Effect of the antioxidant BHT on reducing depletion of chlorothalonil in treated wood after 54 months of ground-contact exposure

Since co-addition of the antioxidant butylated hydroxytoluene (BHT) increased the efficacy of organic biocides in protecting wood in laboratory and outdoor exposure tests, and it was suggested that the antioxidant might also help protect organic biocides against microbial degradation, the latter hypothesis was tested. Stakes were examined that had been treated with different levels of the organic biocide chlorothalonil, with and without two levels of the antioxidant BHT co-added, and exposed for 54 months in ground contact at two locations with high or severe deterioration hazard. Three replicate stakes were removed from each treatment set/location, and chlorothalonil and BHT depletion was measured at three positions in each stake. Biocide depletion was generally lower in the ground-contact sections of stakes that were treated with the two higher chlorothalonil levels and BHT than for matched stakes without co-added BHT, possibly due to BHT protecting the biocide against bacterial and/or fungal degradation. However, a protective effect was only observed in one-half of the ground-contact portion of stakes treated with the lowest chlorothalonil level and in approximately one-half of the above-ground section of the stakes at all three biocide treatment levels. Further studies are needed to definitively determine if BHT can reduce depletion of different organic biocides in above-ground or ground-contact wood. BHT depletion generally ranged from approximately 30–50% after exposure for 54 months. The termite and fungal decay efficacy of the remaining stakes after exposure for 67 months showed that stakes co-treated with BHT and chlorothalonil generally had better fungal and termite ratings than stakes treated with only the biocide.