End-sealed Pinus radiata specimens measuring 235 x 90 x 35 mm were treated using a modified Lowry schedule with permethrin or bifenthrin in white spirit to Hazard Class 2 (H2) retentions. After conditioning, the specimens were exposed at 45o on north facing racks near Melbourne (Australia). Samples of the specimens were taken before installation, and after three and six months exposure. Analyses of permethrin-treated specimens revealed a considerable decrease (14-34 %) in permethrin content in the exposed outer 2 mm after 3 months. Continued exposure for an additional three months did not lead to further significant reduction in the permethrin retention of the outer 2 mm zone. Analysis of the 2-5 mm zone has shown no significant change in permethrin levels. The loss of permethrin is essentially confined to the outer 2 mm zone of the exposed surface which is thought to be caused by UV degradation and/or hydrolysis. Comparatively, bifenthrin exhibited no significant loss in the inner or outer samples, which maybe attributable to its greater chemical stability.

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