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A NYT “Trilobites” article by Monique Brouillette drew my attention to a new paper in Nature Communications documenting, with a variety of evidence, what is the first known flight of any insect across a big ocean. In this case the insect was the ubiquitous Painted Lady Butterfly (Vanessa cardui), the most widespread of all butterflies, and the ocean was the South Atlantic Ocean.

Tomasz Suchan et al. found Painted Ladies on the coast of French Guiana, and, using four different methods, suggest that their most likely origin was West Africa or Europe.  This means that they flew, over a period of 5-8 days, a distance bertween 4200 km (2600 miles) and 7000 sm (4350 miles).  It’s amazing, as butterflies can’t have done that without help: in this case, the wind.

Here’s a Painted Lady (upper side):

Jean-Pol GRANDMONT, CC BY 3.0, via Wikimedia Commons

I worked on long-distance insect movement for several years as a postdoc and young professor, but we marked our Drosophila flies with fluorescent dust and never even tried to find any movement this long.  Click on the screenshot below to go to the Nature paper, or find the pdf here.

First: the discovery (indented sections are from the paper), a group of exhausted Painted Lady butterflies on the coast of South America, found 11 years ago.

Three of about ten observed individuals were captured alive on the beach at ~6:00 am on the 28th of October 2013, apparently arriving after a vigorous flight across the ocean, judging from their damaged wings and resting behavior on the sand. Painted ladies are strong migrators, known for their recurrent trans-Saharan flights and a multigenerational cycle spanning ca. 15,000 km between the Afrotropical and the Palearctic regions. V. cardui is nearly cosmopolitan, but stable populations have not been recorded from South America. The individuals found on the coast of French Guiana should therefore have originated from populations in North America, Europe or Africa.

Thus there was more than one individual, suggesting that they stayed together during the long-distance migration, which is hard to understand.  We can rule out a South American origin of these butterflies simply because stable populations of the butterfly aren’t found on that continent.

Then Suchan et al. used four methods, all of which suggested that the butterflies had a West African or European origin (or both: hatched in Europe, migrated to West Africa, and then crossed the ocean). This means, since they were crossing water, that they had to move long distances without refueling. Although they can cross the Sahara, they can also refuel when doing so.

Here are the methods the authors used:

1.)  Wind.  Apparently wind data (speed and direction) are available for different altitudes from the National Oceanic and Atmospheric Administration. The winds at various altitudes up to 2000 m were inconsistent in direction for the five days before Oct. 26 and the three days after the capture data (Oct. 28-31), but were consistent in direction (east to west) on the 2 days before the capture, which the authors say is “exceptionally favorable for the butterflies to disperse across the Atlantic from West Africa, assisted by winds.

The wind assistance, then, would have been operative for only 2 days, but could have helped on the other days of the 5- 8-day journey if the insects had been at altitudes with the right winds. Is that feasible? They also must have used their own flapping power, but they could not have had the fuel to fly consistently for that entire period on their own.

2.) Genetic affinity.Using RAD-assisted mapping of DNA variation, the authors found that African + European populations, which are similar to each other, were both distinctly different from those of North America.  And the exhausted Painted Ladies found in South America clearly were genetically related to the African + European ones and not to the North American ones, implying that yes, they had crossed the ocean from either Europe or North America.  (It’s also known that October is when Painted Ladies are a high densities in these areas.) Below you can see the genetic clustering. As the key shows, the green butterflies, found on the beach, clearly clustered with both African and European butterflies (red and orangish respectively), but were distinctly different from North American butterflies (blue dots). This is strong evidence that the butterflies found in French Guiana came from across the Atlantic Ocean:

(from the paper): A Principal Component Analysis (PCA) using SNPs with less than 10% missing data per sample and pruned for LD (13 206 SNPs), the variances explained by the two first axes are 6.26% and 5.21%;

3.) Pollen carried by the butterflies. This was a clever idea: the authors did a DNA sequence of the pollen grains found on the bodies of the beach-captured butterflies. (Butterflies pick up pollen when sipping nectar from flowers.) Most of the species found weren’t informative as they couldn’t be classified or represented widely-distributed Neotropical species, but they also found grains from two plant species endemic to the Sahel region of Africa: a narrow biogeographic swath across the subsaharan region. The plants were the Senegal Tea Plant, Guiera senegalensis, and Christ’s thorn jujube, Ziziphus spina-christi, both flowering shrubs, and both found only in the Sahel. They’re in yellow on the bar chart below, both in the bar graphs of pollen frequency and geographic distribution. G. senegalensis was especially highly represented on the butterflies, far more numerous than the pollen of any other species.

Both species were flowering at the time the butterflies were found (flowering season is Aug.-November), also supporting an African origin of the butterflies.

(from the paper) Classification of the obtained ITS2 metabarcoding sequences processed using a denoising pipeline (see Methods), and blasted on curated databases from A PLANiTS83 and B Sickel et al.82 using the SINTAX classifier. In addition to plants present in French Guiana or widely distributed (green bars), two Sahelian endemic plants (yellow bars) were found among the pollen recovered from the bodies of the painted lady butterflies in South America: Guiera senegalensis and Ziziphus spina-christi, the former being especially common. Source Data is available in Supplementary Table S3.

One thing that puzzles me is the existence of “Neotropical pollen grains”. Those would be from the South American tropics, and how did they get on the butterflies? Did they nosh on South American plants after they crossed the ocean? The authors don’t discuss this.

4.) Isotope analysis. I was unfamiliar with this method, but apparently the ratio of isotopes of two elements, strontium and hydrogen, are indicative of the “reproductive habitats” of different areas of the world, and the ratios found in the butterflies’ wings had the highest probability of coming from West Africa and/or Western Europe (Portugal, France, Ireland, and the UK). This also raises the possibility that these butterflies were on their regular migration from Europe across the Sahara, and then were blown off course by the wind. Lacking any direction-finding ability when off course (we also found that this was true of Drosophila), they just kept flapping until they made land in South America. Almost surely most of them would die along the way.

But could they really make it, even if assisted by wind? The authors suggest that they could. Even though they couldn’t flap continuously for 5 to 8 days, they could also glide:

We assessed the feasibility of a transatlantic crossing by estimating energetic requirements and dispersal duration of V. cardui when using different flight strategies.. In the absence of wind-assistance, we estimate that painted ladies could travel a maximum of ~780 km without refueling, far less than the 4200 km distance across the Atlantic. Therefore, the painted ladies must have relied on the easterly trade winds that were present preceding the capture date. Furthermore, even with wind-assistance, painted ladies using an exclusively active flight strategy would travel a maximum of ~1900 km before depleting their energy reserves. Therefore, painted ladies must be using an alternating strategy of active flight and minimum-effort flight (i.e. flapping only to stay aloft and gliding), a behavior that is known from monarchs and other butterflies. Assuming that painted ladies use the same alternating flight strategy as monarchs (with a 15:85 proportion of active:minimum-effort flight) and with the assistance of wind (average windspeed of 7.47 m/s based on trajectories starting 26–28th of October), the painted ladies we captured in French Guiana could have crossed the Atlantic from West Africa in 5–8 days, but only if their starting fat reserves were at least as high as 13.70% of their body mass.

Of course the trade winds blew in a consistent direction only for two days during the crossing, not the 5-8 days suggested by the authors.  And even if the butterflies were loaded with fuel, it’s hard to see how they could make it. But the other evidence convinces me that these individuals did have an African origin, and maybe hatched in Europe. The authors suggest that gliding was important, but I don’t know if it’s been seen in this species of butterfly. And how do butterflies glide, anyway?

The authors suggest that these butterflies hatched in Europe, and give this schematic of their journey (Europe gets a nod since the isotope ratios there are closer than African ones to what was found in the butterflies’ wings):

(From the paper): C  Infographic summarizing the possible natal grounds and dispersal pathway of a flock of V. cardui butterflies across the Atlantic from West Africa to South America, through a non-stop flight of a minimum of 4200 km during 5–8 days. The total flight distance for these individuals could be as long as 7000 km if they developed in Western Europe. Source Data can be obtained online using the provided code (see code availability). Butterfly illustrations by Blanca Martí.

Now the authors note other cases long-distance migration of insects, even one across the Indian Ocean. But most of the other long-distance migration of insects involves en route refueling (except for the dragonfly, and although they don’t give the overwater migration distance for it, Wikipedia says 2500 km).  This butterfly beats the dragonfly by 1700 km.

For example, the dragonfly Pantala flavescens apparently migrates annually across the Indian Ocean. The monarch butterfly (Danaus plexippus) also annually migrates between Canada and Mexico, and tagged individuals have demonstrated flights as long as 4635 km (2880 miles). Recent work using light aircraft and individual radio tracking of death head’s hawkmoths (Acherontia atropos) recorded a remarkable maximum ground speed of 69.7 km/hour (19.4 m/s). These scattered reports of individual feats of migration both in terms of distance covered and flight speed are important: collectively they indicate that trans-oceanic LDD events may be sufficiently frequent to have played an underestimated role in biogeographic dispersal over time (cf. panbiogeography).

The upshot:  Although the evidence varies in strength, the genetic evidence and the pollen-grain evidence alone are pretty convincing that these butterflies came from Africa, and perhaps originally from Europe.  I’m not sure whether they’d be able to colonize South America, as they were exhausted, probably not capable of reproducing, and Painted Ladies aren’t found in South America.

There are still puzzling things about the hypothesis, including where the Neotropical pollen grains came from on the butterflies, and whether Painted Ladies are actually capable of gliding. But the evidence suggests that this kind of long-distance nonstop movement is possible in butterflies, and without refueling. (Painted Ladies are already known to fly long distances when they can refuel.) There’s surely enough data here for lepidopterists to start combing the beaches of eastern South America during October and November!

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