Wetlands are valuable habitats that provide important social, economic, and ecological services such as flood control, water quality improvement, carbon sequestration, pollutant removal, and primary/secondary production export to terrestrial and aquatic food chains. They represent important habitats for a large number of animal and plant species, some of which are threatened or endangered. Wetlands protect adjacent habitats from erosive forces, and have important flood control and water storage functions. Wetlands also have high aesthetic and recreational value which makes neighboring areas highly desirable for human habitation. However, wetlands are also natural producers of mosquitoes and this sometimes creates conflicts with human neighbors. Most frequently, wetlands mosquito production is a “nuisance” issue, affecting the quality of life of nearby residents by instigating generally undesired behaviors. Examples include postponement or cancellation of pleasurable activities, outdoor recreation; irritating mosquito bites and extracurricular activities. Large biting mosquito populations can sometimes also have social, cultural, and economic impacts by limiting community activities.

Mosquitoes can have grave health impacts on the population at large when mosquito-transmitted pathogens such as Westnile virus, eastern equine encephalitis virus, Plasmodium spp. etc., are being amplified and transmitted locally. Mosquitoes can also routinely have serious health impacts on individuals with allergies to mosquito bites. Similarly, mosquitoes can also have significant health impacts on wildlife, livestock, and pets including wild birds, cattle, dogs, and horses.

Mosquitoes are holometabolous insects that occupy two distinct environments within their life cycle. Flying adults are highly mobile and tend to be widely dispersed. In contrast, larvae and pupae are confined to their aquatic breeding habitat, the type of which may be highly specific or not depending mainly on species requirements and oviposition strategy. The location of breeding sites, resting places, blood and nectar sources, coupled with various landscape components such as land cover, hydrologic networks and vegetation height and density may influence mosquito patterns of movement and behavior, ultimately affecting their spatial distribution.

Wetlands provide abundant and diverse habitats for aquatic insects including pest and disease vector species, and they have traditionally been a neglected area of ecological research, particularly regarding mosquitoes. Recent studies in the Parana´ Lower Delta, Brazil, revealed that species composition, richness and diversity of the mosquito community of surface water habitats depend on the environment at the micro and meso scale.

Mosquito Ecology:

Mosquitoes (Phylum: Arthropoda; Class: Insecta; Order: Diptera; Family: Culicidae) genera important to human disease transmission include Aedes, Culex and Anopheles. Each species has distinct habitat requirements, and there are differences between each species. The general pattern of mosquito development is from egg to hatch through four larval instars, to pupation and then emergence as adults. The timeframe for a complete cycle may be as short as five days in tropical and subtropical environments and there are differences between species in their development times.

All mosquito species have an intimate relationship with wetlands. Water is an essential requirement for the larval stages. There are many researchers worked on mosquito ecology. An early reference work is that of Lounibos et al (1985) covering community and population dynamics, ecology, epidemiology and also the role of genetics in the life strategies of the insect mainly mosquito species. Similarly, Service (1993) also worked on the ecology, sampling and modelling of mosquito populations in wetland areas.

In wetlands, the abundances of mosquito larvae are often limited by biotic factors, such as predators and competitors. In addition, the importance of these biotic interactions varies depending on the type of wetlands. That 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.

The eggs are laid in water in rafts of multiple eggs (Culex spp.), on water singly (Anopheles spp.) or singly on damp substrate that will later be flooded (Aedes spp.). In water, the eggs hatch into larvae that go through four instar stages. During this stage they feed on small organisms or decaying material. They breathe air through a siphon that is at or protrudes through the water surface, or, in some species, they attach to plant stems and obtain oxygen directly from the plant tissue. After the fourth instar they pupate and then emerge as adult flying insects. In some species the newly emerged adult female may be able to lay fertile eggs but generally a blood meal is required for protein to produce eggs. For disease vectors the pathogen is picked up during the blood meal and, if it replicates within the mosquito, it may be transmitted later to a victim during another blood meal. In some cases diseases may be passed on directly from the adult female via the egg to the larva and hence to the emerging adult. This is known as vertical transmission. From a wetland perspective it is important to identify the habitats of the immature stages (eggs and larvae) as these habitats are usually wetlands and the focus of larval, and hence wetland, management.

Oviposition sites

The oviposition sites are is an aid to identifying larval habitats as larval survey may miss some of these, such as ephemeral sites when they are dry. Mosquitoes that have received much attention include the aedine nuisance species or vectors of viruses. Eggshells  of Aedes spp. are good indicators of oviposition and can be used to sample at times when larvae are not abundant or are absent, as eggshells are relatively stable both spatially and temporally. Eggshell studies have been carried out for open vegetation such as salt marsh and mangrove forest systems etc.

Mosquito larval habitats

The element common to all mosquito species is the need for water for the larval and pupal stages. Knowing the association between wetlands and mosquitoes is important for mosquito-borne diseases it helps to focus management. Many researchers found that the association between landscape features including elevation and vegetation (mangroves) and malaria vectors in Mexico. Studies from wetland in Iowa, USA shows a significant relationships between immature mosquitoes and habitat characteristics such as water quality etc., (Mercer et al., 2005). Similarly they also identify microhabitats associated with mosquitoes and they used these to estimate the risk of disease transmission. Most of the published research on larval habitats of Aedes taeniorhunchus in mangrove forests only. Although not focusing on larvae, there have been compilations of mosquito species recorded, for instance, in Indian mangrove systems. A detailed report of Mali shows that, malaria vectors found in dry season habitats provided a refuge for mosquito populations and suggested that focusing larval control on those would reduce the wet season populations. Researchers showed that Anopheles Gambiae larvae can survive in wet mud as well as in water and that this explained their survival in an uncertain environment.

Remote Sensing (RS) and Geographic Information Systems (GIS) technologies can help in speed up the process and survey the large areas with cost effectively. These technologies used for disease vector mosquitoes population in different geographic regions. Similarly, researchers also used aerial photographs to identify ephemeral wetlands in urban subtropical areas of Australia; GIS also used in to estimate the spatial distribution of anopheline larval habitats in the highlands of West Kenya, taking into account terrain, land use and surface water. Several studies have mapped the larval habitats of vectors of West Nile virus (WNV) in USA. Spatial models also developed for showing that various mosquito vectors had clear habitat preferences were able to distinguish larval habitats in Wyoming (USA) using Land sat imagery.

The Drawback associated with wetlands: Mosquito borne diseases

The bellow table-4 lists the major disease causing organisms, together with their invertebrate hosts, that are linked to wetlands in Africa region. It is interesting to note that, in addition to malaria, mosquitoes are the vectors of pathogens causing several other diseases. There are 22 mosquito borne viruses have been isolated in Southern Africa, and of these 10 are known to be human pathogens. Four of them (Chikungunya, Sindbis, West Nile and Rift Valley fever) cause serious illness. One of the most pathogenic arboviruses, dengue, is not endemic to Southern Africa but it is spreading in different parts of the world.

Mosquito Management In Wetlands:

Many studies showed that mosquitoes lay eggs in the wetlands and these hatch into larvae, very few larvae are likely to emerge as adults due to predation of the early instars. There needs to be wider recognition that mosquito larvae are an integral component of wetland ecosystems, and providing ecosystem functioning is maintained, then predator–prey relationships will ensure the control of mosquito breeding.

Typha with an accumulation of submerged dead stems and isolated pockets of water are suitable for mosquito breeding. Similarly dense floating mats of Paspalum grass and Perscicaria are also suitable for mosquito breeding but of limited habitat value for many macro invertebrates due to the lack of swimming space and low dissolved oxygen. Mosquito larvae are surface breathers and can survive in anaerobic conditions, however many aquatic macro invertebrate predators are also surface breathers, e.g. notonectid bugs, water beetles, or surface predators, e.g. pond skaters. Predation is best avoided if the mosquito larvae can isolate themselves from predator access. Orr and Resh (1992) found that dense beds of Myriophyllum aquaticum were a primary habitat for Anopheles larvae where they survive in microhabitats. Walton (2002) noted that in the arid south western United States constructed treatment wetlands can increase mosquito production if there is poor water quality and dense coverage of submerged dead vegetation. An abundance of notonectids in the settlement pond at Rosewood probably accounted for low numbers of mosquito larvae. In addition to biological monitoring for vegetation, benthic macro invertebrates and fish, they also monitored mosquito larvae and pupae in the summer months from stations that were 70% vegetation cover.

Biological Control

There are several biological agents that have been shown to cause mortality in mosquitoes including algae, oomycetes, bacteria (Bti, Ls, and recombinant bacteria), microsporidia and gregarine parasites, pathogenic viruses, nematodes, predatory insects including other mosquitoes, copepods, fish and others will be utilized as biological control of mosquitoes.

Water Management

High water flow rates and volumetric turnover rates can negatively influence mosquito production, but the flow rates needed to directly impact mosquito populations are too high to be of use as a sole technique for mosquito control Indirectly, however, water flow and turnover rates can influence mosquito populations by reducing or increasing stagnant pools of organic-rich waters that are attractive to certain mosquito species, particularly several Culex species and by influencing water quality variables that are important for the survival of mosquito larval predators such as larvivorous fish and aquatic invertebrates. Various water manipulation techniques can also be used for vegetation management to eliminate some mosquito producing locations and to enhance predator access to others. In wetlands hydrology is intimately connected with a multitude of ecological processes including vegetation composition; primary production; salinity and oxygen regimes; nutrient cycling; microbial dynamics; sediment transport; plankton, benthos, and nekton composition and population dynamics and many others. Thus, artificial modification of wetlands hydrology should be undertaken with great care as it can severely impact wetland structure and function.

Vegetation Control

Vegetation management, as it relates to mosquito control, is undertaken to create open water areas that are unfavorable for mosquito development or resting habitats  and to increase predation pressure on mosquito larvae. This technique is most relevant to constructed wetlands, and is particularly important to consider during the design phase of the wetlands creation process. Vegetation provides food resources for mosquitoes in the form of plant detritus and also fosters the production of other mosquito food such as bacteria, algae, and protozoa.

CASE STUDY:

Mosquito diversity in Chilika Lake:-

In Orissa 12,266 wetlands have been delineated. In addition, 66,174 small wetlands (< 2.25 ha) have also been discerned. Total wetland area is estimated to be 6,90,904 ha. Inland wetlands dominated the extent of wetlands constituting about 66 per cent. Further, inland natural and man-made wetlands shared approximately similar extents with about 34 and 32 per cent of area under wetlands. Out of 24 per cent of coastal wetlands, the natural accounted for about 20 per cent and the rest 3 per cent is shared by man-made wetlands. The major wetland types are river/stream (223522 ha) comprising about 32 per cent of extent wetlands followed by reservoir/barrage (189972 ha), tank/pond (29301 ha), lagoon (89023 ha), intertidal mudflat (25514 ha) and mangrove (23395 ha) (Figure-1). There are large numbers of small wetlands are present in this state.

Fig-1: Wetland map of Orissa

In Orissa, mosquitoes play a major role as the carriers of various human and livestock diseases in the wetland ecosystems and they are well adapted to survive in various habitats such as ponds, puddles, tree holes, swamps and salt marshes. The hot and humid coastal climate, wide spread paddy fields, small water pools and drains provide favorable breeding grounds for the mosquitoes. Several studies on the mosquito fauna were conducted in the coastal areas of Orissa. The Anopheline vector species were more diverse than those of the Culicine vectors.

Various researchers reported 21 species of Culicines, Mansonia species and Anopheles sundaicus in the coastal areas of Orissa. Environmental changes have greatly affected the diversity and abundance of mosquito fauna of Chilika lake area, including the disappearance of some species. Mosquito vector diversity determines the prevalence of mosquito-borne diseases in an area. Conducive conditions for vector populations, coupled with the prevalence of pathogens, usually result in the transmission of mosquito-borne diseases.

Dash and Hazra reported that there was a higher diversity of the Culicines than Anophelines in Chilika lake area. The reduction of Anopheline diversity in and around Chilika may be associated with the major ecological changes, including the extensive use of insecticides, modified agricultural practices, increased industrial development, natural calamities and other factors. Similarly, filarial cases have been increasing in this area. This could be due to increasing diversity of the Culicine vectors in and around the lake. 

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