Scientists Solve the Mystery of Why Thousands of Flies Stopped by an Oil Rig in North Sea
About 200 miles off the coast of Scotland, engineer Craig Hannah is busy coaxing a tiny hoverfly to enter a tube he has laced with silica gel, mildly frozen to sacrifice the insect. Exhausted after a long-distance journey, the hoverfly readily accepts his invitation. Once the fly gets ensnared in the trap, he quickly seals the tube and parcels it to a laboratory. The scientist tosses it beneath the lens of a microscope. This has been going on since 2021, when Hannah first noticed thousands of these hoverflies on the oil rig. In a new study published in the Journal of Animal Ecology, scientists have finally deconstructed the mystery of why these flies ended up here.
Standing inside the North Sea in Aberdeen, United Kingdom, the oil rig is one of the most isolated places in the world. And although these waters, coupled with the artificial lights, attract many seabirds, fish, and migratory visitors like storm-petrels, dovekies, murres, shearwaters, and snow buntings, to witness thousands of these tiny creatures was nothing short of a surprise. Named “marmalade hoverflies,” these flies are usually seen within the boundaries of gardens. At most, they were thought to travel short distances. But this remote oil plant, never.
After Hannah first spotted the flies dozing on the rig, he reported it to the scientists, who asked him to send samples. Over the next three-year period, he collected and dispatched hundreds of hoverfly samples in tubes for investigation. Back in a laboratory in Penryn campus, Cornwall, scientists extracted pollen from these flies using a variety of methods during the winter of 2024. As it turned out, the orange-black flies were just using the rig as a hotel to stop by, rest, and relax after a long-distance flight during their migration journey.
Previously, it was believed that these insects could only migrate short distances, but this episode shifted that perspective, also representing the “longest recorded example of hoverfly-mediated pollen vectoring,” per The Conversation. About 277 flies were trapped for analysis. Investigation revealed that these hoverflies have the potential of “long-distance pollen transfer,” a status usually assigned to larger species like hummingbirds, monarch butterflies, and orchid bees. The pollen collected from these samples indicated that these flies had travelled long distances, some of them arriving all the way from Spain or parts of central Europe.
The insect migratory routes were deduced from the programming stored in the pollen particles, analysed with metabarcoding, a technique that tests clusters and sequences of DNA to map out a biological barcode of the animal. While they travelled through mountains, forests, seas, and islands, these hoverflies paid brief visits to different plants and flowers. The mystery lay hidden in this “flower passport” that the hoverfly carried within the hairs and the pollen sticking to its body. In addition to curating the “pollen library”, the scientists also studied wind trajectories, both forward and backward, to understand their travel routes.
The identities of species of plants and flowers were catalogued with the help of three databases. The top species these flies seem to have visited on their route turned out to be nettle, black elder, and meadowsweet, followed by daisies, buttercups, and oleaster. The intriguing story doesn’t end here. The forward wind trajectories of some of the hoverflies suggested that after flying away from the oil rig, many of them are likely to make landfall in the Shetland Islands next.
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