Rare beetle found in the ditches at Magor Marsh

King diving beetle, cr Will WatsonKing diving beetle, cr Will Watson

In early October 2013, in Magor Marsh there have been a host of interesting wildlife sightings including otter, grass snakes, water voles and a barn owl, but rarer still is the king beetle. It’s the largest, rarest and most threatened large water beetle in the UK.

It's so rare, it is classed as 'Near Threatened'.

Lurking in the densely matted vegetation at the margins of the reens and ditches, this diving beetle waits to pray on snails that come too close. At nearly 40mm long and stoutly built, it’s a beetle that rightly deserves its regal name.

This beetle will hopefully continue to thrive at Magor Marsh, but elsewhere the greatest threat to this species is posed by the conversion of grazing fen to arable land. Careful maintenance of ditch and reen margins is vital to ensure that the wildlife rich interface between water, land and air continues to feed, protect and nourish the wildlife that lives there.

King diving beetle, cr Rob WallerThe support of highly knowledgeable recorders like Peter Kirby (who recorded this beetle in 2010) and Will Watson who found and identified the king beetle this September, is a huge help in improving our knowledge of the reserve and the species it supports.

Gwent Wildlife Trust is hoping that by working in partnerships with other land owners across the Gwent Levels, it will be possible to improve the way ditches and their margins are managed to benefit wildlife both on and off the nature reserves.

Near Threatened
A great diving beetle
(Bergsträsser, 1778)

A review by Prof Garth Foster

The most recent map is provided by Sutton (2008). Records since 1980 are for South Devon, South and North Somerset, East Sussex, East Kent, East and West Norfolk, Cambs, Hunts, Monmouth and South Lincs. There are older records for East Cornwall, Surrey, South Essex, Salop, Glamorgan, South-west and Mid-west Yorks, and Durham. This is a West Palaearctic species, ranging to North Africa and the Caucasus.

Habitat and ecology
D. dimidiatus occurs in rich fen vegetation in lowland drains and ponds. Beebee (2002) noted its preference for rhynes (Somerset Levels ditches) that were at least partly shaded. Rondelaud (1979) demonstrated that D. dimidiatus will consume the dwarf pond snail Galba truncatula (Müller) both in the field and in the laboratory. In Germany, first instar larval numbers peaked in April and May, with second and third instars in May (Braasch 1989a). Owen (1990) noted a fully fed larva in June at Woodwalton Fen. D. dimidiatus adults trapped in early April in the Broads had fully developed internal genitalia, the males carrying spermatophores indicative of recent copulation (Water beetles of Great Britain 47) (G.N. Foster, unpublished observations). In Switzerland, adult numbers had a single peak in June (Brancucci 1980). It appears that this species breeds in the spring and early summer. This species has been recorded attracted to light (Bembenek & Krause 1969, Kahlberg 2004) and has been noted in flight at dusk. Mark-and-recapture work (Brancucci 1980) has demonstrated movements between three ponds in a 150 metre long row, but not into ponds a further 100 metres away.

This species is concentrated in the Somerset Levels, the Gwent Levels, the Cambridgeshire Fens, Broadland and the coastal fens of Kent and Sussex, with 24 hectad records from 1980 onwards. The records for ponds in Cornwall and Durham probably indicate isolated colonisation events. This species has not been recorded from Askham Bog since 1943, with later Yorkshire records only for Hob Moor in 1952 and the Thorne Moors in 1965, again indicating the potential for this species to lose ground.

The greatest threat to this species is posed by the conversion of grazing fen to arable land, with the associated loss of dykes and fall in water table (Driscoll 1983, 1985).

Management and conservation
A cycle of ditch cleaning to ensure the provision of exposed and richly vegetated - but not overgrown - ditches is essential for the survival of this species on sites such as Woodwalton NNR, Wicken Fen NT Reserve, the RSPB Reserves of Dungeness, Nene Washes and the Ouse Washes, Catfield Fen operated as a reserve of Butterfly Conservation, neighbouring Broadland owned by the Broads Authority, the Somerset Levels and the Romney Marsh.

Published sources
Balfour-Browne (1950), Beebee (1991, 2002), Bembenek & Krause (1969), Blair (1917), Boyce (2004), Braasch (1989a), Brancucci (1980), Carr (1983d), Carr & Philp (1988), Denton (2007a), Drane & Warrington (2009), Driscoll (1983, 1985), Duff (1993), Eve (1966), Eyre & Foster (1984), Eyre, Woodward & Luff (2005), Fitter & Smith (1979), Foster, A.P. (1983, 1984b), Foster, G.N. (1970, 1972, 1973b, 1980b, 1985b, 2004a), Foster & Eyre (1992), Friday (1988a, 1997), Hammond & Merritt (2008), Kahlberg (2004), Lambert (1996), Lohez (2007), Merritt (2007), Nilsson (1982a), Owen (1990), Painter (1994), Rondelaud (1979), Roughley (1990), Sinclair (1982), Skidmore, Limbert & Eversham (1985), Sutton (2002, 2008), Sutton & Wilkins (2005), van de Vijver et al. (1997).