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Friday, August 21, 2015

Mysterious Migration

Wandering Glider, Pantala flavescens

Fall migration season is upon us, and that usually conjures up thoughts of songbirds and hawks using the central flyway through Texas to make their way south to the subtropics and tropics for the winter.  However, birds are not the only ones who migrate, and while much has been said about the complex, annual migration made by Monarch butterflies, the record for the longest insect migration (twice the distance of the Monarch) belongs to a dragonfly species, the Wandering Glider (Pantala flavescens).  In fact, dragonfly migration has been suspected for over 100 years, and up to 50 of the world's 5,200 dragonfly species are thought to migrate (about 16 out of 326 in North America), but not much is known about where they are coming from or where they are going.  

Green Darner, Anax junius

In Texas, there are several species of dragonflies that migrate in addition to the Wandering Glider. They include the Common Green Darner (Anax junius), Black Saddlebags (Tramea lacerata), Variegated Meadowhawk (Sympetrum corruptum), and Spot-winged Glider (Pantala hymenaea).   Cooler nights seem to trigger the dragonflies' journey south, and like birds, they build up their fat reserves before setting out.  They may use the lay of the land as a navigation guide, and some scientists speculate that they have an internal magnetic compass, as those that fly off course and out to sea appear to realize their mistake and reorient themselves.  

Black Saddlebags, Tramea lacerata

Dragonflies migrate during daylight hours, and green darners have been found to break their journeys every three days to rest and feed, using oak and juniper trees as stopover sites.  Like monarchs, the full migration circuit takes multiple generations to complete, as it is the offspring of the generation that traveled south in the fall that is migrating north again in the spring.  

Variegated Meadowhawk, Sympetrum corruptum

Swarms of dragonflies can create one of nature's most impressive spectacles, with tens to hundreds of thousands of individuals streaming southward along lakeshores, mountain ridges, and coastlines.  Even with the origins and destinations poorly known, the migration in the fall is more noticeable than that in the spring, presumably because the spring event occurs over a wider front and a longer period of time.  However, migration is the only explanation for how dragonfly adults appear in early spring in places where their nymphs or larvae from the previous year or years have not yet emerged.

Spot-winged Glider, Pantala hymenaea

The ecological role of migrating dragonflies is another facet of the mystery yet to be resolved. Since several species use the same migration strategies and timing as migratory birds, traveling at the same times and concentrating in the same places, it is likely that certain bird species are exploiting the abundance of dragonflies as a source of fuel for their own migration.  More research is being done to solve these mysteries, most notably the Xerces Society’s Migratory Dragonfly Partnership initiative, which uses “research, citizen science, education, and outreach to understand North American dragonfly migration and promote conservation.”