Sydney University Department of Medical Entomology Westmead Hospital
Saltwater Wetlands
(Mangrove Swamps & Saltmarshes)
Mosquito production & management

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Introduction

Mangrove swamp (click on image to enlarge)

Mangrove Swamp

Mangrove swamps and saltmarshes are often perceived to present a mosquito 'problem', and to be the source of pest populations plaguing nearby residential areas. It is true that there are often pest mosquitoes in and near the various parts of saline wetlands, but the 'problem' is rarely simple and can reflect a very complex situation, with a number of mosquito species involved.

The principal problem species associated with these saline wetlands are Aedes vigilax, Aedes camptorhynchus, Aedes alternans, Anopheles hilli, and Culex sitiens - but their presence and abundance varies with type of wetland, geographic region of Australia, and the temporal and spatial conditions at the site.


The saline habitat

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Flooded Saltmarsh

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Stagnant pool left after flooding

Mosquitoes in well drained saline wetlands are principally found in the areas behind the mangrove communities, on what is called the 'high' marsh, where pools of water in mudflats or saltmarsh vegetation are left by the highest tides (spring tides) of each month, or are filled by rainfall/runoff, and are not flushed by the daily tide movements during the weeks thereafter. In wetlands that are not well drained, mosquitoes are also able to exploit impounded 'stagnant' pools retained within stands of mangroves, and other vegetation on the 'low' marsh, caused by siltation or other blockage of the normal tidal channels and thus not subject to the normal daily flushing.

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Egg of Aedes vigilax on Sarcocornia

Different mosquitoes in the overall saline wetland 'habitat' exploit different 'sub-habitats'. Aedes vigilax, Aedes camptorhynchus and Aedes alternans are associated with the temporary pools created by spring tides or rainfall on the high marsh. Females lay their eggs on drying soil and the base of plants at the edge of depressions on the high marsh, and at the edge of impounded pools in the mangroves. These eggs dry and become dormant; they hatch weeks to months (perhaps years) later when submerged by tide or rain water filling the depressions.

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Larvae breeding in pool

This occurs typically when the highest tides of each month flood the 'high' marsh. When the tides recede, depressions are left filled with water and eggs hatch to mosquito larvae; subsequent high (but lower) tides do not flush out the pools. The larvae which hatch are thus typically in a relatively isolated pool which is protected from predators and flushing. However, it is temporary in nature and may dry out within a week or less, and the species have adapted to develop quickly as larvae and emerge as adults before the water disappears and they perish. Development time is dependent on temperature and food, but generally there is sufficient food and temperature is the principal influence - the higher the temperature, the quicker the development, and sooner the emergence. In summer, larval development in shallow sunlit pools on the high marsh can be as quick as 4-5 days in northern regions.

Other species, such as Culex sitiens and Anopheles hilli, are associated principally with more permanent pools. The females lay eggs that float on the surface of a pool of water; if the pool dries up, the eggs will desiccate and perish, and thus the association with persisting habitats. In saline wetlands, Culex sitiens and Anopheles hilli are found typically in larger marsh pools and impoundments within the mangroves. They may be found with Aedes vigilax, but usually because there is some rise and fall of the pool water level to allow the Aedes eggs to be deposited on drying edges that will later be submerged. Larval development of Culex sitiens and Anopheles hilli is usually a little slower than for Aedes vigilax, often because of the lower temperatures associated with their generally deeper pools, but in northern summers development often takes little more than a week.


The mosquito concern

Of the above-mentioned species, Aedes vigilax is the foremost pest species for most of coastal Australia, with the exception of Victoria, Tasmania, and the far southwest of Western Australia; Aedes camptorhynchus is the major pest in these areas. Both mosquitoes can disperse from a few to many kilometres from the wetlands; indeed Aedes vigilax has been recorded as dispersing more than 50km with wind assistance. Aedes alternans can be a particular pest in certain areas of New South Wales, and also creates problems elsewhere, but it does not appear to disperse much beyond a few kilometres. Anopheles hilli is a northern tropical species that can be a secondary problem in the Northern Territory, and parts of Queensland and northern WA. Culex sitiens is found from mid-NSW around the northern coast to mid-WA, and can be a local secondary pest but it usually does not disperse far from its emergence site.

At least two of the mosquitoes mentioned above, are important vectors (carriers) of arboviruses, particularly Ross River (RR) and Barmah Forest (BF) viruses which can be responsible for a debilitating arthritis that may last months to years. Aedes vigilax and Aedes camptorhynchus are the most important coastal vectors of RR virus in northern and southern Australia, respectively. Culex sitiens is a possible secondary vector of RR virus. Aedes alternans has been associated with RR virus but is thought to be not an important vector. Anopheles hilli is a potential vector of malaria for northern Australia.

Because there is no drug protection/treatment or preventive vaccine available against RR or BF viruses, the reduction of infection risk is best achieved by reducing risk of contact with the vector mosquitoes. This can be done through personal protection measures such as use of protective clothing and topical repellents, and avoidance of saline wetlands during dusk, evening and dawn hours. However, because of the location and scale of habitat often involved, there is usually little that can be done by the individual householder to reduce the threat from saline wetlands mosquitoes. Government authorities, however, can provide for a reduced pest nuisance and lowered risk of infection in their communities by managing the mosquito populations in the wetlands with appropriate methodologies.


The management strategies

There are various options available to reduce mosquito populations in saline wetlands. Habitats can be 'eliminated' (by draining or filling), 'modified' (with water management), or 'treated' (with a control agent to kill the mosquito larvae). The choice is usually determined by a consideration of the target mosquito species, the nature of the habitat, operational factors, and various external environmental and political imperatives.

Proposals for the complete elimination (through draining or filling) of major saline habitats are usually both undesirable and untenable. Modification through impoundment is unlikely to be suitable for many areas as it can have undesirable environmental impacts, but modification to manage water movement, through what is called Open Marsh Water Management (OMWM) or the use of shallow ditches (runnels), with or without the accompaniment of larvicides, can be an acceptable, practical and effective approach. Simply, an environmental management approach should be the core strategy, with judicious use of biorational control agents as necessary during the 'development' phase of the environmental modification process and perhaps thereafter.


Environmental options

Saline wetlands can be 'modified' by restoring full tidal flushing to the mangroves and enabling natural dewatering of the saltmarshes following tidal and rainfall inundation. The former can be achieved by renovating tidal channels and maintaining them in a condition which allows full tidal exchange and precludes the formation of impounded pools within the mangroves. The latter can be most simply achieved through OMWM whereby marsh pools that hold water after highest tides or rainfall are connected for tidal influence using various sized channels and with persisting ponds to support predatory fish, or through runnelling whereby narrow shallow ditches provide tidal influence to the pools. 

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A 'Runnel'

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An effective Runnel

Runnelling modifications generally act against all mosquito species exploiting the marsh pools. OMWM modifications are suitable for reduction of Aedes species, because they eliminate ground surfaces which are alternately wet and dry (which is required for Aedes egg deposition, conditioning and hatching), but they do provide habitat for Culex and Anopheles species unless they support significant numbers of predatory fish.

Mosquito control with OMWM and runnelling is realised through the tidal circulation providing a flushing of the breeding depressions, and also by providing access to the marsh for larvivorous fish. Runnelling can raise rather than lower the water table as occurs with OMWM, and can better maintain the integrity of high marsh (although it may become wetter and more characteristic of low marsh), while OMWM can result in significant changes in vegetation type.

The implementation of either of these habitat modifications will require expert advice on site suitability and construction strategy. Permits from relevant authorities (such as State Departments of Fisheries, Environment, and Planning) will also be required, and these will vary between states.

One important consideration for habitat modification proposals is the possible presence of acid sulphate water/soils in the habitat. These conditions often support mosquito populations in coastal regions and can also produce additional saline habitats for these coastal species in non tidal areas. Runnelling and channelling in areas likely to have acid sulphate influences should not be undertaken without consulting appropriate experts.


Biological options

Predatory fish are the only appropriate and 'available' biological control agent, but there is little reliable information on their effectiveness and little experience in their management for mosquito control in Australia. Some saline habitats could benefit from the introduction of the 'blue-eye' fish (Pseudomugil signifer) which has shown promise in a few areas but, in principle, the successful restoration of tidal flushing should allow a range of potential predators to enter the habitat on a regular basis and reduce mosquito populations without any specific management.


Chemical options

For control of larvae, there is essentially a choice of only two agents:
(i) a commercial product of the bacterium Bacillus thuringiensis israelensis which is relatively specific for mosquitoes and kills larvae when ingested, and;
(ii) an insect hormone analogue called methoprene which is a growth retardant relatively specific for mosquitoes and which prevents the larvae completing development to the adult.

These two products are termed 'biorational agents' as they target aspects of the mosquito biology without impacting unduly on non-target species and the environment in general. The organophosphate pesticide temephos, although relatively specific for mosquitoes and widely used in freshwater wetlands where it kills mosquito larvae by contact, is not suitable for use in saline wetlands because it has detrimental effects on development of some crustaceans.

Bacillus thuringiensis israelensis (Bti) can be useful in both saline and freshwater habitats. There are various commercial products available, in liquid and solid formulations that are variously appropriate for different habitats. The duration of effect against larvae is seldom more than 2 or 3 days, and the agent has only limited effectiveness in polluted water. Because it needs to be ingested, it must be applied within the first few days of the mosquito's aquatic life cycle as it cannot affect mature larvae which have ceased feeding nor the non feeding pupa. Thus, there is a relatively narrow 'window of opportunity' for it to be effectively applied, and often this is limited to the three or four days following the inundation of the marsh depressions. Therefore, timely and accurate surveillance of the habitat and the mosquito populations is usually required.

Methoprene is a growth regulator, and a commercial product has been used extensively in freshwater habitats in the USA and elsewhere for many years, but it is also effective in saline situations. In Australia it has been used on a limited scale against saltmarsh mosquitoes in Queensland. It is available as liquid and sand granule formulations, and also as slow-release granules, pellets and briquettes, and all formulations are now registered for use in Australia. The product must be applied to the larval populations as it will not affect pupae. Larvae will continue to develop after the application but will eventually die in the pupal stage and no adults will be produced. However, because it does not kill larvae, its effectiveness is often difficult to evaluate in the field.

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Aerial spraying
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Backpack spraying

Application of chemical control agents over large areas of saltmarsh can be most effectively undertaken from the air by fixed wing aircraft or helicopters. With experienced pilots using DGPS technology for accurate tracking, and specific delivery systems calibrated for correct dosing, comprehensive coverage and effective treatment of the target habitats can be achieved. Small scale application of control agents to isolated pools on marsh areas, where these cannot be modified to preclude larval 'breeding' (e.g. because of elevation or slope), or cannot be overflown by aircraft (e.g. because of structures or cables), can be undertaken with relatively simple hand-held or backpack applicators (that also must be calibrated to avoid overdosing of the product and any adverse impacts on non-target species which is particularly important in estuarine areas that can be critical fish and crustacean nurseries).


Summary

Mosquito control in saline wetlands ( mangroves and saltmarshes) can be complex, and is usually beyond the capacity of an individual. Notwithstanding the difficulties associated with persuading the various levels of government to undertake mosquito management in these sensitive habitats, the general principles for mosquito control in mangrove and saltmarsh areas can be listed as:

1. Tidal flushing within stands of mangroves should be maintained so that stagnant impounded water does not provide mosquito habitat.

2. Natural dewatering of the surface of the saltmarsh should be maintained so that water does not persist in depressions filled by the highest monthly tides.

3. Structural management (channelling/runnelling) of water flow through the mangroves and onto and off the marsh, providing for natural flushing and dewatering, and access for predators, can be effective in reducing mosquito populations and can be environmentally acceptable.

4. Use of biorational control agents, such as bacterial products and growth regulators, to reduce mosquito populations can be effective and environmentally acceptable.

However, and in conclusion, the issue of mosquito production from saline wetlands must be considered on a case by case basis. Expert advice on the relative nuisance values and health risks, and the acceptable and effective options appropriate for particular sites should be sought from relevant experts and carefully considered.

 

For further information and advice on this topic, including risk assessments of particular sites and further details of management options, please contact Prof. Richard Russell.

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