SummaryChemicals 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 by Pieris brassieae L. (Lepidoptera: Pieridae) females. The oviposition stimulant was isolated using medium pressure liquid chromatography, reversed-phase HPLC, ion-pair HPLC and ion exchange chromatography. Using 1H-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.
Headspace composition, collected from intact cabbage plants and cabbage plants infested with either Pieris brassicae L. or P. rapae L. (Lepidoptera: Pieridae) first instar larvae, was determined by GC‐MS. Twenty‐one volatiles were identified in the headspace of intact plants. Twenty‐two volatiles were identified in the headspace of plants infested by P. brassicae larvae, 2 of which, Z‐3‐hexenyl butyrate and Z‐3‐hexenyl isovalerate, were not detected in the headspace of either intact or P. rapae damaged plants. In the headspace of the latter, 21 compounds were identified, all of which were also produced by intact plants. No significant quantitative differences were found between headspace composition of the plants damaged by one or the other caterpillar species. Major differences between intact and caterpillar‐damaged plants in contribution to the headspace profile were revealed for hexyl acetate, Z‐3‐hexenyl acetate, myrcene, sabinene and 1, 8‐cineole. The larval endoparasitoid Cotesia glomerata L. was attracted by the volatiles emanating from B. oleracea damaged by P. brassicae first instar larvae. C. rubecula L., a specialized larval endoparasitoids of P. rapae, was attracted by the volatiles released from the B. oleracea ‐ P. rapae plant‐host complex. This shows that cabbage plants kept under the conditions of headspace collection produce attractive volatiles for both parasitoids.
Pieris brassicae L. butterflies secrete miriamides onto their eggs. These avenanthramide alkaloids are strong oviposition deterrents when sprayed onto a cabbage leaf. However, these compounds could not be detected in cabbage leaves from which egg batches had been removed two days after deposition and that still showed oviposition deterrency. It was concluded that the miriamides were not directly responsible for the avoidance by females of occupied leaves while searching for an oviposition site. Evidence was obtained that cabbage leaves themselves produce oviposition deterrents in response to egg batches. Fractions containing potent oviposition deterrents could be isolated from surface extracts of leaves from which previously laid egg batches had been removed. The term host marking pheromone that was used previously is not applicable in this case.
The large white butterfly, Pieris brassicae L., a herbivorous pest of crucifers, produces egg-associated chemical markers that inhibit its oviposition. The identification of the marker compounds is reported herein. Separation by means of reversed-phase hplc demonstrated the presence of three active substances, which were identified as tra»j-2- [3-(3,4,5- and trans-2-[3-(3,4dihydraxyphenylpropenoyl)amino]-3,5-dihydroxybenzoic acid [31, using mass and nmr spectroscopy and chemical synthesis. This group of compounds has not been reported from the animal kingdom before. The same compounds are produced by two related Pieris species. This is the first report of taxon-specific compounds affecting butterfly oviposition behavior. The availability, stability, and inhibitory action on colonization of cabbage plants by butterflies make application of these compounds in the protection of cabbage crops feasible and comparable with other environmentally safe crop protection strategies.Females of several herbivorous insect species are known to deposit a marking substance on or near the eggs (1-3). This substance signals to conspecific females (and also to herself if she happens to visit the same site again) that the site is already occupied. This phenomenon constitutes an important element in foraging strategies of herbivorous insects, because it prompts an even distribution of eggs over the available food resources, results in reduction of intraspecific competition, and improves resource exploitation. Because of their important ecological function these host-marking pheromones (HMPs), formerly often labeled as oviposition-deterring pheromones (ODPs), attract much attention. Egg-associated substances also affect related herbivorous species and natural enemies of the herbivores (4-7). Oviposition deterring activity has also been found in feces from larvae (8-10). A more detailed analysis of the ecological role of an HMP requires its chemical identification. Thus far, only a few attempts to identify eggassociated HMPs have been successful (11-13), with a notable example concerning the cherry fruit fly, Rhagoletis cerasi (11).Within the Lepidoptera, several potential uses of HMPs have been reported (14,15). The large white butterfly, Pieris brassicae L., a specialized herbivore of cabbage (Brassica oleráceo) and other cruciferous plants, has been studied in detail (14,16,17). Oviposition by P. brassicae is inhibited when a potential host plant carries conspecific eggs or is sprayed in a methanolic egg wash (14,16). Inhibition of oviposition is especially pronounced when females have a choice between HMP-treated plants and control plants, or when dispersal activity can be manifested (18). Based on a two-choice bioassay, the inhibition of oviposition by egg-associated compounds was quantified (17). Herein the identification and synthesis of HMPs isolated from eggs of P. brassicae is presented. RESULTS AND DISCUSSIONEggs ofP. brassicae, freshly laid on cabbage leaves, Brassica olerácea L. var. gemmifera
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