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Bay Nature magazineApril-June 2008

Why Do Dragonflies Swarm?

by on April 01, 2008

Q: Last October, while hiking on Mount Tamalpais, near Laurel Dell, I saw numerous swarms of dragonflies. Could you tell me more about this phenomenon? Is it seasonal? Or triggered by weather or courtship? [Khiem, San Jose]

A: Those dragonflies were most likely congregating to hunt, catch, and eat abundant insects that were also swarming. This is indirectly weather-related, because dragonflies are active only on warm days. Most need an air temperature of at least 63 degrees to get moving.

Dragonflies and damselflies belong to an ancient insect group called the odonata, some of the earliest flying creatures on our planet. Dragonflies are big, strong fliers. They have very large eyes and hold their wings out straight when resting. Their cousins, damselflies, are smaller and have a weak, fluttery flight. Damselflies have small eyes and rest with their wings held up directly over their thorax.

While driving through the Central Valley near Williams last August, I encountered thousands, maybe millions, of common green darners, our state’s second-largest dragonfly. As far as I could scan through binoculars in every direction for miles and miles, the air was filled with them. These iridescent giants were in constant flight, hunting insects breeding in the standing water and irrigation ditches so prevalent in this fertile agricultural land. On Mount Tamalpais I have seen swarms of common greens but also of the variegated meadowhawk. I suspect the latter may be what you saw. This widespread dragonfly, which grows to about two inches long and varies in color from pink to tan to dull gray, can be active year-round in the Bay Area, even in winter.

The swarming you saw probably had nothing to do with mating. There are over 60 kinds of dragonflies in California and more than 28 in the Bay Area, so mating strategies vary quite a bit among species. In some species, males wait and pounce on females as they fly by, and there is little color difference between males and females. The males of other species, like the western pondhawk, establish territories over a patch of prime real estate (rich with insect prey—freshwater ponds, lakes, streams, rivers, even poorly maintained hot tubs) and chase off rival males and attempt to copulate with every female that comes into their range. These species sometimes show a marked difference between females and males; the males are brilliantly colored with more vibrant facial patterns, which make it easier for them to tell potential mates from rivals.

Two great references are Kathy Biggs’s self-published pocket-size guide Dragonflies of California (Bigg’s Wildlife Pond) and Tim Manolis’s UC Press field guide Dragonflies and Damselflies of California (UC Press).

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Maryann on July 6th, 2012 at 12:00 am

We live in Aquebogue, Riverhead, Long Island New York, Last night about 7PM we saw  for the first time a swarm of 50 or more dragonflies over a cement drivewaay, We were amazed we’ve never seen this before, Thats why I visited your site to find out why, M. McGarry

baynature on July 6th, 2012 at 12:00 am

Wow. That must have been something. How long did they stay?

J.Drake on August 15th, 2012 at 2:49 pm

As I type there is a swarm of dragonflies in my back yard. We usually have two or three, and not always of the same color. This swarm is all blue tails. The swarm was already here when I arrived home at five pm and are still here at 5:42 pm. I am in Weare , N.H. Overcast day, current temp. 77 degrees with a 71 percent humidity. I live in the country and see a lot of wild turkey, deer, and 2 moose in the 12 years we’ve been here..but this is the first time I have seen a dragonfly swarm. J. Drake

Barbara Boudrot on August 15th, 2012 at 10:14 pm

August 15 Deering NH near Deering Reservoir at about 5:30 PM we saw a swarm of about 100 large dark colored dragonflies on the corner of Zoski Rd and Fisher. It looked like they were circling the address sign on the front lawn of the house. I have never seen anything like this. Deering NH borders weare NH.

Wendy C. on August 24th, 2012 at 10:23 am

I was looking out my window last night around 5:00 and saw a swarm of dragon flies. I have never seen so many at on time. They would fly in one spot then move on down the yard and eventually they disappeared. It was in the 80’s and humid last night. I live in Raymond Maine and usually I see a few but never this many, there must have been a couple hundred if not more.

Cale on August 24th, 2012 at 3:55 pm

Very helpful! I am seeing a swarm of about 50 dragonflies right now in my backyard. We have a deck that looks over an acre of grass that leads into woods. No pond.. just grass. Wondering why they’re swarming. Recently I broke an wasp nest.. thought after reading it maybe that had something to do with it. Otherwise, have no idea what they’re hunting. I live in a northwest suburb outside of Chicago, IL.

ST on August 25th, 2012 at 2:31 am

I am from Malaysia and dragon flies are common over here. Today ( August 25th, 2012) was the first time I saw a big swarm of thousands over my house. Is it a coincidence that most entries made in August 2012? Weather was gloomy for the past few days but it wasn’t any different from the usual weather pattern.

Tomcat Jones on August 27th, 2012 at 6:08 am

i also saw a huge swarm of dragonflies hovering above a very tall Ash tree around sunset yesterday. there had to be over 40 of them.

Jennifer on August 30th, 2012 at 5:42 pm

Living in the Twin Cities in Minnesota. Came home to a dragonfly swarm in my neighborhood. I’ve never seen this before. It’s about 7:40 p.m. and it’s in the 80s here. Very cool. I wonder what bugs we have that they like so much?

Renee on August 31st, 2012 at 7:16 pm

We were driving home from working in Madison and about 15 miles from home we started to see them everywhere. We didn’t see dense swarms until we were approaching our home town of Monticello, WI. (7:00) There were thick swarms of close to 100 in half a dozen yards and where the swarms weren’t thick they still seemed like they were everywhere, on every street. They were thickest low to the ground but you could look straight up and see them flying above the houses. There had to be at lease a thousand in our town alone. I’ve never seen anything like it. It’s a warm muggy night and was probably was in the 80’s at 7PM. We started seeing them about 15 miles away from home. I wonder how many were in a 30 mile radius.

MARION on September 1st, 2012 at 3:48 pm

I’m looking at a swarm in my back yard here in NJ.Temp about 72. Where do they come from?

dbunger on September 3rd, 2012 at 5:38 pm

Northern Illinois from about 5pm til 6pm today. It was quite a sight.

Koki on September 16th, 2012 at 4:33 am

New Orleans, LA, swarming in my front yard around 2pm yesterday. I didn’t see standing water nor did I see any gnats, what other reasons could they be here?

Michael on September 23rd, 2012 at 5:07 pm

7pm / sunset at St Simons Island Ga. Lining the beach by the 1000’s

Myra on October 5th, 2012 at 7:45 pm

End of Sept. and I was sitting on the beach on Oak Island , N.C. They came from across the ocean by the 1000’s. I have never seen anything like it .

Alexa on October 30th, 2012 at 1:21 pm

When you see large swarms like that, they can also be migrating–the common green darner is one of the migratory species. I work for an organization that is trying to learn more about this cool phenomenon, and we need help from citizen scientists like you!! Check out http://www.xerces.org/dragonfly-migration/ and http://www.migratorydragonflypartnership.org/index/welcome for more info. You live in a great place to observe migration, since the west coast is a main flyway. We need your observations!

Dan Rademacher on October 30th, 2012 at 1:27 pm

Thanks, Alexa. That sounds cool!

Laurel Paulson-Pierce on June 21st, 2013 at 7:21 am

I found this web site from a science project that is recording and mapping dragonfly swarms and asks for observations to be sent to them.

IlenAllison Aouthern Ill on July 22nd, 2013 at 5:07 pm

Went to water flowers and my back yard there were a lot of dragon flies have never seen this before. I have a small pond in my back yard. It was really something to see. My grandson is visiting and he lives in southtern Mo and has never seen this before either. Just thought would let you know. We looked it up on the internet to see why.

Dave on July 22nd, 2013 at 5:41 pm

There was a swarm of dragonflies beside my driveway this morning – between 7:30 am and 9:00 am. There were between 50 and 100 flying so fast I could not see any detail. They were very impressive . North Vista, CA.

John Dkar on August 1st, 2013 at 12:10 am

July 31 2013 Chengdu,Sichuan,China…my friend and I from Colombia were about to head to the swimming pool at noon when we saw a swarm of hundreds of dragonflies flying in circles next to my window on the 14th floor…there was a weird kinda halo of sunlight lighting our area..it was cloudy day but not for my building..we had something like someone with a torch from the dark clouds lighting up from above to our place and hundreds of hundreds of dragonflies going up n down in circles but not higher than our building..it was unique..lasted 5 or 10 minutes then they smoked out

Alison Hawkes on August 1st, 2013 at 9:22 am

That sounds like a really cool experience!

ladyhd1 on August 6th, 2013 at 10:14 am

august 5, 2013 while walking on the beach in Brigantine, NJ with my friend, we encountered swarms of dragonflies, all sizes…..all the beach goers were wondering what was happening. Kids were swatting at them and hiding under blankets and towels. At 60 years old, I have never experienced anything like this before. We’ve seen plenty of dragonflies before, flying mating, etc., but nothing to this extend. All were heading “west” in off the ocean. Many were very small, others varied in size. Could not get good enough look to determine type/species/color of these dragonflies; it looked as though they were all plain brown. On Monday, I was compelled to get on the internet to try to find out why.

Lisa S on August 8th, 2013 at 1:16 pm

I witnessed this swarm in Brigantine also! Never seen anything like it. it was so cool. I have to admit about the only insect I wouldn’t mind swarming around me is the dragonfly. My nephew caught one and it was more green than brown.

Kath H on August 15th, 2013 at 8:11 pm

On the beach in Ocean City, NJ. Saw a swarm of dragonflies. Like the others, they were flying away from the ocean, inland. Never saw that in my 42 years going toOcean City! Surprised the seagulls didn’t eat them!

Sandy Sullivan on August 22nd, 2013 at 5:44 pm

In front of our house for the last 2 nights a swarm flies around our small front yard for 30 minutes. None of the other houses in the neighborhood have them flying around. St. Charles, MO

Kate on August 25th, 2013 at 6:01 pm

I just saw hundreds of dragonflies swarming at Lake Michigan just north of Chicago at the beach at the end of Judson street. It’s just getting dark. We’ve been watching them for a half hour. Today was in the 90’s and sunny.

Kate on August 25th, 2013 at 6:03 pm

7:30 to dark hundreds of big dragonflies at beach at the end of Hudson street just north of Chicago

tina cleveland on August 27th, 2013 at 6:58 pm

I live in South Dakota and sitting outside my home tonight a swarm of dragonflies were flying overhead an it stretched out for over a mile I thought it was weird and freaky. now I know thanks

Shirley Roseland on August 28th, 2013 at 7:28 pm

August 28, 2013, leaving my mother-in-laws home in Piedmont, SD and came out of the house to a swarm of 50 or more dragon flies, above my car. Warm night in the 70’s, what an awesome site. I have never see so many of them before, and my mother-in-law said all the years she has lived there she has never seen so many.

Joye Morse on September 10th, 2013 at 4:09 pm

This is so cool, it’s 7:05 in Toledo, OH and my husband and I saw our first swarm of Dragonflys, there must be couple hundred or so, we were wondering if it was weather related as it is about 103 degrees, I Think is was about 6-6:30 when when noticed them. My husband just came in from outside and said he thought it is due to a hatch of some kind of flying ants and the dragonflys look to be going after them. Just love the dragonflys

Jeremy F on September 11th, 2013 at 11:15 am

Joye, I’m also in Toledo and last night we had a huge swarm of dragonfly’s in our back yard. Exactly how you describe, they were swarming over the ground and there were ants and flying ants in the grass. I was out picking raspberries and surprisingly didn’t get bit by any mosquitoes! They were doing their jobs! I stood in the swarm for a bit thinking I was the Zen Master and it was neat to hear their little shells clink together as they were masquerading through the yard. Very cool!

Janet on September 11th, 2013 at 3:28 pm

Live in Il. Suburb of Chicago we are looking at a 100 dragonflys in our back yard we just had a passing thunderstorm, they have been here for 30 min. now!

Robin Emrich on September 11th, 2013 at 3:49 pm

I am in Warren, OH and it is 6:45pm and there are hundred swarming in my back yard and over my septic field. I want to mow the septic field as we have a threat of rain tonight, but I am a bit timid about riding through such a massive swarm of them! I have never seen so many at one time!

Bud on September 11th, 2013 at 4:01 pm

My wife and I just witnessed a swarm in the back yard along a fence line. I’d say around 100 or so. First time we have ever seen anything like it. They did seem to be feeding on some type of gnat or midge coming up off the ground. We live ing Hartville, Ohio. I came in to check see if others had seen this. It’s mid 80’s here, expecting thunderstorms later tonight. Very cool to see.

Kathy terela on September 11th, 2013 at 4:45 pm

I am in Girard Ohio and it’s 7:40 pm…there are a swarm of dragonflies and it’s unreal to watch how fast they are flying. There are hundreds of them!!!! It’s very very muggy out and they are so interesting to watch because how fast they are….

Lynn on September 11th, 2013 at 5:04 pm

We are in Dublin, Ohio and have had a swarm for the last two hours of about 50-60. We have never seen this before and it sounds like we are all experiencing a similar thing. We have a small backyard pond and a creek across the street that is very dried up. They must have come from one of those sources. It really is amazing to watch.

Pat Allen, Mich on September 12th, 2013 at 6:08 am

September 10, 2013 I saw dragonflies from Jonesville to Allen along side US 12. Got to my house and they were swarming all over the yard. It was a very hot day. Official high 90 degrees.

Mickey on September 12th, 2013 at 3:30 pm

Just now, at 6:00 on Sept 12, 2013, all of my neighbors are gathered in our back yard as there Re hundreds of blue tail dragonflies swarming around. Looks kinda neat, but have never seen this. We live near Wheeling, WV, in the northern panhandle between PA and OH. I noticed several folks posted similar in OH from yesterday, and wondered if this may be part of that pack on a migration.

Patricia on September 12th, 2013 at 4:03 pm

I live on a small farm property outside of Gambier, Ohio (with two small ponds – half acre or so) and for the last two days we have had thousands of dragonflies swarming over our property. I believe they are the green ones. Yesterday it was around 90 plus degrees; today a little cooler with some rain; very pleasant this evening. My family and I have lived here for 9 years & this is about the fourth time I’ve seen them swarm like this. Love the bugs!

sherry Wallace on September 12th, 2013 at 7:53 pm

WE were at my boys foot ball practice tonight and there were what seemed like thousands of dragon flies sworming around us. Do they harm , Sting, Bite humans??? Ive never seen so many they were not avioding us by any means they were flying directly by us, over our heads nearly bazing our skin as they were passing by . Why so many together ? we were near a few streams . Are they atracted to water . I live in n. easr Ohio

sherry Wallace on September 12th, 2013 at 7:57 pm

Im wodering if the same sworm is traveling through Ohio. Super weird

Bob Slaven on September 20th, 2013 at 2:54 pm

I’m in SE West Virginia. We just had a major rain till about 1:00 and now at 5:30 we get a swarm of a few hundred in my grassy yard. This is a first for me. Moving too fast to tell which ones they are but look like blues.

Sandy on September 20th, 2013 at 7:13 pm

A large swarm came over our deck tonight. It was about 75 degrees and there is a large field near our house. First time my husband and I had ever seen anything like it. Scary and dreamy – all at the same time!

Sandy on September 20th, 2013 at 7:14 pm

BTW, we live near Hurricane, WV (outside of Charleston, WV).

betty barthel on October 15th, 2013 at 7:30 pm

5:30 pm my daughter and grandson and I were standing in the backyard when a swarm of dragonflies came over from west to east we live a few miles from SF Bay.

Mark Rauzon on January 25th, 2014 at 1:21 pm

On October 15, 2013 a migrating ‘swarm’ of Variegated Meadowhawks passed by San Leandro Marina, Berkeley Hills and Half Moon Bay. October is the time to watch for this little-known wonder. http://rauzon.zenfolio.com/p859914566/hc95b00#hc95b00

Charles on January 31st, 2014 at 11:50 am

I’ve never seen them swarm. That must sound unreal.

D Clarke on May 27th, 2014 at 5:21 pm

Hundreds of Dragon Flies swarmed a NJ shore town near Lavallette on Memorial Day. They flew from the ocean towards the Barnegat Bay. It was an awesome site!

Steve Rogers on June 9th, 2014 at 1:11 pm

We had a yard sale this last Sunday 6/8/14 in Dayville Connecticut, early that afternoon my Uncle and I encountered a swarm of at least 1000 dragon flies that appeared suddenly out of nowhere, they swarmed around us and a few costumers as well.. for about 15 mins, none of us had ever seen anything like it..in fact I had never believed that dragon flies swarmed until I visited this web site.. fascinating sight to see though :)

Scarlet on August 8th, 2014 at 11:49 am

Yesterday about 1 pm, I encountered a swarm of 50 in a meadow at Redwood Park, in Arcata CA. Definitely feeding on small insects. Amazing flight patterns, some were greenish, some reddish. Beautiful to see, had to stand in the center for awhile. Magical!

Trish on August 19th, 2014 at 9:02 pm

Visited the 3800+ft summit of Mt Diablo near Danville today. Was greeted by thousands of swarming large dragonflies flying in circular patterns across the visitors center and parking lot. There were none seen below the summit. Remarkable!

maria on August 21st, 2014 at 1:53 pm

Sitting in the yard and could see about 200 dragonflies all along the block very overcast and raining all day about 78 degrees

Tammy on August 22nd, 2014 at 3:51 pm

I just saw my first swarm of them. Have never seen anything like it. It was about 7pm upper 70’s. I am in Devil’s Elbow, Missouri

Ronda W on August 24th, 2014 at 5:51 pm

I’m in eastern Iowa and it’s been extremely hot and humid the last few days. I have a stream in my backyard and today observed hundreds of Green Darners swarming for several hours. What made it even more amazing was watching a mixed flock of insect eating birds (primarily Tree swallows, Bank swallows, Purple martins and Phoebes) swooping in for the feast.

Pam troyer on August 25th, 2014 at 4:54 pm

I live in Solon, Ia. There are literally thousands of them out for 2 or 3 days! They are amazing! We have 2 ponds and a lot of timber. Very cool site to behold. Also have the barn swallows. Hard to get a picture you can actually see them in:)

Romolice Navy on August 26th, 2014 at 4:26 pm

I live in Chicago and we came home and there were dragon flies flying all outside I don’t know where they came from but they looked as if that they were going to attack and I don’t know if they do attack but I was just wondering if there was some way that we can not have them in front of our house or are they waiting to meet or what are they waiting to do I have no idea what is very scary sometimes when you go outside it’s about 85 or 90 degrees now is that the weather that they like maybe it is but I’m a little scared of them cuz they’re so big

john on August 27th, 2014 at 5:09 pm

here is what I think is happening and it may have been mentioned but I don’t have time to read every post…sorry!
Dragonflies have been around for ever…right? Ok, supreme hunter total aerial assaults with insane precision, attacking spawning bugs while they take to their first flight.
These bugs can sense when a spawn is about to happen the larva of the soon to be first fliers must put off a scent because the big-boys were staging for days prior to the final assault in my yard!!
We slowly had an increasingly large amount of large dragonflies in the yard then one day a hatch and it was madness, hundreds I am thinking 1200 at least the air was thick with them and my drive way was covered in half eaten bugs! Awesome!!!!

Karhy Biggs on August 27th, 2014 at 7:57 pm

Wonderful!! They are either amassing to begin migration, &/or feasting on mosquitoes and such in your area! Remember, they don’t bite people, only prey, and they don’t have any stingers so they cannot sting. The Xerces Society is tracking their migration. You might like to help scientists by filling out the form at http://www.migratorydragonflypartnership.org/index/welcome?utm_source=MDP+eNewsletter+Aug+2014+&utm_campaign=MDP+e-newsletter+Aug+2014&utm_medium=email
Kathy Biggs, author of Common Dragonflies of the Southwest

Cherilea Morton on September 2nd, 2014 at 4:51 pm

I just tried to fill out the form and it came back that my e-mail address was already taken. So I sent a e-mail directly that reads: I would like to let you know that for the first time in the 15 years that I have lived in Lake Township Ohio, 7 miles Southeast of Toledo I had a swarm of Dragonflies that covered about 6 acres of land. I had a very heavy and fast rain shower that came through about 5pm and I noticed the Dragonflies about 6:30. I had to call my neighbor, who has lived out here for over 20 years and asked her to look outside and tell me what she was seeing. She was amazed. She had never seen that many at one time either. Usually after a hard rain or when it’s humid like it has been we get swarms of little tiny bugs that fly in a cluster in one spot (I think they are gnats). You literally have to swat your way through them. If Dragonflies were on a feeding frenzy I hope the got their bellies full. They were beautiful. It may not matter, but the other creature that has been in abundance this year around here are the Praying Mantis. Usually we get a few because of all the farm fields, but this year they have been all over.

cheryl on September 2nd, 2014 at 5:24 pm

went to sit in our back yard and there was a swarm of 40-50 dragonflies darting around over our back yard….we are happy to learn they eat mosquitoes , we can barely sit out without being eaten alive by them. the ohio river is about 2 blocks from us, and as others have mentioned it is humid and overcast. i live in louisville, ky

JerryD on September 3rd, 2014 at 5:54 pm

Thousands of dragonflies this evening. I live outside of Sidney. I was able to video record them with my phone as I pulled the car over and watched for about 20 minutes. It was amazing…30+ years living in the countryside, and this is the first time I’ve seen this happen.

peggy b on September 3rd, 2014 at 9:41 pm

live at this one house in cincinnati suburb for 65 years. last night and tonight, about 7 pm til 730 or 8, swarm of dragonflies over our back yard, first time ever seen.also, saw fewer in yard of neighbor on one side but none on the other side. no standing water in our yard. flying high, above house mostly, sometimes most of them fly low to ground.

Deb on September 8th, 2014 at 5:36 pm

Swarms in Green Castle, MO Aug. 8, 2014 temp about 70.

Sean on September 11th, 2014 at 2:32 pm

I live in Laurel, MD and it is now 5:30, cloudy, and about 84 degrees out. Just above my backyard there is a swarm of dragonflies just circling and darting back and forth. Have never seem anything like it. There is no water nearby and just grass and trees.

paige on November 7th, 2014 at 5:17 pm

I’ve always wondered, what temperature do dragonflies live in?

Cathy on July 4th, 2015 at 11:17 pm

July 3, 2015 Never seen anything like it before! A swarm of maybe a 1000+ dragonflies over a 3 hour period. I live on the North Shore of Lake Superior in Canada. We have had torrential rains and cooler weather this last 2 weeks.

Sue Shavers on July 21st, 2015 at 11:40 am

We have an annual swarm at our home. Usually in late June/early July. They are the bright blue ones. They range in size from medium to quite large. Usually there are several hundred and it only lasts for a day or so. I look forward to this each year. We live in central Illinois and the weather is usually in the high 70’s when this occurs.

Shalontee' on August 20th, 2015 at 8:13 pm

I went out on my back deck at about 7:30pm, the temp. was in the 70’s, and there was a swarm of about 50 dragonflies. We do not have any water in our yard, it’s a residential neighborhood, and there were no bugs. They were just flying around in a circle right above the power lines. I live in Omaha, NE.

Carol Moore on September 3rd, 2015 at 3:35 pm

I live in Huntington NY and just came home from work to find a swarm of dragonflies in my front yard and over the roof of my house — I’m 60 years old and I’ve never seen this before in my life!!

joe strickland on September 8th, 2015 at 6:10 pm

on sept 8 walk outside around eight at night and notice swarms of hundreds of fireflys i called my grandson over and he had just came back from uptown and said they were allover the town marquette nebraska im 50 yrs young and have never seen this.

Keith tucker on September 11th, 2015 at 3:20 pm

I live in close to summersville Wv
For past two evenings they has been a massive swarm of dragon flies in mine and neighbors yard . Im talking in the thousands weather is hot & humid ive lived here all my life and have never before seen this .
43 yrs youg

Daisy on September 2nd, 2016 at 3:57 pm

Last night, 28 deg F, sitting on patio in back yard and saw about 8 dragonflies in yard and around vegie garden and before I could even say “look at all the dragonflies” there were thousands all about the same size and brownish colour.
Never witnessed that before, it was “AAAwsome”. Calgary Alberta Canada

Joseph on July 19th, 2017 at 8:16 am

In Florida July 17, living on a pond my Lanni stuck on the screen hundreds on bugs. The bug guy said there from the mosquito family beyond that swarming dragon fly are they eating them?

Nichelle on September 8th, 2017 at 9:42 am

It’s a warm September day with a Cat 5 hurricane coming and a crazy big swarm of dragonflies is swarming the seashore. They seem to be feasting on the gnats that have been active this morning.

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