Sand most people will think "bees". They are correct, for bees are the most important insects involved in pollinating flowers. Rather fewer, however, might guess that the second-most important group is probably hoverflies. Yet hoverflies, according to a study published in 2020, help fertilise 52% of the world's crops and 70% of its animal-pollinated wildflowers.
They may do more than this, though, for some hoverflies migrate. In 2019 Karl Wotton of Exeter University showed, using radar, that they move by the billion to and fro across the English Channel. This suggests the gene pools they stir by spreading pollen may be more than just local ponds. However, recent Chinese work concluded that the hoverflies under study there rarely carried pollen far and that the few which did so bore the pollen of only a few plant species.
To investigate further, Dr Wotton decided to collect hoverflies from a site with no local plants -- meaning any pollen they were bearing must have been gathered far away. The site in question was an oil rig in the North Sea, about halfway between Scotland and Norway. As he and his team report in the Journal of Animal Ecology, during the course of four sampling sessions between 2021 and 2023, they captured 121 insects, such as the one pictured, that had landed on the rig for a rest, and frisked them for grains of pollen they were carrying.
On 111 of their prizes, they found some. These pollen-bearing hoverflies each had, on average, grains from 4.7 plant species. Some carried pollen from as many as 14 sources. Altogether, pollen from more than 100 plant species was carried by at least one of the hoverflies the researchers examined. The most common were nettles and black elder. But crops were also well represented, in the form of a range of vegetables, legumes, cereals, nuts and fruit.
For that to matter ecologically, though, there would have to be plants of the same species in the places where the flies ended up. And, by and large, this seemed to be the case.
Analysis of air currents suggested the flies bearing this pollen came from the Netherlands, Germany and Denmark, and would, had they not been intercepted, have continued onward to Norway or Scotland. Moreover, by conducting so-called wind-trajectory analyses, the researchers could calculate where, on a given day, a captured hoverfly would have ended up.
These calculations suggested that the probable arrival areas often contained plants of the same species as the pollen being carried by the insects. The upshot is that hoverflies, like some romantic go-between, allow plants hundreds of kilometres apart to mate.