Liechti, F.; Haest, B.; Bauer, S.

Flying insects are the most diverse and abundant group of animals using the airspace to travel and spread across regions. Through movements that can span multiple life stages and generations, aerial insect migrations shape the ecosystems where they emerge but also connect ecosystems across sometimes vast distances. Yet even basic data on the abundance of aerial insects is still scarce. Their small sizes and often high-altitude movements make it challenging to describe flyways or measure this transport of biomass within or between regions. We used a remote-sensing approach with vertical looking radars to quantify insect migration across Switzerland. Switzerland harbours complex terrain, with the Alps, Jura mountains, and pre-Alpine lowlands featuring starkly contrasting topography. At three locations within these three main habitats, we recorded a total of 6.6 million medium- to large-sized individual insects during the 8-month study period. Extrapolating this to the whole of Switzerland results in an estimated number of 21 billion insects. Adding the presumed additional proportion of small insects, this translates to a rough estimate of 10,000 tons of insect biomass across Switzerland per year. Our results show that insect migration remains substantial even in topographically complex landscapes and seems strikingly synchronized over a relatively wide area. Mountain passes in particular act as key conduits for insects traversing the Alps, with many crossings occurring at high altitudes and with air temperatures below 10 {degrees}C. Nevertheless, our study is only a first step unraveling insect movements across space and time and their large-scale synchronicity.