UC Riverside

Culex tarsalis, the western encephalitis mosquito, predominantly uses large water bodies as oviposition sites. These water bodies are home not only to macroinvertebrate predators, but larger predators such as fish. The western mosquitofish, Gambusia affinis, has been used worldwide for the control of larval mosquito larvae for several decades.. My research demonstrates that Cx. tarsalis can detect the presence of semiochemicals associated with the western mosquitofish, Gambusia affinis, without the fish being physically present, leading to a decrease in oviposition rate.

Oviposition onto fish-conditioned water was reduced relative to that in control treatments in bioassays using different densities of fish to condition water, in no-choice assays comparing mosquito oviposition on fish-conditioned water with aged tap water controls, and in bioassays using aged tap as well as pond water against Gambusia-associated semiochemicals at different concentrations to simulate “real-world” conditions. Assays with individual females provided responses to fish-conditioned water that were congruous with the results from binary choice experiments using multiple gravid females in replicate cages. Mosquito oviposition onto fish-conditioned water that had been filter-sterilized was reduced relative to filter-sterilized control water suggesting that the semiochemical was still present after bacteria were removed. Female Cx. tarsalis were deterred from ovipositing in the majority of trials conducted, and the response appears to be correlated with the concentration of Gambusia-associated semiochemicals present in oviposition sites.

I identified two non-volatile and three volatile chemical compounds in water in which G. affinis had been held using a variety of chemistry techniques. Oviposition bioassays were conducted using standards of the three volatile compounds identified: dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS) and S-methyl methanethiosulphonate to determine their role in mosquito oviposition behavior. The non-volatile compounds isolated from the Gambusia-treated water could not be fully characterized, but one appears to contain a ring structure similar to that of cholesterol. Female Cx. tarsalis were deterred from ovipositing into sites containing solid phase extracts of Gambusia-treated water. This may indicate that it is the non-volatile compounds that the females are detecting, leading to a decrease in oviposition rate.