Filling in gaps in the dragonfly migration story

Insect migration has long been fascinating to me. I’ve written several posts on this before, including one on migrating moths and one on painted lady butterflies. We’ve long known that many dragonfly species are long-distance migrants, including the large charismatic green darner. Citizen science records have helped document that migration, but there have been many questions about the details. A new study has now cleared up many of those questions, revealing a fascinating story.

Common buckeye butterflies don’t overwinter in Nebraska. We only see them after they migrate northward into Nebraska each year.

Green darner dragonflies (Anax junius) is a common species here in Nebraska and throughout much of North America. For years, we’ve known that the species migrates because people have kept track of when large flocks (herds? squadrons?) are sighted each year. However, green darners can also overwinter in the northern U.S. as aquatic nymphs (their immature stage). The speed of development for an egg apparently depends upon temperature and photoperiod (day length). Some eggs develop quickly into nymphs, which quickly grow and molt into adults. Others develop very slowly and enter diapause (dormancy) over the winter months, emerging the next spring to become adults.

A recently emerged green darner and the empty shell of its nymph form.

The authors of the new study (read it HERE) combined citizen science observations with stable isotope analysis to piece together the migration story of the green darner. What they found was an annual cycle comprised of at least three generations. The first emerges in the southern part of the continent between January and May and migrates as far as the northern edge of the U.S., where the dragonflies lay eggs and die. A second generation emerges in the north and migrates back south late in the year. That second generation includes darners that were born the previous year and overwintered in the north, as well as others that hatched and matured within the same season. When fall migrants arrive in the south, they lay eggs that grow up to form a non-migratory population. The individuals in that population live their whole life in the same general area. The eggs laid by that non-migratory generation become adults that migrate back to the north in the spring.

So to summarize, if you’re a dragonfly whose parents migrated south, you will grow up into an adult that lives its entire life in the south without migrating. However, your kids will migrate to the north where they will lay eggs before they die. Those eggs might turn into adult dragonflies that same year or they might not become adults until the following year. Either way, those new adults will migrate back to the south where they will become the parents of another non-migratory population.

Face to face with a green darner dragonfly.

It’s exciting to better understand the migratory patterns of the green darner. Hopefully, we can get more information on the many other migratory insect species soon. Knowing how and when insects migrate is, of course, fascinating in its own right, but there are also practical conservation implication. That information can inform the way we manage individual sites to ensure management actions don’t interrupt insect cycles at critical points. Even more importantly, understanding migratory patterns helps us consider how climate change might affect them. Temperature shifts and increasing intensity/frequency of droughts are both factors that could potentially have large implications for migratory dragonflies and other insects.

Insect migration is just one of many phenomena we still have much to learn about. On the one hand, it’s invigorating to know there are still plenty of unexplored frontiers out there for scientists studying natural history. On the other, there’s a sense of urgency about getting those data so we can act on them. Either way, I’m grateful for those scientists who manage to find funding for these kinds of projects. They are forging ahead, studying creatures most people ignore, but that play critical roles in the survival of ecosystems and the people who depend upon them.

Photo of the Week – February 1, 2019

It’s till pretty drab and brown outside, so today’s photos are again selected from last summer’s shots. I’m sure I’m not the only one who is glad to look at some color.

We missed out on most of the polar vortex here in Aurora; we only dropped as low as -5 degrees one night, and we’re back up close to 50 degrees today. The misplaced jokes I’ve heard (“heh heh, global warming, am I right?”) reminded me that I’d written a post several years ago about how global warming does, in fact, influence longer and colder temperatures at times during the winter. I looked up the post and was dismayed to see it was almost SIX YEARS OLD. And we’re still arguing (and joking) instead of acting.

Moving on, though, here is some color from last August. I photographed bees and a few other insects on tall thistle (Cirsium altissimum) several different times during that month. (Which reminds me of another previous post, this one on native thistles and their importance to pollinators). Here are some highlights from those August thistle photos.

A crab spider waits for the next pollinator to stop by…
A skipper butterfly on tall thistle at our family prairie.
I think this might be a fruit fly (Tephrellia?) that lays eggs in thistle flowers. Anyone know for sure?