In our native oak species, recurrent loss of vitality and increased mortality create conditions more favourable for infestation with wood-boring insects. The causes are above all climatic changes, dry periods and caterpillars feeding on the oaks, e.g. the caterpillar of the oak processionary moth (Thaumetopoea processionea), the gypsy moth (Lymantria dispar) or the European oak leafroller, also known as the green oak tortrix or green oak moth (Tortrix viridana). Efforts to increase the number of old trees and deadwood in the forest also play a major role. An infestation with wood-boring insects is often associated with physiological damage caused by bark-breeding beetles, especially the two-spotted oak borer, also known as the oak splendour beetle (Agrilus biguttatus F.).
Wood-boring insects with the greatest economic impact
A loss in value of the valuable oak assortments processed for marketing is to be avoided as far as possible.
The wood-boring insects with the greatest economic impact are thus briefly presented below and possible measures to reduce the risk of a loss in value of the heartwood are shown.
Oak Borer or Mediterranean oak borer
Xyleborus monographus F.
This beetle is 2 to 3.5 mm long and attacks damaged standing trees or freshly felled trunks and rootstocks mainly of oak species. The swarming phase of the oak borer begins very early in the year, from the end of March/April (early swarmer).
Starting from one prominent radial passage running into the trunk, a few breeding passages branch off in a ladder-like pattern, mainly running along the annual rings (Fig. 1). The entire system of galleries (forked passages) lies more or less in one plane. The larvae of the oak wood borer feed exclusively on ambrosia fungi, which blacken the walls of the passage as time progresses. The breeding galleries thus stand out clearly from the wood in terms of colour (epithet in German: “little black worm”). The oak borer can produce two generations within one year. The second swarming phase takes place from June to August and the young beetles spend the winter within the brood passages. In addition to the damage presented by the brood tunnels as already described, the infestation can be recognised by the boreholes made by the larger females (length 3 to 3.5 mm), and the appearance of white bore dust. White bore dust can however also be left by sapwood beetles that penetrate less into the heartwood, such as the the oak bark beetle (Xyloterus signatus), so confusion is possible.
Oak pinhole borer
Platypus cylindrus F.
The oak pinhole borer is 4.7 to 5.8 mm long and infests freshly felled or sometimes still standing oaks, as well as other deciduous trees on rare occasions. Mass propagation can occur and is favoured by dry or warmer-than-average years. The swarming phase of the beetle begins comparatively late - at the end of June - and can last until September (late swarmers).
The beetles initially drill in a radial direction towards the heartwood. Once there, the tunnel (gallery) typically branches off and follows the annual rings (Fig. 2).
Side tunnels joined on to this lead in a radial direction back towards the core. The eggs are laid inside the tunnels. The characteristic system of tunnels lies more or less in one plane. The beetles prefer the lower and usually more valuable part of the trunk. The oak pinhole borer produces one generation per year. Since the larvae of this beetle also feed on ambrosia fungi, the passage walls eventually turn black. As they develop, the larvae excavate their pupal cradles vertically into the wood from the mother gallery, creating a ladder-like pattern. This feature is also a good means of recognising the oak pinhole borer. The oak pinhole borer can also be recognised by the bore dust it expels from the holes, which is sometimes compressed into sausage-like forms. The bore dust is usually coarse at first, with relatively long fibres. The comparatively large boreholes of 1.8 to 2.2 mm diameter are also a distinguishing feature.
Saw-horned shipyard beetle
Hylecoetus dermestoides L.
The saw-horned shipyard beetle is 6 to 18 mm long and it is common in Central Europe. They take 1 to 2 years to develop. Their swarming phase lasts from the end of March to July. The species is basically polyphagous and to be found on both deciduous and coniferous trees, but it prefers oak and beech. Here, the eggs are laid on the outside of the trunk in cracks in the bark or wood, and the larvae bore into the wood after hatching. The galleries run in all directions and become steadily larger in diameter as the larvae develop, ejecting the accumulated bore dust from the tunnel as they go. The larvae of the saw-horned shipyard beetle also feed on ambrosia fungi, which is why their galleries also eventually become black. For the necessary cultivation of the ambrosia fungus, certain levels of moisture are required, such as those available in freshly felled timber or fresh rootstocks.
A saw-horned shipyard beetle infestation can be recognised by the clusters of boreholes of varying diameters, as if caused by a shotgun blast with different calibres (Fig. 3). In summer (July/August), striking amounts of white bore dust are visible on the inside and outside of the bark, especially in the case of severe infestation. Under the already loosening bark, typical circles of bore dust can also be seen around the boreholes.
Lymexylon navale L.
The ship-timber beetle is much less common overall than the saw-horned shipyard beetle, and despite its similar size, at about 15 mm long, it differs significantly in its development and way of life. The shipyard beetle is to be found almost exclusively on oak trees. In addition to freshly felled wood and fresh rootstocks, it also infests damaged or dead sections of trunks. This means that it even colonises slightly dried out parts of trunks.
It lays its eggs in dry cracks or other injuries to the tree during the flight period from the end of May to July. The larvae that bore into the wood are initially very thin, which is why they are sometimes known by the epithet “hairworm”. Since the larvae feed on the constituents of the wood and not on ambrosia fungi, they eject no bore dust. They usually take 2 years to develop completely, during which time they create passages which can be up to two metres long, penetrating deep into the heartwood (Fig.4). Sometimes extremely straight, these run more or less horizontally into the centre of the trunk. Now and then the larva blocks the unstained galleries with a very dense plug of bore dust, so that in this area the incised larval passages are difficult to see.
Generally with infestation by the shipyard beetle, there is a risk of the trunk being degraded in value right through to the core, even though no conspicuous signs of infestation are visible from the outside. Higher infestation densities are known to occur at wood storage areas.
Overview of key characteristics of the described wood-boring insects
The diagnosis of the infestation of a trunk with a damaging wood borer is no triviality. Even with experience, a definitive diagnosis can in many cases only be made by taking timber samples and splitting them open.
Risk mitigation measures
One very effective measure that prevents the degradation of the timber with all of the beetle species presented here is to make sure the wood is removed from the forest early enough and processed promptly.
- The swarming phases of the above-mentioned beetles in the course of the year indicate that between March and September, no freshly felled valuable oak assortments should be left in the forest without further protective measures (Fig. 5).
- If the timely removal of the timber before March is not possible, the freshly felled, valuable timber should be stored as far away as possible from stacks of oak timber that have been lying in the forest for a while, from stacks of firewood, and from oak stands with significant numbers of fresh rootstocks and quantities of residual wood left in the stand. The timber should also not be stored near small “islands” of old trees and deadwood or oaks with partially fragile crowns.
- In addition, both forestry companies and wood buyers should check whether timely wet storage of the fresh oak timber is possible in order to prevent the slight drying of the fresh wood that makes it susceptible to infestation by the beetle species.
Plant protection products (PPP)
The use of plant protection products as a last resort to prevent devaluation of the timber is not permitted for all of the wood boring insects presented here. Treatment against the oak wood borer, one of the bark beetle species, is however possible in principle, once the above-mentioned alternative measures have been considered. Apart from the spraying of a plant protection product approved for the purpose, another option worth considering at the moment is the application of StoraNet®. This product is an insecticidal net, and can be used both as a preventive measure to protect against infestation when a risk has been identified, and to prevent bark beetles and wood-boring bark beetles flying out from already infested timber to spread further.
- Please note: this PPP is no longer authorised for use, but existing stocks can continue to be used up until 31.01.2022 (status as of February 2021).
Karate Forst flüssig® is a plant protection product currently approved for the regulation of the saw-horned shipyard beetle.
- Please note: this product is authorised for use until 31.12.2021 (status as of February 2021).
With any application of plant protection products, valid product authorisation is essential (or ongoing sales/usage periods), and it must be ensured that the respective conditions and regulations governing its use are complied with. Depending on the certification, additional conditions must be observed here. An application on timber in which infestation has already been detected is usually no longer effective, depending on the depth to which the beetles have bored into the wood.
With regard to the current far-reaching changes in climate and forest management, further studies are necessary in the future to update our knowledge: on phenology, for example (beginning and end of the swarming phases), and on other questions - especially the potential flight ranges of the beetle species - so that the infestation risk at particular storage sites can be estimated.