Timber borers may not have the same reputation as termites, but they can still cause significant damage. The challenge in managing borers is that they can be difficult to identify and identification is essential in order to determine the necessary borer treatment (if required).
Although termites are perhaps the major cause of damage to timber in service, wood borers can also cause problems, both structural and cosmetic. Borers are also a serious timber pest in live trees and recently felled timber, and their activities in these areas can become apparent in timber in service. Termite professionals will often be called upon to deal with borer damage and provide suitable advice and a management plan. Here we will provide an overview of the range of borers that may be encountered and the damage they cause.
Correct identification of the borer, and understanding its behaviour and life cycle, is the key to determining the course of action and any potential treatment. Like a detective, you need to critically examine the crime scene and gather clues/evidence that may assist in a correct identification.
Finding the offending borer adult either in the act of damaging the timber or loitering in close proximity (maybe numerous dead bodies), is the easiest and quickest path to a correct identification. This can be via the pest manager’s own experience or enlisting the help of an expert in this field. Identification of the adult can also be achieved by looking up reference books or credible websites administered by museums, government agencies, universities or entomological societies. Whichever method is used, it is important to take good quality photos, measure at least the length of the insect and record relevant details with regard to the situation, such as where it was found (e.g. in the roof cavity of a 30-year-old house located in Sydney), what type of timber (e.g. pine trusses) and so on.
Quite often the adult will not be present, and the pest manager will have to rely on circumstantial evidence for a correct identification, such as:
- When was the timber installed (age of the house/timber structure);
- Presence or absence of frass;
- Size and shape of surface holes in the timber;
- Presence or absence of staining around the holes;
- Whether the holes or tunnels are filled with frass/fibres (tight or loose);
- Consistency of the frass (smooth like talcum powder, gritty/granular or stringy);
- Type of timber attacked (pine or hardwood);
- Whether the timber is sound or decayed;
- Situation (weather exposed, semi-exposed or completely weather protected);
- Moisture content of the timber – wet (unseasoned) or dry (seasoned).
The crucial piece of information is whether the infestation is currently active. The presence of frass can be misleading as building vibrations or ants invading the borer workings can produce regular showings of frass even years after borer activity had ceased. One simple method is to mark out an area containing borer holes and record the number of holes present (the homeowner can be instructed to do this). Increasing numbers of holes on subsequent counts indicate that the infestation is still active.
To divide borers into smaller subsets is very difficult because of the multiple overlaps between categories. The best method is to divide borers into two camps depending on whether the borer is attacking:
- Green/unseasoned timber i.e. a standing tree, freshly felled log or milled timber before it dries out, or;
- Dry/seasoned timber i.e. in the process of, or after it reaches its final form, and enters service as flooring, structural member, furniture etc.
Borers of unseasoned timber
Borers that attack unseasoned timber will not initiate attack in seasoned timber. As the timber dries, activity usually ceases but the damage done is still evident. In some cases, the larvae may continue their activity (at a much slower rate) and eventually emerge as adults. As previously mentioned, they will not re-infest but this situation causes much confusion with the homeowner and pest manager alike, especially if the adult emerges years after the timber was installed. In this situation, there is no requirement for treatment, however correct identification is crucial.
Ambrosia beetles
Also known as pinhole or shothole borers. This is a small beetle that is attracted to damaged trees and freshly felled logs where the adults bore round holes into the timber to lay eggs. During the boring process, fungal spores on the body of the adults initiate fungal growth that the newly hatched larvae feed upon, rather than the actual timber. This type of fungus (ambrosia fungus) causes a distinctive dark stain around the hole and tunnels thus giving an easy identification clue. Ambrosia beetles are common throughout the warmer parts of Australia and Asia and were recently discovered in Auckland, New Zealand. One slightly unique example is the polyphagous shothole borer (Euwallacea fornicatus) which is native to Southeast Asia and has now been found in Perth, Australia. Probably a species complex, rather than a single species, they will attack a variety of healthy living trees.
Longicorn beetles
Also known as longhorn beetles, so named because of their long, many segmented antennae. Females lay eggs on damaged or freshly felled trees, usually under a piece of bark or inside a crack. The larvae tunnel under the bark leaving distinctive engraved surface furrows that are often filled with tightly packed, stringy frass. The mature larvae tunnel deep into the log to pupate and eventually emerge as adults via distinctive oval-shaped holes, with no staining. There are no Australian species that attack seasoned timber, however the European house borer (Hylotrupes bajulus) does attack seasoned timber and has been found in Western Australia and parts of Asia. Longicorn beetles associated with timber damage in New Zealand include the two-toothed, burnt pine and huhu longhorn beetles.
Jewel beetles (metallic wood-boring beetles)
So named because of their striking colouration. The female lays eggs on the surface of freshly felled logs (mainly native pines in Australia) and the larvae immediately tunnel deep into the wood to feed, leaving behind stringy frass-filled tunnels. The larvae are also known as flathead borers, as opposed to the roundhead borers (longicorn beetles). The adults chew their way out after 1-2 years, leaving an oval exit hole, with no staining around the hole (Figure 4). They are known to emerge years after the infested timber has been used in service.
Weevils
Known for the distinctive protrusion at the front of the head. The female lays eggs on the surface of damaged or dying trees and freshly cut logs. The larvae tunnel under the bark and then deeper as the timber dries. Life cycle is quicker than other borers, taking months rather than years.
Auger beetles
So named for the precise round drill-like holes they produce. Mainly a pest of freshly felled logs and damaged or dying trees. The female bores into the timber and lays eggs at the end of the tunnel. The larvae hatch and begin feeding on the starch (sugar) found in the sapwood zone. They are generally small to medium sized and have a distinctive cylindrical shaped body with a cowl-shaped head covering. The frass is powdery and tightly packed in the tunnels. A number of auger beetles are associated with timber damage in Australia including the cylindrical, two-spined, and large auger beetle. Although not established in Australian and New Zealand, the lesser auger beetle, Heterobostrychus aequalis, is also found across Asia.
Wood moths
Wood moths attack standing trees and are not a pest of timber in service. The female lays eggs on the bark of a tree and the larvae tunnel through the wood. The larvae do not actually consume the wood they remove; they feed on the soft tissue produced by the tree in response to the damage they do whilst tunnelling. The tunnels may be up to 30 mm wide so the timber is often rejected at the sawmill. Wood moth damage can sometimes be obvious in large timber members such as landscaping timbers, bridge decking, etc.
Wood wasps
The most well-known wood wasp is the sirex wasp (Sirex noctillo) which is a native of Eurasia but has been introduced into southern Australia and New Zealand (Figure 7). It is one of the biggest threats to pine plantations in the southern hemisphere. The female wasp is attracted to sick or stressed pine trees and bores small holes to lay eggs into the wood under the bark. At the same time, she inoculates the wood with spores of a type of fungus that begins to rot the wood, producing fungal mycelium upon which the developing larvae feed. More tunnels are constructed as the larvae get bigger, causing significant timber degrade. In severe infestations, tree death occurs. The mature larvae tunnel close to the surface to pupate and emerge through circular exit holes around 4-7mm in diameter (Figure 8). The damage to the timber is usually discarded in the milling process but some becomes timber in service and is easily confused with beetle induced damage.
The management strategy for borers of green or unseasoned timber, once you have made the correct identification, is one of management of your client. They should be reassured that the damage is already done, and no further damage is expected. No treatment is required, but in the rare case that the adults are still emerging, any rectification work should wait until all adults have emerged.
Borers of seasoned timber
Unlike borers of green or unseasoned timber, seasoned timber borers can attack timber whilst in service and then re-infest to cause maximum damage unless managed.
Powderpost beetles
So named because their frass is a very fine powder, often described as feeling like flour when rubbed between the fingers. Certain criteria must be met before they can infest timber: timber must be hardwood species only (they will not attack pine timbers as they do not contain pores); must have a certain sized pore diameter to allow the egg-laying ovipositor to fit; and only the starch-bearing sapwood is susceptible to attack.
The female will take a test bite of the sapwood to check for starch content and if satisfied, she will insert her ovipositor and lay eggs into the pores. The eggs hatch and the larvae will feed on the starch (sugar) in the sapwood zone until fully grown (2–12 months). The mature larvae will tunnel close to the surface to pupate and emerge an adults (up to 7 mm in length) through round shaped, 1-2 mm exit holes (with no staining around the exit holes). The adults will immediately look for a partner to mate with to begin the next generation. The cycle repeats until there is no starch-bearing sapwood left, which is why they are such a destructive timber pest.
The most common powderpost beetle in Australia and New Zealand is Lyctus brunneus, whilst in Asia, the Malaysian powderpost beetle (Minthea rugicollis and M. reticulata) is the most commonly encountered. Two other borers that are commonly found throughout Asia, which are referred to as powderpost beetles, are the oriental wood borer (Heterobostrychus aequalis), which attacks timber packing cases, boxes, plywood chests and panels, sapwood in furniture and other timber in service, and the bamboo borer (Dinoderus minutus), which attacks a wide range of bamboo and cane products.
Furniture beetles
The common furniture beetle (Anobium punctatum) originated from Europe but now has a global distribution, including Australia, New Zealand and Asia. The furniture beetle is responsible for most of the insect damage found in structural and decorative timber in New Zealand. The adult is approximately 4 mm long, reddish brown in colour, with a cowl-shaped head covering. The female lays eggs in cracks and crevices in timber. Upon hatching, the larvae tunnel into the wood for around 2-3 years and come close to the surface to pupate. The adult emerges through a 2 mm exit hole (with no staining present). The adult female will mate and then periodically shelter within the exit holes and lay eggs to repeat the cycle of attack. The tunnels in the timber are best described as honeycombed and are filled with loose, gritty frass. Under the right conditions, the timber can be so badly attacked that it has lost all structural integrity. They are usually a pest of pine timbers in cooler, moist climates.
Another closely related species is the Queensland pine beetle (Calymmaderus incisus) which is native to Australia and is mostly confined to attacking native pine timbers (hoop and bunya pine). It is very difficult to distinguish between the common furniture beetle and the Queensland pine beetle based on the evidence of activity/damage.
Cigarette beetles
The cigarette beetle (Lasioderma serricorne) is a stored product pest that occasionally infests compressed fibre board (Figure 14). The adults are small, oval shaped, light brown to red brown in colour and approximately 3 mm in length. The female lays eggs on or near an attractive food source and the larvae tunnel through the material. If present in compressed fibreboard, they emerge via 2 mm circular holes (with no staining around exit holes). The life cycle is relatively quick, as little as five weeks, and they are active all year round in warmer tropical climates such as northern Australia and Southeast Asia.
The management for borers of seasoned timber depends on a number of factors. If the affected timber in service is small, it can be placed in a freezer for a number of days (localised heat treatment may also be an option). In some cases, the affected timber may be replaced to remove the infestation (isolated floorboard, architraves, etc.). The application of an insecticide will most likely not penetrate deep enough to arrest the current infestation, but the emerging adult may succumb to the toxicant and not re-infest, thus breaking the breeding cycle and preventing further damage. In severe cases, fumigation may be an option.
Incidental timber pests
There are situations where the evidence of holes or other damage to timber in service can give the outward impression of an active infestation of a serious timber pest that requires immediate remedial treatment. However, a little more detective work may uncover a different story, which could avoid an incorrect diagnosis (not good for your business reputation), and a costly treatment that may not be required.
The first example has already been mentioned – the emergence of a borer that initiated the infestation in the standing tree, freshly felled log, or unseasoned sawn timber. The larvae have survived the processing into the final, timber-in-service article and are in a hibernation-like state because the timber has dried out, only to emerge months and sometimes years later. This situation is probably the most difficult to correctly diagnose and may need the help of an expert in this field.
Occasionally pine timber in service with bark still attached will exhibit numerous small round holes (1-2 mm diameter). If the bark is removed there is little sign of damage underneath, which indicates the activity of the pine bark anobiid (Ernobius mollis). This is not a serious structural pest and is of minor economic importance.
Decayed timber, being soft, can be easily excavated by non-timber pests to provide a comfortable place to nest. Such pests include bees, which can bore multiple round holes thus doing a very good impersonation of a timber borer. Ants will also excavate the soft rotten timber but the presence of ant body parts in the associated frass is enough evidence for a correct identification. Woodlice (slaters) are also occasional invaders of decayed timber.
In summary, timber borers are a significant threat to timber in service and probably don’t receive the same focus or public awareness as termites. As with any pest, correct identification is key to a successful treatment outcome and the help of a good information resource in the form of extra training, reference books, credible website or industry expert will be of great assistance.
Director, Australian Timber & Pest Research Pty Ltd, Brisbane, Australia
