Chemical and Chromatic Bases for Preferential Visiting By the Cabbage Butterfly, Pieris rapae, to Rape Flowers

Scent and coloration of corolla were examined as floral attributes responsible for preferential visiting by the cabbage butterfly, Pieris rapae, to rape flower, Brassica rapa. Floral volatile components that release the flower-visiting behavior of the butterfly were identified by chemical analyses, electroantennography (EAG), and two behavioral bioassays: proboscis extension reflex (PER) in response to odor and attraction to artificial flowers. GC and GC-MS analyses of the headspace volatiles from the flowers revealed the presence of six aromatic compounds, benzaldehyde, phenylacetaldehyde, benzyl alcohol, 2-phenylethanol, phenylacetonitrile, and indole in decreasing order of quantity. Of these, phenylacetaldehyde elicited the highest response in the PER assay. While benzyl alcohol, 2-phenylethanol, benzaldehyde, and phenylacetonitrile evoked moderate responses, the PER-eliciting activity of indole was very weak. In two-choice behavioral bioassays, artificial flowers scented with any one of these PER-active compounds attracted significantly more butterflies than control (unscented) flowers, whereas those treated with indole were almost inactive. The EAG activities of the six chemicals were not high and were about the same at a low dose (1 g), but phenylacetaldehyde elicited a much stronger response from both sexes at higher doses (10 and 100 g). An overall profile of EAG responses at a dose of 100 g was analogous to that of PER performance, suggesting that floral volatiles may be involved in close-range location or recognition of flowers rather than long-range attraction. By spectroscopic and UV-photographic examinations of rape flower, the central part of the corolla was found to absorb UV rays in marked contrast to the other parts, which reflected near-UV rays (_max = 350 nm). This indicates that the flower is endowed with a conspicuous nectar guide that is probably an important visual stimulus for attracting foraging adults of P. rapae. Consequently, the present findings strongly suggest that this elaborate pollination strategy of rape flower, characterized by its good combination of olfactory and visual attractiveness, accounts for preferential visiting by the cabbage butterfly to the flower.     

Floral Scent of Osmanthus fragrans Discourages Foraging Behavior of Cabbage Butterfly, Pieris rapae

Field observations revealed that flowers of Osmanthus fragrans attract few insect species despite their strong scent and vivid coloration (yellowish orange). Floral volatiles of the plant were thus examined for suspected repellency to Pieris rapae, a potential visitor that never visits this flower. The influence of odor components on the foraging behavior of adult butterflies was assessed by two behavioral bioassays: proboscis extension reflex (PER) in response to olfactory stimuli and the frequency of visits to artificial flowers. In the PER bioassays, a binary mixture composed of a test material and 2-phenylethanol (a positive standard compound) was used, and the negative effect of test materials was appraised based on the degree of suppression caused compared with the original PER performance evoked by 2-phenylethanol alone. Isopentane extract of the flower exhibited significant deterrency in the PER test and remarkable repellency in the flower-visiting experiment, thereby suggesting the presence of a repellent(s) in the floral volatiles. Fractionation of the extract by silica gel column chromatography yielded an active fraction (based on PER performance), which, upon GC and GC-MS analyses, was found to consist of -decalactone, -ionone, and linalool oxide (LO) isomers (furanosides and pyranosides) as major components together with small amounts of linalool and -ionone. -Decalactone was strongly deterrent in the PER test, and -ionone and all LO isomers were weakly deterrent, while the other compounds exhibited no deterrency. In the flower-visiting tests, -decalactone showed notable repellency, whereas -ionone had no repellent effect. Among the LO isomers, at least furanosides (cis/trans mixture) and cis-pyranoside were found to be weakly repellent. On the other hand, the butterfly showed strong to weak EAG responses to LO furanosides, cis-LO pyranoside, -decalactone, and -ionone in decreasing order of intensity. The present results clearly indicate that the repellency of a compound is correlated with its deterrent effect on PER but not necessarily with antennal sensitivity.     

Isolation and identification of oviposition deterrents to cabbage butterfly,Pieris rapae,fromErysimum cheiranthoides

Avoidance of some crucifer species by the crucifer specialist,Pieris rapae, has been attributed to the presence of oviposition deterrents in these plants. Studies on one such unacceptable plant,Erysimum cheiranthoides, have resulted in the isolation ofn-butanol-soluble deterrents from the alcoholic extract of foliage. The active fraction contained three cardiac glycosides, which were isolated by reversed-phase HPLC and by open column chromatography on silica gel. Chemical and spectral evidence (UV, [¹H]NMR, and FAB-MS) led to the characterization of these compounds as erysimoside (1), erychroside (2), and erycordin (3). Erysimoside and erychroside were strongly deterrent toPieris rapae, but erycordin was inactive. Both active compounds have the same aglycone, strophanthidin (5) and the inner sugar in both cases is a 2,6-dideoxy hexose to which the outer sugar is attached at position C-4. These structural features, which are absent in the inactive compound (3), may represent specific requirements for oviposition deterrent activity.     

Odor of a papilionid butterfly

Benzaldehyde, phenylacetaldehyde (major component), 2-phenylpropenal,n-heptanal, 6-methylhept-5-en-2-one, and linalool were identified as compounds responsible for the male scent ofAtrophaneura alcinous alcinous. These substances were present predominantly in the wings, and the quantity of them was largest at the inner margin of the hind wing. Female wings also contained some of them in much smaller (except a few components) amounts. The relative proportion of each component exhibited manifests sexual differences.     

Identification of Floral Volatiles Involved in Recognition of Oilseed Rape Flowers, Brassica napus by Honeybees, Apis mellifera

Volatiles from oilseed rape, Brassica napus, flowers were sampled by air entrainment and their relevance to the natural odor profile of the flowers was confirmed by conditioned proboscis extension (CPE) assays with honeybee, Apis mellifera L., foragers. Coupled gas chromatography (GC)-CPE analysis of the air entrainment samples was used to locate key compounds involved in the recognition of B. napus flowers, and the compounds were then identified using coupled gas chromatography-mass spectrometry and comparison with authentic samples. Six regions of the gas chromatograms elicited CPE responses from bees previously conditioned to the total extract, and from these areas 16 compounds were identified that elicited CPE activity from conditioned bees when tested with synthetic samples. Eight of the 16, -pinene, phenylacetaldehyde, p-cymene, -terpinene, linalool, 2-phenyl-ethanol, (E,E)--farnesene, and 3-carene, gave the highest responses. When the bees were conditioned to the total extract of flower volatiles, a mixture of the eight components elicited responses from 83% of the individuals, suggesting that the eight-component mixture accounted for a major part of the CPE activity of the total extract. In addition, a mixture of the three most active compounds, phenylacetaldehyde, linalool, and (E,E,)--farnesene, evoked responses from 85% of the bees after the latter had been conditioned to the eight-component mixture. Thus, these three compounds appear to play a key role in the recognition of the eight component mixture and, by inference, of oilseed rape flowers.     

Constituents of mandibular and Dufour's glands of an australianPolyrhachis weaver ant

Worker ants ofPolyrhachis (Cyrtomyrma) ?doddi collectively discharge the secretions of their large mandibular glands when their nest is disturbed. The major glandular compounds of workers are 6-methylhept-5-en-2-one and phenylacetaldehyde oxime. Other components identified are mellein, 6-methylhept-5-en-2-one oxime, phenylacetonitrile, phenyiacetaldehyde, benzaldehyde, and several alkanes and alkenes. The mandibular gland secretions of queens differ from those of workers only quantitatively. Large queens have considerably more of most components, the small queens have much less of volatile components. There is a pronounced sexual difference: the major components of the male's secretions are octanoic acid and mellein, with geranic acid, 8-heptadecene, 2-methylbutanoic acid, and 9-nonadecene present in lesser amounts. Workers ofP. ?doddi also have unusually large Dufour's glands containing a large array of hydrocarbons, of which tridecane is the major component, but α-farnesene, pentadecane, and heptadecene are also present in large quantities.