Csóka, GY., and Kovács, T. 1999. Xylophagous insects. Forest Research Institute. Erdészeti Turományos Intézet. Agroinform Kiadó, Budapest. 189 pp.
The xylophagous insects in their different developmental stages are eaten by many species of animals. In this chapter we discuss the most characteristic groups of these predators, without the intention of listing them completely. The adults flying at dusk are mainly preyed on by bats. Bats only consume the less chitinized, more easily digestible parts (such as the abdomen) of larger beetles, such as some longhorn beetles for example. The daily food consumption of bats can reach 20-25 % of their body weight. Fragments of xylophagous insects can also be found in pellets of a range of owl species. Wild boars, badgers and foxes frequently look for larger xylophagous larvae in fallen decaying wood. Where they occur, brown bear and martens also feed on these larvae. The xylophages represent a high percentage in the diet of the insectivorous mammals (hedgehogs, moles and shrews). Not only the species known as insectivores feed on them; small rodents, such as forest mice (Apodemus for example) also eat a lot of them.
Although a hidden way of life provides some level of protection from some native enemies, not even the larvae that develop deep in the heartwood are completely safe. The larger woodpeckers, such as the Great Spotted Woodpecker (Dendrocopus major) in the photo are able to excavate large larvae feeding deep in the heartwood.
This is an indirect indication that the larvae are very rich in nutrients and energy, otherwise it would not be profitable to look for them with very high energy investment. Feeding on larvae which are smaller but occurring in large numbers can also be profitable. According to some observations one woodpecker can consume up to 100 bark beetle larvae per day. Larvae developing deep in the heartwood are also threatened by parasitoids. The larvae of the large parasitic wasp Ephialtes mesocentrus develop in larvae of larger longhorn beetles living in hardwood trees such as oak, beech and hornbeam. The female parasitoid, when looking for a host for her offspring, touches the surface of the wood and finds the location of the larval chambers by detecting the very fine tremors and noises made by the longhorn larvae. After detecting the larva she bores with her ovipositor until she reaches the host, up to 3-4 cm deep inside the tree. Then she lays her egg into the cerambycid larva. Her offspring finally will kill the host. Rhyssa persuaria, a parastic wasp of woodwasp larvae, has practically the same life cycle. Unfortunately this species has become rare in the last decades. The smaller parasitoids often attack the host in groups, as can be demonstrated by the pupae of the small parasitoid wasps killed the larva of Currant Clearwing (Synanthedon tipuliformis). The larvae of these small parasitoids often enter via the tunnels made by the xylophagous insect itself. In many cases the parasitoids find these fine holes by detecting the presence of the symbiotic fungus of the xylophages (in the case of woodwasps for example). So the mutualist sometimes "betrays" its partner.
The largest European wasp species (Megascolia flavifrons) has a very special life history. The female uses her sting to paralyse the host (larvae of stag beetle and rhinoceros beetle) and lays a single egg on the surface of it. The wasp larva feeds as an ectoparasite, finally killing the host and leaving its larval skin behind. Unfortunately, as large, old and decaying trees become rare both the hosts and the huge wasp are becoming less and less common.
The larvae developing beneath the bark are preyed on by the larvae of Cardinal Beetles (Pyrochroa coccinea). Both larvae and adults of many rove beetle (family Staphylinidae) are predaceous. Some tiny species of this group are specialised for entering bark beetle galleries and feeding on their eggs, larvae and pupae. The beetle Clerus mutillarius and its relatives are very efficient predators of bark beetles and other smaller xylophagous insects. According to some observations the larvae of a species of this group consumes 10-15 bark beetle larva during its larval development and up to 160 bark beetles in its adult stage. Some of these predators resemble the appearance of their most common prey. The evident advantage of this special mimicry is that the prey will recognise the danger too late and will often not be able to escape from the predator. Ants prey on many insects - including xylophagous insects - up to several magnitudes larger then themselves.