ABSTRACT The objective of this research is to assess the effect of Stemona collinsae extracts on reducing the major cabbage insect pest, the diamond back moth (Plutella xylostella). Stemona collinsae was collected from Phitsanulok province. The root were sequentially extracted with hexane, dichloromethane and methanol. These extracts were tested against the third instar P. xylostella by leaf dipping method. The highest insecticidal activity was observed from dichloromethane extract with the LC
of 0.71%. Following isolation and identification, the major active compound responsible for the insecticidal activity was suggested to be hydroxystemofoline from a molecular peak at m/z 403.2005 [M]
, calcd for C
22
H
29
O
N 403.1995 in the HR-MS spectrum. This compound can be used as a marker for standardization of stemona extracts and formulations for alternative agricultural production systems.