They swarm at mountain summits, along dirt roads, in back yards and over meadows. Some are migrating; some are mating. But mostly they’re doing what dragonflies do — flying around and killing things. 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 pair of Variable Darners (Aeshna interrupta) copulating on my front porch; in this species the stripes are broken — or interrupted.

Swarm Foraging
Dragonflies mature from egg to growing larvae 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. In these feeding swarms, dragonflies can track and follow aggregations of prey species as they shift in location and abundance. They 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. 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).

Migration
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. Pantala is a champion migrant.

Most adult dragonflies live out their lives at a single site – a pond, a bog, a river – over the course of days, weeks or, rarely, a few months. But some live long and migrate like birds. We see it 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 post, video and photos from a Pantala 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 this site.  The Dragonfly Woman is tracking swarms around the continent. If you’re seeing a swarm, please report it to me in the comments section of this post.

References

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.

Pantala flavescens

What’s amazing about today’s flight is its timing. Exactly one year ago, on August 13, 2009, Montpelier hosted another Pantala arrival. I’ve been seeing them in town for the past few weeks, but not in the numbers I noticed today. Must be something about the 13th. Aren’t we lucky!

As I’ve written before in a post about dragonfly swarms, Pantala flavescens is a flying machine. It is found on every continent except Antarctica. This dragonfly can cross oceans, making Pantala the champion flier among dragonflies, in many respect the albatross of insects. Along the way, it feeds on aerial plankton, insects, spiders and smaller organisms floating not on ocean currents but on air currents. Pantala can also store fat, like birds, to power long-distance flight.

Although dragonflies here in the West are sometimes viewed as agents of the devil (“Devil’s Darning Needles”), Pantala 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 Buddhists believe spirits visit their homes.

After spotting a Giant Swallowtail in Loudonville, NY, this May, I fully expected them to make it to Vermont at some point this year. Sure enough one was spotted and photographed in Addison last week. This was all the encouragement I needed to search for one on my own. My first stop was the Bennington Dump. Before you scoff, know that reclamation work at the dump included prairie wildflowers planted by Taconic Tristate Audubon Society. I have found many interesting butterflies and dragonflies at the site.

With no Giant Swallowtails, however, I decided to check a small retention pond where I had found Libellula cyanea (Spangled Skimmer), a rarity in Vermont, a few weeks previous. Almost immediately I spotted a patrolling male Tramea carolina (Carolina Saddlebags). This is a Vermont record so I went about trying to voucher it. A good 45-60 minutes later, I was successful.

This got me curious about a site where I had seen many T. lacerata (Black Saddlebags) earlier this year. I decided to check a spot at South Stream. My luck was good. Three male T. carolina were patrolling and I noticed one tandem pair.

The southern air flow may have made the last few weeks uncomfortable but, they have given Vermont a few nice surprises in return. What’s next?

Once again we slogged through bogs, waded into crystalline rivers and enjoyed the company of colleagues in the great pursuit of dragonflies. The Dragonfly Society of the Americas wrapped up its annual meeting in Orono, Maine, Sunday night. Some of us are heading north to Jackman (in the rain) for a post-meeting field trip that could include rare boreal species, such as Coenagrion interrogatum (Subarctic Bluet) and Somatochlora brevecincta (Quebec Emerald). That damselfly above is a Calopteryx aequabilis (River Jewelwing), a constant companion throughout this meeting. Highlights from Sunday included Mike Blust netting two Ophiogomphus howei (Pygmy Snaketail) from the Machias River and Hal White getting a female Somatochlora incurvata (Incurvate Emerald) from a small spruce bog. After our meeting and cookout Saturday, a few folks nabbed Neurocordulia yamaskanensis (Stygian Shadowdragon) from Pushaw Stream. During this meeting, many of us also got looks at black bear and moose. For a few more images and our preliminary species list

Here are some images. I’ve included a species list below.

Here’s our PRELIMINARY species list (92 species). It now includes reports from the pre-meeting field trip in and around Fryeburg and the post-meeting trip in and around Jackman.