Specialised protagonists in a plant–pollinator interaction: the pollination of Blumenbachia insignis (Loasaceae)

• Analyses of resource presentation, floral morphology and pollinator behaviour are essential for understanding specialised plant‐pollinator systems. We investigated whether foraging by individual bee pollinators fits the floral morphology and functioning of Blumenbachia insignis, whose flowers are characterised by a nectar scale‐staminode complex and pollen release by thigmonastic stamen movements.
• We described pollen and nectar presentation, analysed the breeding system and the foraging strategy of bee pollinators. We determined the nectar production pattern and documented variations in the longevity of floral phases and stigmatic pollen loads of pollinator‐visited and unvisited flowers.
• Bicolletes indigoticus (Colletidae) was the sole pollinator with females revisiting flowers in staminate and pistillate phases at short intervals, guaranteeing cross‐pollen flow. Nectar stored in the nectar scale‐staminode complex had a high sugar concentration and was produced continuously in minute amounts (~0.09 μl·h^?1). Pushing the scales outward, bees took up nectar, triggering stamen movements and accelerating pollen presentation. Experimental simulation of this nectar uptake increased the number of moved stamens per hour by a factor of four. Flowers visited by pollinators received six‐fold more pollen on the stigma than unvisited flowers, had shortened staminate and pistillate phases and increased fruit and seed set.
• Flower handling and foraging by Bicolletes indigoticus were consonant with the complex flower morphology and functioning of Blumenbachia insignis. Continuous nectar production in minute quantities but at high sugar concentration influences the pollen foraging of the bees. Partitioning of resources lead to absolute flower fidelity and stereotyped foraging behaviour by the sole effective oligolectic bee pollinator.

     

Nectar robbery by bees Xylocopa virginica and Apis mellifera contributes to the pollination of rabbiteye blueberry

Honey bees, Apis mellifera L., probe for nectar from robbery slits previously made by male carpenter bees, Xylocopa virginica (L.), at the flowers of rabbiteye blueberry, Vaccinium ashei Reade. This relationship between primary nectar robbers (carpenter bees) and secondary nectar thieves (honey bees) is poorly understood but seemingly unfavorable for V. ashei pollination. We designed two studies to measure the impact of nectar robbers on V. ashei pollination. First, counting the amount of pollen on stigmas (stigmatic pollen loading) showed that nectar robbers delivered fewer blueberry tetrads per stigma after single floral visits than did our benchmark pollinator, the southeastern blueberry bee, Habropoda laboriosa (F.), a recognized effective pollinator of blueberries. Increasing numbers of floral visits by carpenter bee and honey bee robbers yielded larger stigmatic loads. As few as three robbery visits were equivalent to one legitimate visit by a pollen-collecting H. laboriosa female. More than three robbery visits per flower slightly depressed stigmatic pollen loads. In our second study, a survey of 10 commercial blueberry farms demonstrated that corolla slitting by carpenter bees (i.e., robbery) has no appreciable affect on overall V. ashei fruit set. Our observations demonstrate male carpenter bees are benign or even potentially beneficial floral visitors of V ashei. Their robbery of blueberry flowers in the southeast may attract more honey bee pollinators to the crop.     

Assessment of pollen reward and pollen availability in Solanum stramoniifolium and Solanum paniculatum for buzz‐pollinating carpenter bees

The two widespread tropical Solanum species S. paniculatum and S. stramoniifolium are highly dependent on the visits of large bees that pollinate the flowers while buzzing them. Both Solanum species do not offer nectar reward; the rewarding of bees is thus solely dependent on the availability of pollen. Flower visitors are unable to visually assess the amount of pollen, because the pollen is hidden in poricidal anthers. In this study we ask whether and how the amount of pollen determines the attractiveness of flowers for bees. The number of pollen grains in anthers of S. stramoniifolium was seven times higher than in S. paniculatum. By contrast, the handling time per five flowers for carpenter bees visiting S. paniculatum was 3.5 times shorter than of those visiting S. stramoniifolium. As a result foraging carpenter bees collected a similar number of pollen grains per unit time on flowers of both species. Experimental manipulation of pollen availability by gluing the anther pores showed that the carpenter bees were unable to detect the availability of pollen by means of chemical cues before landing and without buzzing. Our study shows that the efficiency of pollen collecting on S. paniculatum is based on large inflorescences with short between‐flower search times and short handling time of individual flowers, whereas that of S. stramoniifolium relies on a large amount of pollen per flower. Interestingly, large carpenter bees are able to adjust their foraging behaviour to drastically different strategies of pollen reward in otherwise very similar plant species.     

Visitation rate of pollinators and nectar robbers to the flowers and inflorescences of Tabebuia aurea (Bignoniaceae): effects of floral display size and habitat fragmentation

Large floral displays favour pollinator attraction and the import and export of pollen. However, large floral displays also have negative effects, such as increased geitonogamy, pollen discounting and nectar/pollen robber attraction. The size of the floral display can be measured at different scales (e.g. the flower, inflorescence or entire plant) and variations in one of these scales may affect the behaviour of flower visitors in different ways. Moreover, the fragmentation of natural forests may affect flower visitation rates and flower visitor behaviour. In the present study, video recordings of the inflorescences of a tree species (Tabebuia aurea) from the tropical savannah of central Brazil were used to examine the effect of floral display size at the inflorescence and tree scales on the visitation rate of pollinators and nectar robbers to the inflorescence, the number of flowers approached per visit, the number of visits per flower of potential pollinators and nectar robbers, and the interaction of these variables with the degree of landscape disturbance. Nectar production was quantified with respect to flower age. Although large bees are responsible for most of the pollination, a great diversity of flower insects visit the inflorescences of T. aurea. Other bee and hummingbird species are highly active nectar robbers. Increases in inflorescence size increase the visitation rate of pollinators to inflorescences, whereas increases in the number of inflorescences on the tree decrease visitation rates to inflorescences and flowers. This effect has been strongly correlated with urban environments in which trees with the largest floral displays are observed. Pollinating bees (and nectar robbers) visit few flowers per inflorescence and concentrate visits to a fraction of available flowers, generating an overdispersed distribution of the number of visits per inflorescence and per flower. This behaviour reflects preferential visits to young flowers (including flower buds) with a greater nectar supply.     

Choice of flowers by foraging honey bees (Apis mellifera): possible morphological cues

Abstract.

• 1 Honey bees foraging for nectar on lavender (Lavandula stoechas) chose inflorescences with more of their flowers open. The number of open flowers predicted whether an inflorescence was visited by bees, inspected but rejected, or ignored. Inflorescences chosen arbitrarily by observers had numbers of open flowers intermediate between those of visited and ignored inflorescences.
• 2 Differences in morphological characters between types of inflorescence correlated with nectar volume and sugar weight per flower so that visited inflorescences had a disproportionately greater volume of nectar and weight of sugar per flower and greater variance in nectar volume.
• 3 Although there were significant associations between nectar content and the morphological characters of inflorescences, discriminant function analysis revealed discrimination on the basis of morphology rather than nectar content.
• 4 Visited inflorescences tended to have smaller than average flowers but bees tended to probe the largest flowers on visited inflorescences.
• 5 Choice of flowers within inflorescences is explicable in terms of the relationship between flower size and nectar content.

     

Solitary and social bees as pollinators of Wahlenbergia (Campanulaceae): single-visit effectiveness, overnight sheltering and responses to flower colour

Solitary bees often form specialised mutualisms with particular plant species, while honeybees are considered to be relatively opportunistic foragers. Thus, it may be expected that solitary bees are more effective pollinators than honeybees when foraging on the same floral resource. To test this, we studied two Wahlenbergia species (Campanulaceae) in South Africa that are visited by both social honeybees and solitary bees, and which are shown here to be genetically self-incompatible and thus reliant on pollinator visits for seed production. Contrary to expectation, the solitary bee Lipotriches sp. (Halictidae) and social bee Apis mellifera (Apidae), which were the two most frequent visitors to flowers of the study species, were equally effective pollinators in terms of the consequences of single visits for fruit and seed set. Both bee species preferentially visited female phase flowers, which contain more nectar than male phase flowers. Male solitary bees of several genera frequently shelter overnight in flowers of both Wahlenbergia species, but temporal exclusion experiments showed that this behaviour makes little contribution to either seed production or pollen dispersal (estimated using a dye particle analogue). Manipulation of flower colour using a sunscreen that removed UV reflectance strongly reduced visits by both bee groups, while neither group responded to Wahlenbergia floral odour cues in choice tests. This study indicates that while flowers of Wahlenbergia cuspidata and W. krebsii are pollinated exclusively by bees, they are not under strong selection to specialise for pollination by any particular group of bees.     

How long to stay on a plant: the response of bumblebees to encountered nectar levels

This field study shows that the number of flowers visited per bee per plant (Anchusa officinalis) increases with the instantaneous nectar level at the plant. Observations during the season showed that a bee visits more flowers per plant of given nectar level, the lower the overall mean nectar level in the study area. These results agree with predictions from a model based on the ‘marginal value theorem’, but with assumptions and constraints adapted for nectar-foraging bees. It suggests that bumblebees assess the nectar level at a plant by sampling one or a few flowers, which is possible because within-plant nectar volumes are correlated. The bees compare encountered gains to an optimal plant switching threshold equal to the overall mean nectar level and leave an unrewarding plant as soon as possible, but continue to visit the flowers on a rewarding plant. However, the bees leave before having visited all flowers due to a searching constraint. The bees’ response to plant nectar levels results in systematic flower visitation, because visitation to recently depleted flowers is reduced, which reduces the variation of the inter-visit time per flower. Systematic flower visitation implies that the overall mean encountered gain per flower is higher than the overall mean standing crop, as predicted by a model of systematic foraging. However, the sampling and searching constraints on the bees’ response to plant nectar levels increase the variation of the inter-visit time per flower, and thereby limit the degree of systematic flower visitation and the effect on the mean encountered gain.     

Bees assess pollen returns while sonicating Solanum flowers

Summary Can bees accurately gauge accumulating bodily pollen as they harvest pollen from flowers? Several recent reports conclude that bees fail to assess pollen harvest rates when foraging for nectar and pollen. A native nightshade (Solanum elaeagnifolium Cavanilles) that is visited exclusively for pollen by both solitary and social bees (eg. Ptiloglossa and Bombus) was studied in SE Arizona and SW New Mexico. The flowers have no nectaries. Two experiments were deployed that eliminated pollen feedback to the bees by experimentally manipulating flowers prior to bee visits. The two methods were 1) plugging poricidal anthers with glue and 2) emptying anthers of pollen by vibration prior to bee visitation. Both experiments demonstrated that bees directly assess pollen harvest on a flower-by-flower basis, and significantly tailor their handling times, number of vibratile buzzes per flower and grooming bouts according to the ongoing harvest on a given flower. In comparison to experimental flowers, floral handling times were extended for both Bombus and Ptiloglossa on virgin flowers. Greater numbers of intrafloral buzzes and numbers of times bees groomed pollen and packed it into their scopae while still on the flower were also more frequent at virgin versus experimental flowers. Flowers with glued andreocia received uniformly brief visits from Bombus and Ptiloglossa with fewer sonications and virtually no bouts of grooming. Curtailed handling with few buzzes and grooms also characterized visits to our manually harvested flowers wherein pollen was artificially depleted. Sonicating bees respond positively to pollen-feedback while harvesting from individual flowers, and therefore we expect them to adjust their harvesting tempo according to the currency of available pollen (standing crop) within Solanum floral patches.     

Better safe than sorry? A Fabaceae species exhibits unfavourable pollen properties for developing bee larvae despite its hidden anthers

Empirical evidence suggests that pollen chemistry plays an important role in shaping the pollen host spectra of many bee species. Although the underlying mechanisms are poorly understood, pollen diets of several plant taxa have experimentally been found to impede larval development of unspecialized bees. The pollen of all plant taxa, for which such a detrimental effect on bee larval development has been observed so far, is freely accessible in the flowers and thus easily harvestable for flower visitors, suggesting that this pollen might be chemically protected in order to reduce its loss to pollen-feeding animals. In the present study, we compared larval performance of five solitary bee species on pollen diets of the two Fabaceae species Onobrychis viciifolia and Lotus corniculatus, which have their anthers concealed inside the flowers, with that on control diets composed of host pollen provisions. As the complex flower morphology of the two Fabaceae species already considerably narrows the spectrum of pollen harvesting bee taxa, which might supersede costly chemical protection of the pollen, we expected bees that usually do not exploit Fabaceae to develop well on Onobrychis and Lotus pollen diets. Larval survival on the Onobrychis pollen diet was successful for all five bee species tested. In contrast, larval survival on the Lotus pollen diet was reduced in three species despite the fact that Lotus flowers are more difficult to exploit for pollen than Onobrychis flowers. We conclude that there is no trade-off between pollen concealment and pollen defence in Lotus and that pollen of morphologically complex flowers with a restricted visitor spectrum is not necessarily an easy-to-use nutritional source.     

Nectar robbing of a carpenter bee and its effects on the reproductive fitness of Glechoma longituba (Lamiaceae)

The floral biology and pollination process of Glechoma longituba (Nakai) Kuprian, a clonal gynodioecious, perennial herb that is widely distributed in China was investigated in natural populations. During visits to the flowers of G. longituba, the carpenter bee—Xylocopa sinensis mainly displayed nectar-robbing behavior with minimal pollination. Nectar robbing behavior began at the onset of flowering and continued for about 3 weeks ending at about the middle of the flowering period. A total of 18.6% floral visits in this period were by nectar robbers, with about 90% of the flowers in the study populations being subjected to two or two nectar-robbing episodes. During controlled experiments, lower pollination efficiency was recorded for X. sinensis compared to legitimate pollinators. Pollination by the robber-like pollinator X. sinensis during the early-mid phase of the flowering season yielded seeds of 16.16%. Although nectar robbing by the carpenter bee seemed to have a slight enhancing effect on seed set in G. longituba, this effect was effectively masked by the more pronounced detrimental effect of nectar robbing. Experiments indicated that nectar robbing by the carpenter bee reduced seed production by more than 26%, largely owing to the consequent shortening of the life span of robbed flowers. Furthermore, 10.43% of the ovules and 13.04% of the nectaries in the robbed flowers were damaged, thus causing a decrease or entire loss of reproductive opportunity in the robbed flowers. In addition, a higher number of pollen grains remained on the androecia of robbed flowers indicating that nectar robbing may have a lowering effect on male fitness in G. longituba.     

Floral biology of the saguaro (Cereus giganteus)

A saguaro cactus (Cereus giganteus) produces an average of 295 flowers per season, each of which produces 286 mg fresh weight of pollen and 543 mg of nectar containing 24% sugar. At 7600 pollen grains/mg pollen, the yearly output per saguaro plant is 6.4×10⁸ grains. Based on the measured saguaro density of 6.56 plants/ha, 553 g/ha of pollen is produced yearly. The enormous variation among individual plants in terms of flower numbers and floral bloom patterns is documented.Honey bees (Apis mellifera L.), the main collectors of saguaro pollen, collect an average of 12.2 mg pollen per foraging trip and can thus harvest 23.5 pollen loads from one flower. An average honey bee colony collects 290 g of saguaro pollen over the season, which is 24.4% of their total intake. Individual colonies exhibit wide variation in pollen collecting activities with some closely tracking the pollen resource and others almost totally ignoring it. The average for seven colonies indicates that even though variation is great the overall trend is toward closely tracking and exploiting the saguaro pollen resource. Based on the pollen productivity of saguaro and a hypothetical 90% pollen harvesting efficiency of bees, the pollen harvest potential of the saguaro environment is 1.72 colony equivalents of pollen/ha and 0.5/ha for saguaro alone. This is the first quantitative reporting of the total pollen productivity and pollen resource utilization for any plant and an opportunistic pollinator.     

Honey bee handling behaviour on the papilionate flower of Robinia pseudoacacia L.

Papilionate flowers, such as those of Robinia pseudoacacia L., show tripping mechanisms that prevent pollen release: only those bees which apply the right force on petals induce pollen to be deposited on their bodies. Apis mellifera is considered a poor visitor of such flowers, since individuals are usually too weak to trip the mechanism. Despite this, the honey bee pays frequent visits to flowers of R. pseudoacacia and produces a much appreciated unifloral honey. We investigated how bees manipulate R. pseudoacacia flowers, whether they contact the plant’s reproductive core and if there is any appreciable difference related to the manipulation of individual flowers. Honey bees showed two strategies for resource collection, namely legitimate visits and robberies. Legitimate visits were more frequent and about 63 % entailed contact with the flower’s reproductive core. We distinguished two behaviours, one to achieve successful positioning on the flower and the other for nectar intake. These behaviours were clearly perceptible and described by different curves of time frequency distribution. From the beginning to the end of anthesis, flowers were classified into four types on the basis of their morphological and phenological traits. Positioning time differed significantly depending on the flower type, with less time needed for more ageing flowers. Time spent in nectar intake was instead highly variable and independent of flower ageing. Selecting the right flower type would appear to lead to obtaining the R. pseudoacacia reward, overcoming species-specific physical inability. Moreover, the role of honey bees as pollinators of R. pseudoacacia is considered. Finally, the relations between petal characteristics and strength needed to trip the mechanism in papilionate flowers is also discussed in the light of nectar foragers.     

Pollination in Jacaranda rugosa (Bignoniaceae): euglossine pollinators, nectar robbers and low fruit set

Nectar robbers access floral nectar in illegitimate flower visits without, in general, performing a pollination service. Nevertheless, their effect on fruit set can be indirectly positive if the nectar removal causes an incremental increase in the frequency of legitimate flower visits of effective pollinators, especially in obligate outcrossers. We studied pollination and the effect of nectar robbers on the reproductive fitness of Jacaranda rugosa , an endemic shrub of the National Park of Catimbau, in the Caatinga of Pernambuco, Brazil. Xenogamous J . rugosa flowers continuously produced nectar during the day at a rate of 1 μl·h⁻¹. Female and male Euglossa melanotricha were the main pollinators. Early morning flower visits substantially contributed to fruit set because stigmas with open lobes were almost absent in the afternoon. Ninety-nine per cent of the flowers showed damage caused by nectar robbers. Artificial addition of sugar water prolonged the duration of flower visits of legitimate flower visitors. Removal of nectar, simulating the impact of nectar robbers, resulted in shorter flower visits of euglossine bees. While flower visits of nectar-robbing carpenter bees ( Xylocopa frontalis , X . grisescens, X . ordinaria) produced only a longitudinal slit in the corolla tube in the region of the nectar chamber, worker bees of Trigona spinipes damaged the gynoecium in 92% of the flowers. This explains the outstandingly low fruit set (1.5%) of J. rugosa in the National Park of Catimbau.     

Carpenter Bee (Xylocopa micans) Risk Indifference and a Review of Nectarivore Risk-sensitivity Studies

This paper presents new results of risk-sensitive foraging studiesof the carpenter bee, Xylocopa micans, and reviews the workto date on risk sensitivity in nectarivores. In the field, nectarivoreschoose among alternative food sources (flowers) that differin the variabilities of their nectar rewards. In the lab, theforaging situation for carpenter bees was experimentally simplifiedby offering the bees a choice between either "low variance"or "high variance" artificial flowers. The two flower typesdiffered in their variabilities but offered the same expectedshortterm rates of net energy gain to test the predictions ofthe short-term rate maximization mechanism. Foragers were testedunder two energy budget conditions, hungry and well-fed, totest the predictions of the z-score model. Individual carpenterbees were indifferent to variability in both nectar volume andnectar sugar concentrations, and their risk-indifference wasunaffected by energy budget. These findings of risk indifferencesupport neither the variance discounting nor the z-score modelof risk sensitivity. Since the low and high variance flowertypes are equivalent for carpenter bees in short-term rate ofenergy gain, there can be no selection on carpenter bees tobe sensitive to variability based on differences in rate ofgain. Studies of risk sensitivity in honey bees and bumble beesusing variance in nectar concentration support this contention.These findings are compared with other nectarivore risk sensitivitystudies in order to highlight the most likely mechanisms underlyingaversion to variation in nectar rewards (short-term rate maximizing,the Weber-Fechner law of perception and learning non-empty flowers)and to suggest future research in the interplay of these threemechanisms.     

Chronobiologic Aspects of a Coadaptive Process: The Interaction of Ludwigia elegans Flowers and Their More Frequent Bee Visitors

Field observations spanning 1 year of the visits of two bee species—Tet-raglossula bigamica (Hymenoptera: Colletidae) and Pseudagapostemon brasil-iensis (Hymenoptera: Halictidae)—to the flowers of Ludwigia elegans (Onagra-ceae) evinced the existence of circadian and circannual components in the rhythms of activity both of plant and bees. In addition to morphologic adaptations to the flower, the bees were temporally adjusted to each other and to the flower's opening and closing mechanisms. At the observation locality (Ribeirao Preto, São Paulo, Brazil—latitude 21 °-21 ° 30'S), the plant bloomed year-round. The flowers of the same population open simultaneously at the onset of the photophase, pollen and nectar being available almost immediately. Withering occurs in the afternoon of the same day. Although many bee species visited the flowers, T. bigamica and P. brasiliensis were the most frequent visitors, but their yearly rhythms were phased opposite to one another. Circadian rhythms were detected in pollen and nectar harvest. Comparisons of acrophases showed significant differences for males' and females' nectar collecting rhythm and for females' pollen harvest rhythm. As for the circannual component, acrophases of all rhythms occur at progressively later times from December to July when the migration is reversed until November. These data strongly suggest that the relationship between L. elegans and T. bigamica and P. brasiliensis is in accordance with a clearly temporal pattern and that the two bee species are temporally separated in nature.     

Bees provide pollination service to Campsis radicans (Bignoniaceae), a primarily ornithophilous trumpet flowering vine

1. Pollination syndromes refer to stereotyped floral characteristics (flower colour, shape, etc.) that are associated with a functional group of pollinators (bee, bird, etc.).
2. The trumpet creeper Campsis radicans, endemic to the southeast and mid‐west United States, has been assigned to the hummingbird‐pollination syndrome, due mainly to its red, trumpet‐shaped flowers.
3. Previous studies demonstrated that the ruby‐throated hummingbird Archilochus colubris is C. radicans' primary pollinator, but anecdotal data suggest various bee species may provide pollination service when hummingbirds are absent.
4. This study characterised C. radicans nectar volume and concentration by time of day. Nectar volume was suitable for hummingbirds, but concentration was higher than typical hummingbird‐pollinated plants (～20% w/w); at ～30% w/w, it approached the concentration expected in bee‐pollinated plants (～50% w/w). We also found substantial amounts of nectar at night.
5. Two C. radicans populations received virtually no hummingbird visits, but the number of bees were markedly higher than in the populations previously described. Interestingly, there were no night‐time visitors despite the large quantity of nocturnal nectar.
6. Based on previously published pollen delivery per visit by various species, this study estimated that cumulative deposition by bees routinely reached pollen deposition thresholds for setting fruit in C. radicans. They are, unequivocally, the predominant pollinators in these populations, thus providing pollination service in the absence of hummingbirds.
7. These results highlight C. radicans as a food source for native bees and add to the understanding of how floral phenotypes can facilitate pollination by disparate functional groups.

     

Pollen in Bee-Flower Relations Some Considerations on Melittophily*

Pollen and nectar are usually lumped together as floral rewards for pollinating bees, but they play totally different roles for flowers and bees (Table 1), as well as in the relationship between them. While flowers are specialized for certain pollinators via nectar, bees specialize on certain flowers via pollen. While flowers need pollen as a prerequisite for pollination, it is the essential larval food in bees. Thus, there is a strong competition between them for pollen. Foraging for pollen must be divided into three phases: uptake in the flower, reloading into and homeward transport within a carrying container. Bees have specializations for transport but hardly any for pollen uptake - and thus for pollination. Bees actively harvesting pollen usually do not pollinate. This only happens as a consequence of contamination of the bee by pollen. From these data a scenario is provided for the evolution of bees and bee flowers. Specialized bee flowers are often characterized by their ability to hide pollen from the bees and at the same time use them as optimal pollinators. If the relationship of bees and flowers is mutualistic at all it is best described as a balanced mutual exploitation.     

Forage quality and quantity affect red mason bees and honeybees differently in flowers of strawberry varieties

Nectar is a vital source of energy for bees and other pollinators and pollen represents the only source of protein in the diet of bees. Nectar and pollen quality and quantity can therefore affect foraging choices. Strawberry, Fragaria × ananassa (Rosaceae), is a flowering crop that requires insect pollination for the berries to develop optimally. The solitary red mason bee, Osmia bicornis L. (Hymenoptera: Megachilidae), occurs naturally but like the eusocial western honeybee, Apis mellifera mellifera L. (Hymenoptera: Apidae), it is also a commercially reared pollinator used in strawberry production. We hypothesized that strawberry nectar and pollen quality would affect the foraging choice of these two types of bees. In this study nectar and pollen quality is represented by various levels of sugar and protein content, respectively, as well as the number of open strawberry flowers in the experimental field, would affect the foraging choice of these two types of bees. Consistent with previous studies, we found significant and major differences between strawberry varieties in proportions of sucrose in the nectar sugar and in pollen viability – a proxy for pollen protein content. All measured parameters had a significant effect on red mason bee visitation frequency. Contrary to expectations, honeybee foraging behavior was only affected by the number of open flowers and not by any of the quality parameters measured. Our findings indicate that red mason bees were capable of assessing nectar and pollen quality and prioritize accordingly. The pattern observed indicates that individual red mason bees changed foraging focus between strawberry varieties depending on whether nectar or pollen was collected. Our results suggest that targeted breeding of varieties toward high levels of nectar sugar and sucrose concentrations and high pollen protein content may increase pollination success from red mason bees and possibly other solitary bees.     

Behavioural differences between male and female carpenter bees in nectar robbing and its effect on reproductive success in Glechoma longituba (Lamiaceae)

Male and female nectar robbers may show significantly different behaviour on host plants and thus have different impacts on reproductive fitness of the plants. A 4-year study in natural populations of Glechoma longituba has shown that male carpenter bees (Xylocopa sinensis) are responsible for most of the nectar robbing from these flowers, while female bees account for little nectar robbing, demonstrating distinct behavioural differentiation between male and female bees in visiting flowers. The smaller male bee spends less time visiting a single flower than the larger female bee, consequently, the male bee is capable of visiting more flowers per unit time and has a higher foraging efficiency. Moreover, the robbing behaviour of female carpenter bees is more destructive and affects flower structures (ovules and nectaries) and floral life-span more than that of the male bee. According to the energy trade-off hypothesis, the net energy gain for male bees during nectar robbing greatly surpasses energy payout (17.72 versus 2.43 J), while the female bee net energy gain is barely adequate to meet energy payout per unit time (3.78 versus 2.39 J). The differences in net energy gain for male and female bees per unit time in nectar robbing are the likely cause of observed behavioural differences between the sexes. The differences in food resource preference between male and female bees constitute an optimal resource allocation pattern that enables the visitors to utilise floral resources more efficiently.     

The superiority of bumblebees to honeybees as pollinators: insect visits to raspberry flowers

Abstract.

• 1 The behaviour and activity patterns of Apis mellifera and of five species of Bombus were analysed in relation to climatic variables and nectar quality on three varieties of unsprayed cultivated raspberry (Rubus idaeus) in eastern Scotland.
• 2 Stages of floral morphology and reward were similar for the three varieties: young flowers offered both nectar and pollen, but medium and old flowers offered nectar only, in diminishing quantities.
• 3 A wide range of insects visited raspberry flowers, but bees were dominant, bumblebees being responsible for about 60% of all visits and honeybees making up most of the remaining percentage. All bees had substantial pollen deposited on their bodies during visits, though few specifically collected it.
• 4 Bombus spp. were found to favour young (receptive) flowers strongly, especially early in the morning when pollen was most abundant: whilst Apis visited unselectively. Bumblebees also foraged over substantially longer periods of the day, and in poorer weather, some being present at most times of observation; and they foraged more quickly in terms of flower visits per minute.
• 5 Bombus carried more pollen on their bodies than Apis, and also deposited more pollen on raspberry stigmas, with B.lapidarius and B.terrestris being particularly effective and also being the most abundant species. All bumblebees also foraged over a longer range, moving between canes and rows more frequently than did honeybees.
• 6 Bumblebees are therefore likely to be substantially more important as pollinators of raspberries than are honeybees, especially as raspberries though moderately self-fertile may exhibit metaxenia. Reasons why Bombus may be the preferred pollinator in most sites of raspberry cultivation are discussed, together with implications for present and future growers.