One of the many species of conservation importance in the Great Fen is the great crested newt. This spring, various ponds in the Great Fen were surveyed, to capture and photograph the newts. This is done to keep track of individual animals and so monitor their movements, gathering data on the populations at nature reserves such as Ramsey Heights and Woodwalton Fen.

This species of newt lives for up to 15 years and every individual has a unique belly pattern. One piece of equipment we can use to survey for newts is the Dewsbury trap, a type of live trap. The survey method involves sinking the traps in a pond the day before and then collecting them the following morning to inspect the ‘catch of the day.’ Newts are not the only wildlife encountered during these surveys; smooth newts, 10-spinned sticklebacks, dragonfly larvae and water beetles are also captured.

One of these additional catches in the traps is the king diving beetle (Dytiscus dimidiatus), the largest ‘diving beetle’ in Britain (at up to 3.8cm in length) and a bit of a fenland speciality.  It is larger than the great diving beetle, the species you may encounter in your garden pond. The ‘king’ is black, with a nice gold trim on either side of the pronotum (the bit behind the head) and the sides of the elytra (wing cases). Capturing a range of species is very valuable as it provides additional data to help evaluate the colonisation of a new habitat, such as the wet farming test beds at the Great Fen. Diving beetles generally, let alone the king diving beetle, are not easily caught using other methods. Using Dewsbury traps has allowed us to record the rapid colonisation of the test beds.

Various species of 'diving beetle' live in the Great Fen. Some you may be familiar with, such as the great diving beetle (Dytiscus marginalis), and others you will be less familiar with, such as the 'king' and the 'wasp' (Dytiscus curcumflexus). The great diving beetle looks very similar to the wasp until you turn it over and see the stunning black and yellow belly markings that gives the wasp its name.

I was out with a volunteer this spring, surveying for newts at Woodwalton Fen. We were having a productive day and had enjoyed hearing our first cuckoo of the year and seen our first large red damselfly. On opening a trap, the volunteer gazed into it and, as they visually sorted the water beetles by size, they gave voice to their thoughts, “so, the smaller ones are probably great diving beetles, and the big one is the king?” I peered in and said, “actually, those little ones are king diving beetles and the big one, well that’s not a king!”

I was staring down on something that was much bigger than all other water beetles we have recorded in the Great Fen. As it paddled around in the trap, we could see the exceptionally long hind limbs, each with a distinctive long, sharp spine protruding from the inside. The next thing to do was to pop this completely black beetle in a clear container and look at its underside. Lo and behold a film of air cloaked its belly, the silver sheen that gives the great silver water beetle its name. While it is aquatic, the great silver water (Hydrophilus piceus) is not a true ‘diving’ beetle, but one of the many ‘scavenger’ water beetles. It can grow up to 4.8cm in size, outstripping all the ‘diving’ beetles.

Once key information had been recorded and photographs taken, our thrilling discovery was safely returned to the pond it came from. Interrogation of various species datasets, on the NBN gateway and iRecord, as well as correspondence with CPERC for their records followed. There are records of this beetle in the Fens from the 1800s and early 1900s. It inhabited Whittlesea Mere before it was drained in 1851. The last record of the beetle in our area seems to have been 1938, at Wicken Fen, by eminent coleopterist, Frank Balfour-Brown.

Where we should expect to find the beetle is particularity interesting. In his book of 1958, Balfour-Brown wrote, “In Cambridge, it used to be common at Whittlesea Mere, long since dried up. As recorded by Stephens and others of the earlier authors but there are few recent records for the county except for Wicken Fen, while in Hunts it has been found only at Yaxley Fen and Mere.”

In the latest New Naturalist book, on ‘Ponds, Pools, and Puddles’ by Biggs & Williams, they recount the decline of this species and draw attention to its contraction in range over the last one hundred years. They write, “Oddly, great silver water beetles are strong fliers, so it remains a mystery why they have been unable to colonise new sites. Perhaps the British climate is currently too cold for them to fly long distances, although in their strongholds they do clearly fly and en masse. For example, in Norfolk in 2018 Mike Hodge found 53 great silver water beetles in a moth trap run in the Norfolk Broads, probably more individuals than most freshwater biologists in Britain see in a lifetime!”

Following a long absence of records, this species is now turning up in moth-traps in Cambridgeshire. In the last five years, great silver water beetles have been recorded in places such as Sutton Gault, by the Ouse Washes, in 2020 and in Over, near the River Great Ouse, it was recorded in both May and October last year. Garth Foster reviews the return of the beetle to Cambridgeshire in the June 2022 issue of Latissimus, the newsletter of the Balfour-Brown Club. The club functions as an international study group on water beetles and maintains joint stewardship of the water beetle recording scheme with the Aquatic Coleoptera Conservation Trust. The record of our exciting find has been submitted through local water beetle expert and Wildlife Trust volunteer, Kevin Rowley.

This species has now reached one of many wetland restoration sites in our region, so it is spoilt for choice on where to colonise. Will the Fens provide a new stronghold and hence a vital stepping stone for further incursions inland? Will we discover it further up the valley of the River Nene?

So remember, it’s big, it’s all black with silver underwear, and its back; get the Dewsbury traps ready, turn on the moth traps, and watch what happens. If you see it let me know. Oh yes, apologies to the king diving beetle, the real KING has returned!

Henry Stanier (Great Fen Monitoring & Research Officer)