Mosquitoes are vectors of various pathogens that cause diseases in humans and animals. To prevent the outbreak of mosquito-borne diseases, it is essential to control vector populations, as treatment or vaccination for mosquito-borne diseases are often unavailable. Insect-specific viruses (ISVs) have previously been described as being potentially helpful against arboviral disease outbreaks. In this study, we present the first in vivo characterization of the ISV Culex Y virus (CYV). CYV was first isolated from free-living Culex pipiens mosquitoes in 2010; then, it was found in several mosquito cell lines in a further study in 2018. For mammalian cells, we were able to confirm that CYV does not replicate as it was previously described. Additionally, we found that CYV does not replicate in honey bees or locusts. However, we detected replication in the Culex pipiens biotype molestus, Aedes albopictus, and Drosophila melanogaster, thus indicating dipteran specificity. We detected significantly higher mortality in Culex pipiens biotype molestus males and Drosophila melanogaster, but not in Aedes albopictus and female Culex pipiens biotype molestus. CYV could not be transmitted transovarially to offspring, but we detected venereal transmission as well as CYV in mosquitos’ saliva, indicating that an oral route of infection would also be possible. CYV’s dipteran specificity, transmission routes, and killing effect with respect to Culex males may be used as powerful tools with which to destabilize arbovirus vector populations in the future.

Arbovirus control strategies often target vector reproductive dynamics, with insect-specific viruses (ISVs) like Culex Y virus (CYV) offering potential as eco-friendly approaches by influencing mosquito reproduction without affecting humans or animals. This study investigated the interplay between autogeny, anautogeny, nutrient availability, and viral infection and their effect on reproductive success in Culex pipiens biotype molestus and Culex pipiens quinquefasciatus. CYV infection had a minimal impact on mosquito reproductive parameters, even after a five-day incubation period. Autogeny enabled Cx. pipiens biotype molestus to produce eggs without blood meals, yet older females (3–5 days post-emergence) showed reduced reproductive success unrelated to nutrient deficiency, as blood-feeding did not restore their egg production. These findings demonstrate that age affects reproductive success in Cx. pipiens biotype molestus but not in Cx. pipiens quinquefasciatus and suggest that CYV has negligible direct effects on mosquito reproduction. This work enhances our understanding of ISV biology and vector ecology, supporting the development of innovative, sustainable arbovirus-control strategies.