Chlorpyrifos (O,O-diethyl[3,5,6trichloro-2-pyridyl]phosphorothioate) was evaluated as a wood preservative insecticide in two long term field tests. In the first project, pine sapwood blocks treated with chlorpyrifos (CPF) were exposed to Coptotermes formosanus using an above-ground test format. CPF is effective in protecting wood from attack by this termite, but its activity appears to decrease with time, possibly because of fungal colonization which degrades or metabolizes the insecticide and leads to subsequent insect attack. In the second project with ground contact field stakes, stakes treated with CPF plus chlorothalonil or diiodomethyl-p-tolylsulfone showed good complementary performance between the two biocides. Long term decay and termite ratings with these formulations show clearly superior performance compared to the biocides by themselves. CPF had a negative effect on the preservative properties of didecyldimethylammonium bromide. Combination with this fungicide resulted in lower decay and termite ratings.

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