The journey of dragonflies
Fall is remarkable for the numbers of bird and insect migrants moving through the Florida Panhandle, from ruby-throated hummingbirds to large, orange monarch butterflies and smaller Gulf fritillaries that flutter in front of cars crossing our many bridges.
Residents have always known that large dragonflies also become very abundant in this season, but few people suspected that they are also long-distance migrants.
The largest and best-known dragonfly in North America is called the common green darner, which was named because it resembles needles that were once used to darn socks. The thorax (where the wings attach) is bright green, but the males also have a sky blue abdomen. Green darners live everywhere from Florida to Alaska. Because the nymphs can overwinter under ice, it is not clear why the adults would need to migrate. However, the northern migration opens a vast breeding area with very abundant insect prey, especially mosquitoes!
However, birdwatchers have noticed swarms of dragonflies moving south from Canada and New England. For example, Frank Nicoletti went to Minnesota’s Hawk Ridge Nature Reserve in 1995 to count small hawks called kestrels. He saw a swarm of green darners moving along the ridge and began to count them. That September, he tallied 1,106 kestrels and an amazing 10,330 green darners. Towards evening, the hawks would often capture and eat the dragonflies.
Mass migrations of green darners only occur during daylight in the fall from late August to October. The flights happen as cold fronts move in from the north with southerly prevailing winds. The cold weather reduces the amount of insect prey that the dragonflies feed on, and greater food availability in winter is probably one reason the dragonflies head south.
Adult green darners weigh about 1.2 grams, about the same as a dollar bill or paper clip, but they are capable of carrying prey that weighs more than they do. A team including Martin Wikelski found that the dragonflies could fly with miniature radio transmitters attached with super glue. He and his colleagues tracked the insects using aircraft; Canadian scientists have also used a network of towers to track the radio signals.
Migratory dragonflies fuel their flights by storing fat equal to one quarter of their body weight. They normally fly for a day or two, and then forage for food before continuing. The insects averaged about 6 miles a day, but the maximum range is about 120 miles a day. The migration continues south for up to two months.
Green darner dragonflies can cruise at 10 mph with peak speeds over 30 mph, so it is likely that mass migrations ahead of cold fronts travel much farther and faster. They would have to cover over 1000 miles to reach Florida from New England or Canada, and new evidence suggests that they can achieve this.
Scientists at the Smithsonian Institute learned that when adult dragonflies emerge from nymphs, their wings are “stamped” with the abundance of certain atoms (hydrogen isotopes), or markers, that differ going from south to north. They found that dragonflies that emerged in the south (with southern “markers”) flew north as temperatures warmed to about 48 ° F. They made their way as far as Canada by late spring, laid eggs to produce a new generation and then died. During the spring trip, the dragonflies do not form large swarms, but travel as individuals or small groups.
Similarly, the second generation of dragonflies that emerged in the north during the summer began making their way south as the temperature cooled in the fall. Their wings showed the northerly isotope pattern, and the migrants could be tracked back to Florida. Upon arrival, they probably mated, laid eggs, and died. By the end of October, the northern markers had disappeared.
The third generation of adults that emerges in the South during the fall does not migrate. They form a resident population that reproduces to produce overwintering nymphs. These nymphs emerge as winged adults in the spring and begin their migration north. Therefore, at least three generations are required to complete the round trip.
More generations may be required, because most adult green darners live 5 to 7 weeks and travel less than 400 miles. It seems likely that eggs laid on the journeys north and south also produce migrants. Taken together, the evidence suggests that migration of green darner dragonflies takes three or more generations, similar to monarch butterflies.
The Migratory Dragonfly Partnership
Several species of dragonflies appear to migrate. Besides the common green darner, other migrants include saddlebag dragonflies, meadow hawks and yellow gliders. A few years ago, a network of citizen scientists was set up to help find out where the dragonflies were going. The Migratory Dragonfly Partnership was established to connect volunteer observers around the United States with research scientists who need help answering questions.
For example, it has not yet been possible to track individual dragonflies over their entire journey, or to learn their destinations. Large waves of green darners have been seen flying from east to west along the Gulf Coast, but no one knows where they disperse. It is possible that some fly south into Mexico.
Another intriguing fact is that, unlike monarch butterflies and birds, no dragonflies live long enough to return north. No one yet knows what cues the newly emerged dragonflies use to head north with no experienced leaders. Also, some have been seen on oil platforms, and it is possible that green darners cross the Gulf of Mexico. That would probably require soaring at higher altitudes, which has never been observed.
One good reason that Franklin County residents should care about dragonflies is that the adults eat up to 15 percent of their body weight in mosquitos and biting flies every day. And dragonfly nymphs eat mosquito larvae. Volunteer citizen scientists are needed to help track dragonfly emergence and migration. Interested people can contact the Migratory Dragonfly Partnership online or contact email@example.com with any questions.
James Hargrove, a retired college professor who lives on St. George Island, can be reached at firstname.lastname@example.org