Individual and synergistic toxicity of cinnamon essential oil constituents against Haemaphysalis longicornis (Acari: Ixodidae) and their potential effects on non-target organisms

Abstract

Plant essential oils (EOs) are usually complex mixtures of volatile compounds, and these compounds can vary in composition as a result of biotic and abiotic factors. Thus, understanding the contribution of the individual compounds particularly their interactions with respect to acaricidal activity is important for the use of EOs as an alternative to synthetic acaricides. Herein, the acaricidal activity of constituents from EOs of cinnamon (Cinnamomum zeylanicum and Cinnamomum cassia) against Haemaphysalis longicornis was investigated using the nymphal and adult immersion tests. Among the test compounds, benzyl benzoate showed the highest toxicity against the nymphs (LC50 = 24.41 µL/mL) and adults (LC50 = 3.99 µL/mL). All combinations at their sublethal concentration (LC30) exhibited synergy. The binary mixture of benzyl benzoate + cinnamaldehyde presented the highest synergy ratio (3.42) while tertiary mixtures of cinnamaldehyde + eugenol + linalool showed the least synergy ratio (1.20). Enzyme assays revealed that benzyl benzoate significantly inhibited AChE activity (p < 0.05). The ecotoxicological assessments of benzyl benzoate and cinnamaldehyde indicate likely minimal effects on non-target terrestrial invertebrate, Tenebrio molitor. On the other hand, they were toxic to non-target aquatic species, Daphnia magna. Based on our results, the acaricidal activity of cinnamon EO against H. longicornis may be linked to the content of benzyl benzoate whose presence may be synergistic to the activity of other compounds.