An arthropod deterrent attracts specialised bees to their host plants

Many bee species are adapted to just a few specific plants in order to collect pollen (oligolecty). To reproduce successfully, it is important for oligolectic bees to find and recognise the specific host flowers. In this study, we investigated the role of floral volatiles used by an oligolectic bee to recognise its host plants. We compared the attractiveness of natural and synthetic scent samples of host flowers to foraging-naïve and -experienced Hoplitis adunca (Megachilidae) bees that are specialised on Echium and Pontechium (Boraginaceae) plants. The investigations showed that naïve H. adunca females are attracted to 1,4-benzoquinone. During their lifetime, bees learn additional floral cues while foraging on host flowers. In contrast to naïve ones, experienced H. adunca females use, in addition to 1,4-benzoquinone, other compounds to recognise their host plants. 1,4-Benzoquinone is an uncommon floral compound only known from the host plants of H. adunca, and is therefore ideally suited to be used as a plant-specific recognition cue. Several arthropods use this compound to deter insect predators. Therefore, 1,4-benzoquinone as an attractant in Echium flowers may have evolved from a primary function as a defensive compound against insect herbivores.     

Echium and Pontechium specific floral cues for host–plant recognition by the oligolectic bee Hoplitis adunca

Flowering plants often have specific floral cues, which allow bees and other pollinators to differentiate between them. Many bee species exhibit specialised associations with flowers (oligolecty) and it is important for them to find and recognise their specific host plants. In this study we compared the visual and olfactory floral cues of different Echium and Pontechium (Boraginaceae) species with the closely related Anchusa officinalis (Boraginaceae). We tested whether plant-specific cues occur in Echium and Pontechium which may allow oligolectic Hoplitis adunca (Megachilidae) to recognise its host plants and to distinguish them from Anchusa non-hosts. Our investigations showed that Echium/Pontechium provides a specific scent bouquet. Furthermore, we identified compounds which were not described as floral scent before ((Z)-3-nonenal and 1,4-benzoquinone). These unique volatiles and the specific bouquet could act as a recognition cue for H. adunca. The corolla colours differed between all species, but were grouped together in the bee colour categories blue and UV-blue and can indicate potential host flowers for H. adunca.     

Perception of floral volatiles involved in host-plant finding behaviour: comparison of a bee specialist and generalist

Specialist and generalist bees use olfactory and visual cues to find and recognise flowering plants. Specialised (oligolectic) bees rely on few host plants for pollen collection. These bee species are suggested to use specific volatiles, but it is unknown whether they have dedicated adaptations for these particular compounds compared to bees not specialised on the same plants. In the present study, we investigated the perception of host odorants and its neuronal substrate with regard to host-plant finding behaviour in oligolectic bees. We reconstructed the antennal lobes (AL) in the Salix specialist, Andrena vaga, and counted about 135 glomeruli and thereby less than the approximately 160 in honeybees. Using calcium imaging experiments to measure neural activity in the bee brain, we recorded odorant-evoked activity patterns in the AL of A. vaga and, for comparison, in the generalist honeybee, Apis mellifera. Our physiological experiments demonstrated that A. vaga bees were particularly sensitive to 1,4-dimethoxybenzene, a behaviour-mediating odorant of Salix host flowers. We found more sensitive glomeruli in the specialised bees as compared to generalist honeybees. This neural adaptation might allow oligolectic A. vaga bees to effectively locate host plants from distances.     

Antennal responses of an oligolectic bee and its cleptoparasite to plant volatiles

Cleptoparasitic or cuckoo bees lay their eggs in nests of other bees, and the parasitic larvae feed the food that had been provided for the host larvae. Nothing is known about the specific signals used by the cuckoo bees for host nest finding, but previous studies have shown that olfactory cues originating from the host bee alone, or the host bee and the larval provision are essential. Here, I compared by using gas chromatography coupled to electroantennographic detection (GC-EAD) the antennal responses of the oligolectic oil-bee Macropis fulvipes and their cleptoparasite, Epeoloides coecutiens, to dynamic headspace scent samples of Lysimachia punctata, a pollen and oil host of Macropis. Both bee species respond to some scent compounds emitted by L. punctata, and two compounds, which were also found in scent samples collected from a Macropis nest entrance, elicited clear signals in the antennae of both species. These compounds may not only play a role for host plant detection by Macropis, but also for host nest detection by Epeoloides. I hypothesise that oligolectic bees and their cleptoparasites use the same compounds for host plant and host nest detection, respectively.Key words: Macropis fulvipes, Epeoloides coecutiens, Lysimachia punctata, oligolectic oil-bee, floral scent, dynamic headspace, GC-EAD, cuckoo bee, host nest findingBees are the most important animal pollinators worldwide, and guarantee sexual reproduction of many plant species.^1,2 This is especially true for female bees, which collect pollen and mostly nectar for their larvae and frequently visit flowers. For finding and detection of suitable flowers, bees are known to use, besides optical cues,^3,4 especially olfactory signals.^5–8 However, c. 20% of bees do not collect pollen for their larvae by their own, but enter nests of host bees and lay eggs into the broodcells.^1,9 The parasitic larvae subsequently feed the food that had been provided for the host larvae. These so called cuckoo or cleptoparasitic bees can be generalistic, indicating that they use species of several other bee groups as host, whereas others can be highly specialized, laying eggs in cells of only few host species.¹ Until now little is known about the cues used by the cuckoo bees for finding host nests. Nevertheless, Cane¹⁰ and Schindler¹¹ demonstrated that parasitic Nomada bees use primarily visual cues of the nest entrance holes for finding possible nests, and olfactory cues for detection of suitable host nests. The chemical cues used by the cleptoparasites originate from the host bee^10,11 and also pollen,¹⁰ the main larval provision. In most bee species, pollen is mixed together with nectar as larval provision, and both floral resources are known to emit volatiles.^12,13 It is unknown, whether cuckoo bees in search for host nests also use volatiles originating from nectar. While the odours of the host bee used as signal by the cleptoparasites, e.g., cuticiular hydrocarbons and glandular secretions, are often species-specific,¹⁴ the chemical cues from the larval provision may just indicate the presence of pollen in the nest without more specifity. As a consequence cuckoo bees could use species-specific host odours to detect nests of a suitable host, and odours released from the larval provision could indicate to them that broodcells are foraged. However, especially those cuckoo bees with oligolectic hosts foraging pollen only on few closely related plant species,¹ may also use the olfactory signals from host broodcell supplies as more specific cue for host nest detection. Thus the same signal from certain flowers may be used for different informations: for the host bee for host plant and for the cuckoo bee for host nest detection.In this concern I tested oligolectic Macropis (Melittidae, Melittinae) and its specific cuckoo bee, Epeoloides (Apidae, Apinae) by using gas chromatography coupled to electroantennographic detection (GC-EAD) on floral scent of Lysimachia (Myrsinaceae). Macropis is highly specialized on Lysimachia, because it is not only collecting pollen from plants of this genus, but also floral oil. Both floral products are the only provision for the larvae.^1,15 Recently, we have shown that the oil bee Macropis is strongly attracted to floral scent of its oil host Lysimachia though the compounds used for host plant finding are still unknown.⁷ Macropis is the only host of Epeoloides, and larvae of this cleptoparasite only feed on the Lysimachia pollen-oil mixture provided for the larvae of Macropis. Worldwide, there are only 2 species of this genus, one in North America and the other in Europe/Asia.^1,16,17 I hypothesized that both bee species respond to specific Lysimachia compounds, which may be used for host plant as well as host nest detection.The measurements with M. fulvipes (F.) and E. coecutiens (F.) antennae demonstrate that both bees, host as well as cuckoo bee, respond to some scent compounds emitted by inflorescences of Lysimachia punctata L. (Fig. 1), a plant being an important pollen and oil source for M. fulvipes. Macropis responded to much more Lysimachia compounds compared to the cuckoo bee, however, two compounds elicited clear signals in the antennae of both bee species: the benzenoid 1-hydroxy-1-phenyl-2-propanone, and the fatty acid derivative 2-tridecanone. Interestingly, both compounds are also emitted from the floral oil of this plant,⁷ and both compounds were also detected in scent samples collected by dynamic headspace in the entrance of a Macropis nest (Dötterl, unpublished data). Therefore, an Epeoloides female being in search for a host nest can detect volatiles emitted from the provision of the host bee at the entrance of a bee nest, and may use these specific compounds for detection of a Macropis nest provisioned with Lysimachia pollen and oil.Open in a separate windowFigure 1Coupled gas chromatographic and electroantennographic detection of a Lysimachia punctata headspace scent sample using antennae of a female oligolectic Macropis fulvipes and a female cleptoparasitic Epeoloides coecutiens bee. (1) 1-hydroxy-1-phenyl-2-propanone, (2) 2-tridecanone.Present results show that an oligolectic oil-bee as well as its cleptoparasite detects volatiles originating from the host plant of the pollen collecting bee, and that oligolectic bees as well as their cuckoo bees may use the same specific signals for host plant and host nest finding, respectively. Biotests are now needed to test this hypothesis.     

Characterization of Salix nigra floral insect community and activity of three native Andrena bees

Salix nigra (black willow) is a widespread tree that hosts many species of polylectic hymenopterans and oligolectic bees of the genus Andrena. The early flowering of S. nigra makes it an important nutritive resource for arthropods emerging from hibernation. However, since S. nigra is dioecious, not all insect visits will lead to successful pollination. Using both visual observation and pan‐trapping, we characterized the community of arthropods that visited S. nigra flowers and assessed differences among male and female trees as well as the chemical and visual drivers that influenced community composition across 3 years. We found that male trees consistently supported higher diversity of insects than female trees and only three insect species, all Andrena spp., consistently visited both sexes. Additionally, Andrena nigrae, which was the only insect that occurred more on female than male flowers, correlated strongly to volatile cues. This suggests that cross‐pollinators cue into specific aspects of floral scent, but diversity of floral visitors is driven strongly by visual cues of yellow male pollen. Through time, the floral activity of two Andrena species remained stable, but A. nigrae visited less in 2017 when flowers bloomed earlier than other years. When native bee emergence does not synchronize with bloom, activity appears to be diminished which could threaten species that subsist on a single host. Despite the community diversity of S. nigra flowers, its productivity depends on a small fraction of species that are not threatened by competition, but rather rapidly changing conditions that lead to host‐insect asynchrony.     

Floral and vegetative cues in oil-secreting and non-oil-secreting Lysimachia species

Background and Aims

Unrelated plants pollinated by the same group or guild of animals typically evolve similar floral cues due to pollinator-mediated selection. Related plant species, however, may possess similar cues either as a result of pollinator-mediated selection or as a result of sharing a common ancestor that possessed the same cues or traits. In this study, visual and olfactory floral cues in Lysimachia species exhibiting different pollination strategies were analysed and compared, and the importance of pollinators and phylogeny on the evolution of these floral cues was determined. For comparison, cues of vegetative material were examined where pollinator selection would not be expected.

Methods

Floral and vegetative scents and colours in floral oil- and non-floral oil-secreting Lysimachia species were studied by chemical and spectrophotometric analyses, respectively, compared between oil- and non-oil-secreting species, and analysed by phylogenetically controlled methods.

Key Results

Vegetative and floral scent was species specific, and variability in floral but not vegetative scent was lower in oil compared with non-oil species. Overall, oil species did not differ in their floral or vegetative scent from non-oil species. However, a correlation was found between oil secretion and six floral scent constituents specific to oil species, whereas the presence of four other floral compounds can be explained by phylogeny. Four of the five analysed oil species had bee-green flowers and the pattern of occurrence of this colour correlated with oil secretion. Non-oil species had different floral colours. The colour of leaves was similar among all species studied.

Conclusions

Evidence was found for correlated evolution between secretion of floral oils and floral but not vegetative visual and olfactory cues. The cues correlating with oil secretion were probably selected by Macropis bees, the specialized pollinators of oil-secreting Lysimachia species, and may have evolved in order to attract these bees.     

Specialist <Emphasis Type="Italic">Osmia</Emphasis> bees forage indiscriminately among hybridizing <Emphasis Type="Italic">Balsamorhiza</Emphasis> floral hosts

Pollinators, even floral generalists (=polyleges), typically specialize during individual foraging bouts, infrequently switching between floral hosts. Such transient floral constancy restricts pollen flow, and thereby gene flow, to conspecific flowers in mixed plant communities. Where incipient flowering species meet, however, weak cross-fertility and often similar floral traits can yield mixed reproductive outcomes among pollinator-dependent species. In these cases, floral constancy by polyleges sometimes serves as an ethological mating barrier. More often, their foraging infidelities instead facilitate host introgression and hybridization. Many other bee species are oligolectic (taxonomic specialists for pollen). Oligoleges could be more discriminating connoisseurs than polyleges when foraging among their limited set of related floral hosts. If true, greater foraging constancy might ensue, contributing to positive assortative mating and disruptive selection, thereby facilitating speciation among their interfertile floral hosts. To test this Connoisseur Hypothesis, nesting females of two species of oligolectic Osmia bees were presented with randomized mixed arrays of flowers of two sympatric species of their pollen host, Balsamorhiza, a genus known for hybridization. In a closely spaced grid, the females of both species preferred the larger flowered B. macrophylla, evidence for discrimination. However, both species’ females showed no floral constancy whatsoever during their individual foraging bouts, switching randomly between species proportional to their floral preference. In a wider spaced array in which the bouquets reflected natural plant spacing, foraging oligolectic bees often transferred pollen surrogates (fluorescent powders) both between conspecific flowers (geitonogamy and xenogamy) and between the two Balsamorhiza species. The Connoisseur Hypothesis was therefore rejected. Foraging infidelity by these oligolectic Osmia bees will contribute to introgression and hybridization where interfertile species of Balsamorhiza meet and flower together. A literature review reveals that other plant genera whose species hybridize also attract numerous oligolectic bees, providing independent opportunities to test the generality of this conclusion.     

Chemical cues involved in the attraction of the oligolectic bee Hoplitis adunca to its host plant Echium vulgare

Host recognition is a key process in oligolectic bees but the mechanisms through which they find and recognize appropriate pollen host plant are not entirely clear. Hoplitis adunca is a monolectic bee collecting pollen only from Echium spp. (Boraginaceae). We aimed to test whether Echium vulgare floral scent plays a major role in the attraction of H. adunca females, and to identify components of E. vulgare scent that may be involved in this specific attraction. We used a combination of behavioral and chemical (GC/GC–MS, PTR-MS) analyses. In order to identify the chemical cues likely to be involved in the specific attraction of H. adunca, we compared the scent of fresh flowers, nectar, pollen, and whole plants of E. vulgare and Anchusa officinalis, another Boraginaceae, which does not attract H. adunca. H. adunca females were attracted to the scent of E. vulgare flowers when offered against a blank or against the scent of A. officinalis flowers. However, H. adunca females were not attracted to the scent of A. officinalis flowers when offered against a blank. The emission spectra of the two plant species differed markedly, as did the emission spectra of various flower components (pollen, nectar and whole flowers) within a species. Pollen presented a low volatile release, but emitted significantly higher amounts of mass 55 (butanal, 1,3-butadiene, or other volatiles of molecular mass 54), and mass 83 (hexanal, hexenols, hexenyl acetate, or other volatiles of molecular mass 82) in E. vulgare than in A. officinalis. Nectar produced a particular emission spectrum with high emission rates of masses 109 and 123. Mass 109 may likely correspond to 1,4-benzoquinone, a volatile specifically measured in E. vulgare in parallel studies to this one. The flower emission spectrum was mainly a combination of the pollen and the nectar scents, although it also contained additional volatile compounds such as those of mass 63 or mass 81. As for terpenes, E. vulgare emitted limonene, longicyclene, junipene, trans-caryophyllene and α-humulene, that were not detected in A. officinalis, and the most emitted monoterpenes were α-pinene, junipene and limonene whereas the most emitted terpenoid by A. officinalis was α-pinene. After identifying these chemical cues, olfactory/behavioural assays with specific volatiles and combinations of volatiles are necessary to understand the chemical interactions of the H. adunca-E. vulgare system.     

Recognition of scent marks in solitary bees to avoid previously visited flowers

Knowing how floral visitors forage efficiently among flowers is important to understanding plant-pollinator interactions. When bees search for rewarding flowers, they use several visual cues to detect the available floral resources. In addition to these cues, bees can recognize scent marks, which are olfactory cues left on flowers foraged by previous visitors. This behavior is well known in social bees, such as honeybees and bumblebees. Although solitary bees do not need to give information about which flowers were foraged to conspecifics, several pieces of evidence have indicated the use of scent marks. However, it is unknown whether the behavior is widely used in many different bee species. We investigated whether four different solitary bees, Colletes patellatus (Colletidae), Andrena prostomias (Andrenidae), Osmia orientalis (Megachilidae), and Tetralonia mitsukurii (Apidae), can recognize flowers that have been foraged previously by visitors within 3 min. All four bees showed rejection responses to flowers foraged by conspecifics. However, our results showed that responses to foraged flowers varied among bee species. The tendency of A. prostomias and T. mitsukurii to reject the foraged flowers was pronounced, while in C. patellatus and O. orientalis it was weak. In both A. prostomias and T. mitsukurii, the rejection rate of flowers foraged by conspecifics decreased as the time lag after the last visit increased. Both bees visited the flowers from which pollen or nectar had been artificially removed. We suggest that A. prostomias and T. mitsukurii would recognize scent marks left by previous visitors, while the other two bees would not recognize them so strongly. It is likely that the decision to use scent marks is dependent either on the richness of resources or on the complexity of floral structure.     

Pollination of Campanula rapunculus L. (Campanulaceae): How much pollen flows into pollination and into reproduction of oligolectic pollinators?

We studied an isolated population of Campanula rapunculus and two oligolectic bee species of Chelostoma (Megachilidae), their main pollinators. The population of C. rapunculus consisted of 2808 plants. Measurements of pollen flow showed that 3.7% of the pollen produced by a flower contribute to pollination, 95.5% was collected by bees for their offspring and 0.8% remained on the styles. Pollen analyses of brood cells of Chelostoma rapunculi revealed that females collected on average 4.9 million Campanula pollen to rear one bee. We calculated that approximately 1588 bees of this species could have been reared at the study site during the studied season. The amount of potentially viable pollen deposited on stigmas was 3.6 to 10.7 times higher than the number of ovules. We discuss morphological features of the flowers which may lower the pollen removal rate per bee visit and consequently cause a high visitation and pollination rate.The study was supported by a joint project CAPES / DAAD (Probral 112/00). We thank NaBu (Naturschutzbund) Bonn and the Untere Landschaftsbehörde Siegburg for the permission to work at the Natural Reserve Dünstekoven.     

Patterns of host-plant choice in bees of the genus Chelostoma: the constraint hypothesis of host-range evolution in bees

To trace the evolution of host-plant choice in bees of the genus Chelostoma (Megachilidae), we assessed the host plants of 35 Palearctic, North American and Indomalayan species by microscopically analyzing the pollen loads of 634 females and reconstructed their phylogenetic history based on four genes and a morphological dataset, applying both parsimony and Bayesian methods. All species except two were found to be strict pollen specialists at the level of plant family or genus. These oligolectic species together exploit the flowers of eight different plant orders that are distributed among all major angiosperm lineages. Based on ancestral state reconstruction, we found that oligolecty is the ancestral state in Chelostoma and that the two pollen generalists evolved from oligolectic ancestors. The distinct pattern of host broadening in these two polylectic species, the highly conserved floral specializations within the different clades, the exploitation of unrelated hosts with a striking floral similarity as well as a recent report on larval performance on nonhost pollen in two Chelostoma species clearly suggest that floral host choice is physiologically or neurologically constrained in bees of the genus Chelostoma. Based on this finding, we propose a new hypothesis on the evolution of host range in bees.     

Intrafloral patterns of color and scent in Capparis spinosa L. and the ghosts of its selection past

Premise

Capparis spinosa is a widespread charismatic plant, in which the nocturnal floral habit contrasts with the high visitation by diurnal bees and the pronounced scarcity of hawkmoths. To resolve this discrepancy and elucidate floral evolution of C. spinosa, we analyzed the intrafloral patterns of visual and olfactory cues in relation to the known sensory biases of the different visitor guilds (bees, butterflies, and hawkmoths).

Methods

We measured the intrafloral variation of scent, reflectance spectra, and colorimetric properties according to three guilds of known visitors of C. spinosa. Additionally, we sampled visitation rates using a motion-activated camera.

Results

Carpenter bees visited the flowers eight times more frequently than nocturnal hawkmoths, at dusk and in the following morning. Yet, the floral headspace of C. spinosa contained a typical sphingophilous scent with high emission rates of certain monoterpenes and amino-acid derived compounds. Visual cues included a special case of multisensory nectar guide and color patterns conspicuous to the visual systems of both hawkmoths and bees.

Conclusions

The intrafloral patterns of sensory stimuli suggest that hawkmoths have exerted strong historical selection on C. spinosa. Our study revealed two interesting paradoxes: (a) the flowers phenotypically biased towards the more inconsistent pollinator; and (b) floral display demands an abundance of resources that seems maladaptive in the habitats of C. spinosa. The transition to a binary pollination system accommodating large bees has not required phenotypic changes, owing to specific eco-physiological adaptations, unrelated to pollination, which make this plant an unusual case in pollination ecology.     

Pollination of <Emphasis Type="Italic">Machaerium opacum</Emphasis> (Fabaceae) by nocturnal and diurnal bees

With plants whose flowers open at night and stay open during the day, nocturnal pollinators may exploit floral resources before diurnal competitors. Moths, bats, and beetles are the most familiar nocturnal pollinators, whereas nocturnal bees as pollinators remain poorly understood. The common Cerrado tree Machaerium opacum (Fabaceae) has white and strongly scented melittophilous flowers, which first open at the night and remain open during the day and, thus, have the potential to be visited by both nocturnal and diurnal bees. We asked: (1) what is the plant’s breeding system? (2) when during the night do the flowers open? (3) what are the visual and olfactory floral cues? and (4) which nocturnal/diurnal bees visit and pollinate the flowers? We show that M. opacum is self-incompatible. Its flowers open synchronously at 03:30 h, produce nectar exclusively at night, and have an explosive mechanism of pollen presentation. The flowers have pure white petals, release strong scents during anthesis, and are pollinated by nocturnal and diurnal bees. We recorded four nocturnal and 17 diurnal species as flower visitors, with females of nocturnal species of Ptiloglossa (Colletidae) being the most abundant. After an initial pollen-releasing visit, only a minor amount of pollen remains in a flower. Several floral traits favor visits by nocturnal bees: (1) night-time flower opening, (2) nectar production at night, (3) almost complete pollen release during the first flower visit, and (4) pure white petals and strong odor production prior to sunrise, facilitating visual and olfactory detection of flowers when light is dim.     

A key role for floral scent in a wasp-pollination system in Eucomis (Hyacinthaceae)

Background and Aims

Floral scent may play a key role as a selective attractant in plants with specialized pollination systems, particularly in cases where floral morphology does not function as a filter of flower visitors. The pollination systems of two African Eucomis species (E. autumnalis and E. comosa) were investigated and a test was made of the importance of scent and visual cues as floral attractants.

Methods and Key Results

Visitor observations showed that E. autumnalis and E. comosa are visited primarily by pompilid wasps belonging to the genus Hemipepsis. These wasps carry considerably more Eucomis pollen and are more active on flowers than other visiting insects. Furthermore, experiments involving virgin flowers showed that these insects are capable of depositing pollen on the stigmas of E. autumnalis, and, in the case of E. comosa, pollen deposited during a single visit is sufficient to result in seed set. Experimental hand-pollinations showed that both species are genetically self-incompatible and thus reliant on pollinators for seed set. Choice experiments conducted in the field and laboratory with E. autumnalis demonstrated that pompilid wasps are attracted to flowers primarily by scent and not visual cues. Measurement of spectral reflectance by flower petals showed that flowers are cryptically coloured and are similar to the background vegetation. Analysis of headspace scent samples using coupled gas chromatography–mass spectrometry revealed that E. autumnalis and E. comosa scents are dominated by aromatic and monoterpene compounds. One hundred and four volatile compounds were identified in the floral scent of E. autumnalis and 83 in the floral scent of E. comosa, of which 57 were common to the scents of both species.

Conclusions

This study showed that E. autumnalis and E. comosa are specialized for pollination by pompilid wasps in the genus Hemipepsis and achieve specialization through cryptic colouring and the use of scent as a selective floral attractant.Key words: Eucomis, Pompilidae, wasp pollination, breeding system, pollination syndrome, pollinator shift, floral volatile, floral filter     

Bumblebees require visual pollen stimuli to initiate and multimodal stimuli to complete a full behavioral sequence in close‐range flower orientation

Flower visits are complex encounters, in which animals are attracted by floral signals, guided toward the site of the first physical contact with a flower, land, and finally take up floral rewards. At close range, signals of stamens and pollen play an important role to facilitate flower handling in bees, yet the pollen stimuli eliciting behavioral responses are poorly known. In this study, we test the response of flower‐naive bumblebees (Bombus terrestris) toward single and multimodal pollen stimuli as compared to natural dandelion pollen. As artificial pollen stimuli, we used the yellow flavonoid pigment quercetin, the scent compound eugenol, the amino acid proline, the monosaccharide glucose, and the texture of pollen‐grain‐sized glass pellets as a tactile stimulus. Three test stimuli, dandelion pollen, one out of various uni‐ and multimodal stimulus combinations, and a solvent control were presented simultaneously to individual bumblebees, whose response was recorded. The results indicate that bumblebees respond in an irreversible sequence of behavioral reactions. Bumblebees approached the visual stimulus quercetin as often as natural dandelion pollen. An additional olfactory stimulus resulted in slightly more frequent landings. The multimodal stimulus combinations including visual, olfactory, gustatory, and tactile stimuli elicited approaches, antennal contacts, and landings as often as natural pollen. Subsequent reactions like proboscis extension, mandible biting, and buzzing were more often but not regularly observed at dandelion pollen. Our study shows that visual signals of pollen are sufficient to trigger initial responses of bumblebees, whereas multimodal pollen stimuli elicit full behavioral response as compared to natural pollen. Our results suggest a major role of pollen cues for the attraction of bees toward flowers and also explain, why many floral guides mimic the visual signals of pollen and anthers, that is, the yellow and UV‐absorbing color, to direct bumblebees toward the site where they access the floral rewards.     

Variation in floral characters,particularly floral scent,in sapromyophilous Stemona species

Flowers or inflorescences often deploy various signals, including visual, olfactory, and gustatory cues, that can be detected by their pollinators. In many plants, these cues and their functions are poorly understood. Deciphering the interactions between floral cues and pollinators is crucial for analyzing the reproductive success of flowering plants. In this study, we examined the composition of the fetid floral scents produced by several Stemona species, including nine S. tuberosa populations from across China, using dynamic headspace adsorption, gas chromatography, and mass spectrometry techniques. We compared variations in floral phenotype, including floral longevity, nectar rewards, pollinator behavior, and flower length and color among the Stemona species. Of the 54 scent compounds identified, the major compounds include fetid dimethyl disulfide, dimethyl trisulfide, 1‐pyrroline, butyric acid, p‐cresol, isoamyl alcohol, and indole. We detected striking differentiation in floral scent at both the species and population level, and even within a population of plants with different colored flowers. Floral characteristics related to sapromyophily and deceptive pollination, including flower color mimicking livor mortis and a lack of nectar, were found in five Stemona species, indicating that Stemona is a typical sapromyophilous taxon. Species of this monocot genus might employ evolutionary tactics to exploit saprophilous flies for pollination.     

Active pollination favours sexual dimorphism in floral scent

Zoophilous flowers often transmit olfactory signals to attract pollinators. In plants with unisexual flowers, such signals are usually similar between the sexes because attraction of the same animal to both male and female flowers is essential for conspecific pollen transfer. Here, we present a remarkable example of sexual dimorphism in floral signal observed in reproductively highly specialized clades of the tribe Phyllantheae (Phyllanthaceae). These plants are pollinated by species-specific, seed-parasitic Epicephala moths (Gracillariidae) that actively collect pollen from male flowers and pollinate the female flowers in which they oviposit; by doing so, they ensure seeds for their offspring. We found that Epicephala-pollinated Phyllanthaceae plants consistently exhibit major qualitative differences in scent between male and female flowers, often involving compounds derived from different biosynthetic pathways. In a choice test, mated female Epicephala moths preferred the scent of male flowers over that of female flowers, suggesting that male floral scent elicits pollen-collecting behaviour. Epicephala pollination evolved multiple times in Phyllantheae, at least thrice accompanied by transition from sexual monomorphism to dimorphism in floral scent. This is the first example in which sexually dimorphic floral scent has evolved to signal an alternative reward provided by each sex, provoking the pollinator''s legitimate altruistic behaviour.     

Pollen partitioning of three species of Convolvulaceae among oligolectic bees in the Caatinga of Brazil

During the rainy season many species of Convolvulaceae bloom simultaneously in the Caatinga of northeast Brazil. In a Caatinga nature reserve we studied pollination and breeding systems of three sympatric species of Convolvulaceae, Ipomoea bahiensis, I.?nil, and Merremia aegyptia, focusing on pollen partitioning among flower visitors and pollen flow. The study showed that only oligolectic bees collected pollen and that these species had different preferences among the three species of Convolvulaceae: pollen of Ipomoea bahiensis, the only self-incompatible species, was collected mainly by Melitoma segmentaria, M.?osmioides, and Melitomella murihirta; pollen of I.?nil by Lithurgus huberi; and that of Merremia aegyptia by Ancyloscelis apiformis and an undescribed species of this genus. Introduced honey bees visited only flowers of Merremia aegyptia, where they were extraordinarily frequent flower visitors. However, they discarded the pollen grains, which led to almost 50% pollen loss. No polylectic bee species compete for pollen with the oligolectic species. Partitioning of pollen diminishes competition for floral resources in this specialized plant?Cpollinator association.     

Olfactory versus visual cues in a floral mimicry system

 We used arrays of artificial flowers with and without fragrance to determine the importance of olfactory and visual cues in attracting insects to a floral mimic. The mimic is a fungus, Puccinia monoica Arth., which causes its crucifer hosts (here, Arabis drummondii Gray) to form pseudoflowers that mimic co-occurring flowers such as the buttercup, Ranunculus inamoenus Greene. Although pseudoflowers are visually similar to buttercups, their sweet fragrance is distinct. To determine whether visitors to pseudoflowers were responding to fragrance we performed an experiment in which we removed the visual cues, but allowed fragrance to still be perceived. In this experiment we found that pseudoflower fragrance can attract visitors by itself. In other experiments we found that the relative importance of olfactory and visual cues depended on the species of visitor. Halictid bees (Dialictus sp.) had a somewhat greater visual than olfactory response, whereas flies (muscids and anthomyiids) were more dependent on olfactory cues. We also used bioassays to determine which of the many compounds present in the natural fragrance were responsible for attraction. We found that halictid bees were equally attracted to pseudoflowers and to a blend containing phenylacetaldehyde, 2-phenylethanol, benzaldehyde and methylbenzoate in the same relative concentrations as in pseudoflowers. Flies, on the other hand, only responded to pseudoflower scent, indicating that we have not yet identified the compound(s) present in pseudoflowers that are attracting them. The ability of insects to differentiate pseudoflowers from true flowers by their fragrance may be important in the evolution of the mimicry system. Different fragrances may facilitate proper transfer of both fungal spermatia and pollen, and thus make it possible for the visual mimicry to evolve. Received: 3 January 1996 / Accepted: 13 August 1996     

Reduced visitation to buzz‐pollinated Cyanella hyacinthoides in the presence of other pollen sources in the hyperdiverse Cape Floristic Region

Many plant species have floral morphologies that restrict access to floral resources, such as pollen or nectar, and only a subset of floral visitors can perform the handling behaviors required to extract restricted resources. Due to the time and energy required to extract resources from morphologically complex flowers, these plant species potentially compete for pollinators with co‐flowering plants that have more easily accessible resources. A widespread floral mechanism restricting access to pollen is the presence of tubular anthers that open through small pores or slits (poricidal anthers). Some bees have evolved the capacity to remove pollen from poricidal anthers using vibrations, giving rise to the phenomenon of buzz‐pollination. These bee vibrations that are produced for pollen extraction are presumably energetically costly, and to date, few studies have investigated whether buzz‐pollinated flowers may be at a disadvantage when competing for pollinators’ attention with plant species that present unrestricted pollen resources. Here, we studied Cyanella hyacinthoides (Tecophilaeaceae), a geophyte with poricidal anthers in the hyperdiverse Cape Floristic Region of South Africa, to assess how the composition and relative abundance of flowers with easily accessible pollen affect bee visitation to a buzz‐pollinated plant. We found that the number of pollinator species of C. hyacinthoides was not influenced by community composition. However, visitation rates to C. hyacinthoides were reduced when the relative abundances of flowers with more accessible resources were high. Visitation rates were strongly associated with petal color, showing that flower color is important in mediating these interactions. We conclude that buzz‐pollinated plants might be at a competitive disadvantage when many easily accessible pollen sources are available, particularly when competitor species share its floral signals.