Present in United Kingdom
Not reportable – See ‘Report a sighting’ below
Scientific name – Platypus cylindrus (P. cylindrus)
Oak pinhole borer is a pest mostly of oak trees, although it will also attack other hardwoods. Its larvae (grubs) can bore deep into the heartwood of stressed oak trees, degrading the timber's appearance, and therefore its value. It is the only borer which, in the absence of fungal decay, will bore into oak heartwood.
Distribution
Oak pinhole borer is present throughout, and native to, the United Kingdom and continental Europe.
Susceptible species
As its name suggests, oak pinhole borer prefers oak (trees in the Quercus genus). However, it will readily attack other hardwood tree species, notably sweet chestnut (Castanea species) and beech (Fagus spp.), the latter particularly when some fissuring of the bark has occurred. It has also been known to breed in ash (Fraxinus spp.), elm (Ulmus spp.) and walnut (Juglans spp.).
It appears to establish only in trees which are severely stressed or already dead, and is not itself responsible for killing trees.
The threat
Oak is economically one of our most important home-grown hardwood timbers, with a wide range of functional and decorative uses. It is one of very few native timbers which can be used outdoors without preservative treatment, and it is widely grown for the hardwood timber market. So although oak pinhole borer in itself is not a threat to oak trees, the presence of its pinholes can spoil the appearance of high-value final products such as veneers and high-quality structural timbers. The consequent reduction in value can therefore adversely affect businesses which grow and process oak timber for the market, and the jobs of people who work in those industries.
At first the tunnels (galleries) are confined to sapwood, so the heartwood will not have been penetrated if the logs are converted to planks within the first year of attack. However, in suitable material, adults and larvae will go on to bore deeply into the heartwood, causing extensive degradation over three to four years.
Timber with pinholes caused by oak pinhole borer is not weakened significantly.
Identification and symptoms
The bored galleries made by the beetles and larvae (grubs) are 1.6mm in diameter, round and regular in cross-section and free from bore dust. Frequently the surrounding wood is stained black or brown by the ambrosia fungi. In early infestations, the tunnels run across the grain but, later, branches run in any direction.
Adult oak pinhole borer beetles are 5-8mm long, rather rectangular in shape, and are pitch-brown to almost black. Compared to oak-feeding bark beetles (Scolytinae), P. cylindrus are usually larger, with an elongated pronotum and a prominent head.
Note: standing oaks in poor health are likely to harbour other secondary, wood-boring insect species which can be confused with oak pinhole borer. These include:
- Two-spotted oak buprestid beetle (Agrilus biguttatus), which can be a significant secondary species in oak trees suffering from dieback. However, it confines its activity to the cambium and outer sapwood. Symptoms of its presence are characteristic D-shaped exit holes through the bark and, sometimes, tarry patches on the bark surface.
- Trypodendron domesticum, another ambrosia beetle which makes pinholes similar to those of oak pinhole borer. However, its tunnelling activity is confined to the sapwood. The bore dust produced by domesticum is always granular.
- Longhorn beetles, which include species of the family Cerambycidae. They tend to attack trees only in the later stages of decline, but some species will feed on the heartwood. The large exit holes are usually oval to circular in cross-section, but the larvae are considerably larger than oak pinhole borer larvae, and the larval tunnels are consequently wider as well.
Report a sighting
There is no requirement to report sightings of oak pinhole borer. Foresters growing oak trees for the hardwood timber market are trained to recognise and manage the pest.
Regulation
Oak pinhole borer is not on the quarantine list because it is already present in the UK, and it is not a pest of standing, healthy trees. There are, therefore, no quarantine controls against it.
Management and control
In the forest
Although spraying logs with insecticide at the felling site is currently (2020) permitted, a more desirable and environmentally friendly means of protecting logs is by synchronising the harvesting operation with the biology of the beetle. This means forest managers should:
- avoid harvesting from June to September, when the adult beetles are flying and colonising logs;
- remove felled timber from the forest as quickly as possible, because logs can be colonised within days of cutting; and
- inspect regularly: it is advisable to carry out regular inspections for signs of attack, even outside the flight period. This is because increasingly warmer winter and spring temperatures might increase the chances of beetles surviving and breeding.
In the yard
Great care needs to be taken if logs are being air seasoned to produce high-value timber. This is because wood left in the round can remain susceptible to attack for up to three years, and debarking is not thought to prevent attack. We therefore advise the following precautions.
- Do not move infested logs into yards during June, July and September, which is the beetles’ flight period.
- Do not allow old branchwood and firewood to accumulate: it might contain oak pinhole borer.
- Inspect stored logs frequently. Any logs with bore dust need to be converted into timber as soon as possible to accelerate the drying process, minimise further degradation, and reduce the risk of spread.
- Spray high-value logs in May and again in mid-July with suitable, approved chemicals to kill adult P. cylindrus as they try to bore into the logs. At yards where there is a danger of infestation, logs might need to be sprayed every May and July for three or four years after the felling date.
Once the beetle has established in logs, early conversion to planks to accelerate the drying process, perhaps combined with kiln drying, is the only option to prevent further activity. Dehumidification, removal of wane and sapwood, and conversion will all help to reduce the moisture content of the wood.
Debarking will accelerate drying, but is not thought to prevent attack. Beetle activity is minimal once the moisture content falls below 30- 40 per cent, but it will not cease altogether until the moisture content reaches 25 per cent or less. At this point the ambrosia fungi on which the beetles depend can no longer survive in the tunnels.
Beetles cannot be controlled with insecticide once they have tunnelled into logs. Chemical treatment is, therefore, only of value in protecting uninfested logs where, with careful timing, it can be used to prevent beetle entry.
Finally, we strongly recommend that anyone buying oak logs from areas where the beetle is known to occur specifies wood which is free from signs of wood borer damage.
Salvage timber
There is no way of preventing beetles from entering severely weakened or dead standing trees, so timber from salvaged trees needs to be treated with extreme caution. We therefore recommend that anyone using salvaged oak trees:
- inspect them carefully for signs of infestation, which are not always easy to spot. They include frass in bark crevices, and entry/exit holes about 1.6mm wide;
- avoid taking material that might be infested into the yard during the beetles’ main flight period; and
- reduce the moisture content as quickly as possible.
Biology and life cycle
Sporadic emergence of mature beetles has been reported to occur throughout the year, but it is thought that only beetles emerging from June to the end of September are able to survive and breed.
The adults are at their most active between mid-July and mid-September, when the males can be found boring into logs and stumps. They appear to be strongly attracted by the smell of fermenting sap, and some logs seem to be more attractive than others. The male excavates the first few centimetres of a gallery, which a female will enter and then re-emerge with the male to mate on the bark surface. After mating the two beetles re-enter the tunnel, this time the female going first. It is the male’s job to push out the bore dust that will now be produced by the female and, subsequently, her offspring.
Tunnelling then proceeds, quite rapidly, in a radial direction. At this stage the bore dust produced is typically fibrous, pale and composed of many short pieces about 0.15 – 0.18mm long. When heaped in bark fissures the fibres have the appearance of piles of wood wool, and this can differentiate oak pinhole borer activity from that of other wood borers, which tend to produce granular frass.
Oak pinhole borers cannot feed on wood themselves. Instead they have evolved a symbiotic relationship with ambrosia fungi. Females have a specialised organ in their thorax, the mycangium, within which the beetle cultivates the ambrosia fungi. As she burrows through the wood, the ambrosia fungi are deposited on the wood and, having been transported by the beetle to a suitable habitat, quickly become established on the tunnel walls and form a thick layer.This layer of fungus provides the nourishment for the adult beetles and the developing larvae.
The female usually lays her first eggs about four weeks after entry, and continues to lay further batches at irregular intervals throughout her 2-3 year adult life. When tunnels are started in late August or September, the first eggs might not be laid until the following spring.
The female continues to extend the gallery system throughout her life, eventually creating a branched system that can reach up to 1.8m in length.
Eggs hatch after two to six weeks into yellowish-white, legless larvae (grubs). As the larvae grow they pass through four or five instars (growth stages), moulting their skin between each stage.
The later instars, particularly the final one, have strong, gouge-like mandibles, which they use to extend the tunnel systems. Larvae tunnel more slowly than adults, and although they ingest wood during tunnelling, they still depend on fungi for their nourishment. Unlike the fibrous bore dust produced by the adult beetles, that produced by the larvae is granular, and cannot be distinguished from the bore dust made by other ambrosia beetles such as Trypodendon.
Fully grown larvae excavate small chambers, inside which they pupate. Newly hatched adults feed on the fungi, and emerge without further boring. The life cycle, from egg to adult, usually takes two years (occasionally one year), and more than one generation of oak pinhole borer might utilise a single gallery system.
Origins and background
The oak pinhole borer is the only indigenous member of the subfamily Platypodinae, and one of the few ‘ambrosia beetle’ species found in Britain. Ambrosia beetles, although wood borers, are not wood feeders: instead, the adults bore into wood and introduce into their tunnels ‘ambrosia’ fungi which grow on the tunnel walls, and these fungi are the main food for the adults and larvae.
Oak pinhole borer was listed in the 1987 British Red Data Books as ‘rare’ in Great Britain. However, its population boomed after the Great Storm of 1987 felled thousands of oak trees. It quickly took advantage of the abundance of breeding material and favourable conditions in southern England and Wales.
Reports of damage to sawlogs rose dramatically in the early 1990s, and numbers have never returned to pre-1987 levels. It continues to be an issue at felling sites and timber yards. This might be a consequence of a continuing supply of breeding material in the form of weakened oaks suffering from chronic oak dieback and acute oak decline.
A similar situation now exists in continental Europe following the severe gales of winter 1999. British buyers have expressed concerns to the Forestry Commission’s Cross-Border Plant Health Service about the presence of oak pinhole borer in oak logs imported from there.
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