Isobutyrylshikonin and isovalerylshikonin from the roots of Onosma visianii inhibit larval growth of the tobacco cutworm Spodoptera littoralis

In the present work, we evaluated the toxicity and growth inhibitory effects on Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) of the root hexane extract from Onosma visianii (Boraginaceae). Furthermore, two naphtooquinone derivatives, namely isobutyrylshikonin and isovalerylshikonin, which were isolated from the extract by column chromatography and identified by Nuclear Magnetic Resonance (NMR) and Mass Spectrometry (MS) techniques, were also assayed for the same target. The O. visianii extract, at 30.0 μg/cm2, led to 100% mortality at day 7, with LD50 of 6.8 μg/cm2 for the same day. On the other hand, isovalerylshikonin was significantly more effective (LD50 = 0.8 μg/cm2) than isobutyrylshikonin (LD50 = 7.3 μg/cm2). The two tested naphtoquinones inhibited the growth of S. littoralis larvae with ED50 of 0.4 and 0.3 μg/cm2, respectively. Furthermore, isovalerylshikonin provide a stronger inhibition of acetylcholinesterase (AChE) enzyme (IC50 = 51.4 μg/mL), than isobutyrylshikonin (IC50 = 162.0 μg/mL). These findings provided new insights into the potential of shikonin derivatives as new valuable botanical insecticides to be used in crop protection.