Jewels in the reeds

My last post - a photographic offering from a local water-meadow site - included a picture of a reed beetle and a promise of something more in-depth on this group of invertebrates. So, here it is...

First of all, what are reed beetles? They are the subfamily Donaciinae within the Chrysomelidae (leaf beetles), and are medium-sized (5-12 mm), fairly elongate and often metallic beetles found generally on waterside vegetation. In Britian there are 21 species within three genera - Donacia, Macroplea and Plateumaris. However, despite being of a fair size with charismatic colouration, some are highly variable in colour whilst true morphological differences between species can be slight. This, coupled with a lack of accurate illustrated keys, meant that there was considerable confusion regarding identification of this group in Britain prior to the publication of keys by Menzies & Cox (1996), and subsequently (currently undergoing revision) Hubble (2010). Knowledge of the group was also greatly enhanced by the publication of an illustrated and annotated atlas (Cox 2007). A visit to an old Hampshire water-meadow in late May 2001 produced many sightings of several species and it is these that are presented and discussed here.

The River Itchen as it runs through the water-meadows.

Apart from being most picturesque, the photo above shows a small path alongside the River Itchen and it is the fringing vegetation (reeds, irises, sedges, Sparganium etc.) for several hundred metres that was the source of the specimens here. Identification of the Donaciinae relies on a range of features - sometimes colour is helpful, but often closer examination is required of features such as pronotal puncturation and sculpturing, femoral spines, antennal segmemts and sometimes dissected genitalia. Therefore, although I have attempted to make definite identifications to species, these were not collected (it was initially a day of voluntary conservation work rather than bug-nerding), and hence - relying on photographs and field observations, despite being in the middle of 'writing the book' - some identifications are tentative. Still, I hope the material is interesting...

A pair of reed beetles on Iris pseudacorus

The pair of beetles illustrated above gives a good indication of their charismatic nature and shows some key features despite them being tricky to ID in the field - the overall shape, metallic colour and 'bumps' towards the front corners of the pronotum (not present in all species). These are (I think) Donacia semicuprea - note the midline of the pronotum with two unequal-sized dimples (fovea; pl. foveae) joined by a groove, and the elytra (wing cases) which are slightly rounded and widened in the front half. The pronotum and elytra are the same copper-bronze colour, and although it is hard to see in this picture the sides of the elytra have a faint greenish tinge - the head is greyish due to short pale pubescence. There are femoral teeth visible which are usually not present, but this is a variable feature and it is not that unusual to find them.

A green reed beetle on a yellow iris flower

A blue reed beetle running along the stem of an umbellifer

Dorsal view of a blue reed beetle

These three images show a quite different colouration - metallic blue and green - and the middle picture tries to illustrate the 'vaulting' (as opposed to relative flatness) of Plateumaris compared with Donacia in side view. Again, this is trickier than it feels it should be, but I think these are all Plateumaris sericea - the variable colour is well-known, and the top picture shows the strong thorn-like tooth on the hind femur. Also, this species can be separated from the similar P. discolor to quite an extent by habitat - P. sericea is found in neutral to high pH areas (as found at the water-meadows which are on chalk), while P. discolor is found in more acidic conditions.

A metallic-green reed beetle

And so to the third and last species that I will be treating individually here. Donacia simplex is arguably the most common British reed beetle and is characterised by the 'frosted' metallic coppery colour, often with a uniform tinge of green (as here), red or grey, plus at least some orange-red on the legs and antennae (faint in the photo, but clearer in the field). Also, the femora are untoothed and it is generally found on Sparganium bur-reeds (this is on a different plant which was among a Sparganium clump).

So, what of the rest? Well, the following pictures are the 'best of the rest' - generally unidentified, or only identified tentatively, due to my unwillingness on this occasion to collect them - some jewels are best left unmined and I had no urge to add them to an entomological collection. Instead they are here to show the varied nature of these splendid beetles. Enjoy!

A reed beetle on a curled iris petal - unidentified, but the pronotal groove and faint elytral green-gold banding suggests Donacia marginata as the usually clearer marking are variable and may be reduced or absent.

Two images of another charismatic specimen - this time with a reddish tinge on the pronotum and front third or so of the elytra.

Two specimens of what I think are the same species with shiny, almost sparkling, reddish-brown to dark brownish-green elytra.

 

And last, but not least - the Donaciinae - feel the love!

References

• Cox, M.L. (2007). Atlas of the Seed and Leaf Beetles of Britain and Ireland. Pisces, Newbury.
• Hubble, D. (2010). Keys to the Adults of Seed and Leaf Beetles of the British Isles (Coleoptera: Bruchidae, Orsodacnidae, Megalopodidae and Chrysomelidae). Test Version. FSC, Preston Montford.
• Menzies, I.S. & Cox, M.L. (1996). Notes on the natural history, distribution and identification of British reed beetles. British Journal of Entomology and Natural History 9: 137-162.

Further sources

For excellent online coverage of the European Donaciinae, including many photos of prepared specimens, see here.