Antennal responses to floral scents in the butterfly Heliconius melpomene

Floral scent, together with visual floral cues, are important signals to adult butterflies searching for food-rewarding plants. To identify which compounds in a floral scent are more attractive and, thus, of biological importance to foraging butterflies, we applied electrophysiological methods. Antennal responses of male and female adults of the tropical butterfly Heliconius melpomene L. (Lepidoptera: Nymphalidae: Heliconiinae) to individual compounds of natural floral scents and synthetic floral scent mixtures were investigated using gas chromatography-electroantennographic detection (GC-EAD). The natural floral scents included those of two tropical plant species, Lantana camara L. (Verbenaceae) and Warszewiczia coccinea (Vahl) Kl. (Rubiaceae), and two temperate species, Buddleja davidii Franchet (Loganiaceae) and Cirsium arvense (L.) Scop. (Asteraceae). The two synthetic floral scent mixtures contained many of the compounds found in the natural scents, but all in equal quantities. Compounds both present in relatively high abundance in the floral scents and detected exclusively in the floral parts of the plant, such as linalool, linalool oxide (furanoid) I and II, oxoisophoroneoxide, and phenylacetaldehyde, elicited the strongest antennal responses, suggesting that they may reflect adaptations by the plant to attract butterfly pollinators. However, other compounds also present in high abundance in the floral scent, but detected in the vegetative as well as floral plant parts, either elicited strong antennal responses, such as trans-beta-ocimene and benzaldehyde, or failed to elicit antennal responses, such as the sesquiterpenes beta-caryophyllene and alpha-humulene from L. camara. The most volatile monoterpene alkenes in the synthetic scent mixtures elicited only low or no responses. Furthermore, the overall antennal responses were stronger in females than in males. The findings suggest that several floral scent volatiles, especially those of exclusively floral origin, are of high biological significance to H. melpomene butterflies. These include compounds of different biosynthetic origins belonging to the benzenoids, monoterpenoids, and irregular terpenoids.