Acaricidal Activity of Bufadienolides Isolated from Drimia pancration against Tetranychus urticae, and Structural Elucidation of Arenobufagin-3-O-α-L-rhamnopyranoside

Chemical characterization of the bulbs of Drimia pancration was conducted to isolate four steroidal saponins (1–4). Earlier, we focused on the structural elucidation of compounds 1–3. Herein, by means of (1)H-NMR, (13)C-NMR, Nuclear Overhauser Effects (NOE), and 2D-NMR spectra, the full stereochemical structure of 4 is reported, and all the (1)H and (13)C signals are assigned. Compounds 1–4 were tested for their acaricidal properties against the two-spotted spider mite Tetranychus urticae. Our results showed excellent activity of compound 1, with an LD(50) (µg/cm(2)) of 0.29 and a LD(90) (µg/cm(2)) of 0.96, whereas compounds 2, 3, and 4 showed moderate activity. Furthermore, the acaricidal and cytotoxic properties of the crude extract were also investigated. Of note, after 96 h of exposure, the acaricidal activity of compound 1 was higher than that of the positive control, hexythiazox. Indeed, for compound 1, LD(50) and LD(90) were 0.29 and 0.96 µg/cm(2), respectively, while hexythiazox LD(50(90)) was 18.7 (132.5) µg/cm(2). Additionally, D. pancration extract, after 72 h, induced a high cytotoxic effect in HaCaT and THP-1 cell lines, with an IC(50) of 7.37 ± 0.5 µg/mL and 3.50 ± 0.15 µg/mL, respectively. Overall, D. pancration can be considered as a green source of novel acaricides effective against mites of agricultural importance, such as T. urticae, pending proper field validation and the assessment of non-target effects on other invertebrate species.