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.