What is something that birds, bats, butterflies, and dragonflies all have in common?
Well, yes, they do fly. But something that doesn’t occur to the typical person not well-versed in these animal types is that all of these creatures migrate. Now that summer is done, the days are getting shorter, and the air is a bit cooler out there, we can expect to see these animals on the move soon.
This blog is the 4th part of a short series on migrating animals. The final topic: dragonflies!
Of all four topics I am covering in this series, scientists seem to understand dragonfly migration the least. This is likely because not much effort has been put into the subject: dragonflies are of not great economic importance, and the best known species that migrates, Common Green Darner, is widespread and abundant and so there is little concern about its future. In general, dragonflies are not very sensitive to water pollution, and can thrive in man-made or naturals wetlands. This is in contrast to some bat species and the Monarch butterfly, which are very specific in their over-wintering habitat selection.
Of the 326 species of dragonflies in North America, about 18 are regular migrants. Besides the Common Green Darner (Anax junius), other migrating dragonflies include the Wandering Glider (Pantala flavescens), Spot-winged glider (Pantala hymenaea), Black Saddlebags (Tramea lacerata) and Variegated Meadowhawk (Sympetrum corruptum).
The dragonfly life cycle
Dragonflies are fascinating creatures, with strange mating, flying, and territorial behaviors. This complexity applies to migration as well, which certainly contributes to the sense that scientists have yet to unravel dragonfly migration.
For example, work done by entomologist R. Trottier in 1971 revealed Common Green Darner could adopt two distinct life-cycles. One group of darners had the standard dragonfly life-cycle: nymphs emerged as adults in June, laid eggs in the summer, and died by late August. The eggs would hatch into nymphs that would overwinter locally in the bottom of streams and ponds and then emerge again the next June. (Side note: Dragonflies spend 1-3 years in the nymph stage, depending on the species).
The other group of the darners did not emerge until late-August, and rapidly disappeared from local ponds and wetlands as they began a migration south. Their children would be the ones to return in early April and continue the generational cycle. In this population, migration is a normal part of the life cycle. In other words, just like the Monarch butterflies, dragonfly migration is a one-way ticket for any given individual. The first generation will travel south, reproduce and die, and the children will head north. They will reproduce and die and their children will go south.
However, not all dragonflies migrate, even within the same species. This is a complication that is not well understood.
Timing and Destination
In 2006, researchers attached micro-radio transmitters to Green Darners and followed them along their migration for 12 days. On average, they traveled 30-40 miles in a 5-7 day period, eventually going an average of 400 miles. Another study recorded a maximum observed distance of 2200 miles.
Like the other animals we have studied, dragonflies rest for several days at a time while on the migration route, so the total migration time can last many weeks. From the Mid-West and Northeast United States, they are able to reach the Gulf Coast states and occasionally Mexico.
Cold nights seems to trigger dragonfly migration, just like with birds. Dragonflies will began their journey south in mid-August, and will continue through the end of October. The dragonflies use northerly winds that follow from cold fronts to speed them on their way, and can be seen traveling in swarms of hundreds of thousands, though they also travel as individuals and small groups.
Like birds and butterflies, it seems like they navigate using some type of internal magnetic compass and using topographic features like lakeshores and coastlines. Another interesting observation in the 2006 study is that dragonflies can alter their migration route considerably (in this case, by 120 degrees) in order to avoid flying over large bodies of water.
Check out this webpage: Migratory Dragonfly Partnership. This partnership is a collaborative effort between universities, dragonfly experts, the federal government, and nongovernmental programs with the goal of learning more about dragonfly migration.
The Partnership welcomes dragonfly observations from citizen scientists!
Thanks for reading!
It has been great fun for me to research these migrations and learn about new things (for me) that I had been wondering about for some time. I hope you enjoyed the series too!
-Paul Steen, HRWC Aquatic Ecologist.