Leaf surface compound fromBrassica oleracea (Cruciferae) induces oviposition byPieris brassicae (Lepidoptera: Pieridae)

Summary Chemicals present on the surface of cabbage (Brassica oleracea L.) leaves were extracted by dipping these leaves for 3 s in dichloromethane followed by a 3 s dip in methanol. When offered in dual choice bioassays using green paper cards as a substrate, the methanol extract stimulated oviposition activity byPieris brassicae L. (Lepidoptera: Pieridae) females. The oviposition stimulant was isolated using medium pressure liquid chromatography, reversed-phase HPLC, ion-pair HPLC and ion exchange chromatography. Using¹H-NMR spectroscopy, the stimulant could be identified as glucobrassicin (3-indolyl-methyl-glucosinolate). When pure glucobrassicin was offered at a dose identical to that in the crude methanol extract, butterflies did not discriminate between these two substrates in a dual choice test. It is argued that a high sensitivity for indole glucosinolates as host recognition factors may confer an adaptive value for these specialist crucifer feeders. The nutritional significance of their precursor tryptophan and the non-volatile nature of the aglycones formed upon enzymic hydrolysis in damaged tissues are proposed as properties of indole glucosinolates that contribute to this possible adaptive advantage.     

Oviposition stimulants and deterrents control acceptance ofAlliaria petiolata byPieris rapae andP. napi oleracea

Summary Differential acceptance of garlic mustard,Alliaria petiolata byPieris rapae L. andP. napi oleracea is explained by their differential sensitivities to oviposition stimulants and deterrents in the plant. Fractions containing the stimulants and deterrents were isolated by solvent partitioning between water and n-butanol and by open-column chromatography followed by HPLC.P. napi oleracea showed no preference when offered a choice ofA. petiolata or cabbage, but was strongly stimulated to oviposit by post-butanol water extracts ofA. petiolata. The most abundant glucosinolate in this extract was identified as sinigrin, which could explain the high degree of stimulatory activity.P. rapae preferred cabbage plants overA. petiolata, and the relatively low stimulatory activity was also associated with the glucosinolate-containing aqueous extract. However, this species was strongly stimulated by a fraction that contained small amounts of glucotropaeolin along with unknown compounds. Deterrents to both species were found in the butanol extract fromA. petiolata, andP. napi oleracea was more sensitive thanP. rapae to these deterrents. Some HPLC fractions from the BuOH extract were strongly deterrent toP. napi oleracea, but were inactive toP. rapae. The ecological significance of these behavioral differences between the twoPieris species is discussed.     

Secondary metabolites of the leaf surface affected by sulphur fertilisation and perceived by the cabbage root fly

Summary. Surrogate leaves treated with methanolic leaf surface extracts of Brassica napus L. (cv Express) plants that received three different sulphur fertilisation treatments showed even more marked differences by the oviposition choice of Delia radicum L. than the potted plants. This confirms that the oviposition preference of D. radicum is mediated by chemical compounds on the leaf surface and that the quality of host-plants in terms of their nutrition status can be perceived by the female insect.The oviposition data were positively correlated with the content of fractionated surface extracts containing either CIF (cabbage identification factor; 1,2-dehydro-3-thia- 4,10,10b-triaza-cyclopenta[.a.]fluorine-1-carboxylic acid) or glucosinolates. Electrophysiological recordings from the tarsal chemoreceptor sensilla C₅ and D_3,4 showed that receptor neurons react to glucosinolate- and CIF-fractions. We found that the chemosensory activity of specific glucosinolate- and CIF-receptor neurons corresponded with the respective behavioural activity in the oviposition choice assays. The responses of D. radicum to glucosinolates in the electrophysiological recordings studies corresponded to the observed oviposition preference on plants or artificial leaves characterised with an higher amount of glucosinolates on leave surfaces. The presented data suggested that CIF and glucosinolates are involved in host-plant preference of D. radicum and are perceived by tarsal chemoreceptors.     

Effects of belowground biota on primary and secondary metabolites in Brassica oleracea

Summary. Soil organisms in direct and indirect interaction with plant roots affect aboveground herbivores, likely by inducing different plant responses. We investigated the combined effects of the root-knot nematode Meloidogyne incognita (in direct interaction with roots) and the endogeic earthworm Octolasion tyrtaeum (in indirect interaction with roots) on the performance of Brassica oleracea. Both earthworms and nematodes increased N uptake and shoot biomass of B. oleracea. Earthworm activity mobilized more soil N than litter N, and herbivory by nematodes tended to increase the microbial biomass in soil. Only the structural class of sulphur containing glucosinolates was affected by the soil organisms. Earthworms decreased glucoiberin concentrations in B. oleracea shoots. Glucoraphanin was affected by an interaction between earthworms and nematodes.     

Secondary metabolites of the leaf surface affected by sulphur fertilisation and perceived by the diamondback moth

Summary. Summary. Oilseed rape, Brassica napus L. (cv Express), plants were grown under three different sulphur regimes: sulphur-free (S₀), normal sulphur (S_n, normal field concentration) and a sulphur-rich (S₊, 2 × concentration of S_n). We performed dual choice oviposition assays with the diamondback moth, Plutella xylostella, using real plants and, for the first time with this insect, artificial leaves sprayed with methanolic leaf-surface extracts. The results mirrored those of a separate study of preferences for whole plants. Females laid more eggs on surrogate leaves that were treated with S_n extracts than on S₀ plants, while only a slight, not significant, difference was observed between extracts of normal and sulphur-rich plants. This shows that chemical compounds on the leaf surface mediate the oviposition preference and that the female insect can perceive the quality of the host-plants in terms of their fertilisation status.Since leaf volatiles are known to be oviposition stimulants, we investigated the effects of leaf-surface extracts on insect olfactory responses using electroantennograms (EAGs). In agreement with the behavioural data, we found that extracts of sulphur-treated plants yielded higher EAG amplitudes than the S₀ extracts. Since the leaf content of the volatiles isothiocyanates is influenced by sulphur nutrition, we analysed the extracts for these compounds. Above the detection threshold of our GC-MS system, no isothiocyanates were found. Thus, other compounds present in the surface extracts must be perceived by the antenna.However, the HPLC analysis revealed 11 different glucosinolates. Progoitrin (2-Hydroxy-3-butenyl) and gluconapoleiferin (2-Hydroxy-4-pentenyl), which belong to the hydroxy-alkene class of glucosinolates, were the most abundant compounds. The total glucosinolate content sharply increased from S₀ to S_n plants, whereas it was slightly lower in _n versus S₊ plants. Since it is known that glucosinolates can stimulate oviposition, it seems likely that the increased content we observed was influencing the insect preference in this study too.