Efficacy of chlorothalonil as a wood preservative against the Formosan subterranean termite

Chlorothalonil (CTL, tetrachloroisophthalonitrile) was both deterrent and toxic to Formosan subterranean termites, Coptotermes formosanus Shiraki (Isoptera: Rhinotermitidae), in laboratory tests using southern yellow pine wafers treated with CTL in oil (AWPA Type A), CTL/chlorpyrifos in oil, or CTL in xylene. The wafers were conditioned by evaporative aging at 40?C for 4 weeks and exposed to termite attack in a modified ASTM 4-week (no-choice) test. Termites were also exposed to CTL in the xylene carrier and solvent-treated pine wafers in a 4-week two-choice test for feeding deterrence. CTL retentions were assayed post-test by x-ray fluorescence, and an average 61 percent decrease in CTL concentration was found from the pretest nominal retentions. In the no-choice test, CTL retentions of 0.13 to 0.15 pcf (assayed post-test) limited wood weight loss from termite feeding to 6 to 13 percent, and retentions of 0.26 to 0.39 pcf CTL resulted in only 3 to 4 percent wood weight loss. In the two-choice test, CTL retentions of greater than or equal to 0.06 pcf deterred termite feeding in comparison to solvent controls, and the highest tested retention of 0.38 pcf limited wood weight loss to 1.5 percent. Termite mortality was positively correlated with CTL retention. In this study, assayed CTL retentions greater than or equal to 0.26 pcf restricted wood weight loss from Formosan subterranean termite feeding to less than 5 percent. A heavy oil carrier was not essential for CTL efficacy.

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