Why do dragonflies swarm?
by Michael Ellis on April 01, 2008 in Ask the Naturalist, Wildlife: Invertebrates, Reptiles, Amphibians
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).
17 comments:
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
Wow. That must have been something. How long did they stay?
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
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.
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.
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.
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.
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.
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?
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.
I’m looking at a swarm in my back yard here in NJ.Temp about 72. Where do they come from?
Northern Illinois from about 5pm til 6pm today. It was quite a sight.
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?
7pm / sunset at St Simons Island Ga. Lining the beach by the 1000′s
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 .
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!
Thanks, Alexa. That sounds cool!