THEY SWARM at mountain summits, along dirt roads, in back yards and over meadows. Some are migrating; some are hunting. But mostly they’re doing what dragonflies do — flying, killing things and having sex. Here’s the scoop on dragonfly swarms.
August is prime time for flights of the large “Mosaic Darners” in the genus Aeshna. “Devil’s Darning Needle” is a colloquial term for these insects. They are the big and marked in pastel blues, greens and yellows.
North America has about 16 members of this genus, all of which can be identified by the shape of the two stripes on the thorax. Pictured here is a Variable Darner (Aeshna interrupta) at a bog in Vermont. Those broken side stripes are diagnostic here in the East.
Dragonflies mature from egg to growing larvae (nymphs) in water — rivers, lakes, bogs, marshes, roadside ditches and backyard ponds. Aeshna, most abundant at lakes and ponds, can undergo a mass emergence from their natal waters to fly as adults in big numbers. And if they’re not having sex, dragonflies spend a good portion of their day flying around and killing things. So much of what we’re seeing now amounts to swarm-feeding or swarm-foraging.
Swarms form around prey species, often small flies. On the wing dragonflies patrol, nab a flying insect, dispatch it with a swift bite and most often land somewhere to finish its meal; sometimes they eat on the wing. In these feeding swarms, dragonflies can track and follow aggregations of prey species as they shift in location and abundance.
Some dragonflies can even locate food using past experience and topographical clues. One researcher in Costa Rica (Young 1980) noticed Roseate Skimmer (Orthemis ferruginea) gathering in good numbers to feed over trays of black-pepper berries fermenting and drying in the sun. The skimmers were feeding on fruit flies and other insects attracted to the berries. When the trays were removed, the Roseate Skimmers departed post haste. But when empty trays were returned to the site, the skimmers returned to investigate, apparently keying on the visual clue as a source for food.
Other causes for swarm formation include dragonflies and their prey gathering in the lee of an otherwise windy setting, for example, or in a sun-dappled clearing in the woods where evening light makes prey easier to locate (Corbet 1999).
August, at least here in the East, is also the month for swarms of an amazing dragonfly called Wandering Glider (Pantala flavescens). About three inches long, Pantala is the color of gold and a flying machine. It is found on every continent except Antarctica. Its long, wide wings allow this dragonfly to cross oceans, making Pantala a champion flier among dragonflies, in many respect the albatross of insects.
It is also steeped in mythology. Pantala, for example, is a symbol of courage and victory in Japan. Large numbers appear in Honshu (the nation’s main island) and Kyushu (the third largest island) around August 15, a date on which Buddhists believe ancestral spirits visit their homes.
On August 13, 2009, Pantala flavescens invaded Montpelier, Vermont (where I live and have an office), probably numbering in the thousands. Exactly one year later, on August 13, 2010, they came again. Thousands of these elegant dragonflies swarmed the city. They cruised the intersection of State and Main. They hovered in the parking lot of the Shaw’s grocery store. And they patrolled in front of the Vermont Statehouse. Few people noticed the Wandering Gliders that day, at least until I went downtown with a net and started swinging. I’ve often seen them in mid August in years since (including now in 2016).
We see dragonfly migration mostly in the fall. In fact, birders often notice migrating dragonflies from hawkwatch sites. One of my best encounters came while birding on Monhegan Island, off Maine’s midcoast, in September of 2009. It was a river of dragonflies, almost entirely Common Green Darners (Anax junius) heading out to sea, southbound for who knows where.
But we’re learning. Researchers are attaching tiny radio transmitters to Anax junius and other migratory dragonfly species in order to track their movements (Wikelski 2006). Others can catch a migrating dragonfly in, say, New Jersey, and by measuring trace concentrations of certain molecules (stable isotopes) in its tissue can learn about where it grew up. It turns out, among dragonflies, you are what you eat. What a dragonfly ate while growing up can give a biologist a clue to where it was born (or hatched in this case) no matter where or when it is caught by a researcher. With tools like these only now are we beginning to learn how far and for what reason dragonflies are migrating. I’ll cover migration in a separate blog post. But here’s a video from a swarm I encountered in Michigan on July 24, 2010.
If you want more, check out The Dragonfly Woman’s blog post on swarms. I heartily recommend her site. The Dragonfly Woman is tracking swarms around the continent.
Corbet, P.S. (1999). Dragonflies: Behaviour and ecology of Odonata. Essex, Harley Books, 829p.
Wikelski, M., Moskowitz, D., Adelman, J. S., Cochran, J., Wilcove, D.S., and May, M.L. (2006) Simple rules guide dragonfly migration. Biol. Lett. (2006) 2, 325–329
Young, A.M., (1980). Observations on feeding aggregations of Orthemis ferruginea(Fabricus) in Costa Rica (Anisoptera: Libellulidae). Odonatologica 9:325-328.