During a trip to Louisiana in mid-July, I decided to show my wife some of my old stomping grounds. We headed north from Bastrop, La., towards Hamburg, Ark. Just before the Arkansas state line we passed through a couple of large clear cuts. In these clear cuts were tens of thousands of dragonflies. They were so thick that I slowed the car to keep from splattering them.

I grew up in northeast Louisiana at a time when clear cutting was a common practice. But in my 30-plus years there, I never saw swarms of dragonflies like these.

By the time we encountered the swarms, my two-and-a-half year old was tiring of her car seat. I didn’t stop to investigate, but I know that green darners, Anax junius, were present.

I did a little Internet research and found that while the phenomenon of swarming dragonflies was nothing new, the reasons behind it aren’t clear.

One report from The American Midland Naturalist describes massive autumn migrations of dragonflies from Chicago in 1978 and Cape May, N.J., in 1992 and at Crescent Beach, Fla., in 1993. The number of dragonflies estimated at Chicago was 1.2 million, for Cape May more than 400,000, and approximately 200,000 at Crescent Beach.

The abstract from The American Midland Naturalist went on to state: “We also document other recent observations of large swarm migrations of dragonflies in eastern North America, review previous reports of this phenomenon, and compare these events to the flights at Chicago, Cape May and Crescent Beach. Records of large dragonfly migrations show several distinct patterns: (1) all reports fell between late July and mid-October, with a peak in September; (2) most of the large flights occurred along topographic leading lines such as coastlines and lakeshores; (3) massive swarm migrations generally followed the passage of synoptic-scale cold fronts; and (4) the common green darner (Anax junius) was the principal species involved in the majority of these flights. Striking parallels between the patterns of seasonal timing, geographical distribution, and meteorological context of dragonfly migrations and those of birds suggest that similar causal factors are involved.”

Another study, one from the University of Illinois, talks about swarms of dragonflies and notes that swarms seem to occur in areas of high prey density. But this study defines a swarm as greater than 50 individuals. It is not uncommon in northeast Louisiana to see 50 or more dragonflies feeding in an area, but as I mentioned earlier I have never seen congregations of tens of thousands.

Some info gleaned from The North American Dragonfly Migration Project notes: “The observations at Crescent Beach suggest that some may reach peninsular Florida, and the Dennis Paulson’s records of seasonal occurrence of green darners in southern Florida is consistent with the idea that migrants arrive there in autumn and at least some of their descendents leave after emergence in spring. Green darners and wandering gliders have been seen flying steadily westward along the Gulf Coasts of Florida and Louisiana, sometimes in large numbers. Also, vast concentrations of green darners, along with other, mostly migratory, dragonflies, sometimes feed along the beaches of Florida from August to early October.”

The problem I have trying to get my mind around those swarms in Louisiana is the term itself. When swarm is used to describe any group from 50 to a million it leaves a lot open to interpretation.

Fifty dragonflies or 100 or 1,000 in a clear cut wouldn’t have piqued my curiosity. But tens of thousands do.

Are there so many dragonflies in northeast Louisiana that a prey-hatch would cause them to congregate in such large numbers or could they be massing for migration? Mid- to late July seems a bit early for migration, but The North American Dragonfly Migration Project cites vast swarms of green darners (and I know green darners were present in Louisiana) feeding along the beaches of Florida in August so, maybe not.

Perhaps answers will be forthcoming as The North American Dragonfly Migration Project gathers more information. To find out about the project do a Google search or email may@AESOP.RUTGERS.EDU.

(Don Hendershot can be reached at ddihen@earthlink.net)