Search

The western mosquitofish, Gambusia affinis(Baird & Girard), has been used worldwide for the control of larval mosquitoes for more than 100 yr. We found that the western encephalitis mosquito, Culex tarsalisCoquillett (Diptera: Culicidae), can detect the presence of G. affinisin oviposition sites based on associated chemicals, leading to a decrease in the number of egg rafts laid. Three volatile chemical compounds were identified in the headspace above the water where G. affinishad been held for 24 h. Oviposition bioassays conducted using standards of the volatile compounds identified (dimethyl disulfide [DMDS], dimethyl trisulfide [DMTS], and S-methyl methanethiosulphonate) found that females reduced oviposition only when low concentrations of DMTS were present, but this response was not consistent across all trials and concentrations tested. DMDS, DMTS, and S-methyl methanethiosulphonate are known bacterial metabolic waste products and may be the source of the compounds. Two nonvolatile compounds of interest were found to be present in the Gambusia-exudate water. After tasting Cx. tarsaliswere deterred from ovipositing onto Gambusia-treated water from which the bacteria had been removed by filtration, indicating that the kairomone may consist of nonvolatile compound(s). One of the nonvolatile compounds isolated from the Gambusia-treated water has a benzene ring structure similar to that of cholesterol but the structure of the two nonvolatile deterrents remains to be fully characterized. Our research shows that three volatile compounds and two nonvolatile compounds are present in water associated with G. affinis(Poeciliidae: Gambusia) and affect the oviposition behavior of Cx. tarsalisin laboratory bioassays.

Considerable previous research has focused on predator-associated semiochemicals and how they affect mosquito oviposition behavior. However, most of this work has been done without taking into consideration either the natural density of the predators or how other semiochemicals in aquatic environments might affect the responses of gravid mosquitoes. The influence of mosquitofish density, source water (tap vs pond), presence of freshly laid egg rafts, and removal of a putative source of semiochemicals (bacteria) on oviposition by Culex tarsalisCoquillett (Diptera: Culicidae) was studied in laboratory bioassays. Culex tarsalisfemales were deterred from laying egg rafts on water that contained semiochemicals associated with Gambusia affinis(Baird & Girard), but this deterrence was not strongly associated with the density of fish used to condition aged tap water. The number of egg rafts laid onto Gambusia-exudate water made with either tap water (density < 1 fish per liter) or pond water was typically ≥ 50% of that onto water that did not house mosquitofish. Gravid mosquitoes tested individually did not reduce oviposition onto Gambusia-exudate water as compared to controls. Likewise, oviposition by females with ablated wings did not differ significantly between Gambusia-exudate water and controls. Oviposition onto filter-sterilized Gambusia-exudate water was reduced relative to unfiltered water, suggesting that semiochemicals deterring egg-laying were still present after bacteria were removed. Taken together, these findings suggest that the responses of gravid Cx. tarsalisto chemicals from habitats containing mosquitofish are complex and the origin of the semiochemicals present in the Gambusia-exudate water needs to be elucidated.

Ammonium nitrogen (NH4-N) is a significant component of municipal and agricultural wastewaters, and nitrogen reduction is an important use of constructed treatment wetlands. The effects of ammonium nitrogen enrichment on resources of larval mosquitoes, larval mosquito abundance, adult mosquito production, and the abundance of related wetland organisms were examined in 0.1-ha replicate treatment wetlands. The hypothesis of a bottom-up effect induced by ammonium addition was not supported by bacterial abundance, mean bacterial cell size, or algal biomass in the water column. There was, however, a significant negative correlation between bacterial cell length and Culex tarsalis Coquillett (Diptera: Culicidae) larval abundance 1 wk later in wetlands enriched with ammonium nitrogen. Larval mosquito (Culex spp.) abundance in southern California wetlands enriched with NH4-N (mean ≈3 mg/liter) was significantly greater than in control wetlands at ambient nitrogen levels (8.3 mg NO3-N/liter, 0.1 mg NH4-N/liter). Adult mosquito production was nine-fold greater and chironomid larvae were significantly more abundant in wetlands enriched with NH4-N than in controls but other censused taxa exhibited no significant trends. Mosquitofish, Gambusia affinis (Baird & Girard), abundance was significantly reduced in enriched wetlands, but other potential mosquito predators were not significantly affected by ammonium enrichment.

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 ≈0.5 km, and females were not recaptured farther than 2 km from the release site. Most marked individuals (=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.