Top-down effect of fish presence on mosquito assemblages in the city of Padova

Throughout history mosquitoes always represented a problem to humans for their ability to act as a vector for many diseases. Due to a combination of globalization and global warming new species of mosquitoes are expanding their geographic range posing a threat to people. In Italy tropical diseases such as West Nile (1998) and dengue (2020) are spreading. In particular in the province of Padova a total of 101 cases of West Nile were recorded in summer 2022. Also, concurrently, a mass fish die-off connected to drought conditions occurred in the city of Padova. To investigate a possible causal connection between the two phenomena, given that fish can be insectivorous, two on-field experiments were set up to explore the presence and extent of a top-down regulatory effect of fish on mosquito assemblages in the urban canals of Padova. The experiment in the wild failed in detecting mosquito larvae, but interesting insights were obtained from the controlled experiment, regarding the species assemblages (in particular, the presence of Culex pipiens was detected even in the city center, together with the expected Aedes albopictus) and their oviposition preference to different degrees of fish cue exposure. A total of 2771 mosquito larvae were collected through the three weeks of the experiment and 2073 of them were identified to the genus or species level. While Aedes individuals were found in all the locations, the Culex mosquitoes appear to be more common farther away from the city center, but where present they showed a marked preference for the water containing dead fish, where they were the most abundant species, but this preference faded as time passed. Conversely Aedes females avoided all kinds of fish cues, with a preference for the tap water environment which continued to be very productive for them throughout the study period. In conclusion it appears to be possible for the city canals to rapidly generate Culex mosquitoes outbreaks after an episode of mass fish death, causing potential public health emergencies in case of drought events and/or channel mismanagement.