Using publicly collected data to study dragonfly swarms: Part 3

Before I get to the buggy stuff today, I’d like to extend a hearty thanks to WordPress for selecting my post on Arizona’s beetles for Freshly Pressed!  It was an unbelievable honor and I am so very thrilled to have been chosen.  The publicity resulted in a bunch of new subscribers to my blog, so I want to give a warm welcome to all of you who recently subscribed!  I hope you like what I have to offer.  And to my subscribers who’ve stuck around for a while, thank you so much for your continued support.  The blogging experience is so much more fun when you have the opportunity to interact with your readers, so a big thank you to all of my readers for injecting an extra little dose of joy into my life!

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Well, most of the dragonflies that going to migrate in the U.S. already have and many of those left behind are nearing the end of their lifespans.  This will therefore be the last of my posts on dragonfly swarming behavior this season!  As promised, today I’ll be making some conclusions about dragonfly swarming behavior from the data that I collected from my readers this summer.  Making conclusions of this sort is ultimately what we biologists strive to do: we try to explain something extraordinary about our world using the evidence we gather via our research.  In this case, my research involved several hundred people from all over the US, Canada, Belize, India, and Mexico, and it’s a lot of fun to go through the data!

Considering I have enough data to write a full-length scientific paper on this topic,  I could write the mother of all blog posts.  But you all don’t want to listen to me ramble on and on and on about this.  (Just ask my husband!  He bore the full brunt of my obsession with this project over the summer!)  Instead, I’ll limit myself to discussing three main conclusions to keep it a decent length.

Conclusion 1: Dragonfly swarms, both migratory and static, are more common in the midwestern and eastern US than they are in the west.

There are several reasons why this might be, but the most obvious explanation is that we just don’t have as much water in my third of the country as the areas where the major swarms took place this summer.  The majority of the reports I received were from areas that had major bodies of water within 10 miles, especially rivers.  Dragonflies are thought to use landscape features for navigation during migratory flights, and indeed most of the migratory swarms reported occurred along major rivers, lakes, or coastlines.  Static swarms also occurred more frequently in areas with a lot of water than in more arid regions.  So, the more water nearby, the more swarming activity an area had.

I think there are two reasons why water in an area would lead to increased swarming activity.  For one, dragonflies are aquatic insects.  They find mates near water, lay their eggs in water, and their offspring spend the majority of their long lives in water.  It stands to reason that areas with lots of water might therefore also have a lot of dragonflies.  And lots of dragonflies means that swarming behavior should be more common, or at least more obvious to human observers because more individuals will be nearby and able to participate.  However, consider also why static swarms form: dragonflies are attracted to locally abundant, swarming prey.  Dragonflies love to eat  insects such as mosquitoes, non-biting midges, and biting midges.  And where you get big swarms of these insects?  Near water!

I think water plays a big role in shaping the distribution of dragonflies in North America.  Take another look at the swarm map video I posted last time if you want to confirm this for yourself.  Notice how many swarms were reported near the Great Lakes, along the Mississippi River, along the Missouri River.  Then take a look at the west, where we just don’t have many big rivers and two of our biggest lakes are saltier than the ocean.  I’ll have to do some fancy analyses to properly test this idea statistically, but my preliminary data suggests that water availability largely explains the patterns in dragonfly swarm distribution reported across North America.

Conclusion 2: Dragonfly swarms are closely tied to weather patterns.

As someone whose pet research project has long been determining the impacts of weather on odonate behavior (I just had a paper published on this topic and I’ll be posting about it soon!), I was overjoyed to learn that weather plays a major role in shaping dragonfly swarming behaviors.  Migratory swarms, as observed by other researchers, usually occured after cold fronts move through an area.  Indeed, in nearly all of the migratory swarm reports I received, the reporter mentioned that it had become dramatically cooler in the area 12-48 hours prior or that a cold front was due to arrive soon after.  My data further suggest that migratory swarms may occur just before major storms hit an area.  The static swarms were also mostly weather related, typically occurring just before or just after storms.

There are tons of things coming into play here, but here’s what I think is happening.  Storms are known to assist in moving dragonflies over long distances (especially the migratory species)  and some species have been observed feeding more heavily just prior to and after storms.  This means that there are more dragonflies in an area that is about to be hit with a storm or just after a storm passes and they’re eagerly looking for things to eat.  Any small swarming prey insects that happen to come out at that time, especially in response to the storm, are going to attract dragonflies and cause static swarms to form.  Storms might also prompt many dragonflies to move from one area to another, using the wind generated by the storm to assist in their dispersal, especially in areas where the density of the dragonflies is very high.  This could be the reason that I had so many reports of migratory swarms that occurred along with storm activity rather than cold fronts.

Weather likely impacts dragonfly swarming behavior directly in these ways, but there are also indirect effects.  Heavy storm activity in an area can cause major changes in the aquatic landscape and these changes have a direct impact on many dragonfly prey species.  Mosquitoes are especially adept at utilizing new and temporary bodies of water and populations of the flies can explode following major storm events.  Increases in mosquito populations promote increases in dragonfly populations as well: more food means more dragonflies are able to live in the area.  In many areas where major swarming activity was reported this summer, significant flooding also occurred days or weeks before the swarming began.  For example, Milwaukee  was bombarded by storms this summer and I’m told many areas of the region flooded.  Mosquito populations skyrocketed in response to the flooding, and then the dragonflies started forming massive static swarms over an enormous area of Wisconsin.  It looks as though storms cause spikes in dragonfly swarming activity in general, but areas that experience major flooding are especially likely to see a massive increase in dragonfly swarming.

Conclusion 3: 2010 was a special year for dragonfly swarms.

Although I know from personal experience that it’s uncommon to see dragonfly swarms, I was quite shocked that about 98% of reporters told me that they had never seen anything like what they’d witnessed before.  Though I say this tentatively because I don’t have enough data to truly support this idea, I believe that 2010 was an extraordinary year for dragonflies in the midwest.  A perfect combination of events seems to have occurred that allowed the dragonfly population to explode.

Let’s take a moment to consider just how many events had to fall into place to allow so very many dragonflies to make an appearance this summer!  Dragonflies often spend over a year in the water as nymphs, so 2010’s dragonfly swarming boom probably began 1-3 years ago.  Conditions in the water had to be just right to allow millions or billions of dragonfly nymphs to survive to adulthood.  This means that the dragonflies likely experienced mild conditions during the winters, the water quality was decent, and that there was abundant prey available for them to eat.  Once the millions or billions of dragonflies emerged from the water and molted into adults, they all required food for continued survival.  Luck was on their side this year as major flooding occurred in several parts of the northern midwest, driving mosquito and midge populations up to abnormally high levels.  Swarming flies attracted dragonflies, so dragonfly swarms formed very often.  I think that this combination of factors, high nymphal survival followed by an overabundance of prey, caused the explosion of dragonflies observed in the midwest this year.  The response of the incredibly high number of dragonflies to the highly abundant prey then in turn led to a much greater than normal level of swarming behavior.

I think that this perfect combination of events probably occurs rarely.  How else do you explain the number of people who said they had never seen anything like it before, the number of reports on television news programs, the number of scientists in the midwest trying to allay the fears of the populace as millions of dragonflies descended on their homes?  That said, I still have 2 questions.  The first: is this really an extraordinary event or does it just seem that way from the reports?  For those of you who are familiar with statistics, my n=1 summer, so I won’t really know the answer to this question until I collect data for a few more years.  The second: how is climate change going to impact this behavior?  If global warming occurs, you might expect to see warmer, milder winters that support explosive populations of dragonflies like the ones witnessed this year, making these sorts of summer more common.  On the other hand, recent studies suggest that the midwest might have colder, more severe winters as climate change occurs, so fewer dragonflies may survive through the winter in the future.  In essence, I need more data collected over more time to begin to answer these questions.  I intend for this to become a long-term research project, one that may occupy a part of my summers for many years to come, so hopefully I’ll have some better answers in the future.


Have you seen a dragonfly swarm?

I am tracking swarms so I can learn more about this interesting behavior.  If you see one, I’d love to hear from you!  Please visit my Report a Dragonfly Swarm page to fill out the official report form.  It only takes a few minutes!



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