Behavioral Responses of Damselflies to Storms

Fountain Creek Park

The pond where I did my research at Fountain Creek Regional Park, CO.

Last week I talked a bit about how weather affects odonate behavior, my favorite topic in biology. Today I’ll go over the study I did to look at these weather related behaviors more closely. Like the little study I did that focused on damselflies and weather in my first college ecology class, this study was done at the wetlands in Fountain Creek Regional Park outside Colorado Springs, CO.  This research was actually part of my undergrad senior thesis!

pond at Fountain Creek Park

My study site. It extended from the cattails on the left side out to the end of the log in the water and from the dock (not visible) to just beyond the log.

I had to work the summer I collected my data, but I went to the wetland most days after work and recorded observations from 4-5PM.  I plopped down on the dock with all of my weather measuring equipment and watched the damselflies in a 5.5 square meter area along the edge of the pond for an hour.  I divided the hour into 5-minute periods and recorded weather data (wind speed and direction, temperature, light intensity, barometric pressure, relative humidity, and whether it was raining or not) for the first minute of each period.  I then spent the remaining 4 minutes counting the number of damselflies that flew within my study area.  Part of that area was filled with cattails and the rest was over open water as you can see in the photo.

Having spent 14 years of my life in Colorado Springs, I can tell you one thing with certainty: in the summer it rains nearly every day between 4 and 5PM.  This meant that I was out watching damselflies during the exact time the storms were blasting over Pike’s Peak and ripping across the plains.  I would sit there watching these phenomenal storm clouds rolling straight toward me with fantastic speed.  Guess who got rained on A LOT that summer?  Me!  I also got hailed on, was sandblasted in high winds, and was once driven running the half mile back to my car when the lightning got a little too close.  However, the clouds moved so quickly (they have to build up a ton of momentum to make it over Pike’s Peak’s 14,115 feet!), the storms didn’t last long, usually 30 minutes at most.  During that time, the weather would transform from hot, sunny, and still to cold, windy, and rainy in the span of a few minutes.  It would usually rain, sometimes very hard, for 10 minutes or so.  Then the storm would suddenly be over and it would become sunny, warm, and still again.  This whole series of events would take place during my hour at the pond.

Now most sane people go inside during storms.  Rain in Colorado is incredibly cold and the storms can be quite powerful with a lot of lightning.  Call me crazy, but I loved curling my whole body into my enormous rain jacket and getting rained on.  I was rewarded for my insanity too because I got to see some things that very few odonate people get to see.

First, I learned that there was a rather distinct pattern of behaviors that was associated with the weather patterns I observed.  The damselflies were most active in sunny, warm, still conditions, the typical weather central Colorado experiences during the summer.  They flew readily into and out of my study area, hunting, looking for mates, mating, and laying eggs.  As soon as a storm approached, you’d see some pretty interesting things.  As the clouds moved in and it became darker and cooler, the number of flights the damselflies made decreased so that fewer individuals flew during a counting period.  As the wind picked up, the activity decreased even further.  Flight activity ceased altogether if it started to rain.  Of all the many hours I spent at the pond, I saw only a single damselfly flying while it was raining, and it was during a very light rain when the sun was still shining.  Most interestingly to me, the damselflies would start to leave the pond when the weather deteriorated sufficiently.  They were displaying pond abandonment behavior.  However, as soon as the storm was over and the sun came back out, the damselflies would return to the pond and resume their normal activity as if nothing had happened at all.  It was fascinating and I am so happy I got to see this behavior!

pond at Fountain Creek Park

My pond at Fountain Creek Park during a light storm.

The flight activity of the damselflies at Fountain Creek Regional Park was clearly affected by the weather, but I was interested in knowing which of the seven weather parameters I measured were contributing to the flight activity I observed.  I used a statistical procedure (multiple regression for those interested) to determine that light intensity, temperature, wind speed (but not direction), and whether it was raining or not were the weather parameters most closely associated with the flight activity that I recorded.  Of these, light intensity showed the greatest association, followed closely by temperature.  Essentially, the brighter and warmer it was, the more damselfly flights you see.

(Brief aside: Remember how I said last time that I didn’t agree with that Russian scientist who thought that barometric pressure was a major player in shaping odonate behavior?  My results didn’t indicate that barometric pressure had any effect.  This coupled with the fact that the Russian didn’t even measure barometric pressure in his study makes me skeptical of his results.)

So four weather parameters were important.  The statistical test confirmed what I’d observed visually, that damselflies flew more readily in good weather than in poor weather.  “Good” conditions were warm and sunny with little or no wind while “bad” conditions were cold, rainy, windy, and dark.  I definitely observed pond abandonment behavior.

The most important question is this: what does all this mean?  I think my data suggest two things:

pond at Fountain Creek Park

My pond at Fountain Creek Park, right after a storm.

1) The damselflies might be able to pick up on cues in the changing weather that alert them that a storm is approaching.  Think about a damselfly, those big wings on a scrawny little body.  If you’re a damselfly, it could be physically dangerous for you to be out in a storm.  Being blown into the vegetation or the water could be deadly, heavy raindrops could impart a significant blow, and evaporative cooling could cause your body to cool down so fast that you can’t escape if the weather gets worse.  Better to leave the pond before a storm than risk getting caught exposed in one.  I think storms are dangerous to odonates, so the pond abandonment behavior that has been so often reported might be a means of protecting them from harm during bad weather.

2) Pond abandonment behavior might be related to roosting behaviors.  Consider these ideas: Damselflies roost in sheltered areas away from the water at night.  Storms usually result in a drop in the light level and temperature, which are the same things that happen as it gets dark at night.  Damselflies disappear from the water before it starts to rain.  It is therefore quite possible that pond abandonment behavior and simple roosting behaviors might be the same thing: odonates returning to their overnight roosts when it gets dark and cools down.  It is likely also advantageous for damselflies to seek shelter during storms, but this could be a secondary benefit, something they gain by completing a behavior that has nothing to do with protecting them from storms.

Are odonates using weather cues to abandon ponds before storms?  Or are they simply returning to their roosts because it’s getting dark?  Are storms dangerous to odonates?  These are some of the endless new questions I had after I finished this project and would like to answer.  I had intended to study this behavior in more depth in grad school, but then I decided to attend grad school in Arizona.  Colorado’s clockwork storms are perfect for studying these behaviors.  Arizona’s wildly unpredictable storms are not.  So, I changed my focus to the water bugs and have studied them ever since.  I will go back to my beloved odonates someday though!  I also decided a while back that my damselfly study was actually pretty unique and could make a real contribution to the scientific literature on odonates.  Ten years after I wrote my undergraduate senior thesis, my data was published.  If you’d like to read more about my study, look at some pretty graphs and whatnot, the citation is listed below.

I am dealing with some heavy things in my personal life at the moment, so I have no idea what I’ll do for the next few posts.  I’m going to let myself be driven by whims for a week or two.  I hope you’ll all check back to see where my whims take me!

Paper citation:

Goforth, C. L.  2010.  Behavioural responses of Enallagma to changes in weather (Zygoptera: Coenagrionidae).  Odonatologica 39: 225-234.

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Unless otherwise stated, all text, images, and video are copyright © 2011 DragonflyWoman.wordpress.com

Weather and Odonate Behavior

Anax junius in flightI’m running a little behind this week, but it’s a new year and I think it’s time to discuss my favorite scientific topic in the world: odonate behavioral responses to weather!  I have been interested in dragonflies for a long time, almost as long as I’ve been interested in insects.  I think they’re beautiful, amazing animals and I am in complete awe of them for many reasons.  When I was an undergrad, I took an ecology class in which we were required to do a personal project, so I naturally gravitated toward dragonflies.  That project turned into a senior thesis for my biology B.A. and eventually became the basis for a scientific paper.  I’ll talk about the results of my study next week.  Today I want focus on what’s known about the behavioral responses of odonates to weather, observations by other scientists who have inspired my work in this field.

As most of you reading this will know, dragonflies are powerful fliers.  However, they still have limitations.  They have massive, broad, flat wings and though they have bulky bodies, they just don’t weigh that much.  So, imagine you’re a dragonfly.  It’s a nice sunny day, but a storm is moving in.  It starts to get cooler, darker, windy.  Are you, as a light-bodied animal with gigantic wings, going to keep flying as the weather deteriorates?

If you answered no, you’re spot on!  Dragonflies and damselflies are known to be strongly affected by changes in weather.   Changes in weather throughout the day can have a significant impact on how a population of odonates behaves.  In fact, weather plays such a huge role in odonate behavior that you see some variation on this sentence in countless scientific papers:

“Data from days exhibiting inclement weather were removed from the data set before analysis.”

There’s a reason so many people do this though.  Though very few researchers have looked specifically at the role weather plays in shaping odonate behavior, these insects are well-known to behave strangely under certain conditions.  If your study is looking at, say, patrolling behaviors in a dragonfly species and a change in weather occurs that disrupts their normal behavior, that data really doesn’t help answer the questions you’re interested in.  You therefore remove that data before you analyze your results because it is not related to your study.  And ultimately, even scientists don’t like to stay outside when it’s rainy, cold, very windy, or otherwise unpleasant.  Scientists just miss a lot of what happens during those inclement weather days.  That’s unfortunate because that’s when some of the most interesting things happen!

Snowman

Snowman from one of our aquatic entomology field trips. Dragonflies would not fly in this weather! Photo by Dennis Suhre.

So what do we know about odonate responses to weather?  Several things.  Let’s consider temperature first.  Odonates are exothermic, like all insects.  If they get too cold, they can’t warm their bodies up enough to fly.  If they get too hot, all kinds of nasty chemical reactions start taking place in their bodies that can result in death if they become severely overheated.  (Long ago, I posted about obelisking behavior in dragonflies, a behavior that helps dragonflies cool down on hot days.)  Changes in temperature are known to influence odonate behavior such as the time of day that a species normally flies, flight activity levels throughout the day, and the postures of dragonflies.

Wind and a palm tree

A palm tree on a windy day.

Wind also plays a role in odonate behavior.  On very windy days, damselflies won’t fly at all, or only in places that are protected from the wind by barriers of some sort.  Dragonflies are able to fly in stronger winds, but even these larger insects have their limits and must stop flying if the winds become too powerful.  Light intensity hasn’t been looked at in as much detail as wind, but it is known to impact fight activity as well.  Activity levels tend to go up and down with light intensity.  More odonates tend to fly on sunny days than on cloudy days.

These are the obvious weather factors.  You can easily observe an increase in wind speed, a drop in the light levels, or a drop in temperature.  What about the less obvious changes in weather like barometric pressure and relative humidity?  Relative humidity isn’t considered by most researchers, so there is very little data available on its effects.  However, one Russian scientist in the 60’s published a paper proposing a link between barometric pressure and dragonfly behavior.  I happen to disagree with him, but I’ll talk about my reasons when I discuss my own study next time.

The Catalina Mountains as a storm rolls in

The Catalina Mountains as a storm rolls in.

Finally, let’s consider rain.  Glorious rain, my favorite weather!  Damselflies almost never fly in the rain.  Some of their larger dragonfly relatives have been observed flying during rain (including my favorite magnificent flier, the wandering glider), but few reports have been made of any dragonflies flying during heavy rains.  In fact, several researchers have observed that dragonflies and damselflies essentially disappear from the water’s edge right before it starts to rain.  This behavior has been named pond abandonment behavior, and it is this behavior that I am most interested in.  Why are they leaving?  Where do they go?  Are they using cues from their environment to tell them when to leave the pond and when to return?  This behavior is fascinating!

This is the sort of information I had to go on when I started my project looking at dragonfly behavioral responses to weather as an undergrad.  The few researchers that have examined the role weather plays in odonate behavior tended to analyze each weather factor separately, sometimes using data from different days.  If you’ve ever watched a storm rolling in, you know that light intensity rarely changes without a change in temperature and the wind often increases before it starts to rain.  Weather factors are clearly linked.  And most of the time you’re not lucky enough to find studies that overtly link weather and behavior to begin with.  The information about this topic tends to be hidden in papers about completely unrelated subjects, often preceded by one of those, “Data from days exhibiting inclement weather…” statements.  I happen across most information about odonates and weather when I’m reading about territoriality or dragonflies using sunny spots in forests as hunting areas.  Lots of odonate researchers know that weather plays a role in odonate behavior (a significant role in fact!), but figuring out exactly how weather does this…  Well, that’s what I am interested in figuring out!

Next up is another Friday 5, but my post early next week will focus on the things I’ve learned in my own work with odonates and weather.  This has been my pet topic ever since that first little study I did in my first ecology class in college – I love this subject!  I hope you’ll enjoy it too!

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Unless otherwise stated, all text, images, and video are copyright © 2011 DragonflyWoman.wordpress.com

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!

dragonfly swarm banner

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.

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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!

Thanks!

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Want more information?

Visit my dragonfly swarm information page for my entire collection of posts about dragonfly swarms!

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Unless otherwise stated, all text, images, and video are copyright © 2010 DragonflyWoman.wordpress.com