Long-range pollen transport across the North Sea: Insights from migratory hoverflies landing on a remote oil rig.

Insect pollinators play a crucial role in the reproductive success of many plant species, with their movement patterns important in shaping gene flow across plant populations. Movements vary greatly from central place foragers that move over relatively short distances to migrants that move over very long distances. Of these highly mobile flower visitors, migratory hoverflies are of high ecological and economic importance as a large group of globally distributed pollinators, capable of connecting distant ecosystems through long-range seasonal movements. However, understanding their role in pollen transport during predominantly high-altitude migration remains challenging due to sampling constraints and the obscured identity of vectored pollen caused by post-migratory foraging from local resources. To address this, we employ ITS2 region metabarcoding to identify pollen species transported by migratory hoverflies during four distinct June or July migration events onto an oil rig devoid of vegetation and found 200 km off the coast of Scotland in the North Sea. Of 121 sampled marmalade hoverflies (Episyrphus balteatus), 92% carried pollen, with metabarcoding of 86 individuals indicating pollen from an average of up to eight plant species per individual (range: 1-14) and 102 species in total across all four events. Dominant pollen sources included common nettle (Urtica dioica), black elder (Sambucus nigra) and meadowsweet (Filipendula ulmaria) alongside visits to vegetable, legume, cereal, nut and fruit species. Backward wind trajectory analysis indicated northward migration in June, originating from the Netherlands, northern Germany and Denmark, over 500 km away. Conversely, migration in late July suggested southward movements from Norway, albeit with lower confidence. Forward trajectory analysis suggested potential destinations following departure from the oil rig including Norway or the Shetland Islands around 250 km away for the June migrations and Scotland for the July event. Our findings highlight the capacity of migratory hoverflies to transport diverse pollen species across extensive distances, underscoring their potential role in long-distance gene flow. Further research is essential to evaluate the ecological and agricultural implications of this phenomenon and its impact on plant communities.