(Natural & Constructed)
|Mosquito production & management|
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Mosquito breeding in freshwater wetlands can be a complex problem and must be considered on a case by case basis. Mosquito management in such sites should aim to minimise population numbers, or reduce them to a tolerable or nonthreatening level. However, different species, various types of habitat and a range of environmental conditions will have to be addressed.
Natural freshwater wetlands (swamps and marshes) provide habitats for a diverse range of mosquito species, including some potential pests and vectors (carriers) of pathogens. They are often not productive of large numbers of mosquitoes, because predators and other factors exert some control, but they must be carefully assessed individually for control requirements.
Constructed (artificial) wetlands, built to manage and treat urban storm and/or wastewater, come in various formats; they may start out as simple vegetated pools but can develop into dense swamps. They have the potential to be more productive of mosquitoes than their natural counterparts, and must be carefully assessed for mosquito productivity and management.
There are many general aspects that are common for natural and constructed wetlands with respect to mosquitoes, but constructed wetlands provide more opportunities for mosquito management than many natural habitats. Therefore, more detail on conditions favouring mosquitoes and options for their management is included below under 'constructed wetlands'.
Swamps and marshes
Freshwater swamps and marshes are often less important as substantial producers of pest or vector mosquitoes than might be imagined, because predators and other factors such as depth and wind-driven wave action in open areas, exert a degree of control over the mosquito larval stages. Nonetheless, these habitats can be ecologically diverse and provide opportunities for many species of mosquito in different parts of Australia, and some species may be produced at nuisance pest level.
Additionally, the freshwater swamp/marsh often provides important habitat for vertebrates which may be reservoir hosts for mosquito-borne disease pathogens, such as wading birds and the arbovirus Murray Valley encephalitis (MVE) virus that causes potentially fatal infections in humans, or kangaroos/wallabies and Ross River virus that causes a debilitating arthritis.
A large number of mosquito species are associated with these wetlands in different parts of Australia, but there are some which are of relatively greater concern depending on the region and circumstances.
Mosquitoes that can transmit malaria, e.g. Anopheles bancrofti and An. farauti in northern Australia, and An. annulipes in northern and southern Australia, can be produced in large numbers from vegetated habitats. In these habitats, Anopheles larvae can find refuge amongst surface algae where they are protected. An. annulipes rarely attacks humans in large numbers but has been responsible for malaria transmission in southern Australia, An. bancrofti can be a major pest near heavily vegetated wetlands and An. farauti is the most serious malaria threat.
Culex annulirostris is often maintained at relatively low levels by predation in many established wetlands, but it is the major vector species for some important pathogens such as Murray Valley encephalitis and Ross River viruses and, even at low population levels, is a cause for concern.
Mansonia uniformis, and Coquillettidia xanthogaster and Cq. linealis, are biologically associated with vegetation and can be produced in large numbers from heavily vegetated sites. These species are important nuisance pests and potential arbovirus vectors in some areas.
Various Aedes species, of greater or lesser importance as pests or disease vectors, can also be associated with freshwater wetlands but the species vary greatly with region and conditions.
Control of mosquitoes in such habitats can be very difficult. Drainage or filling is rarely an option because of the direct and indirect values of the habitat to the general environment, and opportunities to modify the habitat to make it less suitable for mosquitoes are similarly likely to be limited. The use of biorational pesticides can be an option, but the location, size and nature of such habitats, often restrict the application of control agents even when approval to use the agents might be forthcoming from the various authorities.
'Artificial' wetlands are being increasingly constructed to control and treat stormwater and wastewater. They can be similar to natural wetlands at various stages of their development, and can provide permanent habitat for vertebrate reservoirs and mosquito vectors of disease similar in scope and scale to natural wetlands of equivalent size. This issue can be of greater concern in otherwise dry areas where such opportunities have not existed previously, appear only seasonally, or have been eliminated with development.
Compared with natural wetlands, constructed wetlands offer various opportunities to manage the mosquito populations they produce. Overall mosquito management is best achieved with a composite approach, integrating various complementary methods. The wetlands can be designed to make them less favourable for mosquito colonisation, the water and vegetation components can be manipulated to regulate mosquito development, and chemical and biological agents can be used to reduce otherwise uncontrollable populations.
Although mosquito management principles are often incompatible with objectives and operations of constructed wetlands, the health issues are not insignificant. Engineers and other professionals associated with wetland construction should be aware of the various requirements and opportunities for mosquito management within the framework of their objectives for water control and treatment.
Ideally, constructed wetlands should be located away from the community and beyond the flight range of the important local mosquito species. The flight range (dispersal) of mosquitoes varies with species, from less than a hundred metres to tens of kilometres.
Additionally, to minimise mosquito colonisation, constructed wetlands should be sited in open areas where wind action produces surface waves which disrupt larval respiration and inhibit the growth of algae and floating plants that provide protection.
Constructed wetlands contain various zones. Gross pollutant traps and sedimentation zones must be managed to prevent blockages and buildup in pollution which will promote stagnant water breeding. Rock flume riffle zones dissipate energy and prevent scouring; they are also useful in maintaining the integrity of the edges and in oxygenating the water, and low flows can infiltrate the porous ramp and thus reduce pooling. A weir at the downstream end of the wetland can maintain habitat for macrophytes and a reservoir for larval predators.
Sedimentation areas intended to be 'deep', and vegetated treatment zones intended to be 'shallow', provide different opportunities for mosquitoes. Shallow vegetated water typically supports more mosquito breeding; deep pools with steep and deep edges, and no emergent or surface vegetation, provide less suitable habitat for mosquitoes.
Any wetland area, constructed as a shallow vegetated pond, will be a major concern for mosquito breeding. Ponds with simple shapes and a low edge to area ratio are likely to be less productive than those with a greater proportion of edge to area in more intricate designs.
Concrete vertical edges are ideal but these are usually not acceptable for aesthetic reasons. Steep earthen edges or grassy banks are often unacceptable for public safety reasons. The safety issue is becoming more important, but there has to be a recognition of the local mosquito concerns and a compromise considered.
With larger areas, profiling the bottom with the depth greatest at the inflow end is advisable. Higher numbers of mosquitoes can be associated with smaller rather than larger basins, but if the site is too large it may become a shallow marsh supporting large numbers of mosquitoes.
Water quality is an important consideration. Stormwater and rural runoff, not heavily polluted with organic material, may create less problem than sewage or wastewater with greater nutrients for vegetation and mosquitoes. Additionally, predators are less likely to survive in heavily polluted water and pesticides are less effective.
Species inhabiting wetlands may change with an increase in pollution. Additionally, incoming water that is high in chemicals may kill vegetation and predators, and possibly the current mosquito population, but mosquitoes are likely to reestablish more quickly than the predators.
Fluctuations in the water level can be detrimental to some mosquitoes but can promote others. Periodic draining and flooding can effectively interrupt mosquito production. Prescribed times for drying of wetlands must be decided with a view to local mosquito species, vegetation types, seasonal factors, and perhaps other considerations.
Maintaining water movement through the wetland is very helpful in reducing mosquito populations. Riffle zones provide turbulence which is detrimental to larvae, and serves to raise oxygen levels and improve water quality.
Other water management techniques that can be used against mosquito populations include aeration systems that reduce larval survival by disturbing the water surface, and sprinkler systems that inhibit mosquito egg laying. Again, the target species is a critical consideration.
The factors determining mosquito populations in vegetated habitats are complex and vary from site to site and throughout a season. Vegetation protects larvae from physical disturbance and predators, and enhances food resources. Wetlands that do not support vegetation generally do not support large mosquito populations.
Floating vegetation that does not completely cover the water surface may support a diverse mosquito fauna, but mosquito oviposition can be inhibited by floating plants covering the surface - again, the local mosquito species of greatest concern must be considered.
Emergent vegetation, at the margins or in deeper zones, will have areas of exposed water where plants have died or fallen; these 'pockets' of water will be protected from wind and fish, but open to sunlight and thus attractive to mosquitoes.
Dense stands of emergent vegetation produce fewer mosquitoes if periodic harvesting or culling of plants can be undertaken to provide increased water movement and predator access. Chemical control of vegetation may be useful for edge treatments, and may incidentally provide a short term kill of mosquito larvae, but is likely to have a major negative impact on predator populations and in the longer term result in more mosquitoes.
The importance of maintaining clean and deep margins is paramount and the following methods can be useful for reducing mosquito populations:
1. removal of marginal and floating vegetation and debris;
2. maintenance of the integrity and slope of margins;
3. maintenance of depth, by dredging as required;
4. provision for drainage or water level fluctuation to strand vegetation and larvae;
5. installation of sprinkler systems to inhibit adult mosquito oviposition;
6. installation of aeration systems to make surface conditions unsuitable for larvae.
Control agent options
Suppression of mosquitoes may be further achieved by applying pesticides and/or introducing predators to complement or augment the above-mentioned measures.
The principal advantage of chemical control methods is that pesticides can be quickly applied with rapid results at relatively low cost. However, chemical usage should not be viewed as a long term strategy, and should be resorted to only when there are occasional episodes of heavy uncontrolled breeding. Prolonged use will lead to development of resistance in mosquito populations, thereby limiting overall management options. Relatively few chemicals can be recommended for use in wetlands, whether natural or constructed (which usually flow into natural water systems), because of environmental concerns.
An additional concern for wastewater wetlands is that most insecticides are relatively ineffective in highly polluted waters. While less selective pesticides might be required for emergency control, and where a particular situation is not environmentally sensitive, insecticide doses for waters with a high organic content might have to be at least double the recommended rates.
The agents recommended currently are:
1. The organophosphate, temephos, is relatively target specific for mosquitoes and generally suitable for freshwater wetlands. Above recommended dosage rates it can be highly toxic for many invertebrates; it has low mammalian toxicity, but is moderately to highly toxic for fish and birds. Recommended application rates must be followed. Weekly use may be required during summer months, and its effectiveness can be reduced to a few days in polluted or colloidal waters.
2. Commercial products of the bacteria Bacillus thuringiensis israelensis (B.t.i.) and Bacillus sphaericus (B.s.) (the latter is not yet available in Australia). These should not be thought of as biological agents; they are not 'live' organisms but toxic products of a bacterial culture, and don't 'recycle' in the environment. They are environmentally acceptable because of the relative specificity for mosquitoes amongst invertebrates and negligible toxicity for vertebrates. However, B.t.i. has little persistence and mosquito populations can rebound in 1-2 weeks. B.s. has been developed to target Culex species in polluted waters where B.t.i. is ineffective, and B.s. can persist longer and provide control for possibly more than 3 weeks.
3. The insect growth regulator, methoprene, a juvenile hormone mimic that is environmentally benign because of its relative specificity for mosquitoes (at Jan 1998 the agent is not yet registered for use throughout Australia, although it has been widely used in Queensland and also in other states). Larvae are not killed, but adult emergence from the pupal stage is prevented. Slow-release formulations can provide control over some months.
Overall, the effectiveness of the various 'acceptable' agents depends on appropriate formulations and local conditions, and the target mosquito species is of critical concern.
Although, other aquatic insects such as dragonfly nymphs, and various beetles and bugs, can reduce mosquito larval populations to some extent, these predators can rarely be relied upon for adequate control. The habit of some bats and birds to feed on mosquito adults has been exploited at some constructed wetlands by providing roosting or nesting boxes nearby, but it is generally considered they are not particularly useful in effectively reducing large numbers.
Larvivorous fish are the only biological agents available for practical use in wetlands,
and these can be a valuable component of an
integrated control program. Because of undesirable impacts on native fauna by the
introduced 'mosquito-fish' Gambusia
holbrooki, indigenous fish species (such as Hypseliotris spp. and Pseudomugil spp.) should be given
priority in mosquito control programmes but the exotic Gambusia may be difficult to
exclude from wetlands.
Mosquito control in freshwater wetlands is a difficult and complex issue, but control through a management approach is feasible. Notwithstanding engineering objectives for constructed wetlands, the priority guidelines for mosquito management can be listed as:
1. Wetlands should be deep. The perimeter should be regular, free of vegetation and debris, and steeply sloping.
2. Wetlands that are necessarily shallow should be free of vegetation and edges should be steeply sloping. The interior of the pond should be graded for rapid dewatering.
3. Wetlands that are necessarily shallow and vegetated should have structures to provide for flooding and draining. Pools of deep water should be provided for fish survival.
4. Wetlands that are necessarily shallow and vegetated and cannot be periodically drained, should be maintained with substantial populations of predatory fish.
5. Wetlands which cannot be managed with structural, environmental or biological methods should be treated as required with a recommended control agent.
6. Wetlands and the areas surrounding them, whatever above or other measures are implemented, should be subject to periodic surveillance for mosquito species and abundance using appropriate sampling techniques at an appropriate frequency.
In conclusion, the issue of mosquito production by freshwater wetlands, whether natural or constructed, should be considered on a case by case basis. Expert advice on the potential hazards, nuisance values and health risks, and the options for minimising the mosquito populations, should be sought and carefully considered.
For further information and advice on this topic, including risk assessment and details of management options, please contact Prof. Richard Russell.
<http://www.anca.gov.au/environm/wetlands/wettype.htm> (National Wetlands Program - Wetland Types).
<http://www.sainty.com.au/> (Waterplants and Wetlands - Sainty and Associates Pty. Ltd.)
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