by Jörg Ruther, Germany. No.18 Winter 2021
In order to serve the honeybee as a long-term habitat, tree hollows must meet certain criteria. To be able to maintain a climatically stable nest environment, they must provide protection against the weather, especially against heat, cold and rain. It is therefore not surprising that the entrance holes usually face away from the main wind direction, towards the southeast. A semi-shaded location, which does not allow all-day sunlight, also seems to be an advantage. This prevents the change between warm and cold phases from being too abrupt.
As the larvae as well as the stored food are a welcome high-energy food source for other species, the hollow should be located as high as possible and have a relatively small entrance to protect against predators and honey thieves.Furthermore, bees prefer a hollow volume of +/- 30l  for nesting. Here, the amount of winter feed to be stored is probably the decisive factor. It is also an advantage if the width of the hollow allows a stable connection of the honeycombs with the walls . Even if there is a risk of the feed being torn off in winter, this is prevented when the cavity is very narrow. The reason bee colonies choose small >20l as opposed to large >50l hollow volumes is probably due to the warmth of the hollow. A smaller size also has a clear advantage with regard to the development of the varroa mite. Colonies in small hollows swarm more often. This leads to a reduction in varroa infestation by brood interruption .
In addition, tree hollows offer the bee the advantage that it is easier to maintain a homeostatic nest environment in them. By the way the honeycomb structure is aligned, by propolising the inner walls and reducing the size of the flight hole with propolis or a block of honeycomb, the bee is able to actively regulate the inner climate of the hollow and adapt it to its needs.
Development of tree cavities
The development of hollow trees is usually preceded by bark or trunk injuries. These can be of both natural and anthropogenic origin. Originally, bees probably used tree cavities as nesting places which had been created by the hole-excavating activities of woodpeckers. Other possibilities for the development of tree cavities can be pruning wounds, deadwood breakages, storm and snow breakages but also torsion and shear cracks.
Big woodpeckers, the great spotted woodpecker (Dendrocopos major) and black woodpecker (Dryocopus martius) also make holes in healthy trees . Often an initial cavity is created first. This is then expanded as the wood is decomposed by fungi. Other woodpecker species such as the grey, green, small and middle woodpecker prefer trees that have been damaged by rot. Here, the hole entrances are mostly located in the area of rotten, overgrown deadwood or branch wounds .
Branch wounds, dead wood, storm and snow breakage
Depending on the tree species, the size of the injury and its timing, the tree may not be able to compensate for the damage quickly enough. As a result, fungi can colonise the exposed wood and a decay process is set in motion.
Extension of the hollow: wood decomposing fungi
As a result of the injury, the tree tries to close the wound. If this does not succeed, wood-decomposing fungi can penetrate and further enlarge the wound as the wood decomposes. Certain species of wood fungi are specialised in colonising living trees with intact transpiration and assimilation streams. They live parasitically on their host trees and promote cavity development. While decomposition continues inside the trunk, the outer layers of wood usually remain unaffected, so that the tree can remain alive for many years or for decades .
The wood-decomposing fungi, which play a major role in cavity formation, are mainly basidiomycetes and in this section the order of polypores and agaricales (gilles mushrooms). The classification is initially made according to their main occurrence in the tree as stem or root-born fungi. Since bees prefer mostly cavities in the upper stem or crown area, the stem and crown-born wood decomposers are discussed further below. Another subdivision is made according to the type of wood decomposition into white, brown and soft rot decay pathogens. Some of the soft rot pathogens also belong to the ascomycetes .
Even though there are different types of wood decomposition in this group, these wood decomposers have in common that they break down all cell components. First, however, lignin is broken down. This results in a fibrous appearance to the decomposed wood. The decayed wood is often white or greyish in colour and, depending on the type of decomposition, breaks up fibrous or brittle. It is interesting to note in this context that the preferential lignin and hemicellulose decomposition dramatically increases the cellulose’s ability to absorb moisture , . White rot wood therefore has a much higher water absorption capacity than intact wood.
White rot pathogen
Among the most common species of this group, with the potential to form large cavities, are the shaggy bracket (Inonotus hispidus) which can be found on Platanus acerifolia (London plane), Fraxinus exelsior (common ash), Fagus sylvatica (beech) but also on many other tree species, the dryad’s saddle or pheasant’s back mushroom (Poliporus squamosus) with the main hosts Tilia spec. (lime tree) and Acer spec. (maple), as well as the Oak Fire Sponge (Phellinus rubustus). This is found almost exclusively on Quercus robur (common oak).
Brown rot pathogen
The fungi of this group mainly decompose the cellulose-containing components of the wood. What remains is the lignin. The rotten wood is mostly, as the name suggests, brown. The wood acquires a brittle consistency, breaks up into cubes and later decomposes into powder. In contrast to white rot, the decomposition of cellulose and hemicellulose reduces the water absorption capacity , . Chicken-of-the-woods (Laetiporus sulphureus) is the most common species in this group, it is mainly found on Quercus robur (common oak), Castanea sativa (sweet chestnut), Salix spec. (willow) and Robinia pseudoaccia (robinia), but is also found sporadically on other deciduous tree species. The beefsteak fungus or ox tongue, on the other hand, is only found on Quercus robur (common oak) and Castanea sativa (sweet chestnut), although it is mainly found on the trunk of old trees. It can cause extensive rot in the trunk and crown area.
Soft rot pathogen
The wood decomposition pattern of the soft rot pathogens is reminiscent of white rot. However, with this type of rot, the cell walls of the wood fibres are first mined. Only the outer cell walls remain. The decomposed wood has a soft to brittle consistency.
The soft rot decay pathogens usually play a subordinate role in the formation of cavities far from the ground. Some white rot pathogens, especially the Inonotus hispidus, are able to cause additional soft rot and thus overcome the isolation zones of the trees .
Insects play an equally important role in the expansion of the tree cavities. First and foremost, the group of beetles and hymenopterans. As these are mostly unable to break down wood, they depend on the enzymatic decomposition of the cellulose and lignin components by fungi, yeasts and/or bacteria. Furthermore, the micro-organisms involved in the decomposition supply the insect larvae with certain trace elements, amino acids, vitamins, etc. . In particular the family of spider beetles, the stag beetles and the longhorn beetles colonise the wood modified by the wood-decomposing fungi. Their larvae mine the structures and thus provide for a progressive expansion of the cavity. Furthermore, ants are often found who use the caves as a habitat. For this purpose, corridors are excavated in the wood. The larvae are partly fed with the mycelia of the fungi. Also, horntail or wood wasps or the violet carpenter bee (Xylocopa violacea) use exposed and rotten wooden structures in which to lay their eggs and to raise the larvae.
The continuous expansion of the hollow makes it interesting for other vertebrates as well. Birds such as the stock dove, the jackdaw and many owl and songbird species depend on cavities as nesting or resting places. Small mammals such as dormice, squirrels, martens and bats also need them for raising their young or for hibernation. By gnawing and scratching the cavity walls, the tree hollow is constantly expanding , , .
Wood mould body
Due to the continuous decomposition of the fungus-decomposed wooden structures, the cavity volume expands upwards. The space below the entrance is filled with decomposition and nesting materials, as well as carcasses and faeces. The resulting detritus is rich in certain minerals and nutrients and thus provides a habitat for a variety of specialised microorganisms and insects. Many of the creatures adapted to this microhabitat are on the red list of endangered species due to the large-volume tree cavities that have become rare. Here the hermit beetle stands out in particular. Its larvae dig through the wood partially decomposed by fungi at the edge of the hole and thus contribute to a steady expansion of the cavity .
Utilisation and development of a tree cavity
Tree hollows represent one of the most species-rich microhabitats in the forest ecosystem. If, for example, oaks reach 400 years of age, there is a cavity or detritus pocket in almost every tree . Due to the abundance and diversity of the developmental stages of a tree cavity, which is accompanied by a constant change of use, only a brief outline of its formation and development can be given here. Tree hollows are in a permanent state of change or rotation, being used by successive different inhabitants. Often this change is essential for the on-going usability of the cavity.
For example, a tree cavity inhabited by bats can only be used until the bats have accumulated so much excrement on the cave floor that it is no longer possible to fly in and out. Insects adapted to this substrate in turn process the faeces, so that within a short time several centimetres of faeces are broken down and the cave can be used by the bats again .
A similar function to that of insects is performed by tree fungi, which play an important role in the decomposition of organic material and the expansion of cavities. Similarly, the persistent excavating by the woodpeckers and the removal of nesting material or similar by woodpeckers, nuthatches or other species is also crucial for the further usability of the cavity. These examples show the dynamics of use of the tree hollow habitat and the extent to which the various inhabitants are dependent on each other and enable the utilisation cycle to be continued .
Tree hollows and bees
Only very few tree cavities meet the criteria preferred by bees and those that do are highly competitive. As complex as the formation of large hollows is, it follows a certain multifaceted pattern. The result is a preferably dry internal space with a volume of around 30 litres, a residual wall thickness buffering the outside temperatures and offering the tree sufficient resistance to breakage, with a small flying hole facing south-east at a height of around 8m, and moderate sunlight.
Co-evolution in the beehive
In the course of evolution, the honeybee has adapted to a clearly defined ecological niche in the forest ecosystem. All the relationships that honeybees have here are probably so diverse that they could fill several books. But their most important task is certainly their pollination activity. They also provide food for a large number of decomposers. The most prominent example, familiar to every beekeeper, is the wax moth. Although it is sometimes found in bumblebee nests, it lives mainly on the nests of the honeybee and ensures that the organic matter (wax, pupal skin, etc.) is mineralised and returned to the cycle.
As a member of the Cryptophagidae family, Cryptophagus hexagonalis also lives in beehives, where it can be found in the remains of the combs. Here it probably feeds on the colonies of fungi growing on the honeycomb . The same is true for Carpophilus lugubris – this beetle has also been found in honeybee colonies, where the larvae probably feed on fungal residues from the bees’ nests. The adult animals live on pollen, near the hives of growing flowering plants .
If the decomposers are responsible for the breaking down of bee waste products, there are also parasites that specialise in honeybees or their close relatives. The death’s-head hawkmoth (Acherontia atropos), for example, is able to move freely in the beehive without being attacked by the bees because it emits certain scents. Only the sentinel bees take notice of it, but due to its tight-fitting scales they can do little harm to it. Once in the hive, the hawkmoth uses nectar and honey .
Apart from the destructors and parasites, there is probably also a large number of symbionts of honeybees. Due to the fact that wild honeybees have hardly been researched, there is very little data available. If one searches the literature, the Chelifer cancroides (bookscorpion), a pseudoscorpion, is the only species closely related to bees in our latitudes. This observation, however, refers mainly to animals found in hives. However, there is no scientific evidence of book scorpions in wild honeybee colonies. They are mostly found in buildings such as houses or haylofts, where they hunt for other insects and arachnids . Alleochernes wideri, a pseudoscorpion associated with bats, has been found twice in tree hollows inhabited by bees but is not specifically linked to either the bat or the honeybee .
It can be assumed that the honeybee is a key species for a large number of organisms, for which co-evolution makes co-existence obligatory. The loss of natural habitats and the influences of modern beekeeping are likely to have a significant impact on the population density of these organisms. Consistent species protection and the provision of suitable habitats for honeybees would also ensure the long-term survival of these organisms.
All photos by Jörg Ruther and Kapar Clenet.
Jörg Ruther is a beekeeper, alchemist and licensed trainer in traditional Japanese archery Kyudo. In his breadwinning occupation he is a consultant in Artificial Intelligence and Deep Learning for one of the major IT Companies in the market. Helpful in beekeeping, Jörg was trained as a cabinetmaker in his youth, working for the British Forces (RAF) in Berlin as well in the UK and later in the Republic of Ireland. Starting with beekeeping in the mid 80s in the “common” way and having a long break raising children in a big city, he then moved back to the countryside to keeping bees a different way. His beekeeping philosophy is to be sustainable and treatment free and he is researching traditional bee medicine, inspired by spagyric traditions and old European lore, nearly lost over two senseless wars. He studied Alchemy with “The Philosophers of Nature”, founded by the French Alchemist Jean Dubuis who led “Les Philosophes de la Nature”. He is keeping his bees in 45 mm (1 3/4”) hives with a special designed “deep” floor. Also fostering a colony in a tree hive without any interference. Jörg is living with his family, his dogs and bees in a small village in the Lower Rhine region not far from Düsseldorf. Always in active contact and exchange with other treatment-free beekeepers in Germany, Western and Eastern Europe.
Kaspar Clenet was born in 1974 in Krefeld, Germany, on the Lower Rhine. He has been working as a tree expert for about 20 years, focusing on the road safety of urban trees. With the amendment of the Federal Nature Conservation Act in Germany, species protection has become a central theme of his work. Honey bees in tree hollows have unfortunately become a rarity and he encounters them not often but regularly during his work. His work with the bees is driven by the hope that the occurrence of wild honey bees in their natural habitats will become the norm again in the future.
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