JIMMA UNIVERSITY

COLLEGE OF NATURAL SCIENCE

DEPARTEMENT OF BIOLOGY

SCHOOL OF GRADUATE STUDIES

ECOLOGICAL AND SYSTEMATIC ZOOLOGY STREAM

ACADAMIC YEAR 2012/13

SEMINAR PAPER (BIOL652)

TITLE: THE ROLE OF PHYISICO-CHEMICAL AND BIOLOGICAL CONTROLOF

ANOPHELES MOSQUITOLARVAL DEVELOPMENT AND DISTRIBUTION

BY: ABERA HAILU

ID NO: MSC 00204/05

SEMINAR PAPER SUBMITTED TO DEPARTEMENTOF BIOLOGY FOR THE

FULFILLMENT OF THE COURSE SEMINAR (BIOL 652)

ADIVISOR: DELENASAW YEWALAW (PhD)

JIMMA -ETHIOPIA

JUNE/2005E.C

I. Table of Contents

Acknowledgement..........................................................................................................................iv

List of figures

List of Abbreviations and Acronyms...............................................................................................v

Abstract...........................................................................................................................................vi

1. Introduction..................................................................................................................................1

1.1Objectives................................................................................................................................3

1.1.1 General Objective............................................................................................................3

1.1.2 Specific Objectives..........................................................................................................3

2.MaterialsandMethods...................................................................................................................3

2.1 Literature Research Strategy.............................................................................................3

2.2 Selection Criteria...............................................................................................................4

3.Literature Review.........................................................................................................................4

3.1 Anopheles Mosquitoes Ecology, Behavior and Distribution...............................................5

3.2 Anopheles Mosquito larval control........................................................................................8

3.2.1 Means of Biological Control...........................................................................................9

3.2.2 Environmental Modification and Management.............................................................11

3.2.3 Larviciding using Chemicals..........................................................................................12

3.3 Physico-Chemical and Biological Characteristics of Anopheles Mosquito Larval Habitats

.......................................................................................................................................................13

4.Summery.....................................................................................................................................17

References......................................................................................................................................18

ii

List of Figures

Figure 1.The life cycle of Anopheles Mosquito.........................................................................................15

Figure 2. Diverse habitat of Anopheles Mosquito Larvae..........................................................................16

iii

Acknowledgement

I would like to express my acknowledgement of thanks to my Instructor and advisor Dr

Delenasaw Yewalaw for his professional guidance to do this seminar paper. And those

individuals supporting and encouraging me to purse my education next to God.

iv

List of Abbreviations and Acronyms

An = Anopheles

An. albimanus = Anopheles albimanus

An. arabiensis = Anopheles arabiensis

An. culicifacies adanensis = Anopheles culicifacies adanensis

An. dirus = Anopheles dirus

An. funestus = Anopheles funestus

An. gambiae = Anopheles gambiae 

An. gambiae s.l = Anopheles gambiae sensu latu

An. gambiae s.s = Anopheles gambiae sensu strictu

An. maculates = Anopheles maculatus

An. melas = Anopheles melas

An. merus = Anopheles merus

An. quadrimaculatus = Anopheles quadrimaculatus

An. sinensis = Anopheles sinensis

An. stephensi = Anopheles stephensi

An. sundaicus = Anopheles sundaicus

Bs = Bacillus sphaericus

Bti = Bacillus thuringiensis var israelensis

CDC = Centers for Disease Control and Prevention

DDT = Dichlorodiphenyltrichloroethane

Pf = Plasmodiumfalciparum

Pv = Plasmodium vivax

Pubmed = a citation data base developed by US National Library of Medicine National Institute of Health  

v

Abstract

This seminar paper focuses on the role of physico-chemical and biological characteristics of

larval habitats on the development and distribution of anopheline mosquitoes. These mosquitoes,

includes many members of the genera Anopheles.The distribution pattern of adult mosquitoes is

relatedto habitat preferences of the immature stages. Many species of mosquitoes are habitat

generalistwho breed, grow as larvae and emerge from a wide variety of aquatic habitats.The

breeding habitat is crucial for mosquito population dynamics, since it is the location where many

important life cycle processes occur such as oviposition, larval development, and emergence

takesplace. We used different source of information.To control mosquitoes, whether adults or

larvae, it iscrucial to understand the biology and behaviors of the targetspecies. The knowledge

of ecological characteristics ofthe breeding habitats and the environmental factors affecting

mosquito abundance can help in designing optimal vector control strategies. Successful larval

control requires the ability to identify larval habitats and distinguish between sites with high and

low vector populations in a timely manner.

vi

1. Introduction

Malaria is one of the most important vector borne diseases and it is transmitted among humans

by female mosquitoes of the genus Anopheles. Female mosquitoes take blood meals to carry out

egg production, and such blood meals are the link between the human and the mosquito hosts in

the parasite life cycle. The successful development of the malaria parasite in the mosquito (from

the "gametocyte" stage to the "sporozoite" stage) depends on several factors. The most important

is ambient temperature and humidity (higher temperatures accelerate the parasite growth in the

mosquito) and whether the Anopheles survives long enough to allow the parasite to complete its

life cycle in the mosquito host.

Many species of mosquitoes are habitat generalist which breed, grow as larvae and emerge from

a wide variety of aquatic habitats (Carpenter & LaCasse 1955). These species, including many

members of the genera Anopheles. The distribution pattern of adult mosquitoes is related to

habitat preferences of the immature stages. The aquatic larval habitat is an important part of the

Anopheles mosquito’s life cycle and may strongly influence the distribution and abundance of

malaria vectors. These habitats may be natural or man-made, temporary or permanent. Although

an array of mosquito habitats exist, the larval stage is mainly confined to stagnant water pools

and, as such, is quite vulnerable. Adult mosquitoes are difficult to control since they can fly

relatively long distances and survive in a range of microhabitats, including houses, vegetation,

holes in rocks and soil, among others (Gilles &Warrell 1993). Nevertheless, the adult stage has

been the main target in mosquito control for decades (Fillinger et al. 2004). Successful larval

control requires the ability to identify larval habitats and distinguish between sites with high and

low vector populations in a timely manner (Wood et al, 1992).

Larval control through larviciding and environmental management are the main intervention

methods for malaria vector control around the world. Identifying the mosquito larval habitats has

a critical role in each control program. Actually it is difficult to find all potentially breeding sites

of mosquitoes over a large geographic area (e.g. at district level) based on field survey. This

method requires time and money and reduces the efficiency of control program due to missing of

some breeding places. A landscape approach using remote sensing and geographic information

system (GIS) technologies was developed to discriminate between villages at high and low risk

for malaria transmission, as defined by adult Anopheles abundance (Beck LR. et al. 1994).

Mosquito control requires prioritization of the areas in need of pesticide application; this can be

achieved with larval surveillance. One approach to surveillance is to identify key environmental

factors that predict the presence of vector populations, and then use these factors as markers to

predict the presence of significant larval densities. A quantitative description of larval

demography can produce data useful for the development of computer models and evaluation of

control efforts. The biological and physico-chemical attributes of the aquatic environment may

alter adult vector competence. An important target for malaria vector control is the anopheline

larvae.

Geomorphology affects the hydrology of a region; i.e., distribution and seasonal dynamics of

lakes, rivers, streams, and pools. Water quality in these different water bodies is influenced by

rock and soil chemistry, vegetation of the surrounding landscape, and human activities

(Rejmankova et.al 1993). Both hydrology and water chemistry determine the type of aquatic

vegetation present in lakes, pools, and streams. Shallow, quiet water with aquatic vegetation

seems optimal for oviposition and larval development of most mosquito species. Descriptions of

requirements of individual species for specific characteristics of larval habitats have generally

been rather vague.

The characteristics of Anopheles larval habitats are variable a shallow larval habitat with the

presence of algae is a common characteristic of anophelines (Savage et al.1990; Gimnig et al.

2001, Gimnig et al. 2002), although such a correlation is notsystematic. Manysuch larval habitats

consist of animal hoof orfoot prints, orsmall ponds of still water created by irrigation projects

orrainfall. Only a few anopheline species are found in artificial containers (Gilles & Warrell

1993). The characteristics of the larval habitat those are adequate for a given speciesare still

unclear. Many environmental variables can have adirect or indirect effect on

anophelineoviposition (Sumba et al. 2004, Rejmànkovà et al. 2005) as well as on larval

2

distribution, density, and development (Gimnig et al. 2001) and adult fitness (Briegel 1990a,

Briegel 2003).

Physico-chemical factors of the water, such as temperature, salinity, concentration of carbonates

and nitrates have a correlation with the presence or development quality of Anopheles larvae in

pools (Robert et al. 1998, Gimnig et al. 2001).Knowing the soil substrate is most probably of

critical importancein the Anopheles larval habitat. Since the majorityof Anopheleslarval habitats

are temporary due to seasonalrains, the only permanent part of this system is the soil inwhich

most of the biological and chemical components ofthe habitat can persist during the dry season.

1.1Objectives

1.1.1General Objective

- To review the role of physico-chemical and biological factors on the development and

distribution of anopheles mosquito larvae.

1.1.2 Specific Objectives

-To review the role of physico-chemical characteristics of mosquito larvae habitats on

anopheline mosquito larvae development and distribution

- To review the role ofbiological characteristics of mosquito larval habitats on anopheline larvae

development and distribution

2 Materials and Methods

We used journal articles, bulletins, books which are available online and PDF forms,

websitesand personal communication to gather relevant information.

2.1 Literature Search Strategy

This seminar paper was prepared by browsing different websites of pubmed using key words and

phrases such as “mosquito”, “malaria”, environmental factors”, “larval habitat”, “Anopheles

3

mosquito larvae”, ”biological factors”, Physico-chemical characterstics of larval habitats” , and

“mosquito ecology”

2.2 Selection Criteria

We have seen different works done in different parts of the world which are available on internet

as PDF; online which are related to this topic were selected .Then we selected the most relevant

information and pertinent ones.

3 Literature Review

Out of more than 400 described species of Anopheles (White, 1977) some 45 of them are

implicated in the transmission of malaria. Different species of Anophelesare responsible for the

transmission of malaria in specific geographic areas. The density of mosquito population is

dependent on larval ecology. Irrigation schemes, particularly those which used for growing rice,

are preferred breeding sites for An. gambiae s.l. and An. funestus. An.merus and An. melas have

extensive breeding sites within the tidal limits of coastal line (White, 1972, Bryan, 1983; Mbogo

et al., 2003). The malaria vectors play an important role in the transmission of P. falciparum

parasites. These vectors generally have high parasite inoculation rates and are also remarkably

stable in a wide range of bio-ecological and seasonal conditions hence appears to be very

flexible, both in exploiting new man-made environments and in their response to malaria control

activities (Coluzzi et al.,1984).

The adaptability to environmental changes leading to marked contrasts in vector bionomics has

led to the development of various levels of vectorial efficiency for populations of

Anophelesspecies in heterogeneous environments within the same locality and has thus become

important factor in determination of epidemiology of malaria (Toure et al., 1994). Environmental

heterogeneities have arisen mainly as a result of human activities which act as a means of

constant evolutionary challenge as they provide a source of environmental change to which

anthropophilic Anopheles have to respond by developing a highly dynamic vector-host

relationship (Mulla et al., 1990; Mutero et al., 2000).

4

3.1 Anopheles Mosquitoes Ecology, Behavior and Distribution

Several factors significantly affect the distribution of malaria in space and time, between

persons, and the resulting morbidity and mortality. Some of these factors include; the natural

environment through its vector populations, interaction between vector and parasite, parasite

determinants and some of its genetically controlled characteristics, host-biological factors,

behavioral, social and economic elements. Factors pertaining to the natural environment for

example, the availability of the larval habitats for malaria vectors influences the distribution of

malaria in the area. The local rainfall produces rain pools favored by most malaria vector species

for example An. gambiae s.s. and An. arabiensis. The slope of the land and the nature of the soil

are some of the other environmentally related factors, which affect the type of surface water

available and its persistence and subsequently the increase of local malaria vector populations.

The optimal range of temperature and the relative humidity for most malaria vectors is 20-30oC

and 70-80% respectively (Wernsdorfer & McGregor, 1986).

 In wetlands, the abundances of mosquito larvae are often limited by biotic factors, such as

predators and competitors (Blaustein&Karban 1990; Blaustein&Margalit 1996; Stav et al. 2000).

In addition, the importance of these biotic interactions varies depending on the type of wetland.

Wetlands can be divided into three classes’ temporary, permanent and semi- permanent based on

their probability of retaining standing water throughout the year; this in turn determines the types

of species that can live in those habitats and their interspecific interactions (Schneider & Frost

1996; Wellborn et al. 1996; Williams 1996). Permanent wetlands always retain standing water.

In these habitats, predators, including fish and many insects, can complete their life cycles and

reach very high densities. Thus, mosquito densities will be low in permanent wetlands as a result

of predation. Temporary wetlands are those that fill and dry every year. Although some predators

subsist in these temporary habitats (Spencer et al. 1999), most of the more efficient mosquito

predators for example fishes, large insects cannot live in these habitats because of their drying.

Alternatively, mosquito competitors, which can adapt to the predictable yearly drying of these

wetlands (e.g. zooplankton with resting eggs), are often quite dense in the absence of predators.

The number of mosquitoesemerging from temporary wetlands will be low in temporary habitats

5

as a result of strong competitive interactions. This effect could either be because of lower rates of

emergence if high competitor density slows the rate of larval development or because of higher

larval mortality and/or avoidance of oviposition by females in wetlands with high competitor

density.

Semi-permanent wetlands are those that retain standing water in most years, but periodically dry

when precipitation and the water table are particularly low.Jonathan et al (2003) in years prior to

a drying event, predatorswill be common, as in permanent wetlands, because thehabitat has

retained standing water for several seasons, allowing sufficient time for predator colonization. As

aresult, in most years, mosquito densities will be low in semi-permanent wetlands. In years

following a drying event, however, both efficient mosquito predatorsand mosquito competitors

will be rare, as neither group ofspecies are well adapted to drying events. Furthermore, because

mosquitoes have extremely rapid generation times (weeks) relative to their predators and can

readily disperseamong habitats that is mosquitoes should showrapid population increases in

semi-permanent wetlands inyears following a drought event.

The environmental management control methods practices that create unfavorable habitats for

larval breeding. It may also involve the elimination of aquatic habitats. A simple approach is to

fill with rubble, sand, and earth larval habitats of different sizes (Service, 1986). Other

environmental modifications include the removal of overhanging vegetation to reduce breeding

by shade loving mosquitoes such as, An. dirus(Service, 1986). Clearing of bushes can also

eliminate the malaria vector by removing adult mosquito resting habitats. Planting vegetation

along streams and reservoirs make habitats inimical to sun loving An. gambiae. However, this

approach has not achieved much because it is impossible, to fill in all the scattered, small and

temporary collections of water (Service, 1986). Secondly, the environmental changes such as

agricultural irrigation schemes, creation of dams for water reservoirs and road construction or

mining sites may favor the breeding of other species that were previously present in only small

numbers or absent altogether (Service, 1986). Besides, the approach is labor intensive and costly

thus untenable. There is, need to focus on more practical larval control methods.

6

The extent to which environmental heterogeneity affects patterns of vector production that are

important for malaria parasite transmission is unknown (Grillet, 2000). The factors affecting

larval survival and the mechanism controlling adult production are also largely unknown for

even most important vector species. A potentially important target of malaria vector is the

anopheline larva and source reduction through modification of larval habitats was the key to

malaria eradication efforts in the United States, Israel and Italy (Kitron & Spielman, 1989). It is

conceivable that appropriate management of larval habitats in sub Saharan countries particularly

during dry season may help suppress vector densities and malaria transmission (Minakawa et al.,

1999). The understanding of anopheline larval ecology is limited and insufficient to achieve

effective vector control through means of larval control (Oaks et al., 1991).

Since 1930 evidence has accrued that mosquito larvae could utilize dissolved nutrients

(Beklemishev, 1930). Filtered pond or infusion waters supported slow, larval development, the

best growth occurring in waters thought to contain colloidal materials (Hinman, 1932). However,

subsequent acceptance of the idea that mosquito larvae take very little dissipated interest in

dissolved nutrients as a possible natural food resources, even as the burgeoning use of artificial

diet showed that some drinking must be possible. Experiments with holidic diets demonstrated

that mosquito larvae could develop solely by taking dissolved nutrients (Dadd & Kleinjan,

1976). Actual uptake rates of two or three times the larval body weight per day were measured in

osmotic-balance studies of salt water mosquito species in which oral intake of water countered

osmotic loss through the cuticle (Clements, 1992).

Slightly reduced rates of larval imbibing in fresh water species for which the function of taking

in water would be primarily for nutrient ingestion (Aly & Dadd, 1989). That is, mosquito larvae

drink copiously, that drinking rate can be increased by the presence of dilute colloids, and that it

can be regulated independently of other mouth part activities that occur during feeding. Given

these findings, there are natural circumstances when dissolved nutrients, could be an important

food resource for mosquito larvae as recently suggested by (Wotton, 1990). For all suspension

feeders useful nutrients could come through drinking if concentrations of dissolved materials

were high enough. Higher concentration of dissolved organic material may occur adjacent to leaf

and substrate surfaces or near decaying tissue their associated precipitin and bacteria. Such zones

7

would be rich in gelled and colloidal solutions of macromolecular nutrients (Costern et al.,

1987). If such zones have a few percentages of balanced nutrients in solution, they could support

mosquito growth. Howland, (1930) concluded that the abundance of algae in the larval food was

correlated with algal abundance in the habitats and that culicines consumed more algae than

anophelines. The relationships between habitat selection and intrinsic chemical properties of

food and microhabitats have received much attention especially in phytophagus insects (Sota,

1993). For detritus feeders like larva of mosquitoes in aquatic habitats, the habitat water contains

both nutrients and deterrents (Parker, 1982; Fisher et al., 1990; Sota, 1993). Various chemical

properties of the larval habitat related to leaf litter such as pH, and concentration of ammonia,

nitrate and sulphate affect larval development and survival (Carpenter, 1982). Inorganic ions and

organic carbon sources such as nitrogen, phosphorus, sulphur, and carbon, which provide

essential nutritional substances for microbial growth.

3.2 Anopheles Mosquito larval control

A potential target of malaria control is the anopheline larva. This is because the life cycle can be

interrupted before the emergence of adults that bite and transmit malaria parasites. Source

reduction through modification of larval habitats was the key to malaria eradication efforts in the

United States, Italy and Israel (Kitron & Spielman, 1989). The classical method that has been

used to kill mosquito larvae involves the application of oil on water. The oil contains poisons

that presumably affect the nervous system (Wigglesworth, 1976). Anopheles larvae below such

film at 24ºC should be dead in 2 to 3 hours. The mosquito larvae may also die from suffocation,

the oil also reduces the surface tension hence the larvae cannot come out of the water for air.

However, the oil film on the water surface is likely to prevent free exchange of oxygen between

the water surface and the free air thus leading to suffocation of other non-target aquatic

organisms. This factor has prompted the employment of other means of controlling mosquito

larval populations. These methods include: biological control, environmental management,

natural organic larvicides and botanicals or use of plant materials.

8

3.2.1 Means of Biological Control

Biological control is defined as the action of predators, parasites (parasitoids) or pathogens in

maintaining thedensity of another organism at a lower average than would occur in their absence.

Orit is a method consists of the utilization of natural enemies of targeted mosquitoes and of

biological toxins to achieve effective vector management.Biological control of vectors is an

essential and effective means for controlling transmission of several mosquito-borne diseases

such as malaria, filariasis, etc. Chemical larviciding and biological control, particularly using

larvivorous fish, were important to malaria control programs in the early part of the 20th century,

particularly in urban and peri-urban areas (Gratz & Pal 1988).The advantages of biological larval

control agents in comparison with chemical controls can include their effectiveness at relatively

low doses, safety to humans and non-target wildlife, low cost of production in some cases, and

the lower risk of resistance development (Yap 1985).

The use of predatory fish that feed on mosquito larvae was one of the old suggested methods

forcontrolling vector diseases at the larval stages.Predatory fish that eat mosquito larvae,

particularly in the family Cyprinodontidae, have been used for mosquito control for at least 100

years (Meisch 1985).Prior to the 1970s, the most commonly used species was the mosquito fish

Gambusia affinis, a freshwater species native to the southeastern United States.This species was

introduced widely around the world. The practice has since been discouraged as the efficacy is

highly variable and the negative impacts on native fauna of this voracious and aggressive fish

have been quite significant (WHO 1982). The introduction of Gambusia affinis has actually led

to the elimination of native fish from certain habitats (Rupp 1996). More recently, researchers

have evaluated native fish species to identify appropriate local biological control agents. In spite

of widespread recommendations for the use of fish and extensive laboratory data, reports of

controlled field experiments evaluating the effectiveness of larvivorous fish in reducing malaria

transmission are fairly limited.

In rural areas, fish may be appropriate components of malaria control if breeding sites are well

known and limited in number, but use of fish may be less feasible where natural breeding sites

are extremely numerous. Fish may be particularly useful in controlling malaria vectors

9

associated with rice fields (Lacey & Lacey 1990). This practice has proved effective under

certain conditions in California (Blaustein & Karban 1985).

Therefore, tominimize the loss of native species and reduce the variability in effectiveness of

larval control amongdifferent aquatic environments, many pre-application studies were done to

establish the most suitablefish-habitat model. Many studies showed that fish are also highly

effective when the mosquito breeding sites are restricted in number and are well defined.

The two main factors determining the efficacy of the fish are the suitability of the fish species to

the water bodies where the vector species breed and the ability of the fish to eat enough larvae to

significantly reduce the number of infective bites that people receive. The first factor is best

addressed by finding a native fish species that thrives under the conditions present in breeding

sites rather than to change breeding sites to suit the fish, although Wu et al. (1991) recommended

a ditch-ridge system for rice fields to better accommodate the fish. Also, the use of pesticides and

fertilizers can negatively impact fish stocked in irrigated fields (Lacey & Lacey 1990). The

second factor may be strongly influenced by aquatic vegetation, which can interfere with fish

feeding and can also provide refuge for the mosquito larvae. Periodic vegetation removal may be

needed to facilitate the activity of the fish (Dua & Sharma 1994).

In an urban area in Ethiopia, Fletcher et al. (1992) found that the indigenous fish, Aphanius

dispar, effectively suppressed An. culicifacies adanensis breeding in wells and containers

although the experimental design did not allow the researchers to assess the impact on malaria

transmission. Near the Ethiopia-Somalia border, the same researchers observed a locally

developed initiative to control container-breeding malaria vectors, using the indigenous fish

Oreochromis spilurus spilurus (Teklehaimanot et al. 1993).

Other biological control agents include the use ofbacterial agents, fungi, parasites, viruses and

nematodes in controlling the malaria vector. Evaluating the effectiveness of these approaches is

based on two major criteria. It is how efficient the control agent can be in substantially

decreasing the rate of vector transmission and to what extent can this tool be evolutionary

sustainable.

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Two different species of bacteria of the genus Bacillus, B. thuringiensisisraelensis (Bti) and B.

sphaericus (Bs), have been widely demonstrated to be effective larvicides against both

anopheline and other mosquito species. Both Bti and Bs function as stomach poisons in the

mosquito larva midgut. Since the discovery of the mosquito larvicidal activity of Bti spores

(serotype H-14) in 1977, different formulations of Bti have been found effective against larvae of

many mosquito species, including the malaria vectors An. albimanus, An. sinensis, An.

culcifacies, An. sundaicus, An. stephensi, An. gambiae, An. arabiensis, and An. maculatus

(Becker & Margalit 1993; Lacey & Lacey 1990).

3.2.2 Environmental ModificationandManagement

Environmental management involves physical changes to the mosquito larval breeding habitat,

but mosquito suppression can also be achieved through treating the breeding sites directly with

chemical or biological agents that kill the larvae.

Environmental modification involves a physical change (often long term) to potential mosquito

breeding areas designed to prevent, eliminate, or reduce vector habitat. Theprincipal methods of

achieving these changes include drainage, land leveling, and filling (WHO, 1982). Draining

operations include the creation of ditches or drains to keep water moving and to carry the water

used as breeding sites away in a managed way. Drains may be lined or unlined and located at the

surface or subsoil level. Insome instances, marshes have been drained through pumping (Takken

et al. 1990).

As an alternative to complete elimination of wetlands, modification projects could involve the

creation of channels to improve water flow in areas of standing water, filling small ponds or

water-collecting depressions, or changing the banks of water impoundments to reduce mosquito

populations. As rivers and streams can create anopheline larval breeding sites, particularly in

slow-moving pools with heavy vegetation, regarding streams and even straightening riverbanks

may reduce vector populations (Thevasagayam, 1985). Some of these activities require regular

maintenance, whereas others represent permanent changes to the landscape (which may require

substantial initial effort and expense). An important component to environmental modification

11

addresses problems of man-made vector breeding sites associated with water-holding structures

in mini-dams and small-scale irrigation projects. The creation of favorable vector habitat can

often be avoided through careful design (WHO, 1982).

Environmental manipulation refers to activities that reduce larval breeding sites of the vector

mosquito through temporary changes to the aquatic environment in which larvae develop. Water

management activities include changing water levels in reservoirs, flushing streams or canals,

providing intermittent irrigation to agricultural fields (particularly rice), flooding or temporarily

dewatering man-made or (where feasible) natural wetlands, and changing water salinity.

Manipulation of vegetation may also be useful. Planting water-intensive tree species, such as

Eucalyptus robusta, can reduce standing water in marshy areas (WHO, 1982).

3.2.3 Larviciding using Chemicals

Chemical or biological larviciding for the control of malaria vectors is feasible and effective only

when breeding sites are relatively few or are easily identified and treated. That is, it must be

possible to treat enough of the sites to have a significant impact on the adult mosquito population

and on subsequent malaria transmission. Therefore, some vector species such as An. stephensi,

whose larvae are generally restricted to man-made water containers in urban areas may be good

targets for such control methods. Larval control appears promising in urban areas generally,

given the high density of humans needing protection and the limited number of breeding sites.

For example, the vector An. gambiae is capable of breeding in small puddles of rainwater and

may not be controllable in rural areas, where the number of potential breeding sites is enormous.

However, under certain circumstances, populations of this species may concentrate in a few sites

(e.g., construction borrow pits) in urban areas, in which case larviciding may be feasible (Gratz

& Pal, 1988).

As chemical larval control involves application of insecticides to water bodies, contamination of

aquatic ecosystems is a serious problem. Early chemical larviciding programs, using products

such as petroleum oil, DDT, or Paris green, undoubtedly killed many aquatic organisms and may

have caused profound changes in certain ecosystems. Even today, the organophosphate

12

insecticide fenthion is still widely used in spite of its relatively high toxicity to non-target fauna

(Rozendaal, 1997). Even temephos (trade name Abate), which is not acutely toxic to mammals

(Ware, 1989), has been found to harm crabs, shrimp, and zooplankton, leading to the

recommendation in Florida that this chemical not be applied to environmentally sensitive areas

(FCCMC, 1997).

3.3 Physico-Chemical and Biological Characteristics of AnophelesMosquito Larval Habitats

Physical characteristics of larval habitats include water depth, turbidity, and presence of floating

and/oremergent vegetation, light/shadow and temperature. Abiotic/orphysical factors for

example water depth, water temperature, oxygen contentare usually marginally correlated with

larval occurrence. Dominant plant growth forms such as filamentous algae, cyanobacterial mats,

and submersed macrophytes showed the closest association with the larvae of particular

Anopheles species (Rejmankova et al., 1993).

Anopheles gambiae s.l. and An. funestus complex are the most important vectors of human

malaria in sub-Saharan Africa. Production of adults of An. gambiae s.l. occurs in small,

temporary, sunlit, turbid pools of water (Gimnig et al., 2001). Habitats are often created by

human or animal activity wherein larvae are found in small depressions such as foot or hoof

prints, the edges of bore holes and burrow pits, roadside puddles formed by tire tracks, irrigation

ditches and other artificial bodies of water (Gillies & De Meillon, 1968; White, 1972; Minakawa

et al., 1999; Gimnig et al., 2001).

The heterogeneity of soil characteristics is crucial and the role of larval habitat“quality” on the

anopheles mosquitoes (vector) dynamics and distribution (Pfaehler et.al 2006). Identifying

specific soil-relatedfactors underlying larval habitat productivity is a criticalstep towards

predicting how the aquatic habitat quality and associated spatio-temporal variability affects

vector population dynamics. The soil parameters such as total organic matter, organic carbon,

and total nitrogen have arelationship between mosquito developmentalvariables and adult body

13

size. Since organic carbon and organic matter are tightly linked, while nitrogen content depends

on organic matter quantityand the decomposition level of the soil (Singer & Munns,1996;

Sumner, 2000).Surface soils which have a low carbon/nitrogen (C/N) ratio ranging from 8.6 to

12.3, which is an indication of soils with a fast organicmatter turnover.

Larval development duration and adult body size decreasebut pupation rate increases when the

organic content of thesoil substrate increases. This shows there is a relation betweenlarval habitat

quality and mosquito response in terms ofdevelopment time and body size of adult mosquitoes

(Pfaehleret.al 2006) observed that An. gambiae and An. quadrimaculatus grow most rapidly in

habitats where a surface micro-layer (theair/water interface) is enriched with microorganisms

whichmay act as an important source of nutrition for Anopheleslarvae. Timmermann & Briegel

(1993) demonstratedthat larval crowding (i.e., reduced nutrient input perindividual) resulted in

an extended larval developmentalperiod, reduced pupation rate, and reduced adult body size.The

mosquito body size varies according to nutritional status of the breeding habitat.

Mortality during the development of the larval stages is very high. Various studies have reported

that only a small fraction (2–8%) of the larvae that hatched eventually survived to the adult stage

and attributed this to the presence or absence of predators, parasites, pathogens (Fillinger et.al,

2004; Service MW, 1971, 1973 and 1977) or cannibalism (Okogun GRA,2005). Other biotic

factors that may affect survival are predation by sibling species and other interactions between

sibling species (Schneider et.al, 2000).

The figure below shows the life cycle of Anopheles mosquitoes. The female Anophelesafter

mating and blood feeding lays some 50-200 small (1mm long) brown or blackish boat-shaped

eggs on the water surface. Anopheles eggs are white when freshly laid; they turn to brown, then

black respectively as they mature. Viable eggs hatch into larvae within 2-3 days in the tropics,

but in cooler temperate regions they may not hatch until after 4 -7 days or longer (Service, 1980).

The larvae, while on the water surface, lie parallel to the surface to allow air intake and surface

feeding.

14

Figure 1.The life cycle of Anopheles Mosquito

(Source: http://www.cdc.gov/)

The pupal skin splits dorsally and the adult emerges. Careful movements are required to ensure

that the adult mosquito does not fall sideways and be trapped in the surface film. This danger is

particularly acute when the adult is largely out of the pupal exuviate but the terminal appendages

are still not free. Finally the legs become free and spread on the water surface giving stability.

The newly emerged adult inflates its wings, and separates and grooms its head appendages

before flying away (Kettle, 1992). When the progeny of any one egg batch emerge as adults the

males emerge first. The males become ready formatting within 24 hours after emergence such

that by the time the females emerge, the males are competent for mating. Mating is often

preceded or accompanied by swarming in which the males associate over a marker and fly in a

particular manner. Most of the male mosquitoes usually die after mating. The females require a

blood meal for ovarian development, followed by the maturation and oviposition of a batch of

eggs (Gillies, 1955).The percentage of the eggs that form the adults is unknown, but there is

usually heavy mortality, especially among larvae due to predators, disease, drought, and

15

flushing. Larval loss due to predation is one of the factors that reduce the numbers of larvae that

develop into adults.

Tire tracks, Irrigation water

Figure 2. Diverse habitatofAnopheles MosquitoLarvae

(Source: http://www.cdc.gov/malaria/)

According to CDC (2013) report the larvae occur in a wide range of habitats but most species

prefer clean, unpolluted water. Larvae of Anopheles mosquitoes have been found in fresh or salt

water marshes, mangrove swamps, rice fields, and grassy ditches, the edges of streams and

rivers, and small, temporary rain pools. Many species prefer habitats with vegetation. Others

prefer habitats that have none. Some breed in open, sun-lit pools while others are found only in

shaded breeding sites in forests. A few species breed in tree holes or the leaf axils of some plants.

Eighty percent of Anopheles feed on any large mammal (Gillies, 1972). The host preference by a

particular species of mosquitoes is also likely to be influenced by environmental conditions.

Some of the mosquitoes are strictly zoophilic while others are anthropophilic (taking its blood

meal from humans). Of the three species of An. gambiae complex, An. arabiensis and An. merus

are partially zoophilic (taking its blood meal from animals) and partially endophilic (White,

1974).Anopheles gambiae s.s.is primarily endophilic and endophagic whereas An. arabiensis and

An. merusshow some degree of partial exophily and zoophagy (White, 1974;Coluzzi et al.,

1984).Blood feeding by anopheline mosquitoes is essential for transmitting malaria parasites and

characteristic of this behavior can have major implication for the epidemiology of disease.

16

4 Summery

This seminar paper addresses the role of physico- chemical and biological factors on the larval

habitat of Anopheles mosquitoes and malaria transmition. The abiotic factors like turbidity,

temperature, conductivity, pH, and depth wasteland are strongly associated with Anopheles

larval abundance.

The review of this seminar is a useful tool to help in identifying different physic-chemical and

biological characteristics affecting the larval habitat of Anopheles mosquitoes and their control

methods. Knowledge of the ecological characteristics ofthe breeding habitats and the

environmental factors affecting mosquito abundance can help in designing optimal vector control

strategies.Larvae of Anopheles mosquitoes have been found in fresh or salt-water marshes,

mangrove swamps, rice fields, and grassy ditches, the edges of streams and rivers, and small,

temporary rain pools. Many species prefer habitats with vegetation. Others prefer habitats that

have none. Some breed in open, sun-lit pools while others are found only in shaded breeding

sites in forests. A few species breed in tree holes or the leaf axils of some plants.

17

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