A yellow streak catches my eye across the landscape. It is a common clouded yellow (Colias croceus), or in German: ein Wandergelbling, and in Italian: una Limoncella. This butterfly breeds around southern Europe, North Africa and even the far reaches of Afghanistan; but it takes on a mighty journey in summer, fluttering its way to countries as far north as the UK and Scandinavia [1]. Journeys of some individual insect species have already caught our attention and captured our imagination (such as the famous population of monarch butterflies that migrate between Canada and Mexico). However, relatively little is known about the adventures of the great hordes of many other (and especially tiny) insects that travel up and down the globe each year. 

Dr. Will Hawkes from the Centre for Ecology and Conservation at the University of Exeter and his team have been studying insect migrants crossing the Pyrenees’ high mountains on their journey south from countries such as the UK. Their research has uncovered that millions of insects pass through the Bujaruelo Pass each autumn [2]. To find out more about these magnificent migrations we spoke to Will on the topic, and he revealed how novel technologies are being used to uncover some of the mysteries behind these amazing insect migrations, and what we can do to support them [3].

What makes insect migrations so profound? What are they important for and what kind of changes do they have on the ecosystem?

Insect migrations are incredible. The animals are so small, yet they fly so far – thousands of kilometers, easily comparable to the migrations of birds. It is thought that the movement of insects globally is the most important terrestrial movement of biomass in the world – comparable to the movements of marine organisms. They are pollinators (by moving pollen great distances they increase the genetic diversity of the receiving plant population), decomposers (the eggs of just 50 house flies – who are also migratory – can breakdown nearly half a ton of cow manure), pest controllers (just two species of hoverfly migrating into the UK will eat 15 trillion aphids each year), pests, and nutrient vectors which is extremely important. Understanding their movements is vital for understanding how our ecosystems are linked.

Why has insect migration research received relatively little attention in the past compared to other species migrations?

It is very interesting that insect migrations have received little attention. I believe it is because we as humans didn’t believe that such tiny creatures could move so far. Dragonflies cross whole oceans, flies move thousands of kilometers, and I even found evidence of grass flies (2mm long) migrating over 110km from the Middle East to Cyprus! However, we are beginning to realize how remarkable the insect journeys are. I would love everyone to study them so we can protect these wonderful animals.

What exactly is vertical-looking radar and how can it help us to find out interesting things about migrating insects?

Vertical looking radars (or VLRs) are great pieces of kit. They fire waves into the sky above and monitor the echo of these waves as they rebound off the insect. The VLRs are so clever, they can work out the direction of flight, rough shape, and even the wing beat frequency of the insect! I am part of Dr Birgen Haest’s Move in Europe project who have a series of radars across Europe. They can monitor continually at high altitude (something which is very difficult to do without a radar). This allows us to find out how many insects are migrating overhead; previous research has been largely ground based.

Has your research led you to using other kinds of novel techniques to monitor insect migrations?

Yes, in the Pyrenees we needed a way to continuously count the insects moving through the 30 m wide pass of Bujaruelo because there were so many and they were so fast – too fast to count by eye. So we rigged up a phone camera in a waterproof box and coded it to record a 1-minute long video every 15 minutes throughout the day. From this data we were able to calculate that 17 million insects migrate through this pass every autumn, at a peak rate of over 3500 insects per meter per minute! We hope to deploy this migration camera system at other migratory bottleneck locations in the future. 

What are the biggest challenges faced when studying insects migrating across mountain passes?

The weather was quite intense! There was one occasion where there was so much snow, I had to wade 10km to the Pass through drifts up to 2m deep. When I arrived, the wind was so strong it was picking up razor sharp ice shards and the wind chill temperature was down at -22˚C! And another time I was very close to being struck by lightning [while] changing the insect trap in a storm.

Other than the weather, predicting the best spot to monitor insects from at the beginning of the season at a new field site is always challenging.

Fig.1: Weather radars are also used to measure the density of flying insects passing by. (Famartin. Sterling, Virginia, March 2021). Free-licensed media CC BY-SA 4.0 on Wikimedia Commons.

Imagine we wanted to replicate this kind of monitoring in other mountainous regions outside Europe like the Himalayas or the Andes. How could we monitor insect migrations at altitudes far higher than the Pyrenees?

As mentioned already, our little migration camera would be great if deployed at various mountain passes. Especially if we could give it a sim card and so it could send us the footage remotely. Small VLRs would also be useful to count the insects flying above the passes. However, we still would need people on the site to reveal exactly what species of insects are migrating. This part is the part I love most of all. I would so love to go to the Himalayas or the Andes and organize an insect trap to find out what species (and the vital ecological roles that they play) are migrating at huge altitudes. We know they migrate at high altitudes from incidental records from mountaineers seeing dragonflies/butterflies/hoverflies, yet there are no long-term quantitative studies yet.

Which migratory insect species do you find most fascinating and why?

Oh I love all of them! They all have incredible stories. There’s a wasp called Obtusodonta equitoria who lays her eggs in some Noctuid moth caterpillars and has evolved to follow the adult moths on migration. Or there are the incredible globe skimmer dragonflies who migrate across oceans (sometimes carrying tiny Forcipomyia midges on their wings – dragon riders!). Or perhaps the hoverflies who (alongside other diurnal migrants) use the sun as a compass and can choose favorable winds to power their flight… the list goes on, I couldn’t choose!

What are some of the next big questions about insect migrations that you are interested in?

Oh so many questions! Do these insects cross the Sahara? Are trillions of insects migrating towards Siberia each year across the Himalayas? Does the movement of genetic material in the pollen grains by the insects help plants adapt to climate breakdown? How much do animals like birds and mammals and fish rely on insect migrations?
So many.

How can humans help to support insects in their insect migrations?

We have a great responsibility to help, 1) because we have caused their decline but also 2) we as humans rely so heavily upon migratory insects for food security, crop protection, soil health, as well as the fact that the insects support the populations of birds and mammals – nearly everything seems to rely on migratory insects at some point in their life histories. We need to ensure that the insects have safe passageways across their entire migratory routes. This means a concerted global effort to allow habitat linking across vast swathes of agricultural land (allowing wildflowers to thrive and using fewer pesticides). It also means combatting the climate breakdown as the extreme weather is impacting insect populations. Crucially, however, we need people to take an interest. David Attenborough said recently ‘What you don’t know, you cannot love’ and I adore this. By learning about these remarkable little animals, realizing that the housefly or cabbage white butterfly in your garden may have just flown hundreds if not thousands of kilometers to reach you is truly magical. If we take an interest, we will start to care and that will lead to real change, protecting these incredible creatures and the vital roles they play for our human survival.

Some things we can all do are planting native wildflowers, putting pressure on policy makers, eating organically to reduce pesticides, but crucially it is taking an interest which is important, and the rest will come.

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Cover- and preview image: A clouded yellow. July 2012. Free-source photo by Aah-Yeah on Flickr