Deadwood as a habitat for insects

Dead trees, standing or fallen or parts of them (e.g. trunks) or individual dead branches on old trees can all be called deadwood. Belonging to the group of saproxylic insects are not only species which live directly off bark or wood, but all species which at some time or other in their lives depend on deadwood. These are, apart from the actual wood eaters (xylophages), insects, which as second phase colonisers use the holes and tunnels for their own broods, without actually living off the wood substrate.

Belonging to this group are for example, various species of wild bees or wasps or various fly and midge maggots which graze off the fungi and bacteria growing in the drilled out tunnels or feed off dead material and excrement. Tree sponges growing on deadwood shelter specialized beetles and flies. Many predatory and parasitic insects such as predatory beetles or parasitic ichneumon depend on these wood dwellers. Various typical saproxylic groups of insects are portrayed below.

Beetles are not just the most numerous species within the group of insects, but are also the most diversified in deadwood. In central Europe 1340 species are, at some time in their phase of development, dependant on old or dead wood. The over 110 species of bark beetles in Switzerland mainly colonise trees which are already dead. Belonging to the pioneer species of insects which start the wood decomposing, they eat away at the bark and drill holes into it. Unlike the tiny bark beetles which are only millimetres in size, the much larger longhorn beetles and jewel beetles are easier to spot. Their larvae eat into the bark and the wood. Many species of these beetles can often be observed on freshly dead trees or on wood in lumber yards.

The spruce longhorn beetles (Tetropium) or the large musk beetles on coniferous wood for example. The Callidium beetles (Callidium, Phymatodes) prefer coniferous wood which is already dry. These sometimes hatch in firewood which has been stored inside. Jewel beetles are glittering treasures. The worm like tunnels with razor sharp cut edges which can sometimes be seen along wooden fences are the work of their larvae. They are also to be found regularly in lumber yards.

Typical hard wood colonisers are: the antelope or wasp beetles (Chlorophus,Clytus, Xyotrechus) which have similar markings to wasps; opalescent musk beetles (Aromia moschata) and linden burn cow beetles (Scintillatrix rutilans). The smaller longhorn beetles (Anastrangalia, Corymbia, Leptura, Stenurella) often sit on colourful flowers where they feed off the pollen necessary for pubescence. After a two year stage of development the two banded longhorn beetles (Rhagium) pupate in the typical oval "cradles" of wood shavings found under the bark of conifer or deciduous trees. On loosening old bark these wood shaving cradles are, for a long time afterwards, testimony to their makers.

An especially lovely, but very rare longhorn beetle, the alpine longhorn beetle (Rosalia alpina) develops in old splintered beech trunks. This beetle is protected in the whole of Europe and in countries where it still is to be found it is considered to be on the verge of extinction. One of the largest populations of the alpine longhorn beetle in Switzerland is to be found in the lower Prattigau. Isolated beetles have been found around the area of the Walensee. There have been old findings on the Buchberg along the lake of Zurich.

Even Hymenoptera with their fragile wings develop in wood. There are also various wild bees which lay their broods in deadwood. Various leaf cutter bees (Megachile) or the endangered wood bees (Xylocopa) use old beetle tunnels for their broods or gnaw out new ones themselves. The tunnels are then divided into separate cells in which a bee larva develops feeding off stored pollen and nectar.

The digger wasps, mud dauber wasps and spider wasps also use the empty spaces in a similar way. They bring in prey as food for their larvae. The prey is not dead but paralysed. The wasp larvae therefore feed off living but defenceless prey. The wasp larvae can themselves become the prey of the next level of parasitic wasps, for example, the iridescent ruby tailed wasp (Chrysis ignata). And these are then targets of the parasitic chalcidoid wasps. Finally hornets and various fold wasps like to build their nests in cavities in old trees.

There is very little old or dead wood in Switzerland today compared to important historical evolutionary periods of time, especially compared to the last interglacial periods. In the primeval forest of Switzerland approximately 20 to 50 percent of trees were old or dead. In dry habitats and in higher alpine locations there was even more deadwood, because here wood decomposition takes place more slowly. There is no replacement for deadwood habitats which at one time were all over and on hand in large quantities.

Seen from the point of view of landscape ecology the primeval forest provided a sea of trees for thousands of years with only a few isolated tree free pockets. These were river meadows, dry ridges and marshy areas. The forest covered almost all of the landscape and the many inhabitants of old or deadwood did not have far to crawl or fly to find new breeding grounds. That is why most typical forest species are bad flyers. For the species which inhabited open landscapes it was different. They had to wander from clearing to clearing because their old breeding habitats were becoming overgrown. This is why such insects are able to spread out well. Today, the situation in Switzerland is reversed. The mobile open landscape species live in a sea of open agricultural areas, whilst the old or deadwood specialists live in isolated pockets of forests. And as they have difficulty spreading out, they are becoming extinct in our cultivated steppe and managed forests.

Especially in the past two hundred years forests were totally cleaned up. Humans chopped down all old wood and neatly collected deadwood for household firewood. Firewood which is stored in the open can furthermore become traps for insects, because it is a tempting place in which to lay eggs. The resulting larvae are later burnt in stoves.

In Switzerland the so-called red list of endangered old and dead wood inhabitants is only just being compiled. Around 20 percent of all middle European beetle species are directly or indirectly dependent on wood. In Germany 60 percent of these species are classed as being endangered. This is a much higher percentage than any other group of insects. In Bayern seven out of eight extinct beetle species are wood inhabitants and in Austria, out of all up to present extinct beetle species, two thirds are wood species which are dependent on old and deadwood. These figures should also more or less apply for Switzerland and the St. Gallen Rhine valley.

Many of the most spectacular saproxylic beetles are endangered or threatened with extinction, such as the above mentioned alpine longhorn beetle, the great Capricorn beetle, the rhinoceros beetle and the stag beetle. Appropriately most of these are protected in Switzerland (Ordinance on Nature and National Heritage) and in the whole of Europe (Berne Convention).