Flying insects tell tales of long-distance migrations

Every autumn, a quiet mountain pass in the Swiss Alps turns into an insect superhighway. For a couple of months, the air thickens as millions of migrating flies, moths and butterflies make their way through a narrow opening in the mountains. For Myles Menz, it’s a front-row seat to one of the greatest movements in the animal kingdom.

Menz, an ecologist at the University of Bern in Switzerland, leads an international team of scientists who descend on the pass for a few months each year. By day, they switch on radar instruments and raise webbed nets to track and capture some of the insects buzzing south. At sunset, they break out drinks and snacks and wait for nocturnal life to arrive. That’s when they lure enormous furry moths from the sky into sampling nets, snagging them like salmon from a stream. “I love it up there,” Menz says.

He loves the scenery and the science. This pass, known as the Col de Bretolet, is an iconic field site among European ecologists. For decades, ornithologists have tracked birds migrating through. Menz is doing the same kind of tracking, but this time, he’s after the insects on which the birds feast.

Migrating insects, like those that zip through the Swiss mountain pass, provide crucial ecosystem services. They pollinate crops and wild plants and gobble agricultural pests.

“Trillions of insects around the world migrate every year, and we’re just beginning to understand their connections to ecosystems and human life,” says Dara Satterfield, an ecologist at the Smithsonian Institution in Washington, D.C.

Scientists like Menz are fanning out across the globe to track butterflies, moths, hoverflies and other insects on their great journeys. Among the new discoveries: Painted lady butterflies time their round trips between Africa and Europe to coincide within days of their favorite flowers’ first blossoms. Hoverflies navigate unerringly across Europe for more than 100 kilometers per day, chowing down on aphids that suck the juice out of greening shoots. What’s more, some agricultural pests that ravage crops in Texas and other U.S. farmlands are now visible using ordinary weather radar, giving farmers a better chance of fighting off the pests.

Until now, most studies of animal migration have focused on large, easy-to-study birds and mammals. But entomologists say that insects can also illuminate the phenomenon of mass movement. “How are these animals finding their way across such large scales? Why do they do it?” asks Menz. “It’s really quite fantastic.”

To warmer worlds

Animals migrate for many reasons, but the aim is usually to eat, breed or otherwise survive year-round. One of the most famous insect migrations, of North America’s monarch butterflies (Danaus plexippus), happens when the animals fly south from eastern North America to overwinter in Mexico’s warmer setting. (A second population from western North America overwinters in California.) In Taiwan, the purple crow butterfly (Euploea tulliolus) migrates south from northern and central parts of the island to the warmer Maolin scenic area every winter, where the butterfly masses draw crowds of lepidopteran-loving tourists. In Australia, the bogong moth (Agrotis infusa) escapes the hot and dry summer of the country’s eastern parts by traveling in the billions to cool mountain caves in the southeast.

The migrations can be arduous. Each spring, the painted lady butterfly (Vanessa cardui) moves out of northern Africa into Europe, crossing the harsh Sahara and then the Mediterranean Sea before retracing the route in the autumn (SN Online: 10/12/16). Because adult life spans are only about a month, the journey is a family affair: Up to six generations are needed to make the round trip. It’s like running a relay race, with successive generations of butterflies passing the baton across thousands of kilometers.

Constantí Stefanescu, a butterfly expert at the Museum of Natural Sciences in Granollers, Spain, has been tracking the painted lady migrations. He relies on citizen scientists who alert him when the orange-and-black-winged painted ladies arrive in people’s backyards each year, as well as field studies by groups of scientists. In 2014, 2015 and 2016, Stefanescu led autumn expeditions to Morocco and Algeria to try to catch the return of the painted ladies to their wintering grounds.

By surveying swaths of North Africa, Stefanescu’s team confirmed that the painted ladies virtually disappeared from the area during the hot summer months and returned in huge numbers in October. The fliers arrived back in Africa just in time to feed on the daisylike false yellowhead (Dittrichia viscosa) and other flowers. The findings make clear how well the butterflies are able to time their migrations to take advantage of resources, Stefanescu reported in December in Ecological Entomology.

Flying far

Painted lady butterflies embark on one of the world’s most distinctive migrations, traveling thousands of kilometers from Africa into Europe each spring, and back again in the fall. It can take six generations of butterflies to make the round trip journey.

Other insect species are less visibly stunning than the painted lady, but just as important to the study of migrations. One emerging model species is the marmalade hoverfly (Episyrphus balteatus), which migrates from northern to southern Europe and back each year.

Marmalade hoverflies have translucent wings and an orange-and-black striped body. As larvae, they eat aphids that would otherwise damage crops. As adults, the traveling hoverflies help pollinate plants. “They’re useful for so many things,” says Karl Wotton, a geneticist at the University of Exeter in England.

Wotton started thinking about the importance of insect migration after 2011, when windblown midges carried an exotic virus into the southern United Kingdom that caused birth defects in cattle on his family’s farm. Intrigued, Wotton set up camp at a spot in the Pyrenees at the border of Spain and France to study migrating hoverflies. Then he heard that Menz was doing almost exactly the same kind of research at the Col de Bretolet and a neighboring pass. The two connected, hit it off and now collaborate in both the Pyrenees and the Alps.

Funneled by the high mountain topography, hoverflies whiz through the passes like rush hour commuters through a railway station. “We’re talking about an immense number of insects,” Menz says. Millions of flies traverse the Swiss passes each year. Extrapolating to all of Europe, Wootton estimates that many billions of hoverflies are probably migrating. The insects consume billions of aphids that otherwise would have feasted on agricultural crops.

As astonishing as this migration is, most people never notice it. Only at the passes do the hoverflies become noticeable, a never-ending stream of tiny bodies glinting in the mountain light. They ride high on tailwinds and scoot low when the wind is against them. “They fly fast and low and they don’t stop,” Wotton says. “The butterflies are getting turned around like in a tumble dryer, but the hoverflies just shoot straight over.”

Wotton, Menz and colleagues use specialized upward-looking radar to track signals reflecting off of insects passing overhead. The researchers also use traps to catch individual flies to identify the species passing through.

Season after season, the researchers are building up a hoverfly census. By comparing that information with a 1960s survey done at the Col de Bretolet, the team hopes to determine whether species’ numbers have changed over time. Menz says: “I wouldn’t be surprised if they’ve declined.”

Other entomologists have documented sharp drops in the numbers of insects across Europe. In October 2017, a Dutch-German-British research team reported in PLOS ONE that the total insect biomass collected at 63 nature-protection areas in Germany over 27 years had dropped by more than 76 percent.

The paper garnered media headlines around the world as heralding an “insect Armageddon.” That may be overly dramatic. The work covered just one small part of Europe, and the authors could not explain what might be causing the drop, whether climate change, habitat destruction or something else. But if hoverfly numbers are dropping, that would mean fewer are around to eat destructive aphids and to spread beneficial pollen. Hoverflies, which pollinate a wide range of plants, are the second most important group of pollinators in Europe after bees, Wotton says.

Hoverflies also migrate in North America, in ways that are far less understood than in Europe. This month, Menz and Wotton are visiting Montaña de Oro State Park on California’s Central Coast, where last year an entomologist reported spotting a rare hoverfly migration. The researchers hope to see whether the American hoverflies, probably a different species, are moving in the same ways their European cousins do.

Swoop in the destroyers

Not all migrating insects are beneficial. Some are troublemakers that chase ripening crops with the season. Farmers can spray pesticides once insects arrive in the fields, but knowing more about when and where to expect the critters can help growers better prepare for the onslaught.

Weather radar — Doppler data that meteorologists use to follow rain, hail and snow in near real time — is beginning to help. The radar signals reflect off of birds and other animals flying through the air. And although many insect species are too small to be detected in Doppler radar data, researchers are finding new ways to extract the signals of insects and track their migrations as they happen.

John Westbrook, a research meteorologist at the U.S. Department of Agriculture’s Agricultural Research Service in College Station, Texas, has been using weather radar to follow insect flyways in the south-central United States. A 1995 outbreak of two migratory moth species — beet armyworm (Spodoptera exigua) and cabbage looper (Trichoplusia ni) — devastated cotton crops in Texas’ Lower Rio Grande Valley. Westbrook recently dug through the Doppler data from 1995 and was able to pick out the signals of these two species moving during the outbreak, Westbrook and USDA colleague Ritchie Eyster wrote in November 2017 in Remote Sensing Applications: Society and Environment.

Tracking pests

Weather radar data from southern Texas reveal the higher reflectivity of flying crop-eating moths (red). During an outbreak in 1995 that destroyed cotton crops, the moths flew northwest across much of Willacy County (shown) in under an hour. Farmers could use similar radar data to track pests approaching fields.

“Outbreaks are unpredictable,” Westbrook says. “But the weather radar can show where they are occurring.” Modern weather radar contains even more information than 1995 systems did, he notes — and farmers can use that data to their advantage. They may decide to spray heavily where most of the insects are gathering before they spread. Or farmers might stock up on pesticides if a particularly dangerous outbreak is headed in their direction.

 Another way to track destructive insects is to grind them up and test the chemistry of their tissues. As caterpillars grow, they take on a characteristic chemical signature of the environment, with hydrogen, oxygen and other elements fixed in tissues in varying amounts. Analyzing those ratios can reveal the geographic region of a caterpillar’s origin.

Keith Hobson of Western University in London, Canada, and colleagues have been studying the insect pest known as the true armyworm moth (Mythimna unipuncta). It travels between Canada and the southern United States every year, damaging crops along the way. But scientists weren’t sure exactly where the insects originated each year, making it harder to figure out how to manage the problem with pesticides.

In new experiments, Hobson’s team captured true armyworm moths in Ontario throughout the year and analyzed the hydrogen retained within the moths’ wings. Moths captured early in the season had values similar to those seen in Texas waters, while those captured in the summer showed values closer to Canadian waters. The reverse was also true: Adult moths captured in autumn in Texas had Canadian-type values.

It is the first direct evidence that individual moths are making these long-distance round trips, the scientists wrote in January in Ecological Entomology. Further studies could reveal how to better control the pests throughout the growing season, by showing precisely where the insects are coming from and how far they will travel.

The migrating masses

For Menz, Wotton, Satterfield and the rest, the ultimate goal is to go from studying individual species to investigating broader questions of how and why animals move around. That includes exploring how insects alter food webs during migrations across the landscape.

For instance, Mexican free-tailed bats (Tadarida brasiliensis) in Texas and Mexico forage for nocturnal moths, which migrate in very narrow layers in the atmosphere based on how the wind is blowing. “These are like food webs in the sky,” says Jason Chapman, an ecologist at the University of Exeter. “Can bats read the weather patterns and predict where the insects are going to be?”

Similarly, many dragonflies attempt to migrate 3,500 kilometers or more across the Indian Ocean from India to east Africa and back each year, breeding in temporary ponds created by monsoon rains. The dragonfly-eating Amur falcon (Falco amurensis) makes a similar journey, in one of the longest-known migrations for any raptor. If the dragonflies are the reason for the falcon migration, then tiny insects are a major player in this important bird movement.

Insects rule the migratory world by virtue of their sheer numbers. Compared with birds, mammals and other migratory animals, insects are by far the most numerous. Roughly 3.5 trillion migrate each year over just the southern United Kingdom, a 2016 radar study suggested (SN: 2/4/17, p. 12). That means that the majority of land migrations are made by insects.

To Aislinn Pearson, an entomologist at Rothamsted Research in Harpenden, England, studying insects will boost scientific understanding of how animals flow around the planet. “In the next 10 years,” she says, “a lot of the key findings of migration are going to come from these tiny little animals.”

This story appears in the April 14, 2018 issue of Science News with the headline, "Mass migrations: Researchers are asking big questions about animal movements by tracking tiny insects in flight."