Woodwoorm - 'Post Treatment Emergence' or failed treatment?
'Post treatment emergence' - does it exist or is it unknowingly a polite woodwormexcuse for a failed treatment?
First of all, what is meant by 'post treatment emergence'? By definition it is the cutting of exit holes and emergence of wood boring insects through a surface applied spray of a timber preservative having insecticidal properties. It is argued that this is expected following treatment but the surface spray will kill any eggs subsequently laid on the surface and any larvae hatching from those eggs that attempt to penetrate the outer treated surface of the wood. Thus over a period of time 'post treatment emergence' will decline and cease due to the absence of re-infestation: in the case of common furniture beetle (Anobium punctatum) this could take up to five years before the population is eradicated.
A closer look:
Let's take a closer look at the case of common furniture beetle, Anobium punctatum. We'll start by a quick look at the life cycle.
Common furniture beetle starts life as an egg; these are laid on a suitable timber surfaces - but some of these surfaces are cracks and crevices, end grain and, very importantly, down the old emergence holes in pupal chambers and tunnels. The larvae hatch, bore into the sap wood when they remain causing the damage; the larvae are the feeding and growing stages and are by far the longest stage of the insect's life cycle.
At some stage the larva moves close to the surface and constructs a pupal chamber in which it pupates. Between May and August the adult emerges from its pupal skin and chews its way out leaving the familiar 'woodworm holes'. When they cut their way out they do not feed! When emerged they mate and lay eggs, frequently rapidly and very often down the old emergence holes in the pupal chambers and tunnels, i.e.. well below the surface. Indeed, both mating and egg laying can take place within the emergence hole itself.
Spray applied preservatives:
The spray applied timber preservative should contain a contact insecticide such as permethrin or cypermethrin. Basically all the insect has to do is to come into contact with this material - the insect doesn't have to ingest it; if it did it would also kill it. So one could theoretically argue that contact insecticide to have both stomach and contact action. spray
When the wood is sprayed with the preservative it penetrates the surface, probably between 2-6 mm, to leave a protective 'envelope' of insecticide. Any stages initially deluged by the fluid in the 'envelope' will be killed. Those remaining beneath the treatment will survive and continue activity. Clearly these stages will have to come into contact with the insecticidal 'envelope' at some stage for it to be effective. This occurs, at least theoretically, when (a) adults emerge, and (b) larvae hatch from surface laid eggs.
The adults: Adults should be killed as they try to cut their way out of the wood when they cross the 'envelope' containing the contact insecticide; remember they do not feed and must come into contact with the insecticide to have an effect. Therefore the contact acting insecticide should not let them through the treated envelope (non contact insecticides will have no effect on this stage). Indeed, both permethrin and cypermethrin like most pyrethroid derived insecticides, are fast acting. Since it does take time to cut emergence tunnels clearly one would expect these beetles to be affected quite rapidly. If under such circumstances, however, they can completely emerge successfully then they haven't been affected by the treatment: they are then free to mate and lay eggs.
Egg larvae: It is well documented that common furniture beetle mates and lays eggs down old tunnels, often well below the surface. This frequently put the eggs and larvae subsequently hatching beneath the applied treatment, be it a contact insecticide or non contact type insecticide. As such this will lead to successful hatching and larval survival. Hence the infestation continues, and the treatment must therefore be deemed to have failed. After all if you cannot prevent emergence and you cannot prevent egg laying, larval hatching and survival then you have failure.
The evidence for failure of treatments allowing emergence:
The evidence for 'post treatment emergence' likely being synonymous with treatment failure comes from a considerable amount of field research carried out in the 60's and 70's by the Prince's Risborough Laboratory , Dept. of the Environment, 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 'post treatment emergence' was synonymous with treatment failure. And this is with an insect that is potentially more vulnerable to spray treatments than common furniture beetle.
Failure of treatment:
Not withstanding the above, we have received reports of distinct failures following treatment with contact insecticides. Why? Probably on the grounds of insufficient application/uptake of fluid by the wood which leads to low insecticide loadings and, very importantly, poor penetration, i.e., there is less of a protective layer to prevent emergence and the lower penetration makes it far more likely that eggs can be laid beneath the treated envelope.
The evidence exists to show that if beetles emerge from surface treated wood they can successfully mate, lay eggs and subsequent hatching larvae survive to continue activity.
Thus what is typically called 'post treatment emergence' is more likely to reflect treatment failure rather than the interim phase of preventing re-infestation.
(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
(Also see 'The Action of Surface Applied Preservatives against Common Furniture beetle' )
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