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  • Mosquito Control - Freshwater

    Freshwater wetland management Environmental options

    The importance of maintaining clean and deep margins is paramount and the following methods can be useful for reducing mosquito populations:
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    Sprinkler system to prevent mosquito oviposition


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    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.

    Chemical agents
    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.

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    Control of mosquitoes using a backpack sprayer

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    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.

    Biological agents

    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,

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    Gambusia.-.great.to.control.mosquitoes, but.bad.for.the.environment


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    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.

    Summary

    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.

    Related Links
    <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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    http://medent.usyd.edu.au/fact/freshwet.htm#fresh
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