Chrysomelidae

COLEOPTERA > POLYPHAGA > CHRYSOMELOIDEA > Chrysomelidae > Donaciinae

Members of the genus Plateumaris are very similar to Donacia, but with stouter legs and elytra that are a bit more rounded dorsally with the suture between the individual apices inverted and appearing very slightly separated just before the tip.

Although there are four British species, Plateumaris bracata and Plateumaris rustica are rather scarce and are now found in just a few areas and, in fact, from what I can tell have never been recorded from Dorset (VC9) or South Hampshire (VC11). However, both of the more widespread and common species, Plateumaris discolor and Plateumaris sericea are actually two of the most frequently recorded reed beetles in Dorset presumably due to habitat. Across the county border in Hampshire, there have been a few reports of Plateumaris discolor from in and around the New Forest, but it's certainly not as common as Plateumaris sericea.

Current literature and keys continue to recognise four separate species, as does the latest checklist (Duff, 2012), but the two common forms are very similar and could intergrade. Given that I didn't initially appreciate that I had photographed Plateumaris discolor, I spent a bit of time reading up to make sure that I understood how they are distinguished.      

Plateumaris sericea is around 7-10mm long; very variable in colour from metallic gold or bronze, to green, red and vivid blue or even violet; head with convex protruding eyes, a short almost parallel-sided neck and a deep median longitudinal impression; pronotum quadrate with large swellings before the anterior angles, and impressed medially; unlike Plateumaris discolor, the pronotal disc is finely punctured, regular and comparatively smooth, without wrinkles; elytra with rounded shoulders, strongly punctured striae and with the suture inverted towards the apex; the appendages are darker, but usually match the body colour; the antennae are more elongate with the third segment typically around 1.5 times longer than the second, and the fourth a little longer still.

Although usually found in association with Bur Reed [Sparganium sp.] particularly the branched variety [Sparganium erectum], adults may be found on other aquatic plants including Yellow Flag [Iris pseudacorus]. They are typically found in wetlands, pond or lakeside margins with vegetation growing in neutral or basic, rather than acidic, habitat and are most often seen during May and June, although they have actually been recorded both earlier and much later in the year.

Plateumaris discolor averages a little smaller than Platyeumaris sericea at 6.5-9.5mm; superficially similar with the same variation of colour forms, although rarely some individuals may be nearly black; the pronotal disc is strongly punctured centrally and obliquely wrinkled throughout; the appendages are darker as in sericea, but usually reddish around the proximal section of the tarsi and tibia; also, slightly reddish at the base of the antennae, the segments of which are more compact than in sericea with the second more or less the same or only slightly longer than the first, and segments three and four slightly longer than the second.

This species is mainly associated with Sedges [Carex] although adults may also be found on other aquatic vegetation including Bur Reed and Marsh Marigold [Caltha palustris] but, unlike Plateumaris sericea, the habitat will most likely be acidic, such as boggy moorland or around heathland ponds.

In conclusion, the primary features for separating these two species are the appearance and texture of the pronotal disc (smooth v rough) together with the comparative form of the antennal segments (elongate v compact). And, as noted, the habitat (neutral to basic v acidic).

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