Mosquitoes are common inhabitants of natural wetlands, and their presence at approximately the same density in FWS constructed wetlands is to be expected. Mosquitoes should not be a concern for SSF wetlands as long as the system is properly operated with the water level maintained below the top of the bed. Mosquito control is more difficult in polluted waters with a high organic content, as might exist near the inlet end of a FWS wetland. Insecticide doses might have to be at least double the normal amount in this portion of the FWS wetland. Gambusia fish provide effective control during warm weather conditions. An annual restocking of these fish may be necessary in cold climates with low winter temperatures.
The FWS wetland in Arcata, California, successfully used both Gambusia fish and a pupaecide (Altosid®) for mosquito control (Gearheart, et al., 1983). Bacterial insecticides (Bacillus thuringiensis israelensis and Bacillus sphaericus) have been used successfully on a number of wetland systems. The use of Bacillus thuringiensis israelensis was recommended (Tennessen, 1993) for use after trials with several insecticides at wetland systems in Kentucky. The side slopes of the containing dikes should be as steep as possible and any vegetation on these surfaces controlled. The presence of duckweed may also contribute to mosquito control by covering the water surface, but this will also interfere with oxygen transfer from the atmosphere.
At the Sacramento County demonstration constructed wetlands, a comprehensive management plan was developed (Williams et al., 1996). Effective mosquito control was managed over the 5-year project using:
• Vegetation control to reduce stagnant areas of excessive plant growth or lodging of tall plants
• Mosquitofish stocking (2 lb/cell) and deep zones to allow overwintering of the fish
• Regular sampling and analysis of larvae production
• Applications of Bacillus sphaericus or Bacillus thuringiensis israelen-sis whenever the larvae count exceeded 0.1 larvae per dip
No outbreak of adult mosquitoes was detected when this management plan was followed. These strategies have been verified recently (Knight et al., 2003).
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