2012 Dragonfly Swarm Project Year-End Report: Conclusions

Dragonfly Swarm Project logo

It’s time for part 3 of the yearly Dragonfly Swarm Project report! Today, I present the results of the predictions I made last year based on the data you all have contributed over the last 3 years, plus I’m proposing a challenge to you. Last year I made three predictions, so I’ll address those first, but then I want to get some feedback from YOU and see what you think. Ready? Let’s go!

This year was an interesting year in many ways, so the conclusions I thought were going to be so clear-cut and consistent from year to year aren’t necessarily as predictable as expected. That makes for exciting science though! The first prediction was the most straightforward:

Old Prediction: Dragonfly swarming will be most commonly reported east of the Missouri River in the U.S.

This is the one constant in this project each year. The space between the Missouri River and the Mississippi River typically produces several swarms each year, but I think it’s safe to make a new prediction based on the data gathered so far:

New Prediction: US dragonfly swarms are most common east of the Mississippi River.

As I stated last year, I think a lot of this has to do with the amount of water in the eastern US relative to the west. There is more habitat available to dragonflies in the east and likely more dragonfly individuals present in the wetter areas than in the arid west (though I don’t have data to back this up – will be looking through the literature for evidence). To have swarms, you need a lot of dragonflies in one area. You see dragonfly swarms in the west, but there are often identifiable special conditions that concentrate the dragonflies within an area at the time of the swarm. I have a few ideas about that that I’ll get to…

Old Prediction: Most swarms reported will follow flooding or heavy rains.

This prediction was… partially correct. While there were still reports of flooding or heavy rains in 263 of the 705 swarm reports made in 2012, you’ll notice that that’s not even half the reports. In fact, 314 reports indicated that there was no flooding or heavy rains in the area prior to swarming events, which suggests that rains might not play as strong a role in swarm formation as I previously thought. That said, I still think that flooding is an important factor and if you compare the areas of the country where flooding took place in 2012 and the location of swarms, there appears to be a nice correlation between the two. Sadly, I don’t possess the technical expertise to actually show that to you at this time, so you’ll just have to take my word for it. Based on the 2012 data, I have developed a new idea about possible factors in swarm formation, which I’ll discuss later. And I make this prediction:

New Prediction: 40% or more swarms will be observed after flooding or heavy rains in 2013.

I still think heavy rains and floods are a major factor in swarm formation, so I suspect that I will continue to get a lot of swarms reported that occurred with major rains/floods.

Old Prediction: There will be more dragonfly swarms reported from the northern Midwest (Minnesota, Michigan, Wisconsin, Illinois, Iowa, etc) in 2012 than in 2011. Similarly, there will be very few reports of swarms from eastern Ohio, Pennsylvania, Virginia, and West Virginia.

This prediction was based on an idea I had after two years worth of data collection: that areas where there were big weather events and massive dragonfly swarming in one year would not have many reports the following year when that massive swarming was due to flooding.  My idea was that flooding in an area might deplete the nymphal population that would emerge the following year. I made the prediction based on a comparison from two seasons and wasn’t sure it was going to hold true in 2012. However, I crunched a few numbers and made a few maps, and here’s what I discovered. In 2010, there was heavy activity in the north central US, with reports from Iowa, Minnesota, Michigan, Illinois, and Wisconsin making up 36.8% of the total reports for the year. Iowa and Illinois alone made up over 20% of the reports. In 2011, however, those five states made up only 7.8% of the reports. Things picked up in 2012 such that 18.9% of observations were made in the north central US. So, that part of the prediction was correct: swarming activity dipped strongly the year after the flooding in these states and then increased the following year. So far so good! But what about that second prediction? I have numbers if you are interested in them, but the map will show it so much more clearly. In 2011, a massive number of reports were made in Ohio, Pennsylvania, Virginia, and West Virginia, close to half of the reports. The map of the 2011 data in that area looked like this (click images to make them larger), and focus on the four states of interest, that big blotch of nearly solid green on the upper mid-Atlantic states:

2011 Static NE

Clearly, there was a major event happening in OH-PA-WV-VA, and indeed there was a lot of rain and flooding in that area that resulted from Hurricane Irene and Tropical Storm Lee. Lots of flooding meant lots of swarms in 2011. This is that same area in 2012:

2012 Static NE

Now THAT is a huge difference! Clearly there were far more swarms taking place in this region in 2011 than in 2012, so the second part of the prediction held true as well. The sample size is small and it’s hard to make broad conclusions without at least a few more year’s worth of data, but I think the data so far suggest that heavy swarming in a location one year results in low swarming the following year.

That said, there was no obvious and large center of activity this year. In fact, there were only two areas where large swarming events occurred: the New Jersey/southeastern New York area and Colorado. Comparing the 2012 data to 2013 data for Colorado isn’t fair because Colorado is a western state that doesn’t normally have a lot of swarming activity. So, I am going to make this prediction for 2013:

New Prediction: There will be fewer swarms in the New Jersey area in 2013 than in 2012.

Ultimately, however, the event in New Jersey wasn’t one the mega events you all have documented in the last few years, so it might not show the same sort of pronounced dip in activity highlighted in the maps above. Plus, the data from New Jersey is confounded because the eastern migration passes through the state in the fall.  We’ll just have to wait and see what happens!

I have a few ideas about why dragonfly swarms form and why they are important, but in the interest of keeping this post a reasonable length I am going to save them for another post. However, I promised you a challenge, and here it is: what do YOU think? I want to see if you all can come up with exciting, new ideas that I haven’t considered by answering two questions:

1. Why do static dragonfly swarms form? Feel free to list multiple suggestions for why they form at all, in addition to why they form in the locations where they have been observed. And…

2. What roles do you think static dragonfly swarms play in the environment? I.e., why are dragonfly swarms important?

I have my own ideas for about this behavior, but sometimes it’s good to get some fresh perspective.  That’s where you all come in! Feel free to base your answers on your own observations, the information I have shared on my blog, or any other source.  And just to keep things interesting, I’m offering a small dragonfly themed prize pack for a few of my favorite responses. If you want a chance at winning, offer some answers to the questions before 10AM EST Sunday, February 10, when I’ll post the reasons I propose for why these swarms form and why they’re important. I look forward to hearing your ideas!


Unless otherwise stated, all text, images, and video are copyright © C. L. Goforth

2012 Dragonfly Swarm Project Year-End Report: Distribution of Swarms

Dragonfly Swarm Project logo

It was another great year of data collection for the Dragonfly Swarm Project!  I continue to be impressed by the number of people who participate in this project, especially as it’s hard to promote it  in the hopes that someone might see a swarm and most people actually find the project after they’ve already seen a dragonfly swarm.  Still, over 700 reports were made in 2012 and that’s pretty darned good!  So, what does that data tell us?  The next two posts will focus on what we can learn from the data so far.  Today, I present the long-awaited maps (I think this year’s maps took a year or two off my life with all the stress they caused…) and next week I’ll discuss some of the patterns that I’m seeing in the data after three seasons of data collection.  Let’s dive into that data!

Like last year, I’ve split the map data into two videos.  It’s easiest to see changes over time when the images I create are presented as a series, so each map you’ll see in these videos represents the swarms that occurred during the time frame indicated at the top of the screen.  The pushpin colors mean something too: red represents a static swarm and blue represents a migratory swarm.  My apologies that the blue pins are a little hard to see on the map – they show up easily in Google Earth, but not so well in the images it produces.  For the best viewing experience, try watching the videos in full screen mode by clicking on the icon with the four arrows in the bottom right corner of the video player.  You’ll be able to see the changes from week to week more easily that way.

The first set of maps document the cumulative data for the year and show the overall pattern of swarm locations in 2012.  Each week’s new sightings are added to the previous weeks’:

The second set of maps shows the data for each month individually and by swarm type.  The first set of maps are the static swarms and the second are the migratory.  Each map represents only the data for the month indicated at the top of the video rather than showing the cumulative data.  For the migratory swarms, look closely along the southeastern and east coasts.  The pushpins are hard to see against the blue water of the ocean:

As you can see, there were once again more swarms reported east of the Missouri River than west of it.  In spite of the fact that Colorado made it into the 5 states with the most reports in 2012, there just aren’t that many dragonfly swarms in the west and some states (Montana, Wyoming, New Mexico) were entirely unrepresented this year.   Dragonfly swarms definitely appear to be more common in the east than the west.

In 2010, the center of activity was the western Great Lakes states and Iowa and last year it was the Pennsylvania/Ohio area.  This year, the activity was heaviest in the New Jersey area and northern New England coast in the mid to late summer and around the Chicago area in the late summer.   Each year seems to have a different center of activity, and I have a hypothesis for why this happens.  I’ll get to that in the next year-end report!

This year was an odd migration year.  The migration down the east coast has been documented several times in various publications, so it’s a fairly well-established route.  This year, there were very few reports of migratory movements in the east during the typical migration season in late August and September.  In fact, there were hardly any!  While most of the migratory movements reported this year did occur in the usual place, within a few miles of the coastline, the timing was all wrong: most were observed in June and July, much too early for the usual migration.  Again, I have a hypothesis that might explain this, but you’ll have to wait until next time.

The migration along the west coast was also quite weak this year.  That migration has a known set of conditions associated with it, a particular wind direction and a specific temperature.  The dragonfly people in Washington and Oregon were going out this year to the places they usually see migrating variegated meadowhawks on fall days with the right conditions and… nothing!  People were looking, and looking hard, so it seems that it was just a weak year for the migration overall, on both coasts.  I don’t even know how to explain the western migration fail though.  That’s just weird as that one is SO specific and occurs every year almost like clockwork!

Finally, I can say with more certainty that dragonfly swarms really aren’t a rare phenomenon and they happen more often than I’d ever expected when I started this project.  That is in keeping with the last two season’s worth of data.  However, last year I was uncertain whether I would continue to see an increase in the number of swarms reported every year and that has not been the case.  In 2010, I got about 650 reports.  Last year I got over 1100!  This year, I’m back down to 700.  I have a feeling that 600-800 swarm reports per year is normal and that the several hundred reports I got over a 3 week period last year were related to a set of perfect conditions that allowed a massive boom in dragonfly activity right toward the end of the year rather than an increase in participation in the project.  I’ll explain why I think that boom happened in the next part of the year-end report, but I predict that next year we’ll see the same sort of numbers we did this year, barring any sort of odd convergence of conditions that allow another 2011-style reporting boom.

If you’d like to see images of the maps of all the data for the year, I’ve uploaded them to my Yearly Maps page.  There, you can view the maps for static, migratory, and all swarms by year, which should make comparing between years fairly easy.  Click on the images to see a larger version of the map – they’re very tiny on the yearly maps page.

That’s it for this installment, but part three of the year-end report (the conclusions) will be up on Sunday.  It should be pretty interesting!


Unless otherwise stated, all text, images, and video are copyright © C. L. Goforth


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!


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

Palo Verde Beetles

It’s that time of year again!  Time for the palo verde beetles to descend on Tucson and fill the night sky with giant beetles flying around drunkenly looking for mates.  I already wrote a post about how much I love these beetles and promised to write more about their biology.  Today I am making good on that promise!

Meet the palo verde beetle (or palo verde root borer beetle), Derobrachus hovorei:

palo verde beetle top view

Palo verde beetle

I think these beetles are one of the best parts of living in the Sonoran Desert.  Each summer when the monsoons begin, these beetles start to appear.  They’re large, dark brown nocturnal beetles, 3 – 3.5 inches long.  Check out the long, luxurious antennae:

palo verde beetle antenna

Palo verde beetle antenna

Palo verde beetles belong to the beetle family Cerambycidae, the longhorn beetles.  You can see how the family got its common name!  Nearly all members of the group have these long antennae, including several important wood pest species (such as the Asian longhorn beetles).  The palo verde beetle is no exception.

I think the palo verde beetles look rather fierce.  Check out the spikes on the thorax:

palo verde beetle thorax

Palo verde beetle thorax

And the big pinching mouthparts (called mandibles):

palo verde beetle jaws

Palo verde beetle jaws

In spite of their size, their armor, and the powerful jaws, these beetles are largely harmless.  That’s not to say that they won’t flail about wildly and try to bite you if you pick them up, and they can deliver a strong, painful pinch if you’re not careful.  (That’s never stopped me from picking them up!)  Mostly though, the beetles use those impressive mandibles for fighting and/or mating.  I posted a photo of a male and a female palo verde beetle struggling with each other before they mated a while back and jaws were used extensively as the male subdued the female.  In fact, she lost a leg and both antennae in the struggle.  Those strong jaws are also used by males in battles with one another to win females.  The better fighter a male is, the more females he has a chance to mate with.

There’s one thing the jaws aren’t used for though: feeding.  Adult palo verde beetles don’t feed at all and rely on nutrient reserves they ingested as larvae to fuel their adult activities.  As result, their adult lifespans are pretty short, less than a month.  During that month, they fly around (not very well and in the dark – there’s nothing quite like seeing one of these flying toward your head at night!), fight, mate, and lay eggs.  That’s a lot to do for a large flying animal that doesn’t eat!

Once a male finds and mates with a female, the female will burrow into the soil at the base of trees and lay her eggs about a foot down.  When the eggs hatch, the larvae feed on the roots of the trees, focusing on the starches within the roots.  After 2-3 years of feeding and growing, the larvae are enormous and look like this:

palo verde beetle larva

Palo verde beetle larva! This one was over 3 inches long.

The larvae have strong and powerful mouthparts too, essential for cutting trees roots open so they can eat.  When they’ve grown large enough, they pupate underground.  The adult emerges when the monsoons arrive and dig their way up to the surface, leaving large round holes around the base of the tree where they grew up.  Then they go about the serious business of flying around in the dark (scaring a lot of people in the process), looking for mates, and starting the whole process all over again.

Palo verde beetles get their name from the palo verde tree, a gorgeous desert tree with green bark native to the Sonoran Desert.  If you dig up palo verde trees, you will supposedly nearly always find several palo verde beetle larvae happily munching away on the roots.  Because they are root borers and root borers are commonly associated with dead, dying, or unhealthy trees, palo verde beetles are often considered pests.  If you search the internet, you’ll find all sorts of crazy ideas for how to rid your yard of these “dangerous” beetles so that they don’t kill your trees.  It all a bit sensationalistic though!  Palo verde beetles DO eat roots of trees, but consider this: there are millions of palo verde trees in the Sonoran Desert and nearly all of them have several palo verde beetle larvae gnawing on their roots.  If the beetles are really destroying tress, wouldn’t there be fewer palo verde trees around?  Palo verde beetles can cause some damage to trees, especially non-native ornamentals, but usually only in trees that are already having problems.  The best defense against palo verde beetle damage is taking care of your trees!  If you keep young trees healthy by watering them regularly and fertilizing, they will usually be able to withstand palo verde beetle larvae eating their roots quite well.

While I completely understand why people might be scared of these lumbering, giant beetles – they are VERY large after all – I can’t help but love them!  I associate them with lazy, hot summers and the arrival of the much-needed rains.  They’re hilarious to watch flying around.  And they’re stunning!  As proof, I leave you with this last image:

Palo verde beetle side

Palo verde beetle, side view

Love ’em!


Unless otherwise stated, all text, images, and video are copyright © TheDragonflyWoman.com

Friday 5: Bugs on Film!

When I was a kid, I dreaded Sunday dinner.  We’d all sit around the table and my dad would turn the TV around on its rickety cart so that we could watch it during our meal.  We rarely watched TV during meals.  In fact, we watched very little TV period when my dad was home, just movies and the PBS series Nature.  I HATED watching Nature when I was a kid!  I realize the irony of this statement considering I’m now an entomologist and adore nature documentaries, but at the time I really did hate it.  We’d be eating some delicious Sunday dinner (my mom is a marvelous cook!) when Nature would show a parasite that someone had ingested with their food or maggots decomposing a dead raccoon or something.  Not the most appetizing thing to watch during dinner!  Normal families had already eaten their dinners by the time Nature came on, but not my family.  Oh no!  We ate late and got to see Nature in all it’s disgusting glory as we struggled to eat.

But that’s all behind me!  I LOVE nature documentaries now and I can stomach anything during meals, regardless of how disgusting it is.  So, today I give you my five favorite insect documentaries.  In no particular order, they are:

Alien Empire“Alien Empire”, PBS, Nature

This was the first Nature series that I actually appreciated as a youth.  It aired in 1995, about two years after I’d decided I was going to become an entomologist, so it’s kinda old now.  It’s nearly impossible to find and hasn’t ever been released on DVD, but if you happen to own a VCR still and have access to the series on video (maybe it’s at your library?), it’s well worth the time to watch!  My favorite parts of the series (keep in mind these have stuck with me since 1995!) were a segment about an Australian beetle that mistakes brown beer bottles for female beetles and tries to mate with the bottles and a segment about what happens when a mated female fly is trapped in a fridge full of food after the power goes out (LOTS of flies when you open the door!).  The narration is witty and the cinematography was ground-breaking at the time.  It doesn’t compare to today’s cameras, but it was amazing back then and it still looks pretty good.  Check it out if you can!

BugsBugs!  A Rainforest Adventure

Okay, I’ll admit it: I’m only including this one because I don’t feel like Sir David Attenborough should hog the whole list.  I saw this in 3D at a science museum in Boston in 2006.  The videography is really excellent and the footage is impressive.  The narration…  Not so great, even with Dame Judi Dench lending her voice talent to the project.  To say the narration is juvenile is a bit of an insult to kids everywhere!  However, the story that they set up is actually pretty fun.  In the film, you follow a butterfly and a mantid throughout their life cycle from egg to adult.  Then they come together in the end, and if you know anything about mantids you can guess how that encounter goes.  (It’s interesting to read about the moms with kids that were traumatized by the ending of this film in the reviews on Amazon!)  I think this film is aimed mostly at kids based on the narration, but the footage is good enough I’m going to recommend it anyway – and I know I have some really young readers out there too!

LifeLife, BBC

If you haven’t seen Life (or Planet Earth for that matter), you really should.  The cinematography is absolutely stunning.  Seriously, it’s so beautiful I find myself holding my breath at times!  While Life isn’t all about insects and features many other animals throughout the series, there is one whole episode devoted to insects.  It includes one of my favorite insect clips of all time too: a male Darwin’s beetle fighting his way up a tree, battling many males as he tries to win the hand (tarsal claw?) of the fair beetle at the top.  Watching the beetles fall out of the tree makes me laugh every time!  Seriously, this is nature at its absolute best!  If you have a Blu Ray player, this is one of only three films I own that I would absolutely recommend on Blu Ray over DVD.  And, you can choose between narration by Sir David Attenborough, British nature documentarian extraordinaire, or Oprah Winfrey if you prefer an American woman’s touch.  I personally prefer Sir Attenborough because I just can’t resist that British accent, but to each his own.  :)


Ah, Microcosmos!  Some people love it, others hate it.  I love it.  This is probably the most artsy of all insect documentaries and is about as old as “Alien Empire,” but it’s still well worth the time to watch.  There’s next to no narration and very little information provided about the species you encounter.  This isn’t the film to watch if you have a hankering to learn some interesting fact about insects!  However, I don’t think it’s actually meant to be an educational film, rather an attempt to make people appreciate the beauty of insects and their relatives.  The film is gorgeous and the macro technology the filmmakers used still looks great!  My favorite scene involves two slugs doing the horizontal mambo.  Normally I wouldn’t think this was so interesting, but when you see their two bodies curling around one another and the glistening of their slime, all set to a dramatic and operatic score…  Well, that’s another thing entirely!  And who doesn’t love a little good slug porn?  :)  Watch this movie when you’re in the mood to be amazed by the beauty of insects, slugs, spiders, etc.  You won’t be disappointed.

Life in the UndergrowthLife in the Undergrowth, BBC

This is the granddaddy of all insect documentaries as far as I’m concerned!  Another BBC series narrated by Sir David Attenborough, the 5 part series Life in the Undergrowth is all about insects.  Unlike Microcosmos, this series is highly educational.  However, it’s also a ton of fun!  From slow motion video of dragonflies flying to walking sticks that trick ants into dispersing their eggs to predatory glowworms that trap flying insects attracted to sticky bioluminescent silk strands, this series does a phenomenal job of highlighting the amazing world of insects.  The cinematography is great and the series moves along at a good pace, giving you just enough interesting information without leaving you bored.  If you have even a passing interest in insects, you should watch this series!  It’s excellent.

Those are my top 5.  Anyone else have others they’d like to recommend?


Unless otherwise stated, all text, images, and video are copyright © 2011 TheDragonflyWoman.com.  All images in this post from Amazon.com.

Mites on Blue Orchard Bees

A few years ago, I had a job working as a scanning electron microscope (SEM) technician for some bee biologists.  They were looking at how well the blue orchard bee (Osmia lignaria) pollinated almond orchards in California.  At the time, colony collapse disorder (CCD) was starting to cause problems and the Almond Board of California was worried about the impacts the bee losses would have on their billion dollar a year crop.  That’s right – California almonds rake in over a billion dollars each year, and it’s all thanks to bees.  Honey bee colonies are shipped to California from all over the country to pollinate almonds, over a million hives in all.  In fact, nearly half of the commercial hives in the US participate in almond pollination in California!  Needless to say, increases in CCD worried almond growers because fewer bees meant lower yields and decreased profits.  The Almond Board was therefore very interested in using native bees to supplement honey bee pollination, and they funded the project I was involved in.  The goal: to determine if the blue orchard bee was an effective almond pollinator species so that they could supplement and/or replace honey bees in the event that CCD continued to decimate their populations.

My part of the project was to use the SEM to 1) determine how pollens from different almond varieties differed (i.e., the structure of the pollen) and 2) to determine whether the blue orchard bees were visiting more than one tree variety, apparently an essential component of almond pollination, by identifying the pollen types they carried.  I spent hundreds of hours looking at pollen from flowers of various almond tree types and bees that had been captured in almond orchards after being allowed to pollinate flowers.  Basically, I spent hundreds of hours looking at this, over and over again:


Almond pollen, Nonpareil variety. The pollen grain is the wrinkly thing in the middle. The little blobby things in the upper left are fungus or some other junk.

I’m not going to talk about the results at all here because they haven’t been published yet and the information belongs to the USDA, but I did want to talk about one thing I discovered during the project: mites!

There are mites associated with a lot of bees.  The varroa mite is likely involved in CCD in honey bees and causes all sorts of problems.  Other bees have other kinds of mites.  (Check out this fabulous website if you want to see just how many bee-associated mites there are!)  The blue orchard bees apparently have these:


A mite on a blue orchard bee

I was so excited when I found these on the bees! Lots of the bees I looked at had mites on them.  And, if they had any mites at all, they typically had lots of mites:


Mites on a blue orchard bee. There are 7 in this picture!

I looked into the mite-Osmia connection and discovered that these mites are almost surely Krombein’s hairy-footed mite, or Chaetodactylus krombeini.  This mite is really interesting!  First, the mites like the ones  I found on the blue orchard bees are a specialized stage found in some mites called the hypopus or deutonymph.  Because they are immature, they only have 6 legs, not the 8 legs typical of their arachnid brethren (they get the last pair when they become adults):


Mite on blue orchard bee, view of the bottom of the mite

The hypopus is a funky life stage in the mite groups that have it.  Hypopi don’t have definable heads and have no mouthparts, so they don’t feed.  However, they do have giant claws on their legs that are used to grab the hairs of the bees.  If you click on the photo above and look at the tips of the legs (four in the upper right of the photo and two curled up closer to the back), you’ll see the claws.  These claws are necessary because the mites hitch rides on the bees to move them from place to place!  (This lifestyle is called phoresy, and the hypopus is a phoretic stage in this mite.)  Without the claws, they wouldn’t be able to hold on very well.

The mites are hitchhikers as hypopi and generally do not harm the adult bees in this stage.  Once they are taken back to the nest, though, the mites climb off the bees and infest the nest cavities.  There, they feed on the eggs or larvae of the bees or the pollen store left for the bee larva by its mother.  In either case, these mites are a bit of a pest in this species and cause serious damage or death to immature bees.  However, they are also a native pest species and have been associated with these bees for a long time.  Considering the blue orchard bee is still around, the relationship seems to be working out so far!

So the mites are interesting biologically, but I’ll admit that I wasn’t excited about that at all at first.  I only learned it when I was writing up my report anyway.  I was initially more interested in the smashing good looks of these mites!  Look at how cute these little guys are:


Mite on blue orchard bee

All those little wrinkles!  And the stubby little legs!  Adorable!*  Plus, it was exciting to look at one species and find another hidden on it.  Discoveries like these are part of why I enjoy working with the SEM so much.  Science is just so cool!

* I am willing to concede that I am horribly weird for thinking a bee parasite is cute, but I can’t help it.  :)


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

Using Aquatic Insect Tolerance Values: An Example


A highly impaired, effluent dominated stream downstream of a wastewater treatment plant. Photo by Dave Walker.

Last Monday I discussed how tolerance values are assigned to aquatic insects so that water resource managers and scientists can use insects as indicators of water quality.  While simply knowing the tolerance value of an invertebrate can tell you something about that animal and where it is likely to live, combining the tolerance values of a whole bunch of invertebrates can tell you some pretty profound things about the body of water in which you found them.  Today I’m going to walk you through a large study that I did with my former employer, one in which we examined the aquatic macroinvertebrates in five effluent dominated streams in Arizona, to show you how tolerance values can be used to determine the water quality in a body of water.

Arizona isn’t known for having tons of water all over the place.  We have a lot of people in some areas and a whole lot of agriculture, so the demands for water are high.  As urban and agricultural uses grow, the amount of total available water will decrease until there is very little left.  Water resource managers are thus looking to other sources of water to meet the needs of Arizonans and our aquatic wildlife and sport fish.  One possible source of water is effluent.  It’s possible that many of Arizona’s aquatic animals, especially fish, will depend on effluent dominated waters (EDWs) for survival in the future.

water sampling

Me recording data during a sampling trip to an EDW. Photo by Dave Walker.

Soon after I started grad school, the Arizona Department of Environmental  Quality (ADEQ) became interested in classifying and comparing the macroinvertebrate assemblages of five Arizona EDWs to determine a) the water quality at the outfall from the waste water treatment plant (WWTP) and further downstream and b) whether they represented viable habitat for Arizona’s aquatic organisms.  They gave a grant to my former employer, who hired several students to help, including me.  All of us spent many hours working in some really awful water collecting insects, measuring basic water chemistry, collecting water and algae samples, and measuring the physical characteristics of the stream.  We collected from two sites in each of five EDWs, once during the winter and again during the summer.   Back at the lab, I directed a team of people who sifted through the enormous samples, removed all the macroinvertebrates, and then handed them over to me to ID.  Once I had everything identified to genus and counted, I calculated the diversity and the Hilsenhoff biotic index (HBI) for each site during the winter and summer.

For now I’m going to ignore diversity and focus on the HBI results.  The HBI is an index of pollution tolerance that was originally developed by William Hilsenhoff in 1977 and updated in two subsequent publications.  It’s used by aquatic scientists and water resource managers all the time!  It’s simple conceptually: you determine the tolerance values of many aquatic macroinvertebrates (as described in my post on tolerance values), take a macroinvertebrate sample in a body of water of interest, identify and count all the animals in the sample, and calculate the average of the tolerance values for every individual in the sample.  The resulting number tells you the overall average tolerance value of the macroinvertebrates in the stream.  You can then compare the values you get to this chart to see how polluted the body of water is:

Tolerance values

Pollution levels according to the Hilsenhoff Biotic Index. Click to make bigger! From Hilsenhoff 1987.

For the project I was involved with, I calculated the HBI for each site for the five different EDWs.  I’m not going to name the exact streams so I don’t end up getting sued (one particular WWTP wasn’t so thrilled about what we said about their effluent…), but here’s what we learned.  First, the WWTPs with the better treatment processes had lower HBI’s than the ones with lower quality treatments.  For example, in the best WWTP, water is treated using extended aeration, activated sludge, secondary clarification, and ultraviolet disinfection.  The average HBI for all sites and dates combined for this site was 7.23.  At the worst WWTP, treatment consists of filtering out the solids, running the water through biofilters to remove nitrogen, chlorinating and de-chlorinating the water, and then dumping it into the stream.  From another couple of studies I worked on, I know that the water coming out of this WWTP is full of pharmaceutical products, flame and fire retardants, and other chemicals – and it smells terrible too.  The average HBI for this site was 9.75, which is just about as high as it gets!


Sampling at an EDW in southern AZ. Photo by Dave Walker.

There were also some overall trends in the HBI values we calculated for each site and date.  The HBI’s were usually higher near the outfall than further downstream, suggesting that the streambed is acting like a filter or the plants are absorbing pollutants from the streams and improving the water quality as it moves downstream.  For example, in the stream below one of the high quality WWTP’s, the HBI at the outfall was 7.5 but dropped to 6.9 further down.  Also, the HBIs were higher in the summer than the winter, 8.4 and 7.5 respectively in one stream.  The reasons behind these seasonal shifts are complex, but the dissolved oxygen levels in the water played a big role.  Generally, things with high tolerance values tend to be able to survive in much lower oxygen environments than things with low tolerance values, and oxygen levels decrease as water temperature increases.  Thus, invertebrates with tolerance values around 6  were probably just getting by in the winter and couldn’t survive at all in the summer, driving the HBI up during the hot part of the year.

The HBI’s of the five effluent dominated streams ranged from 6.5 at a downstream site in the winter at the best WWTP to 9.8 at a downstream site in the summer at the worst WWTP.  Notice that with the exception of the one instance of an 6.5 HBI that falls into the “fair” category, these streams suffer from extensive organic pollution.  One site earned the HBI of 9.8.  Indeed, we found only three species at that site on that date: bloodworms, drain flies, and sludge worms.  Sounds appetizing doesn’t it?


This EDW looks nice, but it had some pretty nasty water in it. Photo by Dave Walker.

In the end, the HBI values (along with the diversity index we used and our statistics) led us to one undeniable conclusion: none of the EDWs in Arizona are particularly good habitat for aquatic insects.  The oxygen levels are too low and the nutrient and chemical content too high for most macroinvertebrates.  Fish certainly aren’t going to be able to survive in this water over the long term!  In our report we stated that effluent, at least as it is currently treated, is not of sufficient quality to support habitat for most of Arizona’s aquatic organisms and that improved treatment is the only way to make effluent useful for this purpose.  A disappointing recommendation for the water resource managers I think, but it was obvious to anyone who pulled giant handfuls of bloodworms out of a rank, hot, sandy stream when it was 110 degrees outside that this water is far from clean.  In fact, several of the WWTPs recommend that you wash your skin with potable water and soap if you are exposed to effluent.

I’m continuing with the water quality and macroinvertebrate theme next week.  Hope you’ll check back!


For more detailed information about the Hilsenhoff Biotic Index, consider reading William Hilsenhoff’s 1987 paper (might be a little hard to get your hands on if you don’t have access to an academic library…):

Hilsenhoff, W.L.  1987.  An improved biotic index of organic stream pollution.  Great Lakes Entomol.  20:31-39.


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

Mayflies, Damselflies, and Stoneflies: What’s the Difference?

I haven’t done an identification post for a while, so its high time that I write another one!  I find that a lot of people have a hard time distinguishing the aquatic insect nymphs with tails sticking off the back, the mayflies, the damselflies, and the stoneflies.  They’re easy to tell apart once you learn a few basics!  A lot of people have read my post on how to tell the damselflies and dragonflies apart as nymphs, so let’s start with them.

Behold, the mighty damselfly:

damselfly nymph

Damselfly nymph

There are several things to look for that will let you know this is a damselfly nymph and not a stonefly or mayfly.  However, the mouthpart is a dead giveaway!  If you don’t know about the awesome odonate mouthpart, allow me to enlighten you.  Odonates have highly adapted mouthparts that form a long, hinged structure that they can thrust out toward prey to capture it and draw it back to the chewing mouthparts to be eaten.  There are pictures of this structure available on the post linked above and you can see a little part of it sticking out past the head of the damselfly in the image above.  Odonates are the only insects that have this style of mouthpart, so if you have a nymph with tails sticking off the back-end and you can see a long, folded mouthpart under the head, you’re looking at a damselfly for sure.

But perhaps you’re looking at an insect in the water and you aren’t able (or willing) to pull it out to look at the mouthpart – what then?  Well, take a look at the location and structure of the gills:

Damselfly gills

Damselfly gills

The three damselfly “tails” are really gills that they use to help them breathe and swim!  They are always located at the back-end of the insect and they tend to be broad and leaf-shaped with varying levels of pointy-ness.  As you’ll see in a moment, the stoneflies and the mayflies have gills in other locations and do not have broad, leaf-like tails.  If you see gills that look like the image above, you’re looking at a damselfly nymph!

Let’s move along to the mayflies:



You should notice some differences between the mayfly and the damselfly right away.  First, look at the tails:

mayfly tails

Mayfly tails

Nothing broad and leaf-like about these tails!  Mayflies have long, filamentous tails, often longer than their bodies.  They also usually have three tails like the damselflies, but some groups only have 2.  Clearly, the flat-headed mayfly in the photo falls into the latter category.  This causes some confusion when distinguishing the mayflies from the stoneflies, as you’ll see in a moment.  However, if you see 3 filamentous tails, you’ve got a mayfly on your hands!

Now let’s take a look at the location of mayfly gills:

mayfly gills

Mayfly gills

The gills  are always attached along the sides or the bottom of the abdomen in the mayflies, never on the thorax or sticking off the back. If you see gills in another location, you’re not looking at a mayfly.  Mayfly gills tend to be broad and leaf-like as in the damselflies, though they may be fringed or sharply pointed in some groups.  They usually have a pair of gills on nearly every abdominal segment, though the exact placement on the abdomen varies by group.

Now we’re left with the stoneflies:



Stoneflies and mayflies look a lot alike in most cases.  The mayfly in my photos above is a specialized species adapted for living in fast flowing water, but a lot of mayflies are shaped more like the stonefly depicted here.  How do you tell them apart when the body shapes are similar?  Let’s look at the tails first:

Stonefly "tails"

Stonefly "tails"

Stoneflies always have two tails.  Like the mayflies, they’re long and filamentous.  In some species, these tails are very long.  In others, they’re shorter than the length of the abdomen.  They’re never leaf-like.

Let’s check out the location of the gills too.

Stonefly armpit gills

Stonefly gill location

Unlike the damselflies and mayflies, stonefly gill placement is quite variable.  Many species don’t have gills.  Some species that do have gills don’t get them until they’ve matured to some specific point.  Some species have gills on the abdomen, but if they do they’re located only on the first few abdominal segments and never further down.  (This helps distinguish them from the mayflies, which almost always have gills on the 3rd-6th abdominal segments.)  But in most stoneflies with gills, you’ll find them in their armpits, as indicated in the photo.  Stonefly gills are very different from the broad, flattened gills of damselflies and mayflies.  They typically have a round main stalk with multiple branches.  These are called “finger-like” gills for some reason, but I think the structure is rather similar to the boojum tree, just on a smaller scale:

Boojum Tree

Boojum Tree. Photo by Bernard Gagnon, from http://commons.wikimedia.org/ wiki/File:Boojum_Tree.jpg.

I find that people have the most trouble telling the mayflies and stoneflies apart.  If the mayfly has three tails, no problem!  It’s a mayfly for sure.  However, you have to remember those pesky two-tailed mayflies that throw a wrench in the whole system.  Plus, mayflies are notorious for losing their gills.  If you’re working with preserved specimens, sometimes it’s hard to figure out where the gills did or did not attach.  How then do you tell a two-tailed mayfly with no gills apart from a similarly shaped stonefly with no gills?  It’s easy!  Look at the claws on the legs.  Mayflies have one claw on every foot.  Stoneflies have two.  It couldn’t be simpler.

As with any identification, the more animals you see, the easier this gets.  For those of you who have little experience collecting and identifying insects, getting a specimen IDed to order can be a challenge at times!  Remembering the characteristics of tons of insects can be hard too.  I thus present this handy-dandy chart that summarizes the information I covered above:

Mayfly Damselfly Stonefly
Location of Gills abdomen end of abdomen when present, thorax, base of abdomen
Shape of Gills leaf-like, plate-like, or fringed leaf-like finger-like
Style of Mouthparts chewing chewing + hinged segment folded under head chewing
Number of Tails 2-3 3 2
Shape of Tails filamentous leaf-like filamentous
Number of Claws 1 2 2

If you forget the characteristics of the mayflies, damselflies, and stoneflies, use this chart as a quick reminder of what to look for!

Next up: another thrilling edition of Friday 5!  This week’s will feature 5 places I’ve found a particular type of tiny insect in my home.  Check it out to discover where these little beasts may be lurking!


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