd 1967, Hulbert 1978) as:

m* = [xi(x

i-1)/v

i]/x

i

with xithe number of larvae and v

ithe volume in liters, in

the container i withAe.aegypti.

Manipulation of populations

Immatures were manipulated to establish how changesin their number might affect population performance.Fifteen tires and 15 ovitraps were selected at random, andall water was extracted from each container and the watervolume measured. To avoid new hatchings, mosquito eggswere eliminated from the walls of containers. This wasachieved in ovitraps by removing the lter paper and intires by rubbing the interior walls vigorously with a spongeand cloth. Tire and ovitrap contents (liquid and solid) weretransferred to the laboratory. Larvae, pupae, and mosquito

eggs were discarded. Water and detritus from tires andovitraps were mixed separately in two large recipients,one for each container type. Water and organic debris werethen restored in each ovitrap and tire, replacing the originalvolume previously found in each container. This procedureensured homogeneous mosquito food among containers.One of the following treatments (ve ovitraps and vetires/treatment) was assigned to each ovitrap and tire atrandom - Treatment 1: add 1st instar larvae (LI) up to m*estimated from the census (control); Treatment 2: add LI upto 2m* (high density); Treatment 3: add LI up to m*/2 (lowdensity). High and low densities (double and half of naturaldensity) were chosen arbitrarily because of the availability

of experimental larvae.LI were obtained from eggs articially hatchedduring the previous day. On the following days, the waterlevel in each treatment was maintained constant, addingrainwater when necessary. To avoid new ovipositions, theexperimental containers were covered with a mesh ttedwith rubber bands in order to impede the access of gravidfemale mosquitoes. Containers were checked daily. At theend of the immature development, pupae from ovitrapsand tires were removed, counted, sexed, and individuallyweighed in vivo to the nearest 0.01 mg with a SartoriusBP-21 microbalance. After that, they were placed in potsinside cages (one cage/treatment/container type), and

allowed to reach adulthood.

Adult mosquitoes

Adult insects emerged inside cages (30 x 30 x 30cm) supplied with raisins. Males had permanent access tofemales. A human blood meal was offered to females adlibitum. All engorged females were individually isolatedin tubes and labeled with the feeding date, treatment,and container type. Cages and live females in tubes werekept under 16:8 (L:D) photoperiod, around 80% relativehumidity, and 26 2 C, until eggs were fully developed(assessed by eye). Gravid females were dissected and eggs


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were counted.

Analysis

Differences in weight at metamorphosis, developmenttime, biomass, survival, and fecundity among experimentaltreatments were subjected to parametric ANOVA. The mainfactors were Container Type and Treatment, blocked byReplicates (=containers). For development time and weightat metamorphosis, independent ANOVAs were performedfor each sex. Biomass productivity (pooled sexes) wascompared among experimental treatments (Density)considering replicates as blocks in a randomized blockdesign. Survival data (pooled sexes) were transformedusing y= arch sin p

x, with p

x= proportion of survivals in

cohort x = experimental replicate x, and then subjected toANOVA with Replicates as blocks, Container Type, andTreatment as main effects. Differences among treatments infemale fecundity were analyzed by ANOVA with ContainerType and Treatment as main effects. Procedures ANOVA orGLM of SAS (SAS Institute 1999) were used for analysis

of balanced and unbalanced data, respectively, using typeIII sums of squares, followed by Tukeys test (HSD) forcomparisons between means, with a level of signicance= 0.05.

RESULTS

Estimating larval crowding

As a result of the larval census performed before theexperiment, average water volume in ovitraps was 0.15liters and 4.01 liters in tires, with higher variability in tires(coefcient of variation, CV: 23%), than in ovitraps (CV:15%). Population densities differed markedly between

the two types of breeding containers, the density of larvaein ovitraps being higher than in tires (mean density inovitraps= 193.46 larvae/liter; in tires= 68.98 larvae/liter).Experimental densities were calculated based on themean crowding recorded in containers colonized by wild

populations of Ae. aegypti: m*ovitraps

= 301 larvae/liter;m*tires= 93 larvae/liter.

Pupal weight

Weight at metamorphosis in the three treatmentsshowed different patterns for ovitraps and tires (Figures1a, 1b, respectively). Tires produced heavier pupae of bothmales and females. Male pupae were lighter than female

pupae in all combinations container type/treatment. Fifty-two percent of the pupae were male and 48% females,showing a 1:1 sex rate (2= 1.85, df= 1, p= 0.17). Weight ofmale pupae ranged between 0.61 and 3.02 mg (meanSE=1.710.018, n= 595), while female pupae ranged between0.98 and 4.38 mg (meanSE= 2.750.031, n= 549). Bothin males and females, the interaction of Container Type xTreatment affected pupal weight signicantly (males: F=37.09, df: 2, p


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Figure 1. Weight at metamorphosis in ovitraps(a) and tires (b) . Points show mean 2 SE. Means sharing the same letter arenot signicantly different after Tukeys test (p>0.05).

0

1

2

3

4

Control High density Low density

Treatment

Pupalweight(mg)

Males

Females

xy

x

a bb

a

0

1

2

3

4


Treatment

Pupalweight(mg)

Males

Females

bx x

ab

4

6

8

10

12

14

16

Control High de ns ity Lo w de ns ity

Treatment

Developmenttime(days)

Males

Femalesx

x

y

aa

b

a

4

6

8

10

12

14

16

Control High de ns ity Low de ns ity

Treatment

Developmenttime(days)

Males

Females

b

a

b

a

x

yx

Figure 2. Development time in ovitraps (a) and tires (b). Points show mean 2 SE. Means sharing the same letter are notsignicantly different after Tukeys test (p>0.05).

0

100

200300

400

500

600


Treatment

Biomasspro

ductivity(mg)

Ovitraps

Tires

Figure 3. Biomass productivity in ovitraps and tires. Differences among treatments are not signicant after Tukeys test(p>0.05).


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DISCUSSION

This study showed that tness components in Ae.aegypti depend largely on the type of container in whichlarvae develop. The alteration of larval densities wasnot high or low enough to promote signicant effects in

production of nal biomass, female development time,and fecundity. Regarding pupal weight, the variation dueto treatments effects depended on the kind of breedinghabitat. Male development time was signicantly affected

by density and type of container. Tires generated heavierpupae, slower larval development, higher total biomass, andfemales with higher fecundity than did ovitraps. Differencesin immature performance among both types of containerhabitats were motivated by intraspecic competition,which would be more intense in smaller habitats, such asthe ovitraps used in this study. Since the index m* is anestimate of encounters or collisions between individuals, itis appropriate for showing a degree of competition throughmechanical interference among larvae during feedingactivity. Under this assumption, a higher m* could reecta stronger competition. This was shown by differences in

population performances from tires and ovitraps. At highdensities, feeding activity can be hindered by the presenceof other individuals, resulting in a reduction in the efciencyof nutrient exploitation, which is the microscopic biolmattached to plant debris and sediments for many Aedes

species (Clements 1992). Another explanation for negativeeffects of larval crowding is lowered food availability perindividual.Aedes aegypti is very sensitive to low per capitafood availability (Juliano 1998). Density-dependent eventsin Ae. aegypti can be caused by both mechanisms actingcomplementarily.

However, under the present experimental conditions, itcannot be inferred if this competition acts through selectiveresource exploitation for food or space, through interferencecompetitionsensu Schoener (1983), through the existence ofgrowth retardants, or combined effects of those mechanisms.Previous studies on factors altering responses inAe. aegyptitness reached different conclusions. Dye (1982) found

that intraspecic competition in this species is asymmetricwith chemical competition (via metabolites released intowater) being less important than physical (via hamperingamong individuals). In turn, Bedhomme et al. (2005)

showed that both depletion of resources and the presenceof excreted substances by larvae are factors that contributeto competition. Different responses of interference weredetected in two strains ofAe. aegypti by Dye (1984), so

populations from different localities can also exhibit subtlevariations in tness based on geographical origin.

Mortality was higher in tires (85%) than in ovitraps(33%). At least a part of such mortality can be attributed tosome features of the larval environment. It was observedthat water in tires had high turbidity soon after the starof experiment, while ovitraps remained with clean waterlonger. A scarce quantity of leaves was observed in thecontainers, suggesting a shortage of the physical supportfor microorganisms. Such bacteria and protozoa constitutea major nutrient source for larval populations and area limiting resource for container-breeding mosquitoes(Lounibos et al. 1993, Leonard and Juliano 1995, Maci andBradshaw 2000). Other studies also showed high mortalityrates in Ae. aegypti under eld conditions (Southwood eal. 1972, Micieli et al. in press). Regardless of the kind ofcontainer, competence resulted in similar trends in malesand females in pupal weight and development time. A cosdue to competition for food is not sex-related, as reported

by Bedhomme et al. (2005).Data obtained from the previous census showed a

great variability in mosquito numbers among recipientsa characteristic observed in tree-holes commonly used byotherAedesspecies (Bradshaw and Holzapfel 1988, Lonardand Juliano 1995). The great variability among habitaunits masked clear trends related to crowding, which wouldexplain the low R2 in some of the statistical tests. All of theabove suggests that theAe. aegypti population under studywas subjected to intraspecic competition, the intensity owhich is related to quality of the container habitat.

The average potential fecundity (59.8 eggs/female) issimilar to records from Coless and Chellapah (1960) of 51.8eggs/female. The signicant difference between fecundityfrom mosquitoes breeding in tires and ovitraps conrms tha

0

20

40

60

80

100

Control High dens ity Low density

Treatment

Survivorship(%

Ovitraps

Tires

Figure 4. Survival in ovitraps and tires. Points show mean 2 SE. Comparisons between means among ovitraps andtires are signicantly different, but differences amongtreatments are not signicant (Tukeys test, p>0.05).

0

20

40

60

80


Treatment

Fecundity(eggs/female)

Ovitraps

Tires

Figure 5. Female fecundity in ovitraps and tires. Points showmeans 2 SE. Comparisons between means among ovitrapand tires are signicantly different, but differences amongtreatments are not signicant (Tukeys test, p>0.05).


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tires generate adult populations with superior reproductivepotential, although this effect could be compensated by agreater preimaginal mortality.

Events alleviating intraspecic competition, suchas the elimination of a fraction of the population in eachcontainer, can increase the amount of resources availablefor the remaining larvae. As a consequence, the surviving

pupae will be bigger and heavier, and the emerging femaleswill have higher fecundity. In small water receptacles withlow nutrients, removal of larvae can increase numbers ofemerging adults (Agudelo-Silva and Spielman 1984). Thenal effect would be an increased vectorial capacity inthe population of adults. Thus, it is important to carefullyevaluate control measures before they are implemented andto consider if total elimination of all individuals in eachcontainer is feasible. A similar conclusion was suggested byArrivillaga and Barrera (2004) after studying resistance tostarvation inAe. aegypti larvae from Venezuela.

This experiment is the rst report on differences inthe tness of larvae raised both in tires and ovitraps. The

general outcome was the emergence of healthier, althoughless numerous, individuals in larger containers. Reducingthe number of larval mosquitoes per container would leadto a fast recovery of the whole population because femalesurvivors would be able to lay more eggs.

Acknowledgments

I thank Ral E. Campos for assistance during laboratorytasks, logistic support, and critical review of the manuscript.Rodolfo L. De La Sota assisted in performing statisticaltests and providing SAS. Ann Montemayor-Borsingerreviewed a draft of the manuscript. To Alberto Rodrgues

Captulo and Gustavo Spinelli for providing working space.Thanks to an anonymous reviewer for improving the paperthrough valuable suggestions. This research was supported

by Fundacin Antorchas (Buenos Aires) through Grant13783/1-19.

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Differences in Performance of Aedes Aegypti Larvae Raised at Different Densities In