Occurrence of a feeding deterrent in Barbarea vulgaris (Brassicales : Brassicaceae), a crucifer unacceptable to the diamondback moth, Plutella xylostella (Lepidoptera : Plutellidae)

Larvae of the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutel lidae), a crucifer specialist, refuse to feed on the wintercress, Barbarea vulgaris R. Br. (Brassicaceae). The mechanism of this unacceptability to P xylostella was evaluated by a series of no-choice bioassays using leaf disk s. Topical application of sinigrin enhanced feeding of the larvae on the le af disks of a non-host plant, lettuce (Lactuca sativa L.), but not on those of B. vulgaris. Application of the crude methanolic extract from B. vulgar is leaves deterred P xylostella feeding on the leaf disks of cabbage (Brass ica oleracea L.) in a dose-dependent manner. This feeding deterrent activit y was transferred predominantly to the chloroform fraction, but not to the n-hexane or water fractions. These results suggest that the unacceptability of B. vulgaris to P xylostella larvae is mainly due to chemical constituen t(s) extractable with chloroform.