The secrets of butterfly migration, written in pollen

On a warm summer morning in Ieper, Belgium, 66-year-old Sylvain Cuvelier steps into his blooming garden with his 14-year-old granddaughter, hoping to identify and count all the fluttering butterflies. Other days, he helps scientists by netting butterfly samples. Then he records each sighting’s location using GPS, logs them in his Excel database, and sometimes sends the samples to his academic colleagues, who will analyze pollen grains clinging to the insects’ bodies.

Those tiny pollen grains, gathered by citizen scientists like Cuvelier, are helping researchers study a process that until now has been largely inscrutable: the migratory patterns of insects as they move around the globe over the course of multiple generations.

Using pollen, scientists have been able to identify where individual butterflies began their journeys, and even infer the events that likely triggered their migration. The knowledge may help conservationists better understand some of the effects of climate change — not only on the insects themselves, but on their migrations and the ecosystems they inhabit.

A lot of insects spend their whole lives in one place. Many others migrate, like many birds do, to avoid harsh weather, to find food or to breed. Some estimates suggest that trillions of insects migrate across the globe each year, yet scientists know little about where they go or how they get there.

Tracking insect migration is not as straightforward as tracking birds or mammals. With birds, “you can attach a ring to the leg or use radio tracking, and it’s easy to prove that they move from point A to B,” says Tomasz Suchan, a molecular ecologist at the Polish Academy of Sciences in Krakow. But most insects are too small for these techniques to be successful.

In North America, researchers have had some success tracking monarch butterflies, known for their remarkable migration from southern Canada and the northern United States to central Mexico. In the early 1990s, the Monarch Watch citizen science initiative began tagging butterflies around the Rocky Mountains. Over 2 million monarchs have been tagged, with more than 19,000 recoveries reported in Mexico where monarchs congregate to roost for the winter. This has helped biologists to track their migration routes.

Butterflies without such well-defined aggregations are more difficult to track, however. For example, painted lady butterflies often appear in Europe in the fall, sometimes in great abundance. “Then they disappear, and we don’t really know where they go,” says Gerard Talavera, an entomologist at the Institut Botànic de Barcelona.

Some years back, Talavera and his team realized they might be able to track the butterflies indirectly, by studying the pollen that accumulates on their bodies. Every time a butterfly visits a flower for a sip of nectar, it also picks up grains of pollen. If the researchers could identify plants from their pollen, confirm where and when the plants were blooming, and keep tracing them as the butterflies reached different geographic regions, perhaps they could trace the butterflies’ overall journey. “The method is like we put a GPS on them,” Talavera says. “Because we cannot do that, this is the closest we can go.”

Pollen migratory maps

The scientists were able to test the idea in 2019, when painted ladies experienced one of their sporadic population booms. In March of that year, as swarms of the butterflies appeared in the Middle East and the Mediterranean, the citizen scientists netted butterfly samples, then preserved them in an alcohol mixture and shipped them to Talavera’s lab.

There, researchers isolated the pollen grains attached to the butterflies’ bodies and sequenced a particular stretch of the pollen DNA that offers a unique signature for each plant species, a process known as metabarcoding. All the while, citizen scientists kept netting butterfly samples as the population surge gradually spread through eastern, northern and western Europe over the following months, reaching southern Morocco in early November.

Analyzing pollen collected from 264 butterflies from 10 different countries in seven months, the researchers identified 398 different plants they could use to track the butterflies’ movements backward through the year. From this, they found that swarms of butterflies observed in Russia, Scandinavia and the Baltic countries were likely the offspring of butterflies originating from the surge in Arabia and the Middle East. This appears to have spread to eastern Europe, then Scandinavia and then to western Europe, resulting in a noticeable population boom in the United Kingdom, France and Spain. From there, the butterflies may have migrated to southern Morocco, likely continuing on to tropical Africa to complete their annual cycle.

The pollen record even suggested a reason why painted ladies suddenly became so abundant in 2019. Butterflies collected from the eastern Mediterranean, right in the beginning of the population spike, were carrying pollen from plant species found primarily in semi-arid shrublands, grasslands and salt marshes of northern Arabia and the Middle East. Examining satellite images, the researchers noticed that from December 2018 to April 2019, those plants experienced big boosts in growth following a period of unusually heavy rainfall. That burst of growth, the researchers speculate, may have provided ideal conditions for the butterflies to feed and breed, kicking off the population explosion and leaving a ripple effect that affected many generations.

Talavera and his team have used pollen signatures to track other butterfly movements as well. In 2013, for example, painted lady butterflies had been found resting on the coast of South America, in French Guiana. Painted ladies don’t normally live in South America, and it was a mystery where they had come from. A decade later, Talavera’s team sampled pollen from the still-preserved butterfly bodies and found that Guiera senegalensis, a common plant found only in sub-Saharan Africa, was by far the most common type of pollen attached to these butterflies.

By analyzing coastal surveys, wind patterns, pollen and environmental conditions, they confirmed that the butterflies probably crossed the Atlantic in up to eight days’ worth of continuous flight from Africa. This finding marked the first verified instance of an insect crossing the Atlantic.

“The use of pollen metabarcoding to track where each generation of butterflies comes from and how they progress through the cycle is super novel,” says Christine Merlin, a biologist at Texas A&M University and coauthor of an article on the neurobiology of butterfly migration in the Annual Review of Entomology. Because it identifies individual plant species, she notes, this method promises greater precision than the standard method, isotope signature analysis, which tracks regional variations in the insects’ chemical makeup.

While painted ladies serve as a model system for understanding insect migration, researchers say they are confident that this method could be suited for tracking other migrating pollinators that actively visit flowers to collect nectar, including other butterflies, syrphid flies, wasps, beetles and moths.

Tracking migration routes of insects could be of growing importance in the face of changing climate, because such insects can carry fungal diseases in addition to pollen. In fact, Suchan detected many species of fungi in some butterflies. Approximately 1,000 fungi are known to affect insects and over 19,000 can affect crops. Thus, migrating insects could potentially spread these fungal diseases across continents, posing risks to ecosystems and economies. Talavera, Suchan and colleagues hope that using pollen signatures to map changing migration patterns could help to predict where fungal disease outbreaks might occur.

Cuvelier, meanwhile, hopes to continue counting butterflies with his granddaughter. Ecologists will increasingly need more “big data” to understand large-scale phenomena, he says. Without citizen scientists, he says, “it is impossible for researchers to gather such databases.”

Besides, he adds, young people have more to learn from citizen science than just how to catch a butterfly. “They learn about nature,” he says, “and this fosters curiosity in the world.”