The following is an update to an article originally published in the January/February 2003 issue of Southwest Hydrology, written by Bruce Prior, Tucson Water.
(Click here for information on mosquito prevention and West Nile Virus.)
As the Sweetwater Wetlands has developed since construction began in 1996, the City of Tucson has significantly reduced the mosquito population at the site. The site includes 17.5 acres of constructed wetlands, 14 acres of recharge basins, and a recurring mosquito problem. The Sweetwater Wetlands was initially constructed to treat filter backwash water from the City's Reclaim Water Treatment Plant but also has the advantages of providing a wildlife habitat and public education opportunities.
The treatment plant, built in 1984, produces reclaimed water by filtering secondary effluent through granular silica/carbon filters. The filters plug up periodically and plant operators must backwash the filters to clean them. The Sweetwater Wetlands, constructed from 1996-1997, is designed to receive and treat the backwash water produced during this filter cleaning process. The backwash water, high in suspended solids, drains by gravity through settling basins and wetland ponds, and into adjacent recharge basins.
The wetland ponds contain areas of deep, open water 
alternating with zones of shallow water planted with bulrush and cattail. The shallow vegetated zones have, over the years, been prone to produce mosquitoes. The species of mosquito that proliferate in a wetland environment are potential carriers of encephalitis virus. Since the wetland brings together people, birds, and mosquitoes, creating a potential public health risk, the city has undertaken a rigorous mitigation program to control the mosquito population.
Water Department staff monitor mosquito populations weekly at the Sweetwater Wetlands by setting carbon dioxide traps. University of Arizona Entomology Department staff analyze the trap populations and provide species identifications and total numbers. From July 1998 through November 2002, the City contracted to have granular mosquito larvicide applied weekly to the water at the Sweetwater Wetlands using a remote-controlled helicopter. In 2003, Water Department staff switched from the remote-controlled helicopter to a ground-based, hydroseed sprayer for the application of the mosquito larvicide. The larvicide is mixed into a water-based slurry so it can be sprayed from a high-pressure water cannon over the dense aquatic vegetation. Although this method of application improves the amount of larvicide contacting the water along the pond edges, the hydroseed sprayer has proved unable to reach the central areas of the bulrush and cattail.
In 2004, staff contracted for the use of a Kawasaki ARGO to distribute additional larvicide out into the central areas of emergent vegetation. The ARGO is a "tracked Aquatic Watercraft" capable of entering the heavy vegetation and traversing across it. A mechanical spreader distributes the granular larvicide from the back of the vehicle. The combined use of the hydroseeder and the ARGO results in a more complete distribution of larvicide across the thick vegetation The larvicide is specific to mosquito larvae (it is designed to dissolve at the pH of the mosquitoes mid-gut) and is therefore non- toxic to the many beneficial aquatic insects present at the wetlands.
Additionally, during periods of higher trap counts, the city contracts to have the wetlands treated using a truck-mounted aerial fogger that produces an Ultra Low Volume (ULV) mist. The product applied is a synthetic pyrethroid (Anvil 2+2) with a very low toxicity that has been approved by the U.S. Environmental Protection Agency for use in aquatic environments. The mist droplets are micron-sized and the ULV application rate is less than one ounce per acre.
The graph below illustrates how the mosquito population has been brought under control by using these regular abatement procedures since the spring of 1998, when the wetland mosquito population was essentially untreated.
