Individually recognizable scent marks on flowers made by a solitary bee

Volume and density of microglomeruli in the honey bee mushroom bodies do not predict performance on a foraging task

The mushroom bodies (MBs) are insect brain regions important for sensory integration, learning, and memory. In adult worker honey bees (Apis mellifera), the volume of neuropil associated with the MBs is larger in experienced foragers compared with hive bees and less experienced foragers. In addition, the characteristic synaptic structures of the calycal neuropils, the microglomeruli, are larger but present at lower density in 35-day-old foragers relative to 1-day-old workers. Age- and experience-based changes in plasticity of the MBs are assumed to support performance of challenging tasks, but the behavioral consequences of brain plasticity in insects are rarely examined. In this study, foragers were recruited from a field hive to a patch comprising two colors of otherwise identical artificial flowers. Flowers of one color contained a sucrose reward mimicking nectar; flowers of the second were empty. Task difficulty was adjusted by changing flower colors according to the principle of honey bee color vision space. Microglomerular volume and density in the lip (olfactory inputs) and collar (visual inputs) compartments of the MB calyces were analyzed using anti-synapsin I immunolabeling and laser scanning confocal microscopy. Foragers displayed significant variation in microglomerular volume and density, but no correlation was found between these synaptic attributes and foraging performance. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1057-1071, 2017.

Bumblebees can discriminate between scent-marks deposited by conspecifics

Bumblebees secrete a substance from their tarsi wherever they land, which can be detected by conspecifics. These secretions are referred to as scent-marks, which bumblebees are able to use as social cues. Although it has been found that bumblebees can detect and associate scent-marks with rewarding or unrewarding flowers, their ability at discriminating between scent-marks from bumblebees of differing relatedness is unknown. We performed three separate experiments with bumblebees (Bombus terrestris), where they were repeatedly exposed to rewarding and unrewarding artificial flowers simultaneously. Each flower type carried scent-marks from conspecifics of differing relatedness or were unmarked. We found that bumblebees are able to distinguish between 1. Unmarked flowers and flowers that they themselves had scent-marked, 2. Flowers scent-marked by themselves and flowers scent-marked by others in their nest (nestmates), and 3. Flowers scent-marked by their nestmates and flowers scent-marked by non-nestmates. The bumblebees found it more difficult to discriminate between each of the flower types when both flower types were scent-marked. Our findings show that bumblebees have the ability to discriminate between scent-marks of conspecifics, which are potentially very similar in their chemical composition, and they can use this ability to improve their foraging success.

Reconstructing the pollinator community and predicting seed set from hydrocarbon footprints on flowers

The measurement of insect visits to flowers is essential in basic and applied pollination ecology studies but often fraught with difficulty. Floral visitation is highly variable, and observational studies are limited in scope due to the considerable time necessary to acquire reliable data. The aim of our study was to investigate whether the analysis of hydrocarbon residues (footprints) deposited by insects during flower visits would allow reconstruction of the visitor community and prediction of seed set for large numbers of plants. In 3 consecutive years, we recorded bumblebee visitation to wild plants of comfrey, Symphytum officinale, and later used gas chromatography/mass spectrometry (GC/MS) to quantify bumblebee-derived unsaturated hydrocarbons (UHCs) extracted from flowers. We found that the UHCs washed from corollas were most similar to the tarsal UHC profile of the most abundant bumblebee species, Bombus pascuorum, in all 3 years. The species composition of the bumblebee communities estimated from UHCs on flowers were also similar to those actually observed. There was a significant positive correlation between the observed number of visits by each of three bumblebee species (contributing 3-68% of flower visits) and the estimated number of visits based on UHC profiles. Furthermore, significant correlations were obtained separately for workers and drones of two of the study species. Seed set of comfrey plants was positively correlated to overall bumblebee visitation and the total amount of UHCs on flowers, suggesting the potential for pollen limitation. We suggest that quantifying cumulative footprint hydrocarbons provides a novel way to assess floral visitation by insects and can be used to predict seed set in pollen-limited plants.

Hydrocarbon footprints as a record of bumblebee flower visitation

Bumblebees leave traces of cuticular hydrocarbons on flowers they visit, with the amount deposited being positively related to the number of visits. We asked whether such footprint hydrocarbons are retained on flowers for sufficiently long periods of time so as to reflect bee visitation in pollination studies. In laboratory experiments, flower corollae (Primula veris, Digitalis grandiflora) visited by Bombus terrestris workers retained bee-derived nonacosenes (C(29)H(58)) in near-unchanged quantities for 24 hours, both at 15 and 25 degrees C. Additionally, synthetic (Z)-9-tricosene applied to flower corollae of the deadnettle Lamium maculatum was retained for 48 hours in an unchanged quantity. In a field survey, the amount of footprint alkenes on flowers of comfrey (Symphytum officinale) plants was positively correlated with the number of bumblebee visits that those plants had received during the day. Together, these data suggest that flowers retain a long-term quantitative record of bumblebee visitation. The analysis of petal extracts by gas chromatography could provide a cheap and reliable way of quantifying bumblebee visits in landscape scale studies of pollination.

Contrasting responses of bumble bees to feeding conspecifics on their familiar and unfamiliar flowers

Animals exploiting their familiar food items often avoid spatio-temporal aggregation with others by avoiding scents, less rewarding areas or visual contacts, thereby minimizing competition or interference when resources are replenished slowly in patches. When animals are searching or assessing available food sources, however, they may benefit from reducing sampling costs by following others at food sites. Therefore, animals may adjust their responses to others depending on their familiarity with foraging situations. Here, we conducted field experiments to test whether nectar-collecting bumble bees make this adjustment. We allowed free-foraging bees to choose between two inflorescences, one occupied by a conspecific bee and another unoccupied. When bees were presented with flowers of a familiar type, they avoided occupied inflorescences. In contrast, bees visited an occupied inflorescence when the flower type was unfamiliar. To our knowledge, this is the first report suggesting that animals adjust their responses to feeding conspecifics depending on their familiarity with food sources. Such behavioural flexibilities should allow foragers to both explore and exploit their environments efficiently.

The use of heterospecific scent marks by the sweat bee Halictus aerarius

To forage effectively amongst flowers, some bee species utilize olfactory cues left by previous visitors in addition to direct assessment of visual cues to identify rewarding flowers. This ability can be more advantageous if the bees can recognize and use scent marks left by heterospecifics, not just marks left by members of their own species. We conducted field experiments to investigate whether the sweat bee Halictus aerarius avoids visiting flowers of trailing water willow Justicia procumbens emptied by other bee species. We found that H. aerarius rejected the flowers visited by both heterospecifics and conspecifics. They also rejected visited flowers artificially replenished with nectar. Our results demonstrate that social bees outside the Apidae can detect marks left on flowers by heterospecifics but that (on this plant species) they are unable to discriminate against flowers by directly detecting nectar volume. H. aerarius exhibited different rejection rates according to the identity of the previous bee species. We suggest that the frequency of rejection responses may depend on the amount of chemical substances left by the previous bee. In general, the use of scent marks left by previous visitors is almost certainly advantageous, enabling foragers to avoid flowers with depleted nectar levels and thereby improving their foraging efficiency.

Tracing pollinator footprints on natural flowers

Many insects are known to leave lipid footprints while walking on smooth surfaces. Presumably, the deposited substances improve tarsal adhesion. In bumblebees, footprint hydrocarbons also function as scent marks that allow detection and avoidance of recently depleted flowers. I used GC-MS to detect hydrocarbons deposited by bumblebee (Bombus pascuorum) on flowers of Lamium maculatum. In addition to the plants' own cuticular lipids, extracts of corollas that had been visited by bumblebees contained odd-numbered alkenes. The amount of pentacosenes (C25H50) on corollas was linearly related to the number of bumblebee visits, with workers depositing approximately 16 ng per visit (extrapolated to a total of 65 ng of bumblebee cuticular hydrocarbons). Pentacosenes were retained on visited flowers without loss for 2 hr, and probably longer. This and results from flight cage experiments suggest that flower epicuticles retain a chemical record of pollinator visitation, including information on visiting bee species. Continuous footprint accumulation necessitates new explanations concerning the reversibility of "repellent scent marks" of bumblebees.