The Migratory Common Green Darner: An Ephemeral Wetland Winner!

Most of the 380 Common Green Darner Exuviae Collected at the Homer Lake Wetland, July 2 to September 11, 2021

How do you spell success? Certainly in the natural world, one good measure of success is successfully producing the next generation. Breeding in an ephemeral wetland, which is almost certainly going to dry up towards the end of the summer, is a strategy with both risks and rewards. For many of the species that breed here, one of the greatest rewards is the absence of fish. The risk though, as we've been seeing, is that if your young are dependent on the aquatic environment and the wetland dries up before they've matured, you've lost the bet. This is the sad reality for hundreds and hundreds of green frog and bullfrog tadpoles, for example.

For the migratory common green darner, however, it is a different story -- the gamble appears to have largely paid off. In order to better understand how successful these dragonflies were in producing the next generation in the Homer Lake Wetland, from the day I first saw an adult common green darner emerging from the waters of the wetland, I began a regular route around the perimeter of the wetland to collect the cast-off exoskeletons ("exuviae") in order to keep a count. From July 2nd through September 11th I collected 380 common green darner exuviae (pictured above). It is certain that I didn't find every exuviae left behind by the emerging adult common green darners, so this is at best a representative sample of the breeding success of this particular species of dragonfly at this wetland. They appear to be the most successful dragonfly species at the wetland, as measured by the number of naiads emerging as adults (though I did not get a good sense of the numbers of meadowhawks emerging during the month of June, so the ruby or blue-faced meadowhawks may also be contenders for the crown).

What makes the common green darners so successful? Because they are migratory, they arrive at the wetland long before any of the local overwintering populations have emerged. This year I first saw male common green darners patrolling the waters of the wetland in search of females on March 22 and I first saw pairs breeding and laying eggs on April 6th. This gave the naiads three full months to mature and emerge largely during the month of July (235 exuviae collected in July). For the sake of comparison, I saw another migratory species, the Carolina saddlebags, at the wetland on April 9th but did not see any locally emerging species at the wetland until the end of May. This means that the migratory common green darners had a two-month head start over any of the locally emerging dragonflies!

It must though be noted that the common green darners actively breed all summer long, so the offspring of those breeding later in the summer were as unlikely to emerge as adults from the wetland before it dried up as any of the other dragonfly species that emerged locally and began looking for places to breed later in the summer. The photo below is of a common green darner naiad observed on August 4th. At that point, this naiad was only a little over half a centimeter in length. A fully-developed common green darner naiad ready to emerge as an adult could easily be 8x that length, so it's questionable if this little fellow would have been able to make it to maturity before the wetland went dry toward the end of September. I saw pairs of common green darners laying eggs at the wetland as late as September 15th with the wetland being completely dry (the first time) only five days later. In other words, the early common green darners seem to have had a great deal of success, but others that came later to the party were probably as unsuccessful as any of the other dragonfly species that require water over the winter in order to emerge in the Spring.

Young Common Green Darner Naiad (approx. 6mm in length), August 4, 2021