Tucson Audubon Society
Mosquito Abatement at the Sweetwater Wetlands
By Bruce Prior, Hydrologist, Tucson Water

NEWS: Sweetwater Wetlands closure this week
Tucson Audubon has learned that Sweetwater Wetlands in Tucson will be closed Thursday March 6 for the annual controlled burn. The burn mainly will affect the western polishing basin (also known as Towhee, Overlook, Cottonwood and Shoveler Ponds), but the entire wetland area must be closed for safety purposes. Every year a portion of the wetlands are dried and burned to reduce mosquito breeding habitat. The wetlands should reopen as usual on March 7.

For some time after March 7, the burned western ponds will be cordoned off while a liner is put down in some areas. The liner will keep bulrushes from growing in some areas, again reducing mosquito breeding and perhaps also creating more shallow water areas for wading birds. By mid March more information will be available about when water will begin flowing back into the western portion of the Wetlands.

None of this process, except the March 6 burn, will affect access to the eastern polishing basin (including Phoebe Tank and Hidden, Willow, Gazebo, Island, and Keyhole Ponds). Good views of most of the southern settling basins should remain available.

Applications of mosquito larvicide probably will begin again in mid-March, closing the wetlands each Monday during the early morning hours. The wetlands are usually open again by 8 a.m. on those days. This lasts throughout the warm months.

This information comes from Bruce Prior, who runs Sweetwater Wetlands for Tucson Water. He can be contacted if you want further information:

Bruce M. Prior
office (520) 791-2689
voice mail (520) 791-5080/1403
cell (520) 349-5492
fax (520) 791-3293
Bruce.Prior@tucsonaz.gov

Many southern Arizona birders are familiar with the Sweetwater Wetlands. Most, however, are unfamiliar with the Mosquito Abatement Program there and how it has evolved since the summer of 1998. As Arizona’s second West Nile Virus season approaches, it is important for those who enjoy the Sweetwater Wetlands to be familiar with the status of the mosquito control program.

Argo amphibious vehicle at Sweetwater Wetland
Argo amphibious vehicle
(courtesy Bruce Prior)

Controlled burn at Sweetwater
(courtesy Bruce Prior)

The facility is comprised of approximately 18 acres of constructed wetland ponds that include broad areas of shallow water planted with cattail and various species of bulrush. The Wetlands are home to blackbirds, ducks, sparrows, and some warblers. They are also home to mosquitoes that are potential carriers of the Western Equine and St. Louis encephalitis viruses, which have been in Arizona for decades, and West Nile Virus (WNV), which officially arrived in Arizona in 2003. The dominant wetland mosquito (Culex tarsalis) is rarely seen since it is a nocturnal feeder. During the hot days, the insects rest in the dense vegetation where they benefit from higher humidity and shady conditions.

Since people, birds, and mosquitoes are in close proximity at the Sweetwater Wetlands, the City manages the mosquito population to minimize public health risk through an Integrated Pest Management (IPM) program that is vigilant and flexible. The IPM program includes 1) weekly adult mosquito surveillance, 2) weekly applications of low-toxicity larvicides, which kill only mosquito larva while allowing their abundant, aquatic, natural predators to thrive, and 3) annual habitat/vegetation control. The program is not static and continues to evolve due to changing technologies and new products.

In order to be most effective, an IPM program should include regular target-pest surveillance. Rather than perform labor-intensive larval monitoring, Tucson Water staff focuses its resources on weekly adult mosquito surveillance to provide the best gauge for success of the mosquito control program. Carbon dioxide traps, baited with dry ice, use a battery-powered fan to collect unharmed mosquitoes in an attached net. The traps are set at the same locations each time and operated overnight. These sampling events are known as "trap nights." UA Entomology Department staff analyze the trapped mosquitoes and provide total numbers and species identifications. The population data from 1998 to 2004, averaged for each month and shown graphically in Figure 1, is the basis for program management decisions.

Staff have relied on various larvicide application technologies, such as a remote controlled helicopter (1998-2002), a truck-mounted slurry sprayer (2003-present), and a tracked, aquatic watercraft (2004) to distribute granular larvicide over the vegetation. Mosquito larvae consume the granules of biological larvicide (Bti), which dissolves at the pH of the larval mid-gut. The Bti is, therefore, non-toxic to the many beneficial aquatic arthropods that feed on mosquito larvae. Additionally, during periods of higher trap counts when winged adults must be knocked down, the city contracts to have the wetlands treated using a truck-mounted, Ultra Low Volume fogger. The liquid pesticide is a low-toxicity, synthetic pyrethroid (Anvil 2+2) applied at a rate of approximately one ounce per acre. This pesticide is approved by the U.S. Environmental Protection Agency for use in aquatic environments.

Regular visitors are familiar with how the older vegetation becomes well thatched. This summer is a particularly good time to see how strong winds can blow over the taller species of bulrush. The vegetation then dies back in the winter to form mats that block the granular larvicides from contacting the water. Therefore, the Water Department partners with the Tucson Fire Department to annually burn the thickest, most overgrown areas of bulrush and cattail. TFD handles this operation as "controlled burn" training for their Wild Fires teams and is, therefore, able to assure that no more than one third of the vegetation is burned each year. The annual burns must be done in late February to early March when the vegetation is dry and the nesting activity is low.