Abstract

Nine species of local Thai medicinal plant extracts with known insecticidal properties that is,Acorus calamus, Eugenia caryophyllus, Mammea siamensis and 6 species of Stemona(Stemona curtisii, Stemona tuberosa, Stemona burkillii, Stemona kerrii, Stemona unknown 1 and Stemona unknown 2) were screened for the highest insecticidal activity by the brine shrimp lethality test (BST). M. siamensis showed a very strong toxic effect on brine shrimp with the lowest 24 h LC₅₀ value of 0.072 µgml^-1. The purification of its active compound was conducted using chromatographic methods and the BST to select the most effective fraction. The spectroscopic methods were used for the identification of the active compound. Surangin B was finally identified as the active compound. Its insecticidal effectiveness on the 3rd instar larvae of diamondback moth was investigated by topical application and leaf dipping methods in comparison with methomyl. The results indicated that surangin B demonstrated high in both contact and anti-feedant activities than methomyl. According to its side effects on non-target organisms, M. siamensis exhibited no negative impact on earthworm and honeybee. In contrast, it showed a higher toxicity on fish than methomyl. From the results it can be concluded, that M. siamensis might be one of the natural insecticides for the diamondback moth management.

Key words: Plutella xylostella Linn., botanical insecticide, surangin B, bioactive compound.