Saturday, October 30, 2010

Dragons & Damsels: Odonate Overlap

Although killing frosts struck on 17 September, a week before schedule, after this Summer, September was quite a pleasant month. October has been up and down, and not much greenery or insects are in evidence, but the pond bubbler is still on. I usually chicken out in the first or second week of October, but the pump has persevered for 5 summers and probably won’t last that much longer anyway. So, for the benefit of the resident ‘native’ birds (chickadees, red and white breasted nuthatches, house finches, blue jays, and magpies), I’ve kept it going. Soon, though, I will have to give in to the ice gods.
So, what better excuse on a cool, but sunny Saturday, to post about some aquatic insects – and also, to start answering a question that Middle Earth Garden asked back in March – just how similar is the Home Bug Garden fauna to other more natural systems in Alberta? Her question is actually far more interesting than I can attempt to answer. I’d love to spend my summers wandering Alberta and documenting the various insect faunas, but so far no one has offered me a job doing so. However, I do spend many of my weekends at our Moose Pasture and at least for certain groups, I can now make reasonable comparisons between the HBG and the MP. (NB – pictures with blue borders are from the Moose Pasture, no colour borders from the Home Bug Garden.)
 First up is the Odonata. Even the smallest adult odonates are relatively large (or at least long) insects, often very abundant, and not as leery of people as many others. Thus they are more obliging to the photographer and especially friendly for close-focusing binoculars (I like my Pentax UCF XII). BugGuide has many good pictures and even better, for Christmas I got an excellent guide: Dennis Paulson’s Dragonflies and Damselflies of the West (2009, Princeton University Press).
Paulson’s field guide has a distribution map for each species with the species descriptions and pictures – so likely from unlikely species are easily separated. As well as color pictures of mature males and females, plates with line drawings showing male genitalia (claspers) and critical female characters are presented for easy of comparison. Each species has paragraphs with a description and a review of key characters in comparison to similar species. Also, the natural history, habitat, flight season, and a comment on distribution are given. The only complaint I have is that he relies too much on the common, rather than the scientific names – but this complaint may be restricted to entomologists.
Odonate Family Genus species   Moose Pasture  Home Bug Garden
Aeschnidae Aeschna interrupta                +                +
Aeschnidae Aeschna eremita                      +                 -
Corduliidae Epitheca spinigera                   +                -
Libellulidae Leucorrhinia cf hudsonica        +                 -
Libellulidae Leuchorrhinia intactor             +                -
Libellulidae Libellula quadrimaculata       +                +
Libellulidae Sympetrum internum             +                +
Libellulidae Sympetrum danae                  +                +
Coenagrionidae Coenagrion resolutum     +                 +
Coenagrionidae Enallagma cyathicerum      +                -
Coenagrionidae Enallagma ebrium              +                -
Coenagrionidae Nehallenia irene                  +                -
Lestidae Lestes congener                             +                -
Lestidae Lestes disjunctus                          +                 +
Lestidae Lestes dryas                                  -                 +
(excuse the drunken columns - Blogger seems to have trouble with tables. Bold = shared species. Red = HBG only)
As usual, you should take all my identifications with a grain of salt, but in this case if there are any errors, they are probably applicable to both sites. Also, we have many unidentifiable pictures of damselflies, so their actual diversity is probably higher. With damselflies, and I suppose this is ironic, dead males on a pin are the best route to a solid identification.
 So, of the 15 species identified to date, 6 occur both at the Moose Pasture and in our backyard, and only one, the Emerald Spreadwing (Lestes dryas) has yet to be identified at the Moose Pasture. A quick reading of Paulson suggests two reasons for this apparent difference. The Emerald Spreadwing favours shallow densely vegetated ponds (sounds like our backyard pond) and oviposits in live sedges, grasses, and horsetails (the pond is full of sedges). Permanent water bodies are not favourable breeding sites for this species.
Although our backyard pond has only about 2 m square of surface area, it does have lots of emergent vegetation and the bubbler to keep it aerated. At the Moose Pasture, Dancing Elephant Lake (DEL) and its attendant marsh and pond cover about 28 acres. The topography results from stagnant chunks of glacier that were left behind the main retreating mass and formed kettles surrounded by knobs (now covered with aspen) There is much emergent vegetation, and also lots of aquatic marcophytes and some deep areas without vegetation.

The difference between a lake and a pond isn’t well agreed, but usually lakes are expected to be larger and have some area too deep for rooted plants, and DEL seems to just qualify. One of the neighbours calls it a slough, but ‘slough’ is as loosely used as ‘lake’ for everything from an alkaline prairie pothole, to a treed swamp, to a marsh. The only trees in DEL are the skeletons of the willows drowned when the beaver flooded out the meadows that we now call marsh, which to us means it is weedy and shallow enough that getting the canoe through is difficult (The Trunk and Lower Legs). But whatever you want to call it, a host of odonates emerge every year.
The same is true for our backyard pond. Well, maybe ‘host’ is an exaggeration. However, we do have yearly emergences of many damselflies and some dragonflies. It is a rare summer day that one or more of these attractive and useful insects isn't around. They eat other insects, and so we can think of them as good ‘natural enemies” – and if they take any bees, it is only the tiny ones.
And what can we conclude, if anything, from this data? I’ll argue that even a small backyard pond can act as an urban refuge for a subset of the local odonates. This exaggerates the usefulness of the pond for dragonflies – the 4-spotted Skimmer was a one-off (possibly brought in with a plant) and the Variable Darner (Aeshna interrupta) may cruise through your yard even if you don’t have a pond.
 The Cherry-faced and Black (S. danae) Meadowhawks, however, breed in the pond as do several of the bluet damselflies. The Emerald Spreadwing finds a small pond a perfect habitat. In Edmonton, where synanthropic mosquitoes that breed in small accumulations of water are absent, there is no real downside to a backyard pond – the birds love it, the bubbler adds some relaxing white noise, water plants are interesting, and a host of aquatic insects, snails, and other even smaller invertebrates can make a living where dandelions and grass once dwelled.


  1. Great stuff. That's a nice little assortment of odonates you've got there.

  2. The male cherry-faced in the first photo seems to typify the stance they often take when perched. Do you think they are flaunting themselves as a territorial claim, or are they just catching a few rays?

    Love the emergence sequence - well done Heather!

  3. Hi Adrian,

    The perching itself is related to territorial behaviour, but the bum in the air is supposed to be thermoregulation. There is a video of a related Japanese species interestingly named Sympetrum eroticum on You tube: