COLEOPTERA > POLYPHAGA > CHRYSOMELOIDEA > Chrysomelidae > Donaciinae

The members of this subfamily were traditionally known as Aquatic Leaf Beetles, which was a functional name in respect of defining the group as part of the Chrysomelidae (Leaf Beetles) family thereby avoiding any possible confusion between these species and water or diving beetles. However, it wasn't really appropriate for the British fauna; hence the newly adopted term of Reed Beetles.

Of the 21 species recorded in Britain, only Macroplea appendiculata and Macroplea mutica are regarded as truly aquatic as they spend virtually all their time in water submerged on pond weed. All the remaining species in the genera Donacia (15) and Plateumaris (4) occur in similar wetland habitats, but will be found on reeds or on the leaves of marginal or floating plants.

The two Macroplea beetles are mainly dull orangey-yellow or orangey-brown with striped, truncate and apically toothed elytra, and can also be distinguished from their unique final tarsomere that is longer than the combined length of the other tarsal segments. They are rarely seen by casual observers and have never been recoded from my area.

Donacia and Plateumaris are medium-sized, mostly 6-12mm long, elongate, brightly metallic species with mainly bronze-green, bronze-brown or coppery coloured elytra. The Donacia species have a somewhat flattened appearance and elytra where the edges of the wing cases touch each other along the suture thereby leaving a very narrow groove from the base almost down to the tip, which is truncate as though cut off with scissors. The Plateumaris are a bit more robust with somewhat rounded elytra with wing cases that separate slightly just before the tip.

With a bit of care, species can be identified by a combination of form, appearance and colour. 

In respect of the Donacia, current distribution data suggests that only six of the fifteen species are likely to be found across my general area. In recent years, the most frequently recorded from both Dorset (VC9) and South Hampshire (VC11) has been Donacia versicolorea, which is interesting as older records were showing a decline. I have yet to encounter this species. Similarly, with Donacia bicolora, which is not surprising, as it's rarely recorded now. And, also, Donacia vulgaris despite incorrectly featuring it on the previous version of this page. Having taken a closer look at my records, I now believe that there are only three species that I can reliably include.

Donacia marginata averages 8-11mm long; golden bronze to coppery colouration, sometimes a bit greenish; with variable reddish-purple longitudinal marginal stripes along interstices 7-9 and, although sometimes feint, similarly coloured patches either side of the scutellum on interstices 2-5; the elytra is broadest at the shoulders with a slightly depressed area either side of the suture; adults appear from mid-May and can usually be seen right through to the end of August; typically found on Bur Reed [Sparganium erectum] alongside still or slow moving water, rather than faster-flowing streams. 

Donacia semicuprea is a slightly smaller more convex species, usually 7-9mm; the overall colour is generally described as metallic bronze with some green overtones, but it's rather dependent on the angle of view and light; when seen from above the dorsal surface has more of a shiny copper colour with greenish lateral reflections; the colour of the pronotum and elytra contrast with the head, legs and antennae, which are covered with short pale grey pubescence; the head has a well-defined longitudinal impression on the vertex; the pronotum is quadrate with a narrow longitudinal median groove that has a small shallow anterior depression and a larger and deeper posterior pit (fovea), which is an important distinguishing feature of the species; elytra shining and coarsely wrinkled, prominent shoulders and truncate apically; mainly associated with Reed Sweet [Glyceria maxima] and related grasses; adults also found on other pond or lakeside marginal vegetation; most frequently recorded during May and June, but can be active both earlier and later in the year.  

Donacia simplex is much the same size, perhaps a tad larger, up to 10mm; usually coppery-bronze, but often with some green, rarely purple or bluish, reflections; head with convex protruding eyes and long temples; pronotum widest at the rear with the anterior margin smoothly curved; elytra always unicolourous (unlike vulgaris) with broad rounded shoulders, somewhat depressed from behind and evenly tapering from just beyond the middle to the tip, which is truncate as though it's been sheared off; primarily associated with various Bur Reed [Sparganium sp.], but also found on other wetland plants.

And, a few notes on the three species that I still need to find.

Donacia vulgaris (Common Reed Beetle) is typically 5-10mm long; entirely metallic green or coppery, usually with a contrasting purplish, although sometimes more reddish or even bluish, longitudinal band that runs down the elytra on both sides of the suture between interstices 2-4/5; eyes large; pronotum densely punctured with an often-indistinct median groove in the basal half; elytra more-or-less parallel for much of their length before gently curving.

Donacia versicolorea is mostly associated with broad-leaved floating pondweed [Potamogeton natans] and is more likely to be found later in the year during July and August. It's a dark-coloured beetle with bluish or greenish reflections along the margins and down the middle. The most likely location that I'm aware of locally would be on the small ponds at Arne, but there should be no reason why it can't be found on other heathland ponds, such as those on Holt Heath or in the New Forest.

Donacia bicolora is said to have a rather magnificent sparkling greenish-gold colouration. Adults are most likely to be found on the leaves of Bur Reed often with Donacia simplex, but preferring proximity to flowing rather than still waters, which corresponds with the few local records that have been along the Rivers Stour and Frome, and from Bourne Valley NR.

Continues on the next page ....

Updated, Dec.25 (v.3)