Insecticedal activity and chemical compositions of Stemona Burkilli against Spodoptera exigua and Spodoptera litura

The increasing awareness about the environmental and health impact of pesticide use including the increasing demand for food safety and quality led to a higher degree of interest in integrated pest management utilizing biopesticides as one of the tools. Stemona sp. (Stemonaceae), a potential plant, have been widely used for vegetable crops pest control in Thailand especially crucifers. Initial investigations on Stemma burkilli root extracts were shown to possess insecticidal activity. In this research, Stemona burkilli roots were sequentially extracted with hexane, dichloromethane and methanol and bioassayed. The result showed that dichloromethane crude extract (DCE) exhibited the highest toxicity against Spodoptera exiqua and Spodoptera litura by leaf dipping method with LC50 values of 6,204 and 9,589 ppm at 24 hr, respectively. The DCE was further isolated ion silica gel column chromatography and successively purified by preparative HPLC with 100% EtOH elution to give two active fractions. Among the two, fraction 2 showed stronger influence on antifeedant activity in no-choice leaf dip test by giving 80.0 plus or minus 1.07 and 87.9 plus or minus 2.05% feeding reduction against Spodoptera exigua at 17.70 and 44.25 micro g/sq cm, respectively. Against Spodoptera litura, at 15.00 and 30.00 micro g/sq cm, fraction 2 could reduce feeding activity by 80.14 and 83.12%, respectively. Results informed that Spodoptera exigua was more susceptible to the active compound than Spodoptera litura. On the basis of IR, GC-MS, sup1H NMR and sup13C NMR spectral data, the active compound responsible for the insect activity was identified as stemofoline. In addition, it was found that the purified fraction 1, which was effective in causing larvae mortality even less toxocity than compound stemofoline, had EI-mass spectrum identical with stemofoline. Consequently, fraction 1 was suggested to be an isomer of stemofoline. These findings indicated that Stemona burkilli root extract could be applicable as an alternative agent for controlling larvae of Spodoptera exigua, as well as larvae of Spodoptera litura.