Hoverfly Migration: Incredible Journeys

The most remarkable migrants of all

It is the Autumn season birds are leaving our shores heading to their wintering grounds, a phenomenon known as migration. But did you know insects including some species of hoverflies are known to migrate too.

We are probably all familiar with the migration of the Monarch butterfly Danaus plexippus in the Americas and even closer to home in Europe the Painted Lady Vanessa cardui. But a fly no bigger than a 5p migrating such mass distances you might be a bit skeptical of the suggestion.

Why are hoverflies found on Oil Rigs?    credit Google maps

In 2004 the Guardian reported that the people of Essex were alarmed by mass invasions of ‘plagues of wasps’. It turned out to be mistaken identity, as they were all Marmalade hoverflies (Episyrphus balteatus). These hoverflies had come across from Europe; there have since been similar reports over the years. In fact, only a few weeks ago there was a day early October when Harlequin ladybirds Harmonia axyridis were being reported in large numbers up and down the country, possibly a migration of some sort?

Migration History

Mass insect migrations have been known about since biblical times (plague of flies, Exodus 8: 20–32) and even appeared on cave paintings over 17,000 years ago, in Font de-Gaume and Bernifal caves. Insects were transversing the mountain ranges in 1867 as documented by the Italian Vittore Ghiliani.  He specifically noted dragonflies migrating across the Cuneo Alps. But it was in the 1950s when they were first recorded and described, along with birds passing through the Pyrenean Pass of Burjaruelo  With the invention of radar in 1935 there have been many documented large insect migrations. Gila Blend Arizona first confirmed insects in 1949  then in 1954 the Royal Navy detected a desert locust swarm in the Persian Gulf, just as it was in biblical time. http://radarentomology.com.au/observation-campaigns/

A recent report in the Journal of Animal Ecology September 2025 documented hoverflies landing on oil rigs in the North Sea. 

Oil rigs have no flowers so why do hoverflies go there?

Well, it would seem just like the birds in autumn, oil rigs in the North Sea act like a beacon for hoverflies to rest. Maybe they are attracted by the lights but whether true or not the evidence shows that such migrations occur twice a year in May-July and August-October. So, they would seem to be using the oil rigs to migrate across the North Sea, see above. Not only that but according to this article they are transferring pollen from one continent to another, linking geographically isolated populations of plants, transferring vital genetic vigour eg drought resistance.

There are several types of migration:

Spring Migration – to come north to utilise breeding sites and spring growth. These are often completed in phases rather than one direct journey.

Breeding Migration – short moves during the season dependent on food availability, temperature, winds strength both north and south.

Autumn Migration – to go south, avoiding deteriorating weather conditions and resource depletion. More studies need to be carried out to find out if these are direct as in one journey or phased as in the spring migration, current research suggest it’s one journey.

Diagram showing types of migration seen in hoverflies

There are at least 46 species of hoverflies, mentioned in the literature as migrating, including the infamous Marmalade. Below is a graph I’ve created showing the top 40 UK hoverflies using records in the National Biodiversity Network (N.B.N), showing those species that migrate and those not known to migrate. I suspect many species migrate, as most listed below are well-known hoverflies, the less common they are the less is known about their movements.

Graph showing 40 hoverfly species and whether they are known to migrate or not:
Source of migration data ref 8, record number NBN

But hang on a minute, hoverflies have been spotted late Autumn/Winter on a nice warm day. I have seen a Marmalade in a sheltered spot on the 2^nd of January on a mild winters’ day, so obviously not all hoverflies migrate, and you would be correct. There are several papers on the subject indicating that during larval development the larva decides whether to migrate or go into diapause. During the 3^rd instar of a hoverfly’s development, just before adulthood, the larva decides whether it is going to migrate, mind-blowing! I don’t even know what I am doing tomorrow.

A paper published by Doyle 2022, proved experimentally that a hoverfly larva can switch on/off over 1500+ genes: allowing them to become bigger, turning off the development in reproductive organs, switching on genes that allow it to breathe at high altitudes, have better vision, even a gene that reduces signalling from the mitochondria so that they work harder during flight. They did this study by doing a transcriptomic comparison of migratory Marmalades compared with non-migratory Marmalades and found that the development decisions were made in the genes to decide the fate of the soon to be emerging adult.
This means that before winter sets in whilst the hoverfly larva is eating detritus in a rot hole/pool, or munching aphids on my flowers, it is deciding about its future. It must be measuring day-length to know whether to head north or south, forecasting the weather in the months ahead and assessing the host plant food quality. All this information is the decision pathway to deciding if it should move on or stay put in the hope of feeding, breeding and surviving without migrating, how astonishing.

References:

1.Bauer, S., Tielens, E.K. & Haest, B. (2024) Monitoring aerial insect biodiversity: a radar perspective. Philosophical Transactions of the Royal Society B, 379(1904), 20230113.
 
2.Doyle, T., Jimenez-Guri, E., Hawkes, W.L.S., Massy, R., Mantica, F.,Permanyer, J. et al. (2022) Genome-wide transcriptomic changes reveal the genetic pathways involved in insect migration. Molecular Ecology, 31, pp4332–4350.
 
3.Friberg,M., Dahblerus, J., Wiklund, C Strategic larval decision-making in a bivoltine butterly. Pecologia (2012) 169:623-635. DOI 10.1007/s00442-011-2238-z
 
4.Hawkes WL, Weston ST, Cook H, Doyle T, Massy R,Guri EJ, Wotton Jimenez RE, Wotton KR. 2022 Migratory hoverflies orientate north during spring migration. Biology Letters, 18(10), 20220318.
 
5.Hawkes WL et al. 2024 The most remarkable migrants—systematic analysis of the Western European insect flyway at a Pyrenean mountain pass. Proc. R. Soc. B
291: 20232831. https://doi.org/10.1098/rspb.2023.2831
 
6.Lack, E. & Lack, D. (1951) Migration of insects and birds through a Pyrenean pass. The Journal of Animal Ecology, 20, 63–67.
 
7.Reynolds, S.K., Clem, C.S., Fitz-Gerald,B. & Young, A.D. (2024) A comprehensive review of long-distance hover fly migration (Diptera: Syrphidae). Ecological Entomology, 49(6), 749–767. Available from: https://doi.org/10.1111/een.13373
 
8.Wainwright M. 2004 When is a wasp not a wasp? When it’s a hoverfly. The Guardian. See https://www.theguardian.com/uk/2004/aug/03/sillyseason.science.

9. https://theconversation.com/i-surveyed-17-million-insects-flying-through-a-pyrenean-mountain-pass-heres-what-i-learned-about-these-remarkable-migrants-231832