Oak Pinhole Borer (Platypus cylindrus)

Platypus cylindrus (Coleoptera : Curculionidae)

The oak pinhole borer, Platypus cylindrus 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: the adults bore into wood and introduce into their tunnels ‘ambrosia’ fungi which grow on the tunnel walls and serve as the main source of food for the adults and larvae.

Oak Pinhole Borer

Platypus cylindrus has in boomed in numbers in the last 30 years. Listed in the 1987 British Red Data Books under the category ‘rare’, following the hurricane of 1987 it quickly took advantage of an abundance of breeding material and favourable conditions in southern England and Wales. Beetle numbers increased rapidly and in the early 1990’s reports of damage to sawlogs rose dramatically. Numbers have never returned to pre-hurricane levels and P. cylindrus continues to be an issue both at the felling site and in the timber yard. This may be a consequence of a continuing supply of breeding material in the form of weakened oaks suffering from chronic and acute oak diebacks.

Platypus cylindrus appears to establish only in trees that are severely stressed or already dead and is not itself responsible for killing trees.

Life cycle

Adult Platypus cylindrus are 5-7mm long, rather rectangular in shape, and are pitchy-brown to almost black in colour. Compared to oak-feeding bark beetles (Scolytinae), Platypus are usually larger, with an elongated pronotum and a prominent head.

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 successfully.

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, some logs seeming 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, the male following behind. It is his job to push out all the bore dust that will now be produced by the female and subsequently her offspring.

Tunnelling proceeds, quite rapidly, in a radial direction. At this stage the bore dust produced is typically fibrous, pale in colour 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 P. cylindrus activity from other wood borers that tend to produce granular frass. Platypus are incapable of feeding on wood themselves, instead they have evolved a symbiotic relationship with ambrosia fungi. Female Platypus 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 that provides 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 may not be laid until the following spring. The female continues to extend the gallery system throughout her life-time and eventually a branched system is created that can reach up to 1.8m in length.

Eggs hatch after 2-6 weeks into yellowish-white, legless larvae. As they grow they pass through 4 or 5 growth stages (instars), moulting their skin between each stage. The later instars, particularly the final one, have strong gouge-like mandibles, which are used to extend the tunnel system. 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 bore dust of 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 P. cylindrus may utilise a single gallery system.

Damage

As its common name suggests, the oak pinhole borer prefers oak but will readily feed on other hardwoods, notably sweet chestnut and beech, the latter particularly when some fissuring of the bark has occurred. It is also known to breed in ash, elm and walnut. At first the tunnels (galleries) are confined to sapwood and if logs are converted within the first year of attack, heartwood penetration will not have occurred. However, in suitable material, adults and larvae will go on to bore deeply into the heartwood causing extensive degrade over a 3-4 year period. Galleries 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. Timber with pinholes caused by Platypus cylindrus is not weakened significantly, but the appearance of final products will be spoiled, especially in the case of veneers and high quality structural timbers.

Once the beetle has established in timber, early conversion to accelerate the drying process, perhaps combined with kiln drying, is the only option to prevent further activity. Conversion into planks, removal of wane and sapwood and dehumidification will all result in reductions in the moisture content of the wood. Debarking will accelerate drying but is not thought to prevent attack. Once the moisture content falls below 30- 40% beetle activity is minimal, but it will not cease altogether until the moisture content reaches 25% or less. At this point the ambrosia fungi, on which the beetles depend, can no longer survive in the tunnels. Once beetles have tunnelled into logs they cannot be controlled successfully with insecticide. Chemical treatment is, therefore, only of value in protecting uninfested logs where, with careful timing, it can be used to deter the entry of beetles.

Protection

Recommendations are based on the biology of this insect and availability of approved products. They have not been evaluated under field conditions.

Normal Harvesting

While spraying logs with insecticide at the felling site is currently permitted, a more desirable and environmentally friendly means of protecting logs is by managing the harvesting operation in relation to the biology of the beetle.

  • Avoid harvesting from June to September, this is the time the adult beetles are flying and colonising logs.

  • Remove timber from the forest as quickly as possible. Logs can be colonised within days of cutting.

  • Carry out regular inspections. Even outside of the functional beetle flight period, it is advisable to carry out regular inspections for signs of attack since increasingly warmer winter and spring temperatures may increase the chances of beetles surviving and breeding successfully.

In the yard

If logs are being air seasoned to produce high value timber, great care needs to be taken as wood left in the round may remain susceptible to attack for up to 3 years and debarking is not thought to prevent attack.

  • Do not move infested timber into yards during the flight period of the beetle.

  • Do not allow old branchwood and firewood that may contain Platypus cylindrus to accumulate.

  • Inspect stored logs regularly. Any logs with bore dust need to be converted as soon as possible to accelerate the drying process, minimise further degrade and reduce the risk of spread.

  • Spray high value logs in May and again in mid-July with suitable chemicals. This treatment aims to kill adult P. cylindrus as they attempt to bore into the logs. At yards where there is a danger of infestation, logs may need to be sprayed every May and July for three or four years following on from the original felling date.

Salvaged Timber

There is no way of preventing entry of beetles into severely weakened or dead standing trees, so timber from salvaged trees needs to be treated with extreme caution.

  • Inspect carefully for signs of infestation (not always easy to spot) i.e. frass in bark crevices, entry/exit holes approx.1.6mm in diameter.

  • Avoid taking material that may be infested into the yard during the main flight period of the beetle.

  • Take steps to reduce the moisture content as quickly as possible.

In addition to Platypus cylindrus, standing oaks in poor health are likely to harbour other secondary wood boring insect species, notably:

Agrilus biguttatus , the flat headed borer

A. biguttatus can be a significant secondary species in oak trees already suffering from dieback. A. biguttatus confines its activity to the cambium and outer sapwood and so can seriously disrupt transport of water and nutrients in the vessels, adding to the stress on the tree. This can accelerate decline. Signs of colonisation are characteristic D-shaped exit holes through the bark and, sometimes, tarry patches on the bark surface.

Trypodendron domesticum

Another ambrosia beetle, this species makes pinholes similar to those of P. cylindrus but it is rarely of economic importance because its tunnelling activity is confined to the sapwood. The bore dust produced by T. domesticum is always granular.

Longhorn beetles

These include various 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 larval tunnels, and the larvae themselves, are considerably larger than those produced by Platypus.

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Max Blake