OVIPOSITION STIMULANTS IN BARBAREA-VULGARIS FOR PIERIS-RAPAE AND P-NAPI-OLERACEA - ISOLATION, IDENTIFICATION AND DIFFERENTIAL ACTIVITY

The closely related butterflies, Pieris rapae and P. napi oleracea, readily laid eggs on Barbarea vulgaris in greenhouse cages. When offered a choice between cabbage and B. vulgaris, P. rapae showed no preference, but P. napi oleracea preferred B. vulgaris. Bioassays of extracts of B. vulgaris foliage revealed the presence of oviposition deterrent(s) in 1-butanol extracts as well as stimulants in the postbutanol water extracts. However, the deterrent effect was apparently outweighed by the strong stimulatory effect in the whole plants. The postbutanol water extract was preferred over an equivalent cabbage extract by both species, but more significantly in the case of P. napi oleracea. The stimulants were isolated by open column chromatography and HPLC, and the activity was associated with three glucosinolates. P. napi oleracea was more sensitive than P. rapae to the natural concentration of compounds 1 and 3, whereas both species were strongly stimulated to oviposit by natural concentrations of compound 2. Compounds 1 and 2 were identified as (2R)-glucobarbarin and (2S)-glucobarbarin, respectively, and 3 was identified as glucobrassicin, on the basis of their UV, mass, and NMR spectra. When the pure compounds were tested at the same concentrations applied to bean plants, the (2R)-glucobarbarin at 0.2 mg/plant was preferred over a standard cabbage extract by both butterfly species. However, at a dose of 0.02 mg/plant, P. rapae preferred the cabbage extract whereas P. napi oleracea still preferred the (2R)-glucobarbarin. No such difference in response of the two species to the same two concentrations of (2S)-glucobarbarin was obtained. The results indicate a distinct difference in sensitivity of these butterflies to the epimers of glucobarbarin, and the differences in behavioral responses of the two butterfly species depend to a large extent on the concentration of stimulant present.