The Action of Surface Applied Preservatives against Common Furniture beetle.

Common furniture beetle and damage:

Common furniture beetle (Anobium punctatum) feeds on the sapwood of softwoods and European hardwoods; wood also provides a significant protected environment in which the beetle develops.

Life-cycle: Eggs are laid on surfaces but especially on end grain and down old emergence tunnels. The larvae hatch and bore straight through the bottom of the egg into the wood where they remain between 2 - 5 years. The larvae are the feeding and growing stage; these cause the damage. After a period of time the larva comes up towards the surface where it constructs a pupal chamber. In this it pupates and undergoes a significant metamorphosis from a grub like creature to a beetle: the pupal skin splits and the beetle simply chews its way out of the wood; it does not ingest wood on emergence. The result is the superficial 'woodworm holes'.

Mating takes place, usually fairly rapidly, sometimes in the emergence tunnels, and eggs are laid. Most eggs are laid beneath the surface usually in old tunnels/pupal chambers where they are obviously well protected.

Damage: Typically the emerging insects leave small round holes in the wood as the beetle cuts its way out; these are approximately 2mm in diameter. The sapwood frequently has severe tunneling which has a slight tendency to run along the grain. The tunnels are full of loose bore dust - frass- which feels 'gritty' when rubbed between the fingers. Under a good magnifying lens the frass is seen to consist of 'lemon-shaped' pellets.

The Process of Eradication:

So how can we eradicate such a well protected insect? Clearly it is difficult to get at the damaging stages - larvae are buried deep in the wood mostly well below the depth of penetration achieved by a surface spray, let alone a mist/fogging treatment.

The Surface spray:

A surface applied spray of a liquid preservative when applied as directed (1 litre per 3 - 4m²) should give to 2 - 6 mm penetration of the fluid. This provides a thin 'envelope' of protection around the wood. However it only initially kills insects in the treated the zone; a large volume of the wood is left untreated where activity can still continue.

Control can only be achieved when the insects come into contact with the surface treatment. That is when the beetles emerge and/or when the first stage larvae hatch from eggs and penetrate the treatment. At least that is the theory.

Contact insecticides:

So let's take a lot closer look at theory and practice:

Emerging beetles do not feed! They simply chew their way out and do not ingest the wood. Therefore it is necessary to use a contact insecticide; the use of a stomach poison or other material which requires the insect to ingest it will not prove effective against emerging beetles. Contact insecticides such as permethrin or cypermethrin must be used to prevent emergence . The insect only has to come into contact with this treatment when the insecticide should penetrated cuticle, pass round the insects body and affect the sites of action, mostly the nervous system.

When the beetles emerge they pick up insecticide by simple contact as they pass through the treated outer surfaces. This should kill them before they emerge. If the beetles get out they are likely to survive, mate and lay eggs. However, the theory goes on say that if eggs are laid on surfaces then the first stage larvae are killed as they penetrate the treatment, i.e., as they attempt to re-enter the wood.

So the process of eradication is to prevent emergence and to prevent re-infestation. If this is successful then the reservoir of insects stages in the wood must eventually attempt to emerge. If there is no re-infestation the wood will clearly empty of all these stages; eventually this will lead to the eradication of the infestation. This may take two to five years in the case of Furniture beetle and up to around 14 years in the case of Death-watch beetle.

At least this is the theory of eradication!

But what about practice?

All United Kingdom research is based on the use of preservative applied at the rate of 1 litre per 3 to 4m² of timber surface; this is the also the recommended industrial application rate. These rates should provide up to 6mm penetration into the surface of the wood provided it is a permeable timber: a less permeable wood may only allow as little as 2 - 3mm penetration.

In order to get the recommended figure of 1 litre per 3 - 4m² it will require 2 to 3 spray applications on downward and vertical surfaces: a single spray to refusal will only give loadings of around half to a third of that required! Basically, the fundamental necessity of surface applied preservatives is volume - you need volume of material to achieve penetration! On timbers with a moisture range of say 9 -18% as found in the average 'dry' house it is volume that is the all important determining factor that relates to performance!!. A single spray to refusal is highly unlikely to provide that volume, especially on less permeable wood. The less volume of preservative applied the lower the amount of insecticide deposited and, very important, the lower the penetration depth which leads to less protection! Therefore emerging beetles will have less contact with the treatment as they emerge and there is a high likelihood that insects will emerge successfully.

Equally important is that eggs are laid beneath the surface. Where penetration of preservative is poor this means that they are highly likely to be laid beneath the treated 'envelope'. Therefore hatching larvae are not affected because they are beneath the treatment. It is this factor of the eggs and larvae being present beneath the treatments that is frequently overlooked!

So what are the the overall consequences?

With a good application on permeable timber (i.e.. 2 - 3 spray coats) the treatment should kill adults emerging, and on permeable timber should penetrate sufficiently in depth to a level below which the adults may not lay their eggs; in less permeable timber this may not be possible. In this latter case 'compensation' for depth of penetration should be made up by a very high concentration of the contact insecticide in, say the outer 2 - 3mm which should prevent emergence: the chances of re-infestation are then much smaller.

With a poor treatment, i.e., single spray application, this is unlikely to prevent emergence (however, sometimes called 'post treatment emergence'!). If the insects do emerge then they can survive and will lay eggs below the treatment; this will lead to a continued infestation. Evidence that this occurs is given by research undertaken on Death-watch beetle.

In the 60's and 70's the Prince's Risborough Laboratory , Dept. of the Environment, undertook a considerable amount of research on the performance of surface applied preservatives against death-watch beetles(1). Death-watch beetle, Xestobium rufovillosum, is a very close relative of common furniture beetle. Indeed, the behaviour of death-watch beetle actually makes it more susceptible to surface treatments than common furniture beetle. And like Common Furniture beetle it has been recently shown to have the potential to lay eggs beneath the surface in old tunnels(2). After all, it is not surprising that this happens because eggs are significantly more protected in tunnels than on the surface, an evolutionary trait for greater survival.

In the case of death-watch beetle adults were shown to continue to successfully emerge following surface sprays in the early 70's with contact insecticides (See lower figure). Laboratory experiments also showed a specific level of the insecticide necessary to be picked up by emerging insects; this is shown in the figure below as a dotted line. The field data showed that insects that were 'normal' in their behaviour picked up less that this level (Right figure). But those which were collected 'moribund' or dead also picked up less than this quantity. Further examination suggested that these latter insects had mostly died of 'natural' causes rather than insecticide poisoning. Thus, in general terms, the beetles failed to pick up sufficient insecticide on their passage out of the wood. Where the smoke treatments were successful the affected/dead insects had picked up significantly larger amounts of insecticide and had not laid eggs; subsequently the population declined following annual treatments.

Emerged beetles were also shown to successfully mate and lay eggs at a rate similar to those in untreated buildings (greater than 50% mating and egglaying). Furthermore, monitoring of the population following treatment showed no significant decline in subsequent years emergence. The only big drop in emergence was immediately following treatment where pre-emergent adults were directly deluged with the solvent base treatment or affected in the short term by the solvent vapour itself (See fig. opposite and above; 1970 and 1973 - treatments dates. Upper graph - treated 1971).

This research clearly showed that if insects emerged through spray applied treatments they could survive and successfully mate lay eggs and the subsequent populations survive

Why did spray treatments fail against death-watch beetle? Most likely the lack of penetration due to low application rates and the nature of the wood.

It is therefore very important to get a thorough coverage using a product containing a contact insecticide and applying two to three spray coverings to ensure the recommended application rate is achieved.

Non-contact insecticides:

Fairly recently 'environmentally friendly' treatments have been introduced, although restrictions for use under the Control of Pesticide Regulations 1986 remain similar to those for contact insecticide based products. These are based on materials which have no contact action; most, but not all, are recorded to be 'stomach' poisons; basically the spray treatment relies totally on the surface treated timber being ingested. The theory behind their use is that larvae must eat the treated wood when they come towards the surface to pupate, and also the larvae hatching from eggs that are laid must also eat the treated wood as they attempt to re-enter the timber.

The problem with this theory is that (a) larvae can detect chemicals and so pupate beneath the surface treatment, (b) the eggs are mostly laid beneath the surface in many cases below the surface applied treatment, and (c) egg larvae can bore some distances without feeding. So even with a good treatment (two to three spray applied coatings) and even assuming surface eggs are laid, emerging adults can still emerge (they don't feed!) and it is possible for larvae perhaps to penetrate the treated surfaces, but this latter point will, at least in the initial stages following application, depend on what 'co-solvents/solvents' (not organic petroleum solvent) may be present with the non-contact insecticide.

Furthermore, where penetration is poor due to the nature of the wood then even a full application of the spray treatment will not have any effect due to the fact that the insects can still emerge through high concentrations of such materials since they have no contact action and the beetles do not ingest it! And if penetration is naturally poor, eggs can be readily laid beneath the treatment! Indeed, one set of penetration figures quoted for air dry European redwood for a 3 minute dip, the often accepted equivalent of a good spray treatment, is only 1mm, and after 60 minutes this has increased to only 2mm!(2)

With a poor treatment the situation is potentially even worse in that there is still no control on emergence, and even if eggs were surface laid larvae have a very high chance of penetrating the treatment because the material has no contact action. But in reality, however, we would still get emergence, and also eggs laid beneath the surface are not affected by the poison since hatching larvae are beneath the treatment! In this case there is highly likely to be a significant failure of the treatment, especially so where mists/fog applications of non-contact insecticides are applied!!

So even the original theory in relation to the use of non-contact insecticides is somewhat flawed! Indeed, unlike contact insecticides there appears to be very little data, if any, in the public domain regarding the ability of spray applied stomach poisons to effect eradication of a woodworm infestation. Certainly failures of surface applied non-contact poisons are now being recorded, especially from mist/fogging treatments.

Considering all factors, it is likely that non-contact insecticide treatments will always be at a greater risk to failure than those containing contact insecticides since the whole philosophy of the treatment is totally reliant on larvae feeding whereas a contact insecticidal treatment is 'double acting' - it is effective if ingested and, very important, it should prevent emergence and larval activity by its contact action. In the case of the Deathwatch beetle research described above, if surface sprayed contact insecticides showed little, if any, long term effect then certainly a non-contact insecticide would not have proved effective.

So to ensure the greatest chance of a surface applied spray type eradicant treatment being effective:

• Apply the appropriate volume (1 litre per 3 - 4m²); this is very likely take 2 to 3 spray coverings (mists/fog type applications are highly unlikely to acheive such rates on the wood in practice).
• Use a preservative containing a contact insecticide which will not only prevent emergence (provided sufficient depth of penetration has been achieved), it will also stop re-infestation.

Preservative formulations based on Contact Insecticides:

These are the formulations currently available that contain the contact insecticides permethrin or cypermethrin

Solvent based:

The insecticide is dissolved in an aliphatic petroleum solvent such as white spirit or odourless kerosene. When applied the fluid penetrates the wood carrying with it the insecticide. Any insects stages close to the surface will be deluged with the formulation and killed. The solvent subsequently evaporates to leave the protective 'envelope' of contact insecticide.

But some solvent evaporates inwards where the vapour will kill some stages beneath the original liquid front. This results in better initial kill which makes such treatments more suitable for those insects which are difficult to control such as Death-watch beetle (Xestobium rufovillosum) and House Longhorn beetle (Hylotrupes bajulus) because of the better initial kill. However, the solvent vapour affect is short lived.

It is recorded that on air dry timbers solvent based formulations penetrate better than water based (emulsion) type formulations. Unfortunately they present a greater fire, health and environmental risk because of the obviously high propertion of petroleum based solvent which evaporates off once the material has been applied.

Emulsions ('Water based'):

All water-based insecticides containing contact insecticides are emulsions, that is oil/solvent droplets suspended in water: the insecticide is held in the droplets.

Most of these materials are supplied as a concentrate and form the emulsion when added to water.

'Standard' emulsions: These were the first emulsions developed for the control of wood boring insects. The droplets sizes are relatively large, about 1000 manometers (nm) in diameter. This gives the final emulsion a milk-like appearance. These standard emulsions are moderately stable but have a tendency to 'cream' or 'sink' on standing. Such instability of an emulsion if it is severe can lead to problems. For example, it can lead to uneven treatments of floors, etc, where the emulsion is sprayed and the highest proportion of the insecticide is present in, say, the 'cream' that has separated at the top of the drum - in the example opposite, the floor is sprayed and as the 'cream' reaches the bottom of the drum it is taken up and sprayed; this leads to a poor distribution of the insecticide over the floor. When a standard emulsion is sprayed on wood basically we start with the surface film of the material. The next phase is that the water (continuous phase) penetrates the wood; this effectively filters out the oil droplets containing the insecticide; these remain on the surface.

This oily phase carrying the insecticide will only penetrate once the water has evaporated since until this has happened the water occupies the cells/cell walls preventing the 'oily' phase from diffusing. If the oily phase is volatile then there will be a significant reduction in its volume due to evaporation whilst the water itself is evaporating from the underlying wood. The absence of volume of this oily phase remaining once the water phase has evaporated will clearly lead to poor penetration of the insecticide, even with the initial two to three spray applications. Micro emulsions: The micro emulsions are a more recent development. They do not use a petroleum solvent, the insecticide is held in the surfactants or glycols.

Like the older 'standard' emulsions, they are supplied as a concentrate (usually more 'concentrated' than the standard emulsion concentrates) and when mixed with water they form the emulsion, but some are micro-emulsions in the concentrate. The droplets containing the insecticide are very small, usually less than 10nm diameter. When these materials are mixed with water they form a clear, a bright colourless preservative which is almost odourless, and is very stable; micro-emulsions do not separate.

When they are applied to the wood it is likely that the droplets penetrate with the water because of their extremely small size; they are probably not filtered out. This should lead to a better penetration than the old standard emulsions.

Micro-emulsions also have a further advantage in that under current United Kingdom legislation (The Control of Pesticide Regulations 1986), most micro-emulsions have 8 hour re-entry time clearance, i.e., one can re-enter treated areas after a minimum of eight hours following the application of the treatment: the old standard emulsions and solvent based materials require 48 hours before one can re-enter treated areas.

The only 'downside' of a micro-emulsion is that because of their generally 'odourless' nature, complaints are sometimes received from householders believing that the relative absence of smell is indicative of the treatment not having been carried out! (Of course, if a solvent based material was used then they do nothing but complain about the smell - you can't win 'em all!)

Penetration equals performance:

Finally, there is a further reason to obtain good penetration of the preservative into the wood - it will (hopefully) penetrate to sufficient depth to prevent emergence and possibly penetrate below any egg-laying sites, but also to afford long-term protection.

When a preservative containing a contact insecticide has been applied to timber the outer 2mm of the insecticide residue slowly depletes with time. Clearly when this happens if penetration is poor then we will get very limited residual protection notwithstanding the initial problem of mating, laying eggs and larval hatching and survival. This may, in the longer term, allow re-infestation by new incoming insects.

So, in considering all the factors described above, the importance of getting a good depth of penetration cannot be over-emphasised. This is unlikley to be achieved using mists/fogs in that the volume of fluid actually landing on the timbers cannot be guaranteed or controlled.

Conclusions:

The key to performance and success in eradicating Common Furniture beetle (or indeed any other woodborer) is depth of penetration of preservative; this will depend on the volume applied! Indeed, depth of penetration in the case of contact insecticides is probably more important then their concentration.

Without volume of fluid applied you will not achieve good penetration; do not be told otherwise! Where an infestation is known to be active and is required to be eradicated then it is absolutely essential to apply the required volume of spray insecticide treatment, 1 litre per 3 - 4 m², to achieve this goal (ie, as per the manufacturers' recommendations); this will require 2, possibly 3 coarse spray applications on vertical and downward facing surfaces.

It is also advisable, given the natural limitations that one can come across in relation to penetration of preservative, ie, impermeable timbers, to use a contact insecticide rather than a non-contact material: these latter materials have only one phase of action, ingestion; thus if penetration is limited they are unlikely to prove fully effective. Contact insecticides, on the other hand, will not only act if ingested but there is the added very significant benefit of contact action which will restrict/prevent emergence following a fully applied treatment.

Refs:
(1) Coleman, G.R. 'Insecticidal smokes for the conservation of structural timbers.' Oxford Congress: 'Conservation of wood in painting and the decorative arts.' pp 17-23, September 1978.
(2) Ridout, B. 'Timber decay in Buildings' Pub. E & F.N Spon (2000) ISBN 0-419-1820-7

© G.R.Coleman 2000

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