Stemona collinsae fractions on Plutell xylostella
Chromatographic separation of the dichloromethane extract (10 g) (853 g eq to stemona collinsae) on silica gel column yielded 4 fractions (F1-F4) (Figure 2). Bioassays of these fractions showed that F4 was the most toxic (76.67% mortality) at 1.5% concentration, while F1-F3 at the same concentration gave 16.67, 36.67 and 26.67% mortality, respectively. Fractionation of F4 yielded F4.1-F4.5, of which F4.5 at 1.5% concentration resulted in 78.89% larval mortality, whereas F4.1-F4.4 were comparatively less effective (Figure 3). Further separation of F4.5 by column chromatography gave 3 fractions (F4.5.1-F4.5.3). Of the three fractions isolated from F4.5, F4.5.2 showed higher mortality (85%) at 1.5% concentration, the remaining 2 fractions all showed mortality activity. This fraction was further purified by preparative thin-layer chromatography (PTLC), using 20% ethyl acetate in methanol as the developing solvent to give a purified compound (R
=0.65). This compound responsible for the insecticidal activity was suggested to be hydroxystemofoline from a molecular peak at m/z 403.2005 [M]
f
, calcd for C
22
H
29
O
6
+
N 403.1995 in the HR-MS spectrum. The isolated compound was possibly more toxic than two stemofolinetype alkaloids (i)16,17-didehydro-16(E)-stemofoline and (ii) stemofoline obtained from S.collinsae (Jiwajinda et al., 2001). These findings indicated that Stemona collinsae root extracts could be applicable as an alternative agent for controlling of P. xylostella.