The UK is set to receive an influx of millions of painted lady butterflies this summer, according to the charity Butterfly Conservation.

Every year varying numbers of these migrating insects visit the country from southern Europe to breed and lay their eggs on thistle plants.

But a mass immigration on this scale only happens about once every 10 years when conditions on the continent are right. The last time this happened was in 2009 when around 11 million painted ladies arrived and more than 26 million left in the autumn.

It would be amazing to witness another mass painted lady immigration

“It would be amazing to witness another mass painted lady immigration,” says Richard Fox, head of recording for Butterfly Conservation.

“They have been pretty thin on the ground in Britain since 2009; we’ve had five below average years in a row.”

But that’s about to change as Butterfly Conservation is now reporting that the recent warm sunny conditions have led to unusually high numbers of painted ladies gathering in southern Europe. All the signs are suggesting that a mass invasion is imminent, and it’s more of a when and how many than if.

There and back again

Scientists still don’t fully understand the conditions that affect the abundance of painted lady butterflies arriving in Britain. They spend the winter in Africa, spreading northwards across Europe in late spring and early summer, as climatic conditions become favourable and food plants become available for its caterpillars.

In 2009, Fox explains it seemed that higher than average rainfall in North Africa had enabled unusually luxuriant vegetation growth that, in turn, led to a very successful breeding cycle. They always reach Britain, but in years like this they can make it further north.

But it was novel research conducted during this ‘painted lady summer’ that combined radar data and citizen science to finally reveal the route of these long-distance migrants between Africa and the Arctic Circle and back again.

“It can be considerably longer than the famous migration of the monarch in North America,” explains Dr Fox, talking about their 9,000 mile journey.

“It is important to note, though, that the whole migration is never undertaken by any one painted lady. It is a series of separate generations through the year, perhaps as many as five or six.

“However, there is much more to learn about what influences the breeding success of painted ladies in Africa during the winter.”

Back to the future

Dr Fox isn’t sure how many to expect this year, saying: “That’s the one million dollar question!

“If we get a settled spell with winds and air moving up from southern Europe in the next week or so, then we could be in for a massive arrival.”

Another big influx and many new sightings from the public could enable more to be discovered

Butterflies don’t migrate in big ‘clouds’ but typically appear as a stream of seemingly unconnected individuals flying low and fast and all following the same flight paths a minute or so apart.

Fox says that it can be a truly memorable sight.

“In late May 2009 many people reported hundreds of butterflies passing per hour. Lucky observers recorded numbers exceeding 1,000 per hour.”

And like previous years the charity is asking the public to report their sightings on the Migrant Watch website. It will help them to map the arrival, spread and departure of this beautiful black and orange butterfly.

“Another big influx and many new sightings from the public could enable more to be discovered,” he says.

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