Variable working strengths of woodworm treatment fluids.

It may come as a surprise to some but the working strengths of insecticidal timber treatment fluids available for professional use varies enormously.

Currently the most commonly used insecticide for this purpose is permethrin. For many years a working strength of 0.2% permethrin in a preservative’s ready use state has been the ‘norm’ but during recent years we have seen this halved with the introduction of 0.1% by some manufacturers.

The application rate for an insecticidal timber preservative specified by most manufacturers is usually in the region of 1 litre to cover 3.6 m² of timber surface area. It is already known from a practical point of view that due to ‘fluid run off’ from the vertical and undersides of timber it is difficult to achieve this level of application. In many circumstances at least two applications would be necessary to attain this level of application but this seldom happens.

The question has been asked that if 0.1% is now considered by some to be adequate why was 0.2% used for many years in organic solvent (white spirit) formulations where the active ingredient was in solution and not suspended in a micro emulsion. It is acknowledged that in laboratory conditions 0.1% will work but this is very different to a dirty roof with the inherent difficulties of achieving the specified application rate on vertical and underside surfaces. It has been suggested that by reducing the level of active ingredient by 50% manufacturers have reduced the bill of one of their most expensive active ingredients by 50%. In an industry where the manufacturers are suffering from a price depression there could be merit in this argument.

There is no doubt a cost benefit to contractors also but is it proportional to the reduced level of active ingredient that they are purchasing? You will have to draw your own conclusions as to what level of active ingredient you would prefer to use or be used but trying to find this out is fraught with difficulties. Pack sizes vary and so too does the level of active ingredient in the concentrate pack. To help you we have created a table that will enable you to work out the level of active ingredient you are buying or will be used in your property. You will need to find out the concentrate pack size and the % of active ingredient declared on the pack to use the table.

Property Guarantee Administration Limited

Concentrate

pack size

% of active ingredient declared on the concentrate pack label

1.25% w/w

2.5% w/w

5% w/w

5.9% w/w

0.5 litre

0.03%

0.05%

0.10%

0.12%

0.85 litre

0.04%

0.09%

0.17%

0.20%

1.0 litre

0.05%

0.10%

0.20%

0.24%

1.5 litre

0.08%

0.15%

0.30%

0.35%

2.0 litre

0.10%

0.20%

0.40%

0.47%

2.5 litre

0.13%

0.25%

0.50%

0.59%

The ready to use strength expressed in % terms

1.	This table assumes a product density of 1g/ml and dilution with water to make 25 litres of ready to use material.
2.	This table works for most insecticides and fungicides but not borates.
3.	That the final working strength not only depends upon the amount of active ingredient in the concentrate pack, expressed as weight by weight (w/w), but also the pack size.
©Property Guarantee Administration 2004
	Property Guarantee Administration Limited PO Box 956, Bradford, BD4 8WX Tel: 01274 669712 Fax: 01274 667349 Email: This e-mail address is being protected from spambots. You need JavaScript enabled to view it