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 Microspraying preservatives -- myth or magic?

It's getting popular, it’s cheap, quick and easy. Yes, we are talking about misting, fogging, micro-spraying, ULV treatments - call it what you will. So what's it all about?

Basically, the process involves producing a mist of a timber preservative and blowing this into a space where there are timbers, e.g., subfloor areas, roof spaces. This process puts tiny droplets of preservative into the air, some of which land on the timber thereby theoretically providing a suitable protective coating. The advantage is clear - no basic disturbance; floorboards, carpets, etc, can be left and the operator can just sit and wait. And it appears to be quick, simple and clean. Great!

So what's the catch?

Let's start by looking at the conventional spray method of eradicating common furniture beetle (Anobium punctatum). The process involves spraying a volume of timber preservative fluid containing a contact insecticide on to the wood. A coarse low pressure spray is used to insure that, as far as possible, all the fluid applied reaches the wood and not drift away in the air. The fluid penetrates to depths of 3-5mm to provide a protective 'envelope' of preservative around of the wood. The contact insecticide’s function is to kill adults on emergence and any larvae which an attempt to penetrate the treated surface should eggs be laid. Over 3 – 5 years the population in the wood is effectively depleted as emergence and re-infestation are prevented, i.e., the infestation is ‘eradicated’. Or at least that is the theory.

It is very important to get maximum penetration of the preservative in order to afford long-term protection and to attempt to get beneath the level at which common furniture beetle will re-enter old emergence holes and cracks and crevices to lay eggs. Investigations have clearly shown that furniture beetle frequently lays eggs down the old exit holes, sometimes well below the surface. If this occurs below the treated ‘envelope’ larvae survive and infestation continues.

To get penetration into wood you need volume, the more fluid the greater the penetration. Just image wood as a 'sponge' - put a teaspoon of fluid on it and it doesn't go far; put on a cupful and you will get much better distribution.

The usual recommendation is around 1 litre of preservative per 3 square metres of wood surface. If you don't apply sufficient then penetration is limited and problems occur as described above. Furthermore, because the outer 2mm of contact insecticide depletes with time then inadequate penetration will lead to the longevity of the treatment being curtailed: thus the importance of applying a sufficient volume of preservative.

Misting, however, appears to be the antithesis of the standard spray treatment. Here, a mist of preservative is applied into a space surrounded by the wood; the misting is not particularly directed, the droplets basically impinging or drifting onto the wood. Application of preservative to the wood surface is certainly very inefficient, nothing like the 90% of a close applied coarse low pressure spray which would be directed specifically onto the wood. To see the effect just spray an aerosol at glass from say 3'; very little lands, most drifts away: try the same exercise from 6' and there is unlikely to be any detectible residue.

Furthermore, deposits on downward and vertical facing surfaces are likely to be worse than upward facing surfaces. Also note that mist size droplets have very little energy and are very subject to air currents, and in a number of areas there is total reliance on air currents carrying the mist to the wood. All in all preservative distribution is going to be very erratic, and highly likely to be extremely low! If you haven't got volume, you will not get penetration of fluid. Fundamentally, misting is a space spray as used in agriculture/pest control for crawling insects or those which eat the entire structure (eg, leaves) ; a conventional spray is purely designed to put high volume of fluid specifically on to the wood.

 

 

What do we know about performance?

Considering its rise in popularity, surprisingly little data exists for misting. Certainly, application rates of 1 litre per 10 square metres are reportedly specified; this is around 3 times less than a standard spray treatment! Some are even advertising that misting will cut the amount of material used!!

Experience of the use of particulate, non-directed insecticides (smokes) has shown that application rates like this cannot be determined by non-targeted space applications. Usually, they are expressed as application by volume - it is almost pure guesswork as to what is going to reach the various wood faces. And, of course, such systems are potentially open to abuse.

Any permethrin/cypermethrin based product misted at the 1 litre per 10 square metres is going to run into big trouble straight away - at these application rates penetration will be very low and so the material will be subject to significant loss. Remember the recent arguments over the use of a 0.1% permethrin verses 0.2%? Well, using misted permethrin is going to be like using effectively 0.01%-0.05% without the very important factor of penetration. There is no evidence available to suggest that levels of this order surface applied are effective in eradication of common furniture beetle infestations.

However, the borate/glycol formulations appear to be the most popular materials for misting. In current practice they appear to be applied at low rates, and of course, you can't determine the actual application rate applied to the wood from a space spray.

The borates may be classified as a ‘stomach’ poisons - the insect has to eat the impregnated wood for an effect. Thus it can only potentially affect larvae (if it pentrates to sufficient depth), most of which are below the surface of wood: it will not affect adults or their ability to lay eggs. Traditionally applied fluids containing contact insecticides only require simple contact with the insect to kill and unlike borates, both adults and larvae can therefore be affected; eggs are usually not.

Fortunately, there is some data produced on the efficacy of a misting treatment using borate/ glycol mixes. The test involved misting a small roof void and was conducted using sound unaffected timber blocks, these being subsequently challenged with common furniture beetle to determine egg laying and larval survival. The results clearly showed that, at the application rate used (see below), the glycol/borate had no effect whatsoever on egg laying ability of adults, as many eggs at being laid on treated as untreated surfaces. However, the larvae hatching from laid eggs were killed: this would clearly prevent infestation.

This work was undertaken on sound wood, not previously infected old wood with the emergence holes, cracks and crevices etc. So whilst it clearly demonstrated that such treatments could protect sound surfaces from infestation it did not demonstrate that it could prevent re-infestation in old timbers where insects will readily lay eggs down old exit holes, crack, etc, which would not be affected by misting.

Furthermore, in the test the rate of application of the borate preservative was around 1 litre per 1.25 cubic metres of roof space; this took 15 minutes to treat the six cubic metres used for the test. Assuming all the surface area was treated then an application rate equivalent to around 1 litre per approximately 4 square metres was obtained, a figure very close to a standard spray application: it was also much greater than the approximate rate of 1 litre per 10 square metres reported to be in current use! And again volume equals potential penetration and 1 litre per 10 square metres is not volume! Borate/glycol mixes are certainly not cheap, and to apply at a standard rate of one litre per for square metres as per the test makes this type of treatment horribly expensive in material! It also took 15 minutes to treat the 6 cubic metres roof space. At this rate it would take between 2 to 3 hours to treat the average roof! Some reports show some current misting practices to be literally minutes!

It has recently started to become popular to ‘mist’ borates simply dissolved in water, i.e., simple solutions. As with any low volume applied material these simple borate solutions are unlikely to penetrate to any depth; they are more likely to sit in the very outer surface of the wood where the preservative will remain as a crystalline deposit. This is likely to be considerably less effective than even the borate/glycol mixes when misted! Furthermore, whilst the data on effectiveness of glycol/borates on the protection of sound wood against furniture beetle given above is related to a 20% sodium octoborate/glycol material, some materials now on sale claim a 4 – 5% sodium octoborate in water solution will eradicate ‘woodworm’; this is clearly around ¼ of the level known to offer simple protection applied at the same rate (1 litre/4sq m)! Of even more concern is that this concentration is also advised for misting where loadings of the borate will be substantially lower! Such claims probably have little if any substantiation. So, before using such a material it is essential that the claim of eradicant action can be validated and supported by research data.

And there’s another catch. When a borate based misting treatment has been undertaken it is not uncommon for someone to ‘test’ the wood for boron and pronounce that it has been “adequately treated”. What they are actually doing is testing the surface of timbers with a reagent, curcumin, which indicates the presence of the borate on the surface -- it does not reflect that the treatment was undertaken effectively! The presence of a material is not synonymous with suitable levels and effectiveness! Curcumin is relatively sensitive to boron and will show a ‘positive’ even in the presence of very low levels of the material, and it may be expected that some preservative must have reached the timbers in any case. It is still pure guesswork as to what levels have actually reached the surfaces.

 

What can be concluded?

At this stage, whilst there is evidence to show that misting using borate/glycol mixes will prevent infestation of sound wood when applied at the rates similar to that used for a conventional spray treatment, there appears to be no supportive evidence to show that such treatment, be they borate/glycol, simple borate solutions or permethrin based, will actually eradicate an infestation of common furniture beetle, especially at the application rates currently reported used in situ. Consequently, it can only be concluded that the jury must still stay out on this one, especially where relatively low concentrations of simple borate solutions are used.

Just one final thought. How would you know the state of a floor if you haven't lifted floorboards etc. If you didn't and misted for insects and rot was also present, you may be in trouble. And as far as misting goes for rot - forget it!


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