Laboratory and field evaluations of rhodojaponin-III against the imported cabbage worm Pieris rapae (L.) (Lepidoptera: Pieridae).

The activity of rhodojaponin-III (R-III), a grayanoid diterpene compound isolated from Rhododendron molle G. Don flowers, was determined under laboratory and field conditions as an antifeedant, stomach poison, contact toxicant and insect growth inhibitor against Pieris rapae (L.) larvae. The median antifeedant concentration (AFC(50)) values in no-choice leaf disc tests were 1.16 and 15.85 mg L(-1) at 24 h after treatment when tested against third and fifth instars respectively. The median lethal concentration (LC(50)) values in leaf disc tests were 2.84 and 9.53 mg L(-1) at 96 h after treatment against third and fifth instars respectively. R-III showed an almost 30 times higher contact toxicity against third instars than for fifth instars, and the median lethal dose (LD(50)) values for topical application were 1.18 and 34.09 mg kg(-1) at 72 h after treatment respectively. R-III disrupted the development of larvae to pupae or adults with median concentration for inhibiting growth (IC(50)) values of only 1.36 mg L(-1) for third instars and 11.28 mg L(-1) for fifth instars. In field trials, a greater than 80% reduction in the adjusted larval numbers was obtained against P. rapae 14 days after treatment when Rhodo 0.1% EC, a commercial botanical insecticide based on R-III, was applied at both 937.5 and 625 mL ha(-1). These results suggest that further research to develop R-III, and extracts from R. molle, as biorational pesticides or as lead compounds for integrated pest management deserve consideration.