Contact and fumigant toxicity of Armoracia rusticana essential oil, allyl isothiocyanate and related compounds to Dermatophagoides farinae

BACKGROUND: The toxicity to adult Dermatophagoides farinae of allyl isothiocyanate identified in horseradish, Armoracia rusticana, oil and another 27 organic isothiocyanates was evaluated using contact + fumigant and vapour‐phase mortality bioassays. Results were compared with those of two conventional acaricides, benzyl benzoate and dibutyl phthalate. RESULTS: Horseradish oil (24 h LC50, 1.54 µg cm−2) and allyl isothiocyanate (2.52 µg cm−2) were highly toxic. Benzyl isothiocyanate (LC50, 0.62 µg cm−2) was the most toxic compound, followed by 4‐chlorophenyl, 3‐bromophenyl, 3,5‐bis(trifluoromethyl)phenyl, cyclohexyl, 2‐chlorophenyl, 4‐bromophenyl and 2‐bromophenyl isothiocyanates (0.93–1.41 µg cm−2). All were more effective than either benzyl benzoate (LC50, 4.58 µg cm−2) or dibutyl phthalate (24.49 µg cm−2). The structure‐activity relationship indicates that types of functional group and chemical structure appear to play a role in determining the isothiocyanate toxicities to adult D. farinae. In the vapour‐phase mortality bioassay, these isothiocyanates were consistently more toxic in closed versus open containers, indicating that their mode of delivery was, in part, a result of vapour action. CONCLUSION: In the light of global efforts to reduce the level of highly toxic synthetic acaricides in indoor environments, the horseradish oil‐derived compounds and the isothiocyanates described herein merit further study as potential acaricides for the control of house dust mite populations as fumigants with contact action. Copyright © 2011 Society of Chemical Industry