Floral divergence and temporal pollinator partitioning in two synchronopatric species of <Emphasis Type="Italic">Vigna</Emphasis> (Leguminosae-Papilionoideae)

Exclusivity of pollinators, temporal partitioning of shared pollinators and divergence in pollen placement on the shared pollinators’ bodies are mechanisms that prevent interspecific pollen flow and minimize competitive interactions in synchronopatric plant species. We investigated the floral biology, flower visitors, pollinator effectiveness and seasonal flower availability of two syntopic legume species of the genus Vigna, V. longifolia and V. luteola, in ‘restinga’ vegetation of an island in southern Brazil. Our goal was to identify the strategies that might mitigate negative consequences of their synchronous flowering. Vigna longifolia and V. luteola were self-compatible, but depended on pollinators to set seeds. Only medium to large bees were able to trigger the ‘brush type’ pollination mechanism. Vigna longifolia, with its asymmetrical corolla and hugging mechanism, showed a more restrictive pollination system, with precise sites of pollen deposition/removal on the bee’s body, compared to V. luteola, with its zygomorphic corolla and cymbiform keel. There was a daily temporal substitution in flower visitation by the main pollinators. Vigna longifolia and V. luteola had overlapping flowering phenology but the densities of their flowers fluctuated, resulting in a seasonal partitioning of flower visitation. The differences in corolla symmetry and mainly the temporal partitioning among pollinators throughout the day and the flowering season proved to be important factors in maintaining the synchronopatry of V. longifolia and V. luteola.     

Facilitative interactions among co-flowering <Emphasis Type="Italic">Primula</Emphasis> species mediated by pollinator sharing

We investigated flowering phenology, pollinator visitation and visitor community composition in communities of self-incompatible sympatric Primula species in a high-elevation Himalayan ecosystem. Within the tight constraints imposed by short growing seasons in such ecosystems, interactions among co-occurring plants for pollinators may vary from competition to facilitation, depending on the specifics of the system. We found that pollinator community composition changed with elevation in this system: lepidopterans were the dominant visitors at lower elevations (2200–3000 masl), bees (other than bumblebees) dominated at mid-elevations (3000–3800 masl) and bumblebees dominated at higher elevations (3800–4600 masl). However, within an elevation zone, there were no significant differences in pollinators amongst co-occurring Primula species. At a focal study site where multiple Primula species co-occurred, our results showed that even while the overall flowering periods of these species broadly overlapped, the peak flowering periods of different Primula species were temporally segregated. Upon further inferring the nature of interaction amongst co-flowering Primula species, we found that plots with higher Primula diversity (≥?2 species) and density (80–100 individuals) experienced significantly higher pollinator visitation, compared with plots with single species and low flower densities (40–50 individuals). Our results suggest that in this community of sympatric, self-incompatible Primula species, a broadly aggregated, synchronous floral display of multiple species results in pollinator facilitation by attracting a greater number of pollinator visitors. Within this broadly synchronous display, the temporal segregation of peak flowering period of individual species may reduce competition for pollinators and limit heterospecific pollen transfer.     

Foraging habitats and floral resource use by colonies of long- and short-tongued bumble bee species in an agricultural landscape with kabocha squash fields

Bumble bees pollinate and forage on flowers of crop and wild plants in agricultural landscapes. These interactions may depend on landscape patterns and bumble bee traits. We studied the abundance, colony density, and foraging range in long-tongued Bombus diversus Smith and short-tongued B. hypocrita Pérez, and evaluated their visits to flowers of wild plants and cultivated kabocha squash (Cucurbita maxima Duchesne). In forests in a farmland, B. hypocrita workers were trapped more frequently in the canopy. Full-sibs determined by nuclear microsatellite genotypes among workers collected in the farmland showed higher colony density and a larger foraging radius in B. hypocrita (30.8 km^?2 and 848 m) than in B. diversus (8.3 km^?2 and 723 m), respectively. Regarding wild plants, workers more frequently visited shallow flowers in B. hypocrita and deep flowers in B. diversus. These results suggest that bumble bees with different traits forage on different wild flowers in different habitats. Squash flowers were visited by both bumble bee species at similar frequency in the latter period of colony growth when males and new queens appeared. Composition of full-sib workers visiting squash and wild flowers did not depend on the number of collected workers of individual colonies, indicating that foraging on squash flowers was not associated with colony growth. Thus, growth and reproduction of bumble bee colonies may be supported by various wild plants and cultivated squash, respectively.     

Going native? Flower use by bumblebees in English urban gardens

Background and Aims

Although urban gardens provide opportunities for pollinators in an otherwise inhospitable environment, most garden plants are not native to the recipient biogeographical region and their value to local pollinators is disputed. This study tested the hypothesis that bumblebees foraging in English urban gardens preferentially visited sympatric Palaearctic-range plants over species originating outside their native range.

Methods

Twenty-seven surveys of flower availability and bumblebee visitation (Bombus spp.) were conducted over a 3-month summer period. Plants were categorized according to whether they were native British, Palaearctic or non-Palaearctic in origin. A phylogeny of the 119 plant species recorded was constructed and the relationship between floral abundance and the frequency of pollinator visits investigated by means of phylogenetically independent contrasts. Differentiation in utilization of plant species by the five bumblebee species encountered was investigated using niche overlap analyses.

Key Results

There was conflicting evidence for preferential use of native-range Palaearctic plant species by bumblebees depending on which plants were included in the analysis. Evidence was also found for niche partitioning between species based on respective preferences for native and non-native biogeographical range plants. Two bumblebees (Bombus terrestris and B. pratorum) concentrated their foraging activity on non-Palaearctic plants, while two others (B. hortorum and B. pascourum) preferred Palaearctic species.

Conclusions

The long-running debate about the value of native and non-native garden plants to pollinators probably stems from a failure to properly consider biogeographical overlap between plant and pollinator ranges. Gardeners can encourage pollinators without consideration of plant origin or bias towards ‘local’ biogeographical species. However, dietary specialist bumblebees seem to prefer plants sympatric with their own biogeographical range and, in addition to the cultivation of these species in gardens, provision of native non-horticultural (‘weed’) species may also be important for pollinator conservation.     

Floral Sonication is an Innate Behaviour in Bumblebees that can be Fine-Tuned with Experience in Manipulating Flowers

Bumblebees demonstrate an extensive capacity for learning complex motor skills to maximise exploitation of floral rewards. This ability is well studied in nectar collection but its role in pollen foraging is less well understood. Floral sonication is used by bees to extract pollen from some plant species with anthers which must be vibrated (buzzed) to release pollen. Pollen removal is determined by sonication characteristics including frequency and amplitude, and thus the ability to optimise sonication should allow bees to maximise the pollen collection. We investigated the ability of the buff-tailed bumblebee (Bombus terrestris) to modify the frequency and amplitude of their buzzes with increasing experience manipulating flowers of the buzz-pollinated plant Solanum rostratum. We analysed flight and feeding vibrations generated by naïve workers across feeding bouts. Feeding buzzes were of a higher frequency and a lower amplitude than flight buzzes. Both flight and feeding buzzes had reduced amplitudes with increasing number of foraging trips. However, the frequency of their feeding buzzes was reduced significantly more than their flight buzzes as bumblebee workers gained experience manipulating flowers. These results suggest that bumblebees are able to modify the characteristics of their buzzes with experience manipulating buzz-pollinated flowers. We discuss our findings in the context of bumblebee learning, and the current understanding of the optimal sonication characteristics for releasing pollen in buzz-pollinated species. Our results present a tantalising insight into the potential role of learning in floral sonication, paving the way for future research in this area.     

The floral biology and reproductive system of <Emphasis Type="Italic">Mauritia flexuosa</Emphasis> (Arecaceae) in a restinga environment in northeastern Brazil

Studies of the floral biology of the buriti palm, Mauritia flexuosa, have presented conflicting results with respect to the mechanism of pollination, indicating either cantharophily or anemophily. To resolve this question, the floral biology of M. flexuosa was studied in a coastal restinga environment in northeastern Brazil. The reproductive system was studied experimentally, and floral visitors were collected by bagging inflorescences. In this environment, M. flexuosa, a dioecious species, has several gender-specific floral features that function to attract pollinators, especially beetles. The male flowers produce large amounts of pollen as a reward, and male and female inflorescences produce similar odors that attract pollinators to female flowers, which offer only a nectar secretion as a reward. When feeding on the female flowers, the visitors frequently come into contact with the stigmata. To increase the chances of pollination, the female flowers persist longer than the male ones, and the viability of the pollen grain is very high. A curculionid beetle species of the genus Grasidius was found to be an effective pollinator. We suspect that wind also contributes to the pollination of M. flexuosa in the study area, but in a relatively minor way.     

Pollinator-mediated impacts of alien invasive plants on the pollination of native plants: the role of spatial scale and distinct behaviour among pollinator guilds

Alien invasive plant species can affect pollination, reproductive success and population dynamics of co-flowering native species via shared pollinators. Consequences may range from reproductive competition to facilitation, but the ecological drivers determining the type and magnitude of such indirect interactions remain poorly understood. Here, we examine the role of the spatial scale of invader presence and spatially contingent behavioural responses of different pollinator groups as potential key drivers, using the invasive Oxalis pes-caprae and the self-incompatible native annual Diplotaxis erucoides as a model system. Three treatments were assigned to native focal plants: (1) invader present at the landscape scale (hectares) but experimentally removed at the floral neighbourhood scale (pa); (2) invader present at both scales (pp); (3) invader absent at both scales (aa). Interestingly, we found pronounced spatially contingent differences in the responses of pollinators: honeybees and bumblebees were strongly attracted into invaded sites at the landscape scale, translating into native plant visitation facilitation through honeybees, while bumblebees almost exclusively visited Oxalis. Non-corbiculate wild bees, in contrast, showed less pronounced responses in foraging behavior, primarily at the floral neighborhood scale. Average heterospecific (Oxalis) pollen deposition onto stigmas of Diplotaxis was low (<1 %), but higher in the pp than in the pa treatment. Hand-pollination of Diplotaxis with Oxalis and conspecific pollen, however, reduced seed set by more than half when compared to hand-pollination with only conspecific pollen. Seed set of Diplotaxis, finally, was increased by 14 % (reproductive facilitation) in the pp treatment, while it was reduced by 27 % (reproductive competition) in the pa treatment compared to uninvaded populations. Our study highlights the crucial role of spatial scale and pollinator guild driving indirect effects of invasive on co-flowering native plant species.     

Nectar robbery by a hermit hummingbird: association to floral phenotype and its influence on flowers and network structure

Interactions between flowers and their visitors span the spectrum from mutualism to antagonism. The literature is rich in studies focusing on mutualism, but nectar robbery has mostly been investigated using phytocentric approaches focused on only a few plant species. To fill this gap, we studied the interactions between a nectar-robbing hermit hummingbird, Phaethornis ruber, and the array of flowers it visits. First, based on a literature review of the interactions involving  P. ruber, we characterized the association of floral larceny to floral phenotype. We then experimentally examined the effects of nectar robbing on nectar standing crop and number of visits of the pollinators to the flowers of Canna paniculata. Finally, we asked whether the incorporation of illegitimate interactions into the analysis affects plant–hummingbird network structure. We identified 97 plant species visited by P. ruber and found that P. ruber engaged in floral larceny in almost 30 % of these species. Nectar robbery was especially common in flowers with longer corolla. In terms of the effect on C. paniculata, the depletion of nectar due to robbery by P. ruber was associated with decreased visitation rates of legitimate pollinators. At the community level, the inclusion of the illegitimate visits of P. ruber resulted in modifications of how modules within the network were organized, notably giving rise to a new module consisting of P. ruber and mostly robbed flowers. However, although illegitimate visits constituted approximately 9 % of all interactions in the network, changes in nestedness, modularity, and network-level specialization were minor. Our results indicate that although a flower robber may have a strong effect on the pollination of a particular plant species, the inclusion of its illegitimate interactions has limited capacity to change overall network structure.     

Micromorphological and histochemical attributes of flowers and floral reward in <Emphasis Type="Italic">Linaria vulgaris</Emphasis> (Plantaginaceae)

The self-incompatible flowers of Linaria vulgaris have developed a range of mechanisms for attraction of insect visitors/pollinators and deterrence of ineffective pollinators and herbivores. These adaptive traits include the flower size and symmetry, the presence of a spur as a “secondary nectar presenter,” olfactory (secondary metabolites) and sensual (scent, flower color, nectar guide—contrasting palate) signals, and floral rewards, i.e. pollen and nectar. Histochemical tests revealed that the floral glandular trichomes produced essential oils and flavonoids, and pollen grains contained flavonoids, terpenoids, and steroids, which play a role of olfactory attractants/repellents. The nectary gland is disc-shaped and located at the base of the ovary. Nectar is secreted through numerous modified stomata. Nectar secretion began in the bud stage and lasted to the end of anthesis. The amount of produced nectar depended on the flower age and ranged from 0.21 to 3.95 mg/flower (mean?=?1.51 mg). The concentration of sugars in the nectar reached up to 57.0%. Both the nectar amount and sugar concentration demonstrated a significant year and population effect. Pollen production was variable between the years of the study. On average, a single flower of L. vulgaris produced 0.31 mg of pollen. The spectrum of insect visitors in the flowers of L. vulgaris differed significantly between populations. In the urban site, Bombus terrestris and Apis mellifera were the most common visitors, while a considerable number of visits of wasps and syrphid flies were noted in the rural site.     

Bee pollination and evidence of substitutive nectary in <Emphasis Type="Italic">Anadenanthera colubrina</Emphasis> (Leguminosae-Mimosoideae)

Anadenanthera colubrina (Vell.) Brenan (Leguminosae-Mimosoideae) is a widely-distributed tree in seasonally dry tropical forests of South America that was classified previously as lacking nectaries. However, some studies have stated that its flowers produce nectar, while others analyzed the composition of unifloral honey produced from A. colubrina flowers, raising the question about nectar production in the species. We studied the pollination and reproductive biology of A. colubrina var. cebil (Griseb.) Altschul in a natural population in the Caatinga, northeastern Brazil. Reproductive phenology, sexual system, floral biology, resource, and pollinators were investigated. We analyzed the breeding system through controlled pollinations for addressing its dependence on pollen vectors for reproduction. Anadenanthera colubrina flowered in the dry season, flower heads are heteromorphic, with staminate flowers at the base and perfect flowers at the apex of the inflorescence, characterizing andromonoecy. Anthesis is diurnal. We observed small drops of nectar at the apex of the petals of some flowers per inflorescence. Together with observations on flower visitor behavior and histochemical tests, we propose that A. colubrina produces floral nectar at the apex of the corolla, characterizing a substitutive nectary (sensu Vogel). This is the first record of substitutive nectary in the Mimosoideae and the first record of andromonoecy in the genus. Bees were the main pollinators (higher frequency), although other insects such as wasps, butterflies, and small beetles were also observed collecting nectar and/or pollen. The species is self-incompatible, thus depending on insect pollen vectors, mainly bees, for reproduction.     

Diversity and evolution of pollinator rewards and protection by <Emphasis Type="Italic">Macaranga</Emphasis> (Euphorbiaceae) bracteoles

Flowering plants have modified their floral organs in remarkably diverse ways to optimize their interaction with pollinators. Although floral organs represent a major source of floral diversity, many plants also use extrafloral organs, such as bracts and bracteoles, in interacting with pollinators; however, the evolutionary dynamics of non-floral organs involved in pollination are poorly studied. The genus Macaranga is characterized by protective mutualisms with ants that potentially interfere with pollinators on flowers. Macaranga flowers lack perianths and, notably, bracteoles serve the dual function of rewarding pollinators and protecting them from guarding ants; in one group of species, bracteoles provide a nectar reward to generalist pollinators, while in another group, bracteole “chambers” protect thrips or hemipteran pollinators that use these structures as feeding and breeding sites. We examined the diversity and evolutionary dynamics of inflorescence morphology in Macaranga, focusing on bracteoles. We recognized three inflorescence types based on examination of herbarium materials: Discoid-gland, which possess disc-shaped glands on the bracteole surfaces (including all the generalist-pollinated species); Enclosing, in which bracteoles cover flowers (including all the thrips- and hemipteran-pollinated species); and Inconspicuous, in which bracteoles are small, narrow or absent. Ancestral state reconstruction indicated that inflorescence morphologies have changed multiple times in the genus. These findings suggest that morphological changes in non-floral characters (bracteoles) of Macaranga species have occurred as frequently as in the floral structures of many flowering plants. The multiple evolutions of the Enclosing bracteoles, which protect pollinators, might have been facilitated by pollination interference from mutualistic ants.     

Exploring the predation of UK bumblebees (Apidae, <Emphasis Type="Italic">Bombus</Emphasis> spp.) by the invasive pitcher plant <Emphasis Type="Italic">Sarracenia purpurea</Emphasis>: examining the effects of annual variation,seasonal variation,plant density and bumblebee gender

Invasive carnivorous plant species can impact the native invertebrate communities on which they prey. This article explores the predation of native UK bumblebees (Bombus spp.) by the invasive pitcher plant species Sarracenia purpurea and discusses the potential effect of S. purpurea on native bumblebees. Specifically, it evaluates whether the extent to which bumblebees are captured varies (i) over successive years, (ii) across June and July, (iii) with density of distribution of pitchers or (iv) with bumblebee gender. Pitcher contents were examined from an established population of Sarracenia purpurea growing in Dorset, UK. Results show that the total extent to which bumblebees were captured differed over the years 2012–2014 inclusive. A 1-year study in 2013 showed that more bumblebees were caught in July than in June and more bumblebees were captured when pitchers grew at high density. Results from 2013 also showed that more pitchers caught more than one bumblebee than would be expected based on a normal probability distribution and that this phenomenon affects female and male bumblebees equally. We discuss possible reasons for these results including that the bumblebees may be using S. purpurea as a resource. Further work is required to establish the exact underpinning mechanisms and the relative roles of plant and bumblebee behaviour within the relationship. Such interaction complexity may have consequences for consideration in invasive carnivorous plant management.     

Structure of the stigma and style of <Emphasis Type="Italic">Callaeum psilophyllum</Emphasis> (Malpighiaceae) and its relation with potential pollinators

The family Malpighiaceae, particularly in the Neotropic, shows a similar floral morphology. Although floral attraction and rewards to pollinators are alike, stigmas and styles show more diversity. The stigmas were described covered with a thin and impermeable cuticle that needs to be ruptured by the mechanical action of the pollinators. However, this characteristic was only mentioned for a few species and the anatomy and ultrastructure of the stigmas were not explored. In this work, we analyze the morphology, anatomy, and ultrastructure of the stigma and style of Callaeum psilophyllum. Moreover, we identify the potential pollinators in order to evaluate how the disposition of the stigmas is related with their size and its role in the exposure of the receptive stigmatic surface. Our observations indicate that Centris flavifrons, C. fuscata, C. tarsata, and C. trigonoides are probably efficient pollinators of C. psilophyllum. The three stigmas are covered by a cuticle that remained intact in bagged flowers. The flowers exposed to visitors show the cuticle broken, more secretion in the intercellular spaces between sub-stigmatic cells and abundant electron-dense components inside vacuoles in stigmatic papillae. This indicates that the stigmas prepares in similar ways to receive pollen grains, but the pollinator action is required to break the cuticle, and once pollen tubes start growing, stigmatic and sub-stigmatic cells release more secretion by a granulocrine process.     

Abundance,body size,and morphology of bumblebees in an area where an exotic species, <Emphasis Type="Italic">Bombus terrestris</Emphasis>, has colonized in Japan

An exotic bumblebee species, Bombus terrestris, has colonized in Japan and becomes dominant in some local communities. We examined the effects of land use and bumblebee abundance on the number and body size of bumblebees collected using window traps in a lowland area in the southern Ishikari district, Hokkaido. In 2004, we collected 922 bumblebees of six species using 70 traps at 17 sites. A statistical model fitted to the data demonstrated that dispersion from commercial B. terrestris colonies used in greenhouses positively affected the number of B. terrestris caught by each trap. This exotic species was abundant in sites where paddy fields were prevalent, but three native species, B. hypocrita, B. ardens, and B. diversus, were abundant in sites where farms and woodlands were widespread. The local abundance of B. terrestris was not associated negatively with the number and body size of native bumblebees. Thus, we did not find any competitive interactions between exotic and native bumblebees although habitat conditions seem to be common determinants of the bumblebee populations. A morphological analysis showed that B. terrestris had intermediate tongue length between B. hypocrita and B. ardens.     

Association of flower color with pollen reward may explain increased bumblebee visitation to the Scotch broom yellow morph

Given that pollinators usually visit flowers for hidden rewards, they need to rely on floral traits that indicate reward status (“honest signals”). However, the relationship between pollination, honest signals, and floral rewards is little documented in natural conditions. The Scotch broom (Cytisus scoparius) is an invasive shrub with polymorphism in the color of its flowers that can be yellow, orange, or red. In three areas dominated by the Scotch broom, we described the abundance of the floral morphs and estimated bumblebee (Bombus terrestris) visitation rate. We examined whether bumblebee visitation to the floral morphs was related to pollen reward. We collected flowers and classified their stamens according to their function: reward or pollen export. Then, we measured anther size and estimated pollen quantity. The yellow morph was more abundant and more visited by bumblebees than the orange and red morphs. The yellow flowers did indeed offer more pollen than the other morphs and this occurred only for rewarding anthers, suggesting that bumblebees could use yellow color as an honest signal to visit the most rewarding flowers. We discuss whether innate and/or learned preferences of bumblebees can explain why the yellow morph is more visited, pollinated, and abundant, while the other morphs are maintained at a lower frequency. This is one of the few field works that shows that variation in intra-specific floral traits is associated with variation in floral reward and pollinator visitation rate, helping to understand the foraging preferences of pollinators and the coexistence of floral morphs in nature.

Clinical trials registration: Not applicable.

     

Convergence of anti-bee pollination mechanisms in the Neotropical plant genus <Emphasis Type="Italic">Drymonia</Emphasis> (Gesneriaceae)

The neotropical plant genus Drymonia displays a remarkable variety of floral shapes and colors. One feature that is particularly important to coevolution with pollinators involves the variable shapes and widths of corolla tubes. To evaluate the evolutionary context for changes in corolla shape, we constructed a phylogeny of 50 of the 75 species of Drymonia using molecular markers from plastid (trnK-matK) and nuclear regions (ITS and ETS). Mapping tube shapes on the phylogeny supports open, bell-shaped (campanulate) corolla shape as the ancestral character state for Drymonia, with multiple independent origins of constriction in the corolla tube. Corollas with constrictions take one of three tube shapes: a constricted flower opening and throat with a large, expanded pouch on the lower surface (hypocyrtoid); a constricted flower opening and throat lacking an expanded pouch on the lower surface (urceolate); or a constricted opening and throat where the sides of the corolla appear laterally compressed. Fieldwork demonstrates euglossine bees (mostly Euglossa spp. and Epicharis spp.) visit campanulate corollas while hummingbirds visit corollas that are constricted. Results support eight independent origins of constricted corolla tubes from ancestors with campanulate corolla tubes: 3 hypocyrtoid clades, 3 laterally compressed clades, and 3 urceolate clades (one of which represents a shift from a hypocyrtoid ancestor). Constricted corollas are associated with shifts from the ancestral condition of poricidal anther dehiscence, which presents pollen to pollinators in multiple small doses, to the derived condition of longitudinal anther dehiscence, which presents all pollen to pollinators simultaneously. The association of hummingbird pollination with constricted corolla tubes suggests that narrowing evolved as a barrier mechanism that prohibits the visitation of flowers by bees.     

Bee- to bird-pollination shifts in <Emphasis Type="Italic">Penstemon</Emphasis>: effects of floral-lip removal and corolla constriction on the preferences of free-foraging bumble bees

Plants might be under selection for both attracting efficient pollinators and deterring wasteful visitors. Particular floral traits can act as exploitation barriers by discouraging the unwelcome visitors. In the genus Penstemon, evolutionary shifts from insect pollination to more efficient hummingbird pollination have occurred repeatedly, resulting in the convergent evolution of floral traits commonly present in hummingbird-pollinated flowers. Two of these traits, a reduced or reflexed lower petal lip and a narrow corolla, were found in a previous flight-cage study to affect floral handling time by bumble bees, therefore potentially acting as “anti-bee” traits affecting preference. To test whether these traits do reduce bumble bee visitation in natural populations, we manipulated these two traits in flowers of bee-pollinated Penstemon strictus to resemble hummingbird-adapted close relatives and measured the preferences of free-foraging bees. Constricted corollas strongly deterred bee visitation in general, and particularly reduced visits by small bumble bees, resulting in immediate specialization to larger, longer-tongued bumble bees. Bees were also deterred—albeit less strongly—by lipless flowers. However, we found no evidence that lip removal and corolla constriction interact to further affect bee preference. We conclude that narrow corolla tubes and reduced lips in hummingbird-pollinated penstemons function as exploitation barriers that reduce bee access to nectaries or increase handling time.     

Pollination biology and breeding system in five nocturnal species of <Emphasis Type="Italic">Oenothera</Emphasis> (Onagraceae): reproductive assurance and opportunities for outcrossing

The capacity to produce seed, both by selfing and outcrossing, or mixed mating strategies, is considered a mechanism for overcoming unpredictable pollinator availability. In the present study, we investigate breeding system, insect visitations and the role of insect visitors in the pollination of five species of Oenothera subsect. Oenothera. Field experiments showed that autonomous selfing occurs at bud stage, prior to the opening of the flower. Control flowers showed similar seed set to hand-pollinated flowers, whereas emasculated flowers and those subject to open pollination set fewer seed. These results are consistent with the hypothesis that the investigated Oenothera exhibit a great capacity for autonomous selfing and that selfing is selected in order to provide reproductive assurance. Although flowers were visited mostly by nocturnal lepidopterans, these insects did not precipitate pollination and are thus considered nectar thieves. Conversely, analysis of pollen loads and behavior during foraging by diurnal insect visitors revealed that honeybees and bumblebees are the probable pollinators. We conclude that production of flowers capable of autonomous selfing at bud stage, followed by anthesis and opportunities for outcrossing, probably improves the invasive potential of these Oenothera in Europe, together with a rapid increase in their populations, even when pollinators are scarce.     

Low interspecific pollen transfer between invasive aquatic <Emphasis Type="Italic">Ludwigia grandiflora</Emphasis> and native co-flowering plants

Showy invasive alien plants are often integrated in the diet of generalist pollinators and because of the lack of co-evolvement with the native plant community, a high amount of interspecific pollen transfer (IPT) can be expected. We investigated pollinator switching and magnitude plus distance of IPT between the alien aquatic Ludwigia grandiflora and the native Lythrum salicaria in both directions in uninvaded and invaded sites with a different relative abundance of L. grandiflora (% cover of the alien plant: no cover; low cover: <5%; high cover: 50–75%). A field experiment was conducted to include both pollinator interspecific movements and tracking of IPT, using fluorescent dye as a pollen analogue. Despite a substantial overlap in pollinators between L. grandiflora and the native L. salicaria, less than 10% of the observed flights were interspecific. Similar results were found in dye transfer patterns. The proportions of stigmas with conspecific dye were always higher than the proportions of stigmas with heterospecific dye for L grandiflora and L. salicaria. There were no differences in conspecific dye loads for L. salicaria between uninvaded and invaded sites. Conspecific pollen loss (native CPL) and heterospecific pollen deposition (alien HPD) were in general low and species-specific. The distance of HPD ranged respectively from 1.7 to 39 m and from 0.3 to 54.8 m in the low cover and high cover sites while CPL ranged respectively from 6.40 to 68.02 m and from 0.60 to 40.18 m in the low cover and high cover sites. We can conclude that, in this system, CPL and HPD will play a minor role in pollinator-mediated interaction. Furthermore, interspecific competition for pollinators will cover a larger distance than just neighboring individuals. Our results suggest the necessity to consider the combined effect of insect visitation, pollen deposition, relative alien abundance, distance and seed set when investigating pollinator-mediated interactions of invasive plants.     

Intersexual mimicry and flowering phenology facilitate pollination in a dioecious habitat specialist species, <Emphasis Type="Italic">Myristica fatua</Emphasis> (Myristicaceae)

Myristica fatua is a dioecious specialist species restricted to the endangered, freshwater Myristica swamp forests in the Western Ghats, India. Earlier studies have alluded to pollination by deception in members of the Myristica genus, and thus we examined the pollination ecology comprising floral biology, flower production, flower visitors, and reproductive success in M. fatua and inferred the potential strategies that could permit such deception in this habitat specialist tree. Male flowers provide pollen rewards for an extended period of time while female flowers are rewardless and both sexes are visited by generalist insects, mainly by honeybees and stingless bees. Bee visits were significantly more frequent and longer on male than on female flowers as bees collected pollen from male flowers. We found that flower production patterns create a preponderance of males compared to females in the swamp populations. Using a model of honeybee color vision, we found the distance between the color loci of male and female flowers and based on minimum visual angle subtended by these flowers, we suggest that the two floral sexes cannot be discriminated by bees. Bees are likely deceived by the perceptual similarity of rewardless female flowers to pollen-offering male flowers and pollination is the consequence of foraging errors made by pollinators that encounter largely male–rarely female flower mosaics as they forage among clump-distributed M. fatua trees in the swamp habitat.