The fact that significant photodegradation of pyrethroids can occur in use has been established for over 20 years. It is known, mainly from experience with tsetse fly control, that such degradation leads to marked reductions in efficacy. This realisation has lead to extensive investigation into ways of stabilising this group of important insecticides. Pyrethroids are widely used as the insecticidal component of pre-treatment wood preservatives, in addition to being important components of remedial treatments. However, studies on the longevity of pyrethroids in wood are limited and work to date has concentrated almost exclusively on the effects of temperature and humidity, either in soil contact or remedial treatment situations. The current study reviews experience with pyrethroid degradation in other end-use areas. Such findings are also considered in the context of preliminary exposure trials which suggest significant losses of synthetic pyrethroids from timber during a construction period. In the UK, water-borne emulsions of insecticide and fungicide mixtures are being increasingly used in preference to light organic solvent preservatives, prompted by ever stringent VOC-emission limits. This change in carrier-system, the situation in mainland Europe for some time, is likely to exacerbate any photodegradation problems with pyrethroids since emulsions tend not to penetrate as deeply into the timber as solvent-borne preservatives. Such implications are discussed, not least because the requirements for standard pre-conditioning and minimum penetration requirements may need to be revised for photo-labile active ingredients.

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