The chemical basis of host-plant recognition in a specialized bee pollinator.

Many pollinators specialize on a few plants as food sources and rely on flower scents to recognize their hosts. However, the specific compounds mediating this recognition are mostly unknown. We investigated the chemical basis of host location/recognition in the Campanula-specialist bee Chelostoma rapunculi using chemical, electrophysiological, and behavioral approaches. Our findings show that Ca. trachelium flowers emit a weak scent consisting of both widespread and rare (i.e., spiroacetals) volatiles. In electroantennographic analyses, the antennae of bees responded to aliphatics, terpenes, aromatics, and spiroacetals; however, the bioassays revealed a more complex response picture. Spiroacetals attracted host-naive bees, whereas spiroacetals together with aliphatics and terpenes were used for host finding by host-experienced bees. On the intrafloral level, different flower parts of Ca. trachelium showed differences in the absolute and relative amounts of scent, including spiroacetals. Scent from pollen-presenting flower parts elicited more feeding responses in host-naive bees as compared to a scentless control, whereas host-experienced bees responded more to the nectar-presenting parts. Our study demonstrates the occurrence of learning (i.e., change in the bee's innate chemical search-image) after bees gain foraging experience on host flowers. We conclude that highly specific floral volatiles play a key role in host-flower recognition by this pollen-specialist bee, and discuss our findings into the broader context of host-recognition in oligolectic bees.

Experience-dependent plasticity in the mushroom bodies of the solitary bee Osmia lignaria (Megachilidae).

All members of the solitary bee species Osmia lignaria (the orchard bee) forage upon emergence from their natal nest cell. Conversely, in the honey bee, days-to-weeks of socially regulated behavioral development precede the onset of foraging. The social honey bee's behavioral transition to foraging is accompanied by neuroanatomical changes in the mushroom bodies, a region of the insect brain implicated in learning. If these changes were general adaptations to foraging, they should also occur in the solitary orchard bee. Using unbiased stereological methods, we estimated the volume of the major compartments of the mushroom bodies, the neuropil and Kenyon cell body region, in adult orchard bees. We compared the mushroom bodies of recently emerged bees with mature bees that had extensive foraging experience. To separate effects of general maturation from field foraging, some orchard bees were confined to a cage indoors. The mushroom body neuropil of experienced field foragers was significantly greater than that of both recently emerged and mature caged orchard bees, suggesting that, like the honey bee, this increase is driven by outdoor foraging experience. Unlike the honey bee, where increases in the ratio of neuropil to Kenyon cell region occur in the worker after emerging from the hive cell, the orchard bee emerged from the natal nest cell with a ratio that did not change with maturation and was comparable to honey-bee foragers. These results suggest that a common developmental endpoint may be reached via different development paths in social and solitary species of foraging bees.

Behavioral foraging responses by the butterfly Heliconius melpomene to Lantana camara floral scent.

Floral color has been shown to influence flower selection by butterflies, but few studies have investigated the role of floral scent. In this study, adults of Heliconius melpomene L. (Lepidoptera: Nymphalidae: Heliconiinae) were tested in two-choice bioassays to investigate their ability to distinguish floral scent of the butterfly pollinated plant Lantana camara L. (Verbenaceae) from other plant scents. The relative importance of floral scent vs. color was also studied. Butterfly foraging behavior was measured as probing with proboscis. This probing, on floral models varying in scent and color, was timed. When given a choice of floral and vegetative scents of L. camara, newly emerged butterflies preferred floral scent, indicating an innate response to floral scents. When butterflies were conditioned to L. camara floral scent by offering the scent with yellow color and sugar water, yellow color elicited stronger feeding responses than did the floral scent. However, the floral scent of L. camara was preferred to that of the novel species Philadelphus coronarius L. (Hydrangiaceae). The floral scent of L. camara was dominated by tepenoid compounds, while that of P. coronarius by fatty acid derivatives, thus demonstrating totally different compositions. It is concluded that, while H. melpomene butterflies often use visual floral traits when selecting which flowers to visit, floral scents elicit behavioral responses that initiate and maintain foraging on flowers.

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.