The invertebrate fauna of wood-decay
Red cardinal beetle
||There are more than 1700 different invertebrate
species in Britain and Ireland which are dependent on decaying wood in order
to complete their life cycles (Alexander, in prep. The invertebrates of
living & decaying timber in Britain & Ireland). This represents about 6% of
the entire British invertebrate fauna - wood-decay is a major resource! That
means more than 1700 different life styles, since each species has very
particular requirements. These statistics really bring home just how diverse
a habitat wood-decay can be.
The keys to understanding the ecology of these invertebrates is to develop
an understanding of the two key processes involved:
- the aging process of woody plants
- the process of wood decay.
|What do they feed on?
||Very few invertebrates possess the necessary
gut enzymes to break down the principle components of wood - cellulose and
lignin. Most rely on fungi and/or micro-organisms to convert these compounds
into more digestible materials. The exceptions to this - species which can
digest cellulose - include goat moth Cossus cossus, longhorn beetles (Cerambycidae),
bark beetles (Scolytidae) and the very rare beetle Lymexlon navale.
The most important wood for wood-decay invertebrates is of course the living
tree, for it is the living tissues which generate the wood which will
ultimately decay. Dead wood has a limited existence, it decays and is
ultimately re-cycled. Conservation of wood-decay communities requires
conservation of a diverse age structure of woody plants in order to ensure
continuity of wood-decay habitats.
|Invertebrates of Heartwood
||The single most important wood-decay resource
for invertebrates is a large standing living tree with columns of decay in
In the early stages of decomposition, white-rotted heartwood is commonly fed
upon by larvae of lesser stag beetle Dorcus parallelepipedus and rhinoceros
beetle Sinodendron cylindricum, forming characteristic large and convoluted
galleries. White-rot decay supports a very wide range of some of our most
colourful insects. Larvae of the beautiful red net-winged beetle Platycis
minutus develop in relatively soft moist heartwood, especially beech and
ash. The brassy tortoise beetle Thymalus limbatus develops beneath loose
bark on decaying broad-leaved timber, especially oak, and in the later
stages of white-rot decay when the heartwood is dry and soft. Melandrya
caraboides is a large black beetle with a metallic green sheen and develops
in relatively soft moist dead heartwood of boughs, trunks and stumps of
various broadleaves, but especially ash and beech. The larvae of the bright
green beetle Ischnomera cyanea develop in relatively soft white-rotting
heartwood of a great variety of broadleaves. The larvae of the spectacular
tiger or feather-horned craneflies Ctenophora spp also develop in soft moist
Red-rot supports a quite different fauna, including some of the most
colourful click beetles. The hairy fungus beetle Mycetophagus piceus feeds
directly on the mycelium of the sulphur polypore fungus, deep inside the
decaying trunk - it is itself eaten by larvae of the rare click beetle Lacon
querceus in Windsor Forest, but nowhere else in Britain. Another very rare
heartwood click beetle, the bright red Ampedus cardinalis, also develops in
red-rotten heartwood of old oaks, in smaller boughs as well as trunks. Like
the Lacon it is an active predator, feeding on the larvae of developing
beetles and flies. The red-rot itself is bored by a number of small dark
beetles, notably Dorcatoma chrysomelina and Anitys rubens. The larvae of the
stiletto fly Pandivirilia melaleuca (Therevidae) is a particularly
aggressive predator living in very dry powdery red-rotten heartwood of oak.
Scenopinus niger is another scarce fly the larvae of which are specialist
predators on dermestid and probably other beetle larvae in dry red-rotting
heartwood of various broadleaves.
The larvae of the rare noble chafer Gnorimus nobilis develop in wood mould
within hollowing old trees; often associated with old fruit trees, but also
in oaks and willows; it is a speciality of the lower Thames, Severn and
||Many of these insects gain access to the
hollowing interior of old trees through patches of heartwood which have been
exposed to the air through physical damage to the bark, eg through lightning
strikes or damage caused by the collapse of a neighbouring tree. The dull
brown beetle Ptilinus pectinicornis bores in exposed dry heartwood of old
broad-leaves, making the small pin-holes which are so common in areas of
exposed heartwood. As with so many of these wood-decay beetles, the female
attracts males by release of a pheromone and the males have feathery
antennae designed to maximise the sensory area. The females bore breeding
passages into the solid outer heartwood to lay eggs, preferring standing
tree trunks over fallen ones; only a few females actually leave the old
breeding site to initiate new infestation. The bright red and blue beetle
Tillus elongatus is a specialist predator of Ptilinus pectinicornis larvae,
entering the pin-holes in the exposed heartwood and exploring the galleries
below for occupied burrows. Their larvae hunt nocturnally within the
galleries and actively explore the trunk surfaces for new prey. Tomoxia
bucephala is an example of a species with decay-feeding larvae which is
unable to excavate access for itself but uses the vacated Ptilinus galleries
to get in to the decay.
|Later stages of heart-rot
||In both red- and white-rot the end product is a
black wood mould which accumulates in the bottom of the hollow trunk as the
fungus works its way into the upper trunk and main boughs. Some of Britain's
rarest insects develop in this medium of relatively constant temperature and
humidity, protected from the outside world by the surrounding living trunk
The darkling beetle Prionychus ater is one of the most widespread
specialists here. Amongst the rarer species are many of the bright red
Ampedus click beetles and the famous violet click beetle Limoniscus
violaceus - rare throughout its European range and one of Britain's very few
legally protected beetles. The larvae of most of these species appear to
develop in hollow trees which have been occupied by cavity-nesting birds
such as jackdaw, stock dove or owls. The decayed heartwood is not rich in
nutrients, and inputs of bird droppings, feathers, bones, etc, may provide
an important source of minerals, etc, which promote successful development.
Similar conditions can sometimes develop beneath loose bark on the trunks
and main boughs and Prionychus ater can also be found developing in this
situation, as well as the much rarer Prionychus melanarius.
The later stages in the decay process of timber are not essentially
dissimilar to other decaying organic matter and certain wood-decay
inhabiting invertebrates may also be found in other decaying matter. A good
example is the click beetle Denticollis linearis, a widespread species
developing in decaying timber, but it also develops in peat on moorland.
Decaying timber eventually supports what is essentially a soil fauna,
dominated by millipedes, woodlice and centipedes.
||There are two ant species which form their
nests in the decaying heartwood of trees: brown tree ant Lasius brunneus and
jet ant Lasius fuliginosus. Wood is macerated by their jaws and hardened by
secretions from the mandibular glands to create the nest itself and
intricate passageways are developed through the heartwood to access points
in the outer trunk from whence the workers forage over the leaf canopy for
food. These ants are of considerable interest for the wide range of other
insect species which live specifically in their nests and which are
associated with their runs. Good examples are the rove beetles (Staphylinidae)
of the genus Zyras which live in the runs and nests of jet ant.
|Other guest species
||Other species appear to be associated rather
more with the galleries of wood-decay insects than with the wood-decay
itself. The rare beetle Aeletes atomarius is usually found in the burrows of
lesser stag beetle Dorcus parallelopipedus in moist crumbly decaying
heartwood, although also recorded with Sinodendron cylindricum and brown
tree ant Lasius brunneus.
Many bees and wasps exploit the exit holes of wood-boring insects and other
cavities in timber as nest sites. Hole-nesting digger wasps are good
examples. These have their own specialist parasites including certain
Sarcophagidae flies such as the rare Macronychia polyodon and Macronychia
||Most of the above discussion relates to
hollowing trunks, but similar processes also occur in smaller cavities in
the trunk and in the branchwood. These are termed rot-holes and are
particularly favoured by Diptera for the moister conditions which prevail.
Such rot-holes are open to the elements and particularly rain, creating
different conditions to those found in the centre of the main trunk. The
gunge which accumulates in these cavities is favoured by a whole host of
hoverflies (Syrphidae), moth flies (Psychodidae), wood gnats (Mycetobiidae),
long-headed flies (Dolichopodidae), etc. One rare hoverfly Pocota personata
is known to develop mainly in rot-holes which are high in the canopy. As
with the decaying heartwood, these include species which feed directly on
the decaying mulch, others are scavengers, predators and parasites.
Water-filled rot-holes even support a specialist freshwater fauna including
the copepod Moraria arboricola, non-biting midges (Chironomidae) such as
Metriocnemus martinii, and mosquitoes and gnats such as Anopheles plumbeus.
The last develops in water-filled holes on mature trees; the eggs are laid
on the sides of tree holes just above waterline and hatch only when flooded.
The pale orange beetle Prionocyphon serricornis also develops in
water-logged hollows in old trees, especially favouring those hollows
amongst roots at the base of the trunk; the larvae are aquatic, feeding on
the detritus from the dead leaves which accumulate in the cavities.
||The decay already discussed relates to the
activities of the fungal mycelium - the feeding part of the fungus. The
fruiting bodies themselves provide specialist habitats for another huge
array of invertebrate species.
The larvae of the small moth Morophaga choragella (Tineidae) feed in
galleries excavated within the fruiting bodies of various wood-rotting
fungi, especially Inonotus and Ganoderma spp, pupating either in the fungus
or in deadwood. Elodia ambulatoria is a rare tachinid fly which specialises
in parasitism of tineid moth larvae, mainly Morophaga choragella.
The hard black fruiting bodies of the fungi Daldinia concentrica and
Hypoxylon spp are remarkably favoured by insects. The precise species of
fungus appears not important, rather the hard black medium provided: the
scarce fungus weevil Platyrhinus resinosus develops in Daldinia concentrica
on ash trees as well as Hypoxylon fragiforme on beech, and other beetles
behave similarly, eg Biphyllus lunatus, Litargus connexus, and Mycetophagus
More typical bracket fungi also have their specialist fauna, eg Inonotus
hispidus is where the beetles Triplax russica and Orchesia micans develop,
I. radiatus for Abdera flexuosa, and sulphur polypore appears to be the key
larval habitat for the beetles Eledona agricola and Hallomenus binotatus.
These bracket fungi are fairly long-lived, but even the short-lived brackets
of beefsteak fungus Fistulina hepatica are used by a number of species which
can develop from egg to adult relatively quickly. The soft-bodied fruits of
oyster mushroom Pleurotus ostreatus, for instance, are favoured by the
bright red and blue or black Triplax beetles.
Amongst the many interesting variations on the theme is the small black
beetle Dorcatoma ambjoerni which has only been found in the fruiting bodies
of the bracket fungus Inonotus cuticularis actually inside hollow beeches
rather than growing externally. The larvae of the platypezid fly Agathomyia
wankowiczii develop in galls under brackets of Ganoderma applanatum. So far
only the galls have been found in Britain and it is currently known from
only six localities in the south-east. It is presumed to be a recent
establishment from the continent as the galls have only been noticed in
recent years and yet are very conspicuous.
Two slugs are particularly associated with old wood-pastures - ash-black
slug Limax cinereoniger and slender slug Limax tenellus. The latter is the
rarer of the two and a beautiful animal especially when feeding on say black
buttons Bulgaria inquinans, when the contrast of the bright yellow slug and
the deep matt black of the fungus is very attractive.
|Heart-rotters as keystone
||Basically, the heart-rotting fungi are keystone
species - a large number of other species are completely dependent on the
conditions which they create. And not just invertebrates. Where would
woodpeckers and bats be without hollowing trees?
||The above species are all opportunistic,
colonising suitable wood after fungi have become established. A small group
of beetles go one step further and carry their fungal food with them and
inoculate the freshly dead timber themselves! The beetle Hylecoetus
dermestoides develops in dead timber and root stumps of hard and softwoods.
The eggs are laid in batches in wood crevices, in rough bark or in
boreholes. The fungal spores are in the eggshells and the larvae feed on the
ambrosia fungus which develops on the walls of larval galleries. The other
ambrosia beetles known from Britain are all in the bark beetle family (Scolytidae).
||The outer parts of the trunk and branches - the
bark - also has a whole succession of invertebrates associated with it.
Freshly dead or even dying bark is rapidly colonised by bark beetles (Scolytidae)
and with them come a series of scavengers which exploit the beetle
galleries, plus predators and parasites. The ant beetle Thanasimus
formicarius is a specialist predator of bark beetles, and larvae of the
long-headed flies (Dolichopodidae) Medetera spp are also found in the
burrows of bark beetles and other beetles on whose larvae and pupae they
feed. The adult flies are very characteristic of the surfaces of exposed
heartwood on the trunks where they court, mate and catch their prey.
Larvae of the oak jewel beetle Agrilus pannonicus tunnel in and under thick
oak bark where dying and dead. Its main refugia are ancient woodlands and
pasture-woodlands, but this beetle spreads more widely on occasion. Numbers
were already building up in the south-east when the Great Storm of 1987 gave
the species a bonanza of freshly dead oaks. This event was closely followed
by Oak Dieback Disease, which has boosted the species ever further to the
point that it is now being implicated in this worrying oak condition, albeit
on very questionable grounds.
Larvae of Wood Snipe Fly Xylophagus ater develop beneath bark on branchwood
of a wide variety of dead broadleaves in the early stages of decay. They
feed on the larvae of larger beetles such as longhorn beetles and possibly
other insects although it is not known quite how they overcome their large
prey - these are by no means defenceless since they have large jaws used for
breaking up hard wood! It is suspected that they use their large toughened
beak to puncture the waterproof coating of a potential prey larva and then
retire and wait for the prey to loose vigour as it dehydrates and then to
come back for the kill once the victim is more or less moribund. It has even
been suggested that the dense rings of wood-dust with which longhorn beetles
surround themselves before pupating is intended as a defensive stockade to
protect this vulnerable stage specifically from Xylophagus. The larvae of
snakeflies such as Xanthostigma xanthostigma are more conventional predators
which pursue more defenseless prey beneath the bark.
Many fungi exploit dead wood on the outer surfaces of the tree or on smaller
branchwood and it is not surprising that there are many further
invertebrates which specialise on them. Good examples are species such as
the beetles Phloiophilus edwardsii and Tetratoma ancora which develop in the
fruits of the fungus Phlebia merismoides. A number of scarce species develop
particularly in the dead lower limbs of trees, branches which have become
shaded out by the tree's own canopy, presumably feeding on fungi such as
Stereum spp. Examples include the beetles Tetratoma desmaresti and Abdera
Britain's largest false scorpion Dendrochernes cyrneus lives beneath loose
bark on tree trunks. It is mainly a southern species and prefers timber
heated by the sun. It not only feeds on small wood-decay invertebrates but
also explores the outside surfaces of the trunk on calm warm summer evenings
- exploiting the epiphyte communities as well as the saproxylics!
||Four species of cobweb beetle have larvae which
live in the crevices beneath dead bark on the trunks of large old living
trees, or under the dry loose bark of dead standing trees, where they are
associated with the webs of bark-frequenting spiders. They feed on the
remains of insects eaten and left over by the spiders. The larvae are
covered with long bristles which protect them from the jaws of the spiders.
They pupate within the larval skin, which splits along the back and affords
protection for this vulnerable stage in their life cycle. Ctesias serra is
the most widespread and to be found on just about any ancient tree in the
countryside. In contrast Trinodes hirtus is a rarity found only on ancient
oaks in old wood-pastures
|Deadwood lying in water
||Deadwood lying in water provides yet another
range of niches to be exploited by invertebrates. The beetle Cyanostolus
aeneus only occurs under bark on trunks and boughs which have been saturated
with water, occurring along rivers and streams subject to spates. Larvae of
the non-biting midge (Chironomidae) Orthocladius lignicola specialise in
submerged rotten wood, and the rare hoverfly Chalcosyrphus eunotus develops
in deadwood which is semi-submerged in freshwater.
||So far discussion has focused on the aerial
parts of the trees. Decay in the roots is a further important habitat for
invertebrates. The most famous species is the Stag Beetle Lucanus cervus,
the larvae of which develop in moist decaying wood below the soil surface,
especially the decaying roots of old stumps, but also in the base of
fenceposts. The rare metallic green hoverfly Caliprobola speciosa develops
in wet-rot in underground roots of beech stumps, and Criorhina spp and
Xylota spp hoverflies also specialise in decaying roots.
|Importance of nectar sources
||The requirement of the adults of some of the
insects which develop in wood-decay for access to blossom is widely
appreciated. Nectar provides an energy-rich food which can rapidly be
assimilated and used to fuel flight, and pollen is a protein-rich food which
aids egg production. Flowering trees and shrubs are by far the most
important sources, although other plants can also be very popular, notably
hogweed and Angelica. Hawthorn provides the classic insect blossom, partly
due to its flowering in late spring when so many wood-decay insects are in
the adult stage. But really, blossom can important right through the season,
and the presence of species such as sallow, holly, privet, rowan, crab
apple, wild pear, guelder rose, bramble, and so on, are all beneficial. Even
elder, with its poor reputation amongst entomologists, can be important for
a select few species - it is particularly favoured by the nationally scarce
beetle Aderus oculatus, for instance, which develops in red-rot in old oaks.
||Today's fauna is unique in time. The fauna of
the ancient Wildwood of Britain and Ireland would have been particularly
species-rich. Extinction has been a continuing process, in relation to a
variety of factors but particularly fluctuating climate and as a result of
the activities of people since prehistoric times. The fossil record includes
many species which nowadays require visits to continental Europe to see
them: Rhysodes sulcatus is a relict species of primary, wholly undisturbed
forest, ie before it has been disturbed by human activity, and is most
recently known in Britain at c3000 BP. The click beetle Porthmidius
austriacus appears to have become extinct during the Neolithic period. There
is evidence for the presence in Britain of another species of stag beetle
Platycerus caraboides up until the Bronze Age. And so on.
Even many of our currently rare species were once much more widespread here.
The rare chafer Gnorimus variabilis, for instance, used to occur in old
trees on Tooting Common, London, now so long gone that none of the residents
can remember there ever having been old trees there! It is currently
confined in Britain to Windsor Forest
An interesting quirk is the story of the lime bark beetle Ernoporus
caucasicus which was described in Britain from fossil remains found in the
Somerset Levels long before anyone noticed that it was still alive and well
and widespread across the Midlands!
But it is not all decline. Species have been colonising too, although mostly
in response to the activities of people! Many species have been accidentally
introduced through commerce and others through the introduction of exotic
plants for gardens and hothouse collections. Some of today's commonest
wood-decay insects came originally from long away. Good examples are the
weevil Euophryum confine and the small fungus beetle Cis bilamellatus which
originate from New Zealand. The jewel beetle Agrilus sulcicollis is one of
the latest arrivals, having been expanding its range across Europe in recent
years and was first noticed in Britain in 199 - did it fly the channel or
did it hitch a lift on a timber lorry?
Some species have established themselves in Britain firstly within
buildings, but with global warming are not being able to live out of doors.
An example is Alphitobius diaperinus, known as the Lesser Mealworm Beetle
for many years as it has been exploiting stored products and especially deep
litter poultry houses, but now its other name of Black Fungus Beetle is
becoming more appropriate as it colonises old trees in the countryside.