Emergence patterns of Culex mosquitoes at an experimental constructed treatment wetland in southern California.

The emergence patterns of mosquitoes inhabiting a 0.1-ha experimental wetland in southern California were monitored using emergence traps during the late summer and autumn of 1996. Culex erythrothorax was the largest contributor to emerging populations, comprising 94% of the total emerged adults with an average emergence rate of 59 adults/day/m2. None of the Culex species exhibited a pattern of emergence associated with water depth (range: 5-60 cm). Culex quinquefasciatus and Cx. tarsalis did not show a pattern of emergence associated with the inflow-outflow gradient; however, Cx. erythrothorax emerged in higher numbers along a transect at the middle of the wetland than from near the inflow and outflow. Additionally, the number of emerged Cx. erythrothorax was positively correlated with the density of vegetation below emergence traps. The comparatively large number of adults emerging from the middle of the wetland was most likely caused by a trade-off between an increasing gradient of resource abundance and a decreasing gradient of toxic compounds from the inflow to the outflow of the small wetland.

Dispersal, survivorship, and host selection of Culex erythrothorax (Diptera: Culicidae) associated with a constructed wetland in southern California.

Three mark-recapture studies were carried out at a constructed wetlands facility in San Jacinto, CA, to examine the dispersal and population ecology of the most abundant host-seeking mosquito, Culex erythrothorax Dyar, collected in carbon dioxide-baited traps. Recapture rates were 0.3, 7.4, and 13.9% for August, September, and October, respectively. The mean distance traveled per night was approximately 0.5 km, and females were not recaptured farther than 2 km from the release site. Most marked individuals (> or = 99.5%) were recaptured within 0.5 km of the release point. Marked individuals were recaptured for 33 d after release. Horizontal estimates of survival calculated using recapture data were 0.89, 0.87, and 0.84/d for August, September, and October, respectively. Temporal differences in the recapture rate were attributed to the effects of blood meal acquisition on host-seeking activity versus effects of mortality and strong developmental site fidelity on weekly recapture rates. Partially engorged females collected by CO2-baited traps at the wetland fed predominantly on cattle indicating that host-seeking females were using hosts at dairies surrounding the wetland and were returning to the wetland for resting before seeking an additional blood meal. Estimates of the gonotrophic cycle length and survivorship (vertical estimates) were problematical because of the low parity rates for females collected by CO2-baited traps. Limited dispersal and long survival of Cx. erythrothorax are important factors in the development of large populations at constructed wetlands.

Effect of marsh design on the abundance of mosquitoes in experimental constructed wetlands in southern California.

The species composition and abundance of larval mosquitoes were studied in the vegetated regions of 2 types of experimental constructed wetlands: one-phase marshes, which have continuous vegetation throughout the marsh, and 3-phase marshes, which have 2 vegetated regions separated by a region of comparatively deeper open water. Larvae of Culex spp. were significantly more abundant in one-phase marshes than in 3-phase marshes. Larval populations in one-phase marshes also contained proportionately more older larval instars (stages III and IV) than did populations in 3-phase marshes. Mortality rates of larvae increased during the summer and were higher in 3-phase marshes than in one-phase marshes during the initial 6 wk of the study. Differences in mortality rates between marsh types were related to predator abundance during the first 6 wk of the study and thereafter were not strongly associated with predator populations. An infusion of decaying bulrush (Schoenoplectus californicus) stimulated oviposition by gravid Culex stigmatosoma more than by gravid Culex quinquefasciatus and Culex tarsalis. Culex erythrothorax was the most abundant host-seeking species collected in CO2-baited traps; however, larvae were rarely collected during routine dip sampling and egg rafts were never collected in oviposition studies.

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.