CHASING the DRAGON
Thanks to citizen scientists and new technologies, we are finally learning about the Common Green Darner, an amazing migrating dragonfly
Susan Blayney started out watching birds. Next it was butterflies. Now it’s dragonflies. Come spring the retired former registered nurse will be out walking her 62-hectare, old-farm property near Fenelon Falls in Ontario’s Kawartha Lakes region, keeping her eyes peeled for the return of the common green darners — the first dragonflies of the season. “In the morning, if you go out before the sun is strong and they get the energy to fly, you can find them in the grass,” says Blayney. “They’re cold-blooded creatures, so they have to warm up.”
Don’t let the name fool you. As dragonflies go, the common green darner (Anax junius) may be, well, common, but it’s far from ordinary. Common green darners are among North America’s largest dragonflies, as big as hummingbirds — about eight centimetres long from the tip of their striking iridescent green heads and thorax to the end of their bright blue (male) or dull red, green or brown (female) abdomens, with an eight- to nine-centimetre wingspan. Despite their size, their larvae develop from egg to emergent adult sometimes in a matter of weeks, compared with up to four years for other species. And they are equally fast in flight, reaching speeds of more than 55 kilometres per hour. Incredibly, like all dragonflies, green darners are carnivores that catch their prey — moths, mosquitoes and flies — on the wing, using their amazing directional flight control and spiny legs to haul it in.
Perhaps most awesome, however, is that green darners are also formidable distance fliers, one of just 16 dragonfly species in North America (out of more than 300) that regularly migrate south in autumn and return north in the spring. However, while they’re known to migrate — green darners gather and depart in swarms in the fall, and they are seen wintering in the southern U.S., Mexico and even into Central America — the details of that migration and their annual life cycle have been shrouded in mystery.
Are the green darners in the south the same dragonflies that leave Canada in the fall, for example? Are they, like birds, the same ones that show up in Susan Blayney’s fields the next April? Or, like monarch butterflies, do they cycle through several generations, each completing a leg in the journey, before making it back?
Such questions reflect how little is actually known about the annual cycle for many migratory insects. It’s a knowledge gap that scientists say hampers the development of coordinated conservation action to protect these migrations and, in the case of dragonflies, the sensitive wetland habitats on which they depend.
“We have records of common green darners going as far south as Guatemala and Belize and, I think, even Honduras,” says Colin Jones, provincial arthropod zoologist with the Ontario Ministry of Natural Resources and Forestry’s Natural Heritage Information Centre. “So, we’ve known that they’ve gone that far. But we didn’t really know what they were doing down there.”
Now, a major new research study, initiated by the Migratory Dragonfly Partnership, a collaboration of scientists, educational institutions, NGOS and government agencies in Canada, the U.S. and Mexico, has put those questions to rest.
To do it, a team of five researchers — three from the Migratory Bird Center at the Smithsonian Conservation Biology Institute in Washington, D.C., and two from the Vermont Center for Ecostudies in White River Junction, Vermont — conducted stable-hydrogen isotope analysis on 852 green darner wing samples from eight countries spanning 140 years. They then combined that analysis — which can determine the approximate latitude of the water body where each dragonfly hatched — with 21 years of citizen science data tracking the timing of the darner’s seasonal migrations against daily average temperature.
Their conclusion: the charismatic insect’s annual cycle involves at least three generations of dragonflies — the first making the full trip north, the second making the return flight back, and a third staying put in the south. But there’s also some mind-bending overlap. While many of the second-generation nymphs emerge as adult dragonflies just weeks after hatching here in the north, others enter a state called diapause, overwinter as nymphs and don’t emerge until the following spring. From a timing standpoint, the green darner’s movement north and south is strongly linked to air and water temperature.
In all, the results at once confirmed suspicions, dashed old assumptions and gave scientists a few new wrinkles to consider.
Kent Mcfarland is a conservation biologist and one of the two Vermontbased researchers on the common green darner study. Though he’s an accomplished veteran scientist, and the isotope work he led on this project is cutting-edge research, he sounds a lot like amateur naturalist Susan Blayney as he describes his anticipation of the green darner’s return each spring. “I like to go to the local beaver ponds in April, when the ice is first coming off. And the first time I see them buzzing around the pond, I’m like: ‘Cool, that thing probably came from really far south,’” says Mcfarland.
At the same time, he neatly articulates the critical significance of their study’s findings. “Being able to put this first cut on it, to be able to see the minimum and maximum migratory lengths of some of these individuals is pretty amazing. And then to be able to actually sink into what’s going on with some of these generations and where they’re occurring was pretty new for me,” Mcfarland says.
According to the team’s research, the green darner story unfolds with the first generation of adults in the annual cycle originating in the south between February and May. Those dragonflies then migrate north a minimum of 650 to 700 kilometres. This group includes the early arrivers that Mcfarland and Blayney see in their locales. Based on previous estimates of daily migration rates, it takes the dragonflies 50 to 60 days to make these trips, though favourable winds may speed that up. Once in the north, they breed, and the females lay their eggs in aquatic vegetation in quiet ponds and pools.
The offspring that hatch in the north grow as larvae under water and then crawl onto land when they’re ready to emerge from their final larval skin as adults (a process called eclosion), making up the second generation. However, there are two distinct cohorts within this group. The first to emerge are those that, as noted, began as eggs laid the previous year but then overwintered in diapause rather than hatching the same year. These green darners emerge as adults between May and July. The second cohort is made up of young that emerge from eggs laid the same year. Their numbers peak later, in September. It’s believed the reason that some don’t eclose the same year, but overwinter as larvae, is due to water temperature. If it drops in late summer before they are far enough along in their larval development, they enter diapause, biding their time until the waters warm again next year.
The second generation is the one that migrates south, starting as early as July and August, peaking in early fall. The study samples revealed that these dragonflies fly at least 500 to 850 kilometres before they stop to breed and die. Their offspring, which hatch in the south, make up the third generation. Unlike the others, this group of adult dragonflies are non-migratory. Their offspring, born early in the new year, are the ones that head north. And the cycle repeats.
The isotope analysis that enabled researchers to deconstruct this life cycle is one that, to date, has been used most commonly to determine approximate origin locations for migratory birds. It is based on the fact that the chemical makeup of precipitation varies over the continent with latitude, explains Michael Hallworth, one of the study authors affiliated with the Migratory Bird Center in Washington. “The unique signature gets incorporated into ponds and food webs, so that when different organisms are eating or developing in those ponds, that information gets incorporated into their tissue,” he says. “When the dragonflies emerge as adults, they hold that chemical signature. And wherever they are captured, that signature lets you place it back roughly where it was growing that tissue.”
It took Mcfarland and another colleague two years to gather the green darner wing samples for this study. They needed a large number to get representation from all seasons over the entire eastern half of North America. They captured and collected some themselves, enlisted citizen science volunteers to gather others, and got the balance from specimens held in archived collections in different countries (which explains the enormous age range in their data set). Canadian samples were obtained from the Royal Ontario Museum, the University of Guelph, the Canadian National Collection of Insects in Ottawa and Ontario’s Natural Heritage Information Centre.
Dragonflies — whose existence goes back more than 300 million years to before dinosaurs — respond faster to climate change than most any other organisms
“The museums were a little tricky at first, because in order to sample these things, we had to destroy the tissue sample. It gets burned up so we can find out how much deuterium [hydrogen isotope] is present,” says Mcfarland. But because they’re a common species and they only needed a “minuscule chunk off of the end of one wing,” none of the insitutions turned them down.
The fact that the research team on the green darner study had more experience studying birds and bird migration than insects reflects a significant scientific crossover. Jones, from the Natural Heritage Information Centre and a member of the Migratory Dragonfly Partnership steering committee that initiated this project, says their isotope expertise was essential. “When we first started the partnership, we said maybe there’s a way we can do the same technique with dragonflies. They worked on it, and they figured it out.”
Given the extent to which other forms of tracking technology have revolutionized scientific understanding and knowledge of bird migration in the past decade, the prospect of further cross-fertilization and new revelations about the green darner and other dragonflies seems likely. “It’s just starting to blossom now, which I think is fantastic,” says Hallworth.
By way of example, he cites how weather radar is now used to detect mass movements of both birds and insects. But the real holy grail for dragonfly researchers would be the development of radio and GPS transmitters that could be attached to dragonflies, enabling scientists to trace their movements in precise detail — directions flown, daily distance travelled, stopover locations and so on. A decade ago, in the case of birds, this sort of gear, including batteries, was heavy enough that it could be used only on large birds. Since then, miniaturization has made it possible to do radio and GPS tracking with birds as small as warblers and thrushes.
“Everyone’s hope — we talk about this every year it seems at the dragonfly meetings — is that the radio transmitters will get smaller, and then we’ll be able to put them on smaller dragonflies,” says Jessica Ware, a Canadian entomologist who is an associate professor at Rutgers University in New Jersey and secretary of the World Dragonfly Association.
It does already seem possible for the largest dragonflies, however. In 2005, Ware’s former PHD adviser, now a Rutgers professor emeritus, Michael May, proved it with a small study that fixed radio transmitters with eyelash adhesive and superglue to the thorax of 14 common green darners during autumn migration. The darners, which have big flight muscles in their thorax, were able to fly normally, and May and his team tracked them with radio receiver antennas for several days until they were out of range (140 kilometres). The results showed the green darners flew south and navigated water bodies much like birds.
Mcfarland envisions a day in the near future when more dragonflies will be tracked this way. “We can know what they’re doing all the time. It would be amazing.”
While documenting the green darner’s three-generation annual cycle stands as the main takeaway from the present study, temperature’s influence on nymph development and the timing of the adult migration — and the fact that this latter information was drawn mainly from citizen science observations — can’t be overlooked.
As climate continues to warm, the impact could include faster nymph development, a shifting migration schedule and the green darner’s expansion into areas farther north. Whether these outcomes will be positive, negative or just different will need more study and time to say.
Rutgers’ Ware highlights the importance of such research, noting that dragonflies — whose existence goes back more than 300 million years, pre-dating dinosaurs — respond faster to climate change than most organisms, “so they will probably be the first to start changing where they go and their routes.” She cites the example of a close green darner relative, the emperor dragonfly (Anax imperator), which is found in central Asia, Africa and, increasingly in the past 20 years, Europe. “It used to be just found as far north as Tunisia, in northern Africa, but with climate change allowing warming temperatures, it invaded as far north as Sweden and now has established populations there,” says Ware.
All of which means the more average citizens who, like Susan Blayney, move on from watching birds and butterflies to dragonflies, the better. (Blayney, for her part, has honed her skills to the point where she now leads public dragonfly counts around her region.) And once they start watching, Mcfarland encourages people to report sightings to online databases like inaturalist or the dragonfly- and damselfly-specific Odonata Central.
“We couldn’t have done our work without a whole bunch of data from people looking for them all the time,” he says. “So, I always tell people, every time they see something like a common green darner, if they can put it in one of the crowd-sourced databases, it’s going to be useful one day for sure.” a
Green darners are formidable distance fliers, one of few dragonfly species in North America known to migrate between Canada and as far south as Central America