Airborne Mobs on the Hunt or in Migration
Updated September 3, 2019
They swarm at mountain summits, along dirt roads, in back yards and over meadows. They flash pastel blues, carmine reds and sacred golds. And for the most part, they’re doing what dragonflies do: fly around, kill things and have sex. Some are migrating.
Many of these dragonfly swarms represent a localized rite of summer, which is prime-time for flights of the large “Mosaic Darners” in the genus Aeshna. These impressive dragonflies are big and marked in blues, greens and yellows. And most aren’t migrating — they’re killing.
North America has 15 members of this genus, all of which can be identified by the shape of the two stripes on the thorax (the thick segment where the wings and legs are attached). Note, for example this Variable Darner (Aeshna interrupta) at a bog in Vermont on 17 Aug 2012. Those broken blue side stripes are diagnostic here in the East. Compare it to the Blue-eyed Darner (Rionaeschna multicolor) on the wing above, which I photographed in Arizona on 8 April 2010.
All dragonflies mature, from egg to growing nymph, in water: rivers, lakes, ponds or wetlands (even roadside ditches). Aeshna, most abundant at lakes and ponds, can undergo a mass emergence from their natal waters to fly as adults in big numbers. If they’re not having sex, dragonflies spend a good portion of their day flying around and killing things. So much of what you’re seeing in summer or fall amounts to swarm-feeding or swarm-foraging.
Swarms form around prey species, often small flies. On the wing, the 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, it seems, 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 and September, at least here in the East, is also prime time for 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, Wandering Glidersinvaded 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. 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 2019).
Although about 16 of the 326 or so dragonfly species in the U.S. migrate in one way or another, we’re tracking the movements of five classic migrants. Learn about that at the Migratory Dragonfly Partnership. Or check out news of the multi-generational migration of Common Green Darners (Anax junius), covering more than 600 km (373 miles) on average, with some individuals flying more than 2,500 km (1,553 miles).
Finally, see a swarm for yourself in my video from Michigan on July 24, 2010.
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.