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 gamma-decalactone, beta-ionone, and linalool oxide (LO) isomers (furanosides and pyranosides) as major components together with small amounts of linalool and alpha-ionone. gamma-Decalactone was strongly deterrent in the PER test, and beta-ionone and all LO isomers were weakly deterrent, while the other compounds exhibited no deterrency. In the flower-visiting tests, gamma-decalactone showed notable repellency, whereas beta-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, gamma-decalactone, and beta-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.