Insects are perhaps the most remarkable of migrants. They move in their trillions and are known to travel thousands of kilometres using celestial cues and wind patterns to guide and power their migration. However, despite the impressive nature of this phenomenon, very little is known (when compared to knowledge of other migrants such as birds) about exactly which insect species migrate, how many they number, where they are going, and the ecological roles insect migrants play.
This thesis focuses on Europe, studying the assemblages and quantifying the numbers of migratory insects moving along European flyways. Firstly, I consolidate the existing research on the poorly understood but likely ecologically vital world of Dipteran migration.
Secondly, I present empirical work on the springtime insect migrations in Europe. I study the assemblages and number of insects migrating to Cyprus from the Middle East and add further detail to the Eastern European insect flyway. Additionally, in Cyprus I study the impacts of insect mortality while on migration. Springtime orientation behaviour in hoverflies was also studied in Cornwall and the Isles of Scilly, UK.
Finally, the latter sections of the thesis are empirical chapters on the autumn migration of insects in Western Europe. Four years of study took place in the Pyrenees focussing on diurnal assemblages, numbers, and meteorological predictors of insect migration in the region. Additionally, the nocturnal insect assemblage was monitored alongside the interactions with their bat co-migrants.
The research carried out in this thesis reveals that the Diptera dominate migratory assemblages (86% in Cyprus, 89% in the Pyrenees). Meteorological conditions of temperature and wind direction are highly important for the occurrence of insect migration at the migratory hotspots of Cyprus and the Pyrenees, albeit for different reasons in each site. The same meteorological conditions were shown to predict nocturnal insect migration in the Pyrenees. The activity of migratory and sedentary bat species was shown to be closely correlated with numbers of migratory nocturnal insect migrants in the Pyrenees. Environmental conditions and vegetation growth levels in source locations were shown to have major consequences for the numbers of insects emerging in the springtime, and hoverflies were shown to use wind directions and an innate compass sense (in the absence of wind) to orientate northwards while on spring migration. Mortality was found to differ amongst the migratory insect assemblage with high mortality in beetles during long-distance sea crossings, however evidence of butterfly and hoverfly mortality highlights the cost of migration for all taxa.
Collectively, this work increases our knowledge of insect migration in Europe. Baselines have been created for future comparison studies, crucial for understanding the future impacts of climate change on these most remarkable of migrants.