Antifeedant and growth inhibitory effects of the limonoid toosendanin and Melia toosendan extracts on the variegated cutworm, Peridromasaucia (Lep., Noctuidae)

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Abstract
Antifeedant and growth inhibitory effects of toosendanin, a limonoid allelochemical from the bark of the trees Melia toosendan and M. azedarach, were determined for the variegated cutworm, Peridroma saucia, using different bioassays. Toosendanin significantly deterred feeding of 2nd and 4th instar larvae in diet choice and leaf disc choice bioassays, respectively. Refined bark extracts containing 60–75% toosendanin were less deterrent than the pure compound in both bioassays. However, the 60% extract significantly inhibited growth of neonate larvae in a dose‐dependent manner when incorporated into an artificial diet, with an EC50 (concentration reducing growth by 50% relative to controls) of 42.3 ppm at 7 days. Extracts containing 60% and 75% toosendanin were more inhibitory than equivalent amounts of pure toosendanin in this chronic growth bioassay, suggesting the presence of other minor constituents which must be significantly more inhibitory, though less deterrent to feeding, than toosendanin itself. Nutritional analyses of 4th instar larvae following both oral and topical administration of toosendanin or the 75% extract confirm that both materials possess a combination of antifeedant and growth inhibitory properties.