Every spring, the UK receives swarms of insect visitors from mainland Europe—among them two species of particularly beneficial hoverflies. For the first time, researchers have quantified the sheer scale of their migration and the ecological benefits they bring to Britain.
According to the study, published today (June 13) in Current Biology, the insects consume up to 10 trillion crop-gobbling aphids during their stay, in addition to pollinating British wildflowers and crops. The researchers also report that numbers of migratory hoverflies have been relatively stable over the last decade, and not in decline like many other pollinator species.
“We often think about bees as our major pollinators, but actually the hoverflies and other species of flies are important, and these sheer numbers suggest they’re [very] important,” remarks Helen Roy, an entomologist at the UK-based Center for Ecology and Hydrology who has collaborated with one of the study’s coauthors in the past but wasn’t involved in the current study.
Given the well-documented declines in many resident pollinator species, University of Exeter ecologist Jason Chapman began to wonder how beneficial migratory insects were faring. To observe the hoverfly migrations, he and his colleagues at other institutions in Europe and China turned to vertical-looking radar (VLR) data—a specialized form of radar that Chapman has used in previous studies to study insect migrations.
These radar systems emit upward-facing radar beams designed to detect small insects flying at high altitudes—between 150 and 1,200 meters above ground. The researchers filtered the data to pick out two species of interest based on their body mass, shape, and speed: Episyrphus balteatus, commonly known as the marmalade fly, and Eupeodes corollae. As larvae, these two common European hoverflies consume aphids, and as adults, they pollinate plants.
Between 1 billion and 4 billion of them fly in and out of southern Britain annually, the team estimated—around 70 times more than painted lady butterflies, which undertake similar migrations. Consistent with other reports, the first hoverflies reach British shores around May. After breeding for several generations, British-born hoverflies start returning to mainland Europe in August and September.
While the numbers of migratory hoverflies varied slightly over the years, Chapman and colleagues found that the overall population was quite stable. The results closely mirrored trends recorded through the hoverfly recording scheme, a British citizen science project that makes annual counts of the insects.
“There is evidence . . . that migratory hoverflies in the UK are doing better than non-migrants,” Bill Kunin, an ecologist at the University of Leeds who wasn’t involved in the research, writes to The Scientist in an email.
It may be that migratory insects as a whole are faring better than sedentary species, Chapman says. “This [trend has] also been seen in [migratory] butterflies and moths that have been studied as well.” One possible reason for this is that migratory species tend to be generalists, pollinating multiple flower species and eating different types of aphid, granting them an insurance policy in case their target species aren’t available once they arrive.
VLR approaches could help fill a key knowledge gap in insect monitoring, says Roy: while there’s a good deal of data on the distribution and diversity of insects in the UK, there tends to be little information on abundance. Being able to estimate the abundance of insects in the air column could prove useful in future studies, she says.
However, Kunin points out that there are limits in accuracy when it comes to using radar technology to pick out specific insect species. “They don’t know that these creatures they’re picking up that are the size of marmalade flies, and the shape of marmalade flies, are in fact marmalade flies.” Nevertheless, the authors have convinced him that most of what they’re seeing are the hoverflies they set out to detect, adds Kunin, who says he has plans to collaborate with Chapman.
Both Roy and Kunin are reassured to know that the two migratory hoverfly species are faring well. Their roles as pollinators and in pest control may become increasingly important as other beneficial insects dwindle, they note.
The majority of aphid species the hoverflies eat are likely ones that damage cereal crops, since they’re the most common type in southern Britain, Chapman says. The team didn’t calculate the flies’ pollination services specifically, but given their large numbers—which rival those of managed honeybees during the summer—they likely play a key role in pollinating crops and wildflowers.
And Chapman thinks the migratory hoverflies could be doing more than local pollination and pest control. His team also calculated the insects were capable of shuttling billions of pollen grains from Britain to Europe, which could be significant in allowing gene flow between British and continental populations of plants. And because the insects—whose bodies contain nitrogen and phosphorus—often die in places where they were not born, they could transfer nutrients too, thus aiding the Earth’s nutrient cycles.
“These aerial movements of insects—and birds and bats too—may be a parallel to the movements in the ocean of plankton and nutrients,” Chapman says. “Everybody knows how important [oceanic migrations] are, but we’ve not really thought about aerial movements in the same way. We’re trying to make the point that with these insect migrations being so abundant, that they may be quite important.”
K.R. Wotton et al., “Mass seasonal migrations of hoverflies provide extensive pollination and crop protection services,” Curr Biol, doi:10.1016/j.cub.2019.05.036, 2019.