Evaluation of vegetation management strategies for controlling mosquitoes in a southern California constructed wetland.

The abundance of mosquito larvae and adult production were measured in 3 vegetation treatments and 2 species of emergent macrophytes in replicated wetland mesocosms (12 x 80 m). During the 8-wk study, no significant differences were found in abundances of larvae and emerging adult mosquitoes among the vegetation treatments: 100% of the surface area in emergent vegetation, 50% of the surface area in emergent vegetation in 5-m-wide rows, and 50% of the surface area in emergent vegetation in 10-m-wide rows. Mosquito larvae (predominantly Culex tarsalis and Anopheles hermsi) were significantly more abundant in inundated bulrush (Schoenoplectus californicus) than in inundated cattail (Typha sp.). Adult emergence from vegetated zones containing bulrush also was significantly greater than from cattail. The failure of reduced emergent vegetation coverage to provide a significant reduction in mosquito production from the vegetated zones of the wetlands might have been caused by favorable conditions for mosquito oviposition and larval development after vegetation management and by the ineffectiveness of mosquito predators in emergent vegetation. A 50% reduction of vegetation did not significantly reduce the water quality of the wetland effluent; however, narrower rows (<5 m wide) of vegetation may be required to reduce mosquito production from vegetated regions of the treatment wetlands. Even though the abundance of mosquito larvae in open water is typically less than in emergent vegetation, creation of open-water zones in shallow treatment wetlands (<1 m depth) by drying the wetland followed by removal of emergent vegetation with heavy equipment is unlikely to provide a significant long-term reduction of mosquito production.

Effectiveness of control measures against mosquitoes at a constructed wetland in southern California.

The effectiveness of larvicide and adulticide treatments against mosquitoes at a constructed wetland in San Jacinto, California was assessed with larval surveys, trapping of emerging adults, and collections of host-seeking females by carbon dioxide-baited traps. Bacillus thuringiensis var. israelensis (Bti, Bactimos pellets) applied at a rate of 19 kg/ha did not demonstrably affect Culex larval and emergent adult populations. Larval populations in the seven marshes of the wetland decreased from approximately one third-fourth instar larva/dip to undetectable levels following two applications of Bacillus sphaericus (Vectolex CG) at a rate of either 19 or 23.6 kg/ha. The largest decline in the number of adult mosquitoes emerging per day from vegetated regions of the wetland occurred after B. sphaericus treatments. The Culex erythrothorax host-seeking population declined about 80-fold during September beginning three weeks after the first treatment with B. sphaericus; however, the Culex tarsalis host-seeking population did not decline abruptly until mid-October 1997. This result suggests that immigration of females from other developmental sites might be an important factor influencing the Cx. tarsalis host-seeking population at the wetlands. Safety concerns required that insecticide applications were carried out during daylight hours, and two daytime applications of adulticide (Pyrenone) in early August were ineffective against mosquitoes resting in the thick vegetation.