In or out? Exploiting citizen science to explore urban mosquito indoor host-seeking behaviour

Introduction and objectives: Traditional mosquito surveillance in Europe largely relies on outdoor sampling, leaving a significant gap in understanding the indoor behaviour of species of public health relevance. In this study, we focused on two major arbovirus vectors in Europe, the native Culex pipiens, typically exhibiting endophagic behaviour, and the invasive Aedes albopictus, considered as predominantly esophagic in its native range. We explored potential differences in their indoor and outdoor host-seeking activity of the two species by analysing georeferenced photographic records submitted by citizen scientists through the Mosquito Alert (MA) project in Italy and Spain. Materials and methods: Mosquito photographic records collected from Italy and Spain and validated by MA experts were classified as taken indoors or outdoors, based on information reported by the citizen scientist while uploading the record in MA application for smartphones. Relative frequencies of Ae. albopictus and Culex spp. (treated here as Cx. pipiens due to the negligible occurrence of other species of the same genus) were analysed using a binomial regression Model. Specifically, the probability of a record being classified as indoors (success) was modeled as a function of mosquito species, biogeographical region, and time of the year. Potential country-level differences and sampling effort variations were accounted for. A random effect for year was included to control for temporal variation. Results, Discussion and Conclusion: A total of 12,706 georeferenced photographic records of adult mosquitoes were collected in Italy and Spain between June to October of three consecutive years (2021-2023), 37% of which outdoors and 63% indoors. Notably, considering data from both countries combined, Ae. albopictus accounted for 95% and 64% of the outdoor- and indoor-records, respectively. The Model results consistently indicated a higher likelihood of recording Ae. albopictus outdoors than Culex spp., aligning with the known hostseeking behaviours of the species. Notably, models also suggest that the likelihood of recording Ae. albopictus indoors increases over time, from approximately 50% in June to nearly 75% in October, suggesting a relevant unexpected Ae. albopictus indoor host-seeking activity. In contrast, Culex spp. showed a consistently high probability of indoor records across all biogeographical regions and the entire sampling period, likely reflecting the stable endophagic behavior and adult diapause typical of Cx. pipiens. These findings, if confirmed, may have public health implications (as they highlight need of personal protection also in the indoor environments) and emphasize the power of integration of citizen science data into traditional monitoring frameworks to study mosquitos’ features whose characterization has so far been constrained by the limitation of conventional monitoring approaches.