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Rare beetle drops out of the sky at RSPB Ynys-hir!

Last modified: 17 December 2014

Calosoma inquisitor

Image: Dr Roger S. Key

A rare beetle, called the ‘inquisitor’, has been discovered for the first time at Cwm Llyfnant, part of RSPB’s Ynys-hir reserve near Machynlleth in mid Wales.

The rare beetle (full Latin name Calosoma inquisitor) and also known as the ‘Caterpillar-hunter’ or ‘lesser searcher beetle’, was discovered in ancient woodland – a key habitat for the species - in the Cwm Llyfnant area of the reserve. 

The survey which took place this summer has been part-funded by Natural Resources Wales (NRW) and is part of a wider survey which is investigating special communities of insects in the Dyfi Valley. These insects are one of the main reasons for the designation of the area as a Site of Special Scientific Interest (SSSI).

Consultant Ecologist Dave Boyce and NRW Invertebrate Ecologist Adrian Fowles made the discovery, though neither had to dig around under logs or rotting leaves to unearth the rare find. As Dave Boyce explains: “We were discussing management specifically for these special insects, when out of the sky an inquisitor beetle, one of Britain’s rarest beetles,  flew into a clearing and landed right next to us!”

Wales is thought to be the last remaining stronghold in the UK for this creature, with a particularly strong population at RSPB Carngafallt reserve in the Elan Valley, another site with lots of ancient oak trees. 

The inquisitor beetle was once widespread in England and Wales but its population has halved in the last 25 years, and it is now only found in small, isolated pockets of woodland in England, and a handful of sites in mid- and north Wales.

RSPB Ynys-hir reserve manager Dave Anning said: “This is a wonderful first for the reserve. We knew the woodland here was important for lots of wildlife, and to have this survey prove that, is wonderful news.”

Adrian Fowles believes that the discovery is a good sign, he said: “Finding this beetle suggests there is a breeding population within the valley, which is encouraging news.”

The ideal habitat for the inquisitor beetle is ancient oak woodland, and with more than 40% of the UK’s total upland oak forests found in Wales, it’s easy to see why Wales is a valuable and special home for this creature.

The cause of the decline of the inquisitor beetle is not fully understood but it is thought it may relate to a lack of appropriate management of woodland, including a lack of grazing.

Adrian Fowles added: “Discoveries like this when we are monitoring help us further our knowledge about the significance of Welsh oakwoods. They also highlight the importance of the conservation efforts to manage and protect these key areas of our environment and help prevent the loss of biodiversity.”

These beetles are nocturnal and adults are often found climbing the trunks of oak trees to feast on the caterpillars of woodland moths like the carpet and pug species in the canopy. It is possible that there's been a reduction in their food as many widespread moth species are also declining.

This work is part of the RSPB-NRW Strategic Partnership and looks at exploring what insects call ancient woodlands their home, especially those woodlands with veteran trees and an abundance of deadwood. 

For more information about this project please contact

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