The most frequent insect
pest in wood in buildings in the UK is the common furniture beetle,
Anobium punctatum, better known as woodworm.
It has a 5 year life cycle,
maximum. We need to look at wood preservatives and their performance
- wood treatment is always aimed at breaking the egg laying cycle -
so what is that cycle and how does it influence insecticide treatment?
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
The spray applied timber
preservative can 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 has both stomach and contact action. Using an alternative
Insecticide, like a Boron based product, only kills the 'grub' or lava
when it eats wood. It acts as a stomach 'poison' by affecting the enzymes
in the gut. The advantage of this type of treatment is that it does
NOT kill flies or spiders - only wood borers - therefore leaving them
for the Bats. Boron based products are also superior because that have
no vapour - so the active part - the Borate - never leaves the timber,
giving permanent protection.
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.
It is well documented that
common furniture beetle mates and lays eggs down old tunnels, often
well below the surface. Rarely, this can cause hatching beneath the
applied treatment, be it a contact insecticide or non contact type insecticide,
if the treatment was inadequately applied (two good coats to refusal).
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.
- guides to insects, rots, moulds and damp
Wood Boring Insects, Rots and Moulds: full colour: BRE - £55.00
Buy BRE Book
Dampness in Buildings: Coleman - Reprint - £9.99