… a blog written by Sarah Hawkes, Buglife’s Natur am Byth! Scarce Yellow Sally Conservation Officer. (view this page in Welsh)
My tale begins in mid-September on those last warm, Indian summer days when looking around at the autumnal flowers in your garden, your window box or the park – wherever there is a flower or two full of nectar – you can see a tiny, feeding, Marmalade Hoverfly or larger Eristalis hoverfly or perhaps another smaller fly. The females (and they mostly are females pregnant with eggs who make the journey) are feeding themselves up ready for a long flight.
Suddenly, one morning here in North Wales there will be a signal – perhaps a chill in the air, perhaps day length falls below a certain level – but whatever it is (and it could well be different for each species or even each population) that day every one of that species in your garden will lift off and fly high to catch the winds south. Migrating insects use winds, but do not rely on them, they have a direction in mind and head for it exclusively rather than getting blown at random.
‘My’ hoverfly from my garden will head south, crossing the English Channel and fly across France and Spain. When she gets to the Pyrenees, she will choose to fly over a pass in the mountains rather than flying too high over the peaks.
It was this concentrated migration through the mountain passes that first revealed the mass movement of insects to scientists. A couple of bird researchers, Mr. and Mrs. Lack, holidayed in the Pyrenees in the 1950’s to watch birds on migration. Whilst there, they gradually realised that there was something even more incredible happening. Thousands of insects were also cresting the cols with the birds and heading south. The Lacks recorded then that here were perhaps ‘the most remarkable migrants of all’.
Meanwhile, after struggling against headwinds over the col my little hoverfly slips down the southwestern flanks of the mountains and continues across Spain to the south coast near Gibraltar, at the shortest sea crossing to Africa. For now, the story ends here, having travelled at least to the Mediterranean basin. But it is likely she will continue far beyond. There is much work still being done by researchers to find out where they go but as an indication from personal observation in December 2022, I saw Marmalade Hoverflies at an oasis in the Sahara and we found a Painted Lady butterfly corpse on top of an extinct volcano in the desert.
Wherever she and her babies spend the winter, on the return journey there is no long single flight, and my hoverfly will never come back to my garden. Instead, she will die after laying her eggs in the warm south.
Some generations later, the northern hemisphere spring begins to stride northwards and my hoverfly’s descendants follow by instinct, having no parent to tell them where to go. They take advantage of the new growth and less crowded habitat at the front of the ‘Green Wave of Spring’ to lay eggs near a newly hatched aphid colony.
And so it goes on across the plains of Europe: The eggs hatch, eat the aphids that mum has kindly spotted and laid her eggs amongst and the next generation of adults lifts off to fly a little further to the next flush of aphids a few miles or so north, following Spring on and on until, generations later a relative (probably? possibly? so many research opportunities!) of my original hoverfly arrives back in my garden.
Implications
This is by no means just a magical story. From the planetary point of view, migration is a fundamental part of the engine that keeps our ecosystems operating.
Think about the steps involved. A hoverfly drinking nectar from a flower in my garden, having grown up there and now carrying eggs, will fly a few thousand miles south where she lays her eggs and then dies, leaving her body (fed on nutrients from my garden) to decompose perhaps in a garden of a family living in a desert oasis, or perhaps even further, south of the great Sahara itself. A bit of my garden goes to Africa every year, and this has been happening for thousands of years! A wonderful, newly discovered connection with people and places I will never know, and it is not just one way. The nutrients in that far off garden will travel northwards with the children of my hoverfly. Slowly, year by year, north and south are constantly intermingling. Importantly, the journey north feeds the whole distance wherever the insects breed, feed, poo and die.
Pollination is another benefit of this migration tale. On the way north insects moving to find new pastures carry with them pollen collected during their last meal. Whether they are flies or any other insect, if they eat nectar as adults the chances are they will pick up pollen as they do so. They may transport this genetic plant material 50 miles north across the sea (e.g. crossing from Iraq to Cyprus) or a mile or two to the next flowery meadow. On the way south, insects and birds may stop to refuel which creates an opportunity for autumn flowering plants to spread their genes southwards.
Some illustrations of the importance of this whole cycle will help here: There is a violet in southern Spain, Viola cazorlensis, with a population existing in very small patches that were once thought to be relict populations going down a genetic plughole. When the DNA was studied, it was remarkably variable within each pocket-sized population. The theory is that this little violet has evolved with pollinators to flower at times of migration and be pollinated by a migrating moth, the Hummingbird Hawkmoth (Macroglossum stellatarum).
In the Pyrenees there is a native bat (Tadarida teniotis) that has evolved its whole breeding cycle to take advantage of the autumn migrational influx of food. Most bats pup in spring as the insect populations burgeon. The insect population bonanza in the Pyrenees happens during September and October and there the resident bats pup in early September so that their young can fatten up in time for the winter hibernation!
Another amazing adaptation is, it is now thought, that the plants of permafrost areas are there only because of the nutrient transfers to soils from migrant species of all types that arrive in summer to breed.
What can we do right now to keep this cycle going and support migrating insects?
All of us can start right now by making our spaces welcoming and encouraging the cycle to work where we are, with space for invertebrates. You can let some of your garden or window box go wild and see what comes, plant some native flowers, build a bee hotel or a hoverfly lagoon, and crucially, don’t use pesticides or herbicide.
Most important of all, ‘Notice’ the insect visitors to your space and find out about them. Take a photo, talk about them to your friends and on social media and enjoy being part of nature.
If you haven’t already, read the first part of our blog on Insect Migration.
Main Image Credit: Marmalade Hoverfly (Eipisyrphus balteatus) © Will Hawkes
