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Many species of flies migrate for hundreds or thousands of kilometres, often in vast numbers, a review of the evidence shows. These largely overlooked migrations mean flies may play a key role in many ecosystems, in particular by carrying pollen for much longer distances than other pollinators such as bees.

This role could be especially important as the world gets hotter, says at the University of Exeter in the UK, by allowing plants to acquire genetic variants from further afield that may help them survive drought, say, or to better resist pests and diseases that thrive in warmer conditions. 鈥淲e don鈥檛 know this for certain, but it might allow plants to adapt to a warming climate,鈥� he says.

Hawkes became interested in insect migrations while researching his PhD on the island of Cyprus in the Mediterranean. 鈥淭he insects were coming towards us across the sea from the Middle East, and they were getting channelled up this narrow gully,鈥� he says. 鈥淥ne day, the peak rate of insect arrival to this 100-metre-wide area of land was 6000 insects per metre per minute 鈥� and they were all flies. We had to hide behind our car doors because there were so many insects going through.鈥�

This made Hawkes think that fly migrations might be more common and important than was realised. Now, he and his colleagues have searched through the scientific literature and brought together all the evidence for this.

In scientific terms, flies are insects with a single pair of wings. This includes mosquitoes and midges, but not dragonflies and mayflies.

The review concludes that even though few studies have looked at fly migration and little is known about it, it is very common. 鈥淚n terms of the number of individuals, flies are definitely the main insect migrants,鈥� says Hawkes.

Hoverflies are the family of flies for which the team found the most evidence of migration in various species.

For instance, female marmalade hoverflies (Episyrphus balteatus) migrate from as far north as Scandinavia to Spain and even North Africa during the autumn, where they lay eggs. During spring, the hoverflies migrate back more slowly over the course of several generations, following the wave of flowering plants.

A 2022 study showed that some individual American hoverflies (Eupeodes americanus) . This is the longest confirmed fly migration, says Hawkes.

Mosquitoes are another fly family for which there is much evidence of migration. For instance, says Hawkes, some migrate between India and Japan.

Some flies, including the common drone fly (Eristalis tenax), have been found to travel over 100 kilometres carrying pollen of at least eight plant species.

When travelling long distances, flies rely on winds to carry them. 鈥淲hat they are really good at is choosing winds,鈥� he says, though it is unclear how they do this. They may fly as high as 1 or 2 kilometres above the ground while migrating and have been observed in mountain passes at altitudes as high as 5.4 kilometres.

In addition to their major role in pollination, migrating flies may also be important in terms of pest control, decomposing waste 鈥� including farm slurry 鈥� and transferring nutrients, the review concludes.

For instance, species such as the marmalade hoverfly are estimated to eat 16 trillion aphids in the UK each year, says Hawkes, while a 2016 study found that the flight of insects above southern Britain alone .

at the Natural History Museum in London, the author of several books on flies, says that the review shows fly migration is more common and more important than was realised.

鈥淚 think at the moment it would be hard to quantify the total impact of these migrations. More research into gut contents, pollen analyses, pollination studies, genomic analyses to understand population distributions, and life history studies would help us determine the impact flies are having,鈥� she says. 鈥淛ust in terms of the sheer numbers involved, though, their impact must be significant and thus exceptionally important in terms of conservation and food security.鈥�

The review also looks at evidence that the numbers of many species of migrating flies are plummeting, mainly due to habitat loss. For instance, a study in Germany reported a 97 per cent fall in the number of hoverflies migrating through one region over the past 50 years.

But fly populations can recover, says Hawkes. 鈥淚f we give them the chance with more habitat, then they can really come back quite quickly.鈥�