Wood boring insects

Beetles Beetles are the largest group of animals, having about 400,000 species in 500 families. They are extremely diverse with species adapted for almost any kind of environment and to feed on any food source, including many agricultural, horticultural and forestry plants and products. Many species are also beneficial predators of other insect pests.

Beetles have the basic insect anatomy of head, thorax and abdomen, but are characterised by a hard exoskeleton, hard forewings and abdomen with numerous hard plates. They also have the typical insect lifecycle of egg, larva with several stages of growth (instars), pupa and adult.

Woodworm is a generic term used for a number of species of wood-boring beetle and refers to the larvae of the beetles, which feed on wood after hatching from the egg, creating tunnels in the process. They only emerge from the timber after pupating and developing into adults, creating the characteristic holes in the wood surface.

This characteristic also leads to the misconception that the holes can be treated with insecticide to kill the beetle, when in fact it is pointless as the hole signifies that the beetle has left!

The main beetles that cause damage to structural timber and wooden fittings, furniture and items in buildings are classified into three groups, commonly called: deathwatch (Anobiidae family), powderpost and false powderpost beetles (Bostrichidae family).

Deathwatch beetle

Several genera and species of beetle in the Anobiidae family are called deathwatch beetle. The species well known in Europe, and the source of the name, is Xestobium rufovillosum. In the US, however, the Pacific deathwatch beetle Hemicoelus gibbicollis is the most common in California, while in other parts of the world Xyletinus spp and Ptilinus ruficornis are called deathwatch beetles.

The beetles get their name because of their habit of tapping on wood, usually at night, to attract a mate, which in European folklore was associated with the ‘grim reaper’ tapping on his scythe to announce an impending death.

Deathwatch beetles mainly infest moist and partly decayed softwood timber that has moisture content over 14%. They are more likely to be found in damp buildings or areas of buildings with poor drainage or water leaks. They are unlikely to be present in buildings with central heating where the moisture content is low.

The beetle larvae fill their tunnels with frass that has a slightly gritty texture, but less coarse than false powderpost beetles. The exit holes can be of varying sizes, but are larger than powderpost beetle holes.

Common furniture beetle, Anobium punctatum

The furniture beetle only infests dry, seasoned sapwood of hardwood and conifer trees. It causes more damage in structural timbers and joinery than in furniture. The females lay their eggs in cracks in the surface of wood or inside old exit holes. The eggs hatch into larvae in a few weeks and bore into the wood. The larvae mature over 3-4 years, eating their way through the timber. After pupating near the surface of the wood the adults make an exit hole 1-1.5 mm in diameter. An active infection is shown by the presence of wood dust around holes. As the larval stage lasts several years there can be an active infestation with no visible signs during this period. The actual time of development to the adult depends on type of wood, temperature and humidity.

There are more than 700 species in the Bostrichidae family, which includes a number of common wood and also food pests in both temperate and tropical regions.

False powderpost beetles

The false powderpost beetles infest hardwoods and some softwoods. Tropical timber and bamboo transported in international trade are often infested with the beetles. The common products infested with the beetles include flooring, panelling, furniture and other hardwood items.

The females do not lay their eggs on the surface of the wood, but bore a ‘gallery’ into the wood to lay their eggs in pores or cracks. The larvae of this group pack their tunnels with a coarse, gritty frass, which distinguishes them from other types of wood borer.

Powderpost beetles

This group has recently been reclassified into a new subfamily of the Bostrichidae beetle family, called Lyctinae. It was previously in a separate family, Lyctidae, so some descriptions still refer to this old classification. There are eleven species of the genus Lyctus recorded in the Global Biodiversity Information Facility.

Lyctus brunneus, is the most common species found throughout Europe and has also been recorded in South Africa, Australia and Japan (Global Biodiversity Information Facility, www.gbif.org). In the US, three species were reported by the University of California Integrated Pest Management Program to be the most common in California: old world lyctus beetle, Lyctus brunneus; southern lyctus beetle, L. planicollis; and the western lyctus beetle, L. cavicollis.

Powderpost beetles tend to attack the sapwood of certain hardwoods and bamboo that have large pores in the wood in which they can lay their eggs and that have a high starch content, such as oak, ash, walnut, mahogany. Imported tropical hardwoods are often infested with the beetles. Hardwoods with smaller pores such as birch and maple are rarely infested and softwood, which is from conifers and has lower nutrients, is never infected. The beetles also prefer dry wood, feeding on wood as low as 8% humidity.

The starch content reduces with the age of the timber, therefore powderpost beetles are rarely found in old timber. They are more common in new homes and recently manufactured hardwood items such as window and door frames, panelling, flooring, plywood and furniture. Powderpost beetles are unlikely to infect structural timbers as these tend to be made from softwood. Wood that has a finish, such as paint, varnish, or wax will not have a new infection, but the beetles can already be present when it is processed and can still emerge later.

These beetles are called powderpost because the larvae produce a characteristic fine, dust-like frass with the consistency of flour or talcum powder, as they feed on the wood and bore the tunnels. This distinguishes them from other types of woodworm.

The adult females lay their eggs on the surface of wood or in cracks. The life cycle ranges from three months to over one year, depending on temperature, humidity, and the nutritional quality of the wood.

Old house borer, house longhorn beetle

The old house borer, Hylotrupes bajulus, is native to Europe but has been spread to many areas worldwide through the trade in timber and wood products. It is a pest of seasoned sapwood of softwood timber. It is more often found in new homes, infesting younger timber, rather than timber in old buildings. The life cycle takes up to 10 years, depending on environmental conditions and the nutrient content of the wood, which decreases with age.

Infestation in homes is caused mainly by using timber already containing the eggs or larvae. They feed on the wood, mainly near the surface, until maturing into the adults. The larvae are larger than many wood boring larvae, up to 2.5 cm long and they cut larger exit holes 6-10mm in diameter.

Anoplophora longhorn beetles (FERA)

The Asian longhorn beetle (Anoplophora glabripennis) and citrus longhorn beetle (Anoplophora chinensis) are native to East Asia but have been introduced to other areas of the world, most significantly Europe and North America. The Asian longhorn beetle is a pest of hardwood trees in roadside plantings and plantations in China and has now become a pest in Europe and the US. In the US it tends to attack Acer species. The citrus longhorn beetle is a pest of over 100 species of tree and shrub.

In the UK and the US the early discoveries of the beetles were related to plants imported from China and Korea. Both species are listed in the EU Plant Health Directive, while in the US the Department of Agriculture and the Forest Service ask for notification if the Asian longhorn beetle is spotted.

The adult female of the Asian longhorn beetle chews a small pit in the bark to lay each egg. The larvae feed within the vascular layers of the tree: the citrus longhorn in the lower part of the tree and the Asian longhorn in the upper trunk and branches. The adults make a round exit hole about 10mm in diameter.

Wharf borer

The wharf borer, Narcerdes melanura, has been found in many temperate countries including the UK, Australia, New Zealand, France, Japan and all states of the US except Florida. The beetle is found where there is moist decaying wood, such as in docks, harbours, jetties and along rivers and coastal areas. The females lay eggs on the rotten timber and the eggs hatch in 5-11 days. The larvae burrow about 1 cm below the surface, then tunnel into the wood to feed. The larval stage can last from two months to two years, depending on conditions. The adults do not feed, living only a few days to mate and find a new site to lay eggs.

The wharf borer is a ‘secondary pest’ as it feeds on already rotten timber, but its tunnels can further weaken the timbers.

Bark beetles are now regarded as a specialised section of the weevil family, the Curculionidae, in the subfamily Scolytinae, which has about 6,000 species in 220 genera.

Bark beetles can be pests of tree crops — in forestry and agriculture — and logs in storage or transport. They introduce fungi that attack the wood, killing the tree or introducing decay in stored logs. In the US, where many native species are important pests of conifer trees, the US Forest Service has also recorded 53 invasive species that are thought to have been introduced through international trade.

In natural forests, bark beetles are important in recycling weakened and dying trees and creating habitats for other insects and fungi. They also play a major role in making patches where new trees can regenerate, creating a more diverse forest ecosystem.

Some species, however, break out into epidemics in forests with high densities of single species, killing trees over large areas. This also greatly increases the risk of fire, as happened in California in September 2015, where bark beetles are thought to have increased mortality in trees already stressed by the long-term drought. More than 40,000 acres of forest went up in flames in 12 hours, the Los Angeles Times reported (14 Sept 2015: www.latimes.com/local/lanow/la-me-ln-bark-beetles-valley-fire-20150914-story.html).

The female lays the eggs in the phloem layer, the moist inner layer of the bark next to the sapwood. Some species mine a short chamber in the bark for depositing the eggs or also mating. The larvae feed on the phloem layer, leaving characteristic patterns of tunnels in the outer wood of the tree.