Knock‐knock, who is inside? Exploiting citizen science data to shed light on Aedes albopictus indoor presence in southern Europe

Animal behaviour in human‐dominated landscapes is difficult to study at large spatial and temporal scales, especially for small species characterised by elusive movements. Citizen science can help overcome these limitations by turning routine human–animal encounters into observations of where and when species are active. Here we exploited the Mosquito Alert citizen science platform to study the indoor versus outdoor presence of Aedes albopictus (generally considered as predominantly exophagic/exophilic) in contrast with sympatric Culex pipiens . We analysed 21,433 expert‐validated photographic records of Ae. albopictus and Culex spp. from Italy and Spain (2021–2024) classified by Mosquito Alert contributors as taken indoors or outdoors. We combined descriptive statistics with generalised linear and additive mixed models to (i) quantify species‐specific probabilities of indoor records and (ii) estimate the relative contribution of Ae. albopictus to indoor mosquito records. We complemented the citizen science results with two small‐scale indoors versus outdoors entomological collections carried out in Italy in August–September 2024. Overall, 56% and 91% of Ae. albopictus and Culex spp. records were classified as indoor, respectively. The modelled probability that an Ae. albopictus record referred to a specimen photographed indoors averaged 56.3% (95% CI: 49–63) and increased from 40% to 50% in late spring to ~60% in late autumn. In contrast, the probability of Culex spp. indoor records remained consistently >90%. Among indoor records, the probability that a photographed specimen was Ae. albopictus averaged 61.8% (95% CI: 58.7–64.9) and peaked at 73.4% (95% CI: 70.6–76.0) in midsummer. Results from the small‐scale field assessments show observed daily mean female Ae. albopictus indoor ratios of ~22% and ~27% in the two study sites. These results provide a first estimate of indoor presence of Ae. albopictus in its invasive range in Europe. Even accounting for the biases associated with this citizen science data, results show that the probability of coming into contact with Ae. albopictus indoors is not negligible, with relevant implications for epidemiological studies, vector control and public health. Moreover, the study highlights the potential of citizen science platforms to act as observatories for wildlife living in close association with humans, revealing behavioural patterns not assessable otherwise. Read the free Plain Language Summary for this article on the Journal blog.