To understand the reason for post-treatment emergence of wood-boring-insects in remedial treated timber, it is essential to consider both the composition and limitations of the treatment formulation and the biology of the insect involved, since these factors are inter-related. Treatments of this kind are usually carried out by conventional surface spraying techniques, using either solvent-based or water emulsion-based insecticides. In this form of treatment, the penetration of the insecticide, and its subsequent distribution within the timber after evaporation of the carrier, results in the formation of a highly insecticidal sheath of wood.

Treatments of this type are aimed at destroying as many beetles and larvae in the wood as possible, and preventing re-infestation by any survivors which might subsequently emerge. The majority of beetles and larvae present in infested timber are killed within a short period of time by a treatment of this kind.

However, in this form of surface treatment it is not possible to achieve deep penetration into the wood by the preservative, and therefore a proportion of the timber after treatment is still non-toxic to the infesting insects. Under such conditions the final eradication of the insect depends upon its habits and life cycle, and the effect on these of the treatment fluid....

Like all beetles, the common furniture beetle (Anobium punctatum) and the death watch beetle (Xestobium rufovillosum), - those which are most commonly encountered in building timbers, have a life cycle consisting of four successive stages - egg, larva, pupa and adult.

On emergence from infested wood the adult females, after mating, lay their eggs in old exit holes or cracks in the surface of rough sawn wood. The eggs of Anobium punctatum and Xestobium rufovillosum normally hatch in about four to five weeks. The newly hatched larvae of Anobium immediately start to bore into the wood, whereas those of Xestobium crawl on the surface of the timber before commencing to bore into it.

Since the whole of the larval life of these insects is spent in the wood, it is possible, due to variations in the growth rate within a species, to have larvae at different stages of development present in wood even though emergence of adult beetles has taken place.

Under normal circumstances the larvae of Anobium take between three and four years to reach maturity before pupating and eventually emerging as adult beetles. It has been reported that five years and commonly much longer is required for egg-to-adult in the case of Xestobium.

When the larvae of these insects have reached maturity they direct their galleries towards the outside of the wood, and immediately beneath the wood surface construct pupal chambers. Here after a short period of time, usually a matter of weeks, they change into pupae and thence to beetles.

It follows from this that in the early stages of a surface treatment of infested wood, only those insects that come into direct contact with the treatment fluid, or which are present in that area of the wood which contains a lethal concentration of the biocide, will be killed. Since it is the habit for the larvae of these insects on reaching maturity to tunnel towards the outer surface of the wood, any survivors from the initial stage of treatment will, on entering this phase in their life cycle, have to enter the highly toxic outer sheaths of wood.

If they manage to survive this second stage in the process of eradication, then the third and final stage takes over. In this stage, eradication is achieved because the female beetles can only lay their eggs in or on the outer highly toxic sheath of treated wood.

In controlling outbreaks of Anobium, remedial treatments using formulations that contain a powerful contact insecticide such as Permethrin are highly effective.

When dealing with an infestation of Xestobium, treatments are effective only if the insect can be reached; and since the larvae of this insect often tunnel well below the surface of the heartwood, it is important to choose a treatment formulation with good penetrating properties.

Infestations of Xestobium are often found in old, large-sectioned hardwood timbers where some fungal decay has taken or is taking place. In such situations, paste-type emulsions that contain both a contact insecticide and a fungicide are of considerable advantage. When applied to the surface of timber at adequate loadings, such treatments will achieve exceptionally deep penetration, and thus greatly assist in the preservation of abnormally large or concealed timbers. The inclusion of a fungicide in the formulation further helps control the infestation by reducing the spread of fungal decay in the wood.

In circumstances such as these, a single treatment cannot always be relied upon to destroy all stages of the insect in the wood, and the phenomenon of post-treatment emergence is sometimes observed, particularly in the case of long-standing infestations. If, however, the treatment has been carefully and thoroughly applied, structurally unsound or heavily infested timbers removed and the conditions favouring attack eliminated, then a high degree of reliance can be placed in this form of treatment. It is, nonetheless, important that periodic examinations are carried out by experts to establish whether or not there is a necessity for further action. This is particularly true in the case of long-standing infestations.