A hidden biomass flow - 10,000 tons of insects fly over Switzerland

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. Significance StatementInsect abundance is of great importance for most terrestrial food chains, including human agriculture. While ground-based studies have elucidated many relationships within trophic systems, very little is known about the effects of their large-scale movements through the airspace. We show how insect movements, dispersal and migration, are spatially and temporarily correlated across a wide area of complex terrain and provide a rough estimate of the insect biomass involved. Temperature is a crucial factor in the extent of activity; however, our results show that low temperatures (<10{degrees}C) do not prevent massive insect migrations when crossing mountain ranges. Our results demonstrate the large, coordinated flow of insect biomass across the landscape, which needs to be studied in much more detail to better understand the spread of pests and beneficial insects, their impact on local communities, and the transmission of diseases.