Contact toxicity of six plant essential oils in combination with ivermectin against Sarcoptes scabiei (Linnaeus) (Astigmata: Sarcoptidae)

To screen for efficient and safe lead compounds for botanical acaricides, this study evaluated the in vitro contact activity of six plant essential oils (EOs) (Lemon, Ageratum, Eucalyptus, Zanthoxylum, Angelica, and Ligusticum chuanxiong) against adult Sarcoptes scabiei using a filter paper contact bioassay. Results revealed that all tested EOs demonstrated time- and dose-dependent acaricidal effects. Ligusticum chuanxiong, Ageratum, and Angelica EOs showed the strongest activity, with 24 h LC50 values of 1.292 mg/mL, 1.144 mg/mL, and 1.010 mg/mL, respectively. The synergistic effects between each EO and ivermectin were evaluated using the synergistic mortality index (SMI). SMI (IVM) analysis revealed that Ligusticum chuanxiong EO (3.761) showed the strongest synergistic effect, followed by Eucalyptus EO (2.880) and Lemon EO (2.556). All six EOs exhibited significant synergism with ivermectin, and the combined treatments showed markedly stronger acaricidal activity than single applications. This study indicates that Ligusticum chuanxiong is a highly promising source for the development of natural acaricides. Their application as synergists can effectively improve the acaricidal activity of ivermectin, reduce dosage requirements, and delay the development of drug resistance.