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Powder-post beetle (Lyctus brunneus) identification guide

Adult powder-post beetles are generally 4-5 mm long and range in colour from light brown to reddish-brown. Adult beetles usually emerge from the infested wood between May and September, with bore holes 1.5mm to 2mm in diameter.

Identification

The adult beetle is 2.5-8 mm long (mostly 4-5 mm), somewhat elongate and relatively flat. The colour varies from light brown to reddish-brown. The pronotum (the plate that covers the upper part of the thorax - the middle body section of the beetle) is rectangular. It is usually darker than the wing cases.

The head is distinct from the other body parts, the antennae are clubbed and about the same length as the thorax. The larvae are cream coloured with brown head and jaws and three pairs of small jointed legs. On hatching, the larvae are about 0.5 mm long and straight-bodied but later become C-shaped. The bore holes of emerging adults are 1.5mm to 2mm in diameter.

Distribution and habitat

Originally from the tropical and subtropical regions, the powder-post beetle is now found in many countries. In the UK, it is constantly imported in wood from other parts of the world.

It is a serious pest of seasoned wood and it is to be found chiefly in wood-yards, carpenter’s workshops and houses. It attacks the sapwood of wide pored hardwoods such as oak, chestnut, ash and elm, provided the sapwood has sufficient starch content (greater than 3%).

As the wood ages the starch content declines due to bacterial action. After around 10-15 years starch levels drop so that infestation/activity is no longer possible. Dead softwoods are immune to the beetle.

Life cycle

The female lays up to a maximum of 220 eggs; these are pushed into large vessels in exposed end grain. The larvae hatch and feed on the sapwood for one to two years.

Indoors, the length of the life-cycle can vary, depending on environmental conditions and the nutritional condition and type of wood. Normally it takes around one year but under ideal laboratory conditions it can be as short as 10 to 12 weeks. However maximum life cycles between 2½ years to in excess of 4 years have been reported.

Adult beetles usually emerge from the infested wood between May and September. They can fly well and are attracted to light at night; during the day they hide in cracks and crevices. Beetles can often be seen emerging from infected wood and can be collected for identification.

Damage and control

The sapwood is often entirely disintegrated leaving only a very fine flour-like dust (frass). Where tunnelling exists it tends to run parallel with the grain. Emergence holes are about 1-1.5 mm in diameter, and like the damage, restricted to the sapwood. Initially on upward facing surfaces the holes are surrounded by little 'volcanoes' of frass.

If damage is minor on a few sapwood edges, one can use an 'injector' can of wood preservative to squirt into the holes.

As structural timber is made of mostly heartwood, damage is only decorative, so no treatment is recommended.

As an alternative to fumigation, oxygen-deficient atmospheres (0.4% oxygen) kill all eggs, larvae and pupae in 6 days and all adults in three days. Heating the wood to 50°C for 15-24 hours also kills 100% of all stages.

Preventative measures aimed to reduce or completely deplete the starch in green timber have minimised the infestation: ringbarking (‘girdling’) of the standing tree; storing logs in the round with bark intact; storing logs under water; and special kiln treatment of converted stock.