The deadwood invertebrates

Holly weevil illustration by Sonia Hensler

It’s apparent that a great deal of the articles on our blog seem to revolve around a single aspect of our work as rangers here at Trelissick. This oft-omitted subject – a source of great fascination, discussion and affection for our small countryside team – concerns the retention and protection of our dead and dying trees and the remarkable, obscure ecosystems that are entirely reliant on this twilight phase of the woodland cycle.

Within the woods at Trelissick, it is not difficult to spot substantial accumulations of deadwood that we have deliberately left in place. Organisms that feed upon this wood are called ‘saproxylic’ and a large proportion of those that are rare or threatened are often completely confined to this relict form of habitat. A great deal of these species have rigid requirements for their very existence and can only be found where management has made concessions for the continuity of mature trees and deadwood. You can find out more about this in our previous blog post here.

European hornets (Vespa crabro) feeding on a sap run caused by the fungus Ganiderma resinaceum
Hornets feeding on sap run in the Trelissick countryside
European hornets (Vespa crabro) feeding on a sap run caused by the fungus Ganiderma resinaceum

Trees that are dead or decaying play a crucial role in the functioning and fecundity of woodland ecosystems through their direct effect on biodiversity (the sheer number of different species present). Within a single such tree, crevices and rotting cavities fill with detritus and provide food and shelter for an enormous range of invertebrates such as scarce beetles, centipedes and wood lice. These insects in turn provide food for birds such as the tree creeper and lesser spotted woodpecker and bats such as the noctule. Furthermore, tree wounds and seepages (where rainwater gets into the sap) afford feeding sites for hornets, hoverflies and butterflies.

We would like to introduce you to a selection of these secretive creatures that we have here on the estate:

Illustration of lesser stag beetle by Sonia Hensler

Lesser stag beetle

These beetles are the smaller cousin of the larger (and more famous) stag beetle: The larvae of this insect depend upon dead and rotting wood for both food and shelter (especially ash, beech and apple wood) and the adults can be found, during the summer, hiding around dead wood and leaf litter during the day. At night these beetles emerge to fly and are often attracted to bright lights.

Illustration of large fruit tree bark beetle by Sonia Hensler

Large fruit tree bark beetle

Trelissick is the only site in Cornwall where this beetle has been recorded. The larvae of these wood boring creatures develop in the galleries between the sapwood and the bark of freshly dead or dying Rosaceous trees; mainly pear, plum, apple and hawthorn. Scolytus mali is therefore usually found in traditional orchards and hedgerows containing fruit trees. This recording was noted in one of the old orchards we have on the wider estate.

Illustration of snail killing fly by Sonia Hensler

Snail killing fly

Another invertebrate of which the only recording in Cornwall has been here at Trelissick. This fly has been observed at Roundwood and is mainly associated with ancient woodlands, especially those situated near water, making Trelissick a perfect habitat. The voracious larvae of this fly feed on various types of land snail. Although fairly widespread across Britain, these are the first such records for the fly in Cornwall.

Illustration of death-watch beetle by Sonia Hensler

Death-watch beetle

The death-watch is a wood boring beetle, rare in Cornwall and rich in sinister folklore. Its morbid presence can be noted throughout the full spectrum of arts and literature as an ill omen, symbolic of imminent death. It is the beetle’s characteristic behavior that has bestowed that moribund moniker. In order to attract a mate, these insects create a tapping sound, often heard in the beams or rafters of old buildings during summer. As a result, this species is associated with sleepless nights and the vigil kept beside the dying or the dead. The superstitious have even come to view the appearance or sound of a death-watch as the harbinger of impending doom!

" ...within ye hear No sound so loud as when on curtain’d bier The death-watch tick is stifled."
- Keats
Illustration of hornet longhorn beetle by Sonia Hensler

Hornet longhorn beetle

How apt that this beetle, in its Cornish colours, resides with us on the estate. It is rare across the UK but best known in Cornwall within the Fal and Helford catchment areas. The soft larvae of this longhorn develop over several years in the totems of dead trees, stumps and cordwood. They particularly favour oak, beech and chestnut. When ready, they emerge as adults in July and August to visit the flowers of elder, angelica, bramble, broom and scabious, flying with a loud - almost mechanical - hum.

Illustration of oak longhorn beetle by Sonia Hensler

Oak longhorn beetle

This bright red and violet/black beetle is commonly called the violet tanbark beetle. In Europe, this is one of the most common longhorn beetles but sadly, within the UK, it is rare and recorded most frequently on newly cut stacks of oak. The first recording of them in Cornwall was here at Trelissick. These beetles have a one year life cycle with oak as the preferred host. The larvae feed under the bark of dead branches and trunks, finally surfacing as adults from April to June. Unlike other longhorn beetles, Phymatodes will readily lay their eggs in freshly cut timber.

The oak longhorn beetle often appears during the winter months in firewood when kept for several days in heated rooms so keep an eye out before putting logs in the fire! They are not, however, considered a danger to wood used in buildings because it has become too dry for their particular preferences.

Unfortunately, this beetle’s vivid claret colour may also have contributed to its scarcity. Many gardeners in the UK often assume any all-red beetle is a lily beetle (Lilioceris lilii) and often kill them.

Important vertebrates

Several notable bat species are also reliant on old trees and the two listed below have been recorded at Trelissick. We are about to have a bat survey carried out and will have a more definite idea of which species reside at Trelissick when it is complete…

Illustration of Noctule bat by Sonia Hensler

Noctule bat

The noctule is one of the largest species of bat in the UK. It is predominantly a tree-dwelling species, often roosting in old woodpecker holes or those created by rot. It emerges in the early evening, making characteristically steep dives to catch flies, beetles and moths.

Illustration of Barbastelle bat by Sonia Hensler

Barbastelle bat

Barbastelle bats are veteran tree specialists and are one of Britain’s rarest mammals. Recently, we were privileged enough to be part of the first recording of these elusive creatures on the Fal. They are medium sized with a peculiar pug-shaped nose and amber-brown fur, feeding mainly on moths during the dark hours.

All of these species listed here are really just a small proportion of the total that resides at Trelissick in direct consequence of our strict deadwood policy. Hopefully these covert creatures will help demonstrate why we are so relentlessly passionate and outspoken on this subject!

- The National Trust ranger team, Trelissick and North Helford