Protection effect of the anthraquinones, cassiatorin and aurantio-obtusin from seeds of Senna tora against cowpea weevil attack

Objective: To explore the potential insecticidal, ovipositor deterrent and antifeedant effects of ethyl acetate extract of the seeds of Senna tora (Syn. Cassia tora) against cowpea weevil (Callosobruchus maculatus). Methods: The activities were evaluated using standard protocols. In these bioassays, the cowpea seeds were used directly as an insect feed. The activity of the extract and isolated compounds were tested at concentrations of 100, 200 and 300 μg/mL and compared to neem oil and cinnamaldehyde (as standard positive controls). Phytochemical analysis of the ethyl acetate extract was done through a number of chromatographic techniques and the structures of the isolated compounds were established through comprehensive spectroscopic analysis including 2D-NMR and ESI-MS studies. Results: Fractionation of the active ethyl acetate extract resulted in the isolation of one known anthraquinone, aurantio-obtusin (1) and a novel compound that was named as cassiatorin (2). Compounds 1 and 2 showed comparable insect antifeedant properties with the positive controls while their insecticidal and ovipositor deterrent effects were far superior to the standard controls. Conclusions: It is thus concluded that Senna tora extracts and the isolated compounds (1 and 2) may be employed in the postharvest management of stored cowpea seeds and as other crop protectants.