Foraging strategies of insects for gathering nectar and pollen, and implications for plant ecology and evolution

The majority of species of flowering plants rely on pollination by insects, so that their reproductive success and in part their population structure are determined by insect behaviour. The foraging behaviour of insect pollinators is flexible and complex, because efficient collection of nectar or pollen is no simple matter. Each flower provides a variable but generally small reward that is often hidden, flowers are patchily distributed in time and space, and are erratically depleted of rewards by other foragers. Insects that specialise in visiting flowers have evolved an array of foraging strategies that act to improve their efficiency, which in turn determine the reproductive success of the plants that they visit. This review attempts a synthesis of the recent literature on selectivity in pollinator foraging behaviour, in terms of the species, patch and individual flowers that they choose to visit.

The variable nature of floral resources necessitate foraging behaviour based upon flexible learning, so that foragers can respond to the pattern of rewards that they encounter. Fidelity to particular species allows foragers to learn appropriate handling skills and so reduce handling times, but may also be favoured by use of a search image to detect flowers. The rewards received are also used to determine the spatial patterns of searches; distance and direction of flights are adjusted so that foragers tend to remain within rewarding patches and depart swiftly from unrewarding ones. The distribution of foragers among patchy resources generally conforms to the expectations of two simple optimal foraging models, the ideal free distribution and the marginal value theorem.

Insects are able to learn to discriminate among flowers of their preferred species on the basis of subtle differences in floral morphology. They may discriminate upon the basis of flower size, age, sex or symmetry and so choose the more rewarding flowers. Some insects are also able to distinguish and reject depleted flowers on the basis of ephemeral odours left by previous visitors. These odours have recently been implicated as a mechanism involved in interspecific interactions between foragers.

From the point of view of a plant reliant upon insect pollination, the behaviour of its pollinators (and hence its reproductive success) is likely to vary according to the rewards offered, the size and complexity of floral displays used to advertise their location, the distribution of conspecific and of rewards offered by other plant species, and the abundance and behaviour of other flower visitors.     

Statistical modeling of insect behavioral response to changes in weather conditions in <Emphasis Type="Italic">Brassica napus</Emphasis> L.

Understanding specific meteorological factors that affect the foraging activity of pollinator insects can provide valuable information to ensure appropriate levels of pollination of crops. This research was carried out to verify the spectrum of pollinating insects and their foraging behavior in relation to specific meteorological variables in the Brassica napus (rapeseed) crop. Data collected consisted of the following observations: number of visits by each pollinator, floral resources collected by Africanized honeybee Apis mellifera L., number of flowers visited by Africanized honeybees in 1 min, number of flowers visited by Africanized honeybees in one specific plant during the timed observation of 1 min, and the time taken by the honeybees to visit each flower. In this study the analyses were made through different generalized linear models. The Africanized honeybees were the most abundant pollinating insects (88%) and most visitors were observed collecting nectar (90%). The Africanized honeybees visited a total of 12.9?±?1.40 flowers in 1 min and 2.96?±?1.09 flowers of a single plant in 1 min. The time the honeybees spent on the rapeseed flowers was 4.2?±?1.6 s. The number of floral visitors correlated closely to the variation of abiotic factors, especially temperature, relative humidity, wind speed, and solar radiation. Africanized honeybees were more active at different times of the day as compared to the other floral visitors and they foraged more intensely on the Hyola 433 hybrid than on the Hyola 61 hybrid. The Hyola 61 hybrid was more attractive to Diabrotica speciosa and Lepidoptera.     

Reversal Learning and Risk‐Averse Foraging Behavior in the Monarch Butterfly,Danaus plexippus (Lepidoptera: Nymphalidae)

Learning ability allows insects to respond to a variable environment, and to adjust their behaviors in response to positive or negative experiences. Pollinating insects readily learn to associate floral characteristics, such as color, shape, or pattern, with appetitive stimuli, such as the presence of a nectar reward. However, in nature pollinators may also encounter flowers that contain distasteful or toxic nectar, or offer highly variable nectar volumes, providing opportunities for aversive learning or risk‐averse foraging behavior. Whereas some bees learn to avoid flowers with unpalatable or unreliable nectar rewards, little is known about how Lepidoptera respond to such stimuli. We used a reversal learning paradigm to establish that monarch butterflies learn to discriminate against colored artificial flowers that contain salt solution, decreasing both number of probes and probing time on flowers of a preferred color and altogether avoiding artificial flowers of a non‐preferred color. In addition, when we offered butterflies artificial flowers of two different colors, both of which contained the same mean nectar volume but which differed in variance, the monarchs exhibited risk‐averse foraging: they probed the constant flowers significantly more than the variable ones, regardless of flower color or butterfly sex. Our results add to our understanding of butterfly foraging behavior, as they demonstrate that monarchs can respond to aversive as well as appetitive stimuli, and can also adjust their foraging behavior to avoid floral resources with high variance rewards.     

Floral visitors and ant scent marks: noticed but not used?

1. Bee behaviour when visiting flowers is mediated by diverse chemical cues and signals, from the flower itself and from previous visitors to the flower. Flowers recently visited by bees and hoverflies may be rejected for a period of time by subsequent bee visitors. 2. Nectar‐thieving ants also commonly visit flowers and could potentially influence the foraging decisions of bees, through the detection of ant trail pheromones or footprint hydrocarbons. 3. Here we demonstrate that, while naÏve bumblebees in laboratory trials are not inherently repelled by ant scent marks, they can learn to use them as informative signals while foraging on artificial flowers. 4. To test for similar activity in the wild, visitor behaviours at the flowers of Digitalis purpurea Linnaeus, Bupleurum fruticosum Linnaeus, and Brassica juncea (Linnaeus) Czernajew were compared between flowers that had been in contact with ants and those that had not. No differences were found between the two treatments. 5. The use of chemical foraging cues by bees would appear to be strongly dependent on previous experience and in the context of these plant species bees did not associate ant scent mark cues with foraging costs.     

Does Invasion of Exotic Plants Promote Invasion of Exotic Flower Visitors? A Case Study from the Temperate Forests of the Southern Andes

Habitat disturbance, particularly of human origin, promotes the invasion of exotic plants, which in turn might foster the invasion of alien-interacting animals. Here we assess whether the invasion of exotic plants – mostly mediated by habitat disturbance – facilitates the invasion of exotic flower visitors in temperate forests of the southern Andes, Argentina. We recorded visit frequencies and the identity of visitors to the flowers of 15 native and 15 exotic plant species occurring in different highly disturbed and less disturbed habitats. We identified three alien flower visitors, the hymenopterans Apis mellifera, Bombus ruderatus, and Vespula germanica. We found significantly more visitation by exotic insects in disturbed habitats. This pattern was explained, at least in part, by the association between alien flower visitors and flowers of exotic plants, which occurred more frequently in disturbed habitats. However, this general pattern masked different responses between the two main alien flower visitors. Apis mellifera exploited almost exclusively the flowers of a subset of herbaceous exotic plants that thrive under disturbance, whereas B. ruderatus visited equally flowers of both exotic and native plants in both disturbed and undisturbed habitats. We did not find any strong evidence that flowers of exotic plants were more generalist than those of native plants, or that exotic flower visitors were more generalist than their native counterparts. Our results suggest that alien plant species could facilitate the invasion of at least some exotic flower visitors to disturbed habitats. Because flowering plants as well as flower visitors benefit from this mutualism, this association may enhance, through a positive feedback, successful establishment of both exotic partners.     

Persistence of flower visitors and pollination services of a generalist tree in modified forests

Flower‐visiting insects provide essential pollination services to many plant species. It is thus of critical importance to understand the effects of anthropogenic landscape modification on these animals. Particularly at the landscape scale, we still lack information on how flower visitors are affected by different intensities of human disturbance. In this study, we chose six representative types of forest modification across a heterogeneous South African landscape. At 36 study sites we observed insect visitation to Celtis africana flowers in two consecutive years. This generalist tree species has small unspecialized flowers which we found to be pollinated by a diverse array of insects as well as by wind. Visitation rates to flowers of C. africana differed significantly among the six forest types and between two study years. Visitation rates were enhanced in modified forests, facilitated by a high abundance of feral honeybees (Apis mellifera). Fruit set in C. africana showed significant positive associations with insect visitation and with the diversity of flower visitors, but was only weakly predicted by forest type. Our findings imply that even though forest modification can strongly alter flower visitors, pollination services for trees with unspecialized flowers may persist at a landscape scale. We advise conservation managers to maintain modified forest fragments in addition to natural forests as these may contribute to sustain pollination services in human‐modified landscapes.     

Flower Visitors in a Natural Population of Arabidopsis thaliana

Abstract: Arabidopsis thaliana is commonly regarded as a self-pollinated plant species. One of the many surprises in population genetic studies of the species was the observation of distinct traces of recombination in the DNA sequences that may be the result of rare outcrossing events. We studied flower visitors in a natural population of the species. Solitary bees, diptera and thrips are among the most frequently observed insects among the surprising diversity of insects visiting flowers of A. thaliana. Assuming that every visit equals an outcrossing event, the outcrossing rate was estimated to be 0.84 %. This value falls between estimations of outcrossing rates from molecular data and those of artificial systems. Despite the rather low rate of flower visitation, A. thaliana can no longer be regarded as a completely self-pollinated plant species in the wild. This observation may explain recombination events observed in molecular analyses. Possible pollen transfer between populations due to the mobility of the observed insects should be considered in population genetic analyses.     

Models of optimal foraging and resource partitioning: deep corollas for long tongues

We model the optimal foraging strategies for 2 nectarivore species,differing in the length of their proboscis, that exploit thenectar provided by 2 types of flowers, differing in the depthsof their corollas. When like flowers appear in clumps, nectarivoresmust decide whether to forage at a patch of deep or shallowflowers. If nectarivores forage optimally, at least one flowertype will be used by a single nectarivore species. Long-tonguedforagers will normally visit deep flowers and short-tonguedforagers shallow flowers, although extreme asymmetries in metaboliccosts may lead to the opposite arrangement. When deep and shallowflowers are randomly interspersed, nectarivores must decide,on encounter with a flower, whether to collect its nectar orcontinue searching. At low nectarivore densities, the optimalstrategy involves exploiting every encountered flower; however,as nectarivore densities increase and resources become scarce,long-tongued individuals should start concentrating on deepflowers and short-tongued individuals on shallow flowers. Therefore,regardless of the spatial distribution of flowers, corolla depthcan determine which nectarivore species exploit the nectar fromeach flower type in a given community. It follows that corollaelongation can evolve as a means to keep nectar thieves at bayif short-tongued visitors are less efficient pollinators thanlong-tongued visitors.     

Significance of flower exploding pollination on the reproduction of the Scotch broom, <Emphasis Type="Italic">Cytisus scoparius</Emphasis> (Leguminosae)

The significance of single flower visits by pollinators on the reproductive success of the Scotch broom, Cytisus scoparius, with explosive flowers was investigated. Unexploded flowers (intact flowers) produced no fruit, implying that autonomous selfing and apomixis do not occur and that the explosion of flowers by insects is necessary for the fertilization of flowers. The fruit set in the natural condition was intermediate between that in artificial selfing or exploding and that in artificial outcrossing. Seed set and the ratio of seeds to ovules did not differ significantly between the natural condition and artificial outcrossing. The fruit set by a single flower visit from Apis mellifera was similar to natural conditions. Apis mellifera was the most frequent visitor and the number of flowers tripped open per hour was the highest among all the flower visitors. Approximately 75% of pollen on a flower disappeared in a single flower visit by A.mellifera. The small amount of pollen remaining on already-exploded flowers appears to be the reason why A.mellifera do not frequently visit already-exploded flowers. The higher seed set by outcrossing was believed to contribute to the seed set in both natural conditions and single flower visits by A.mellifera, and did not differ between them. Therefore, at least in Japan, A.mellifera appears to be the most important pollinator for C.scoparius, resulting in a high outcrossing rate with each single flower visit.     

Floral resource partitioning by social wasps (Hymenoptera: Vespidae) and bees (Hymenoptera: Apoidea) in an area of caatinga in Brazil

Wasps and bees are important components of flower visitor guilds in the Neotropical region. The abundance of social wasps and bees, collected during flower visits, was used to calculate niche breadth and niche overlap indices, in order to characterize the utilization of the floral resources by those insects. Samples were taken monthly, for 13 months (156h), by two collectors which captured the specimens on flowers using entomological nets, while walking along a way in an area of caatinga at Itatim, State of Bahia, Brazil. Wasps and bees heavily visited a few sources of floral resources, but the most visited plants by each group were different, and few plant species were important for both groups, resulting in low niche overlap between bees and wasps. The niche overlap among wasp species was generally higher than among bee species. The general overlap, which takes into consideration all species together was low (< 30%).     

Excised or intact inflorescences? Methodological effects on parasitoid wasp longevity

The development of accurate and repeatable experimental techniques is a cornerstone of any research program. Indeed, the first stage in developing a conservation biological control program typically involves ranking the suitability of various plant species as food resources for the target species of natural enemy in the laboratory or glasshouse. Herein the choice of flower presentation method is a highly relevant consideration. It is unclear whether excised flowers with their peduncles submerged in water will generate similar effects on the life history traits of a natural enemy compared with those using flowers remaining intact on a rooted plant. Either method has been used in 86 previous studies, yet none has quantified this effect. It is possible that both plant nectar content and production are altered as a result of changes in the physiological condition of the excised flowers. A laboratory test was designed to assess the influence of flower presentation method (excised or intact inflorescences) and different types of nectar (artificial and natural) on the longevity of the wasp Aphidius ervi, an important parasitoid of aphids. Distinct differences were revealed in the suitability of the nine flower species and three control treatments on parasitoid wasp longevity, with buckwheat being the most suitable plant. However, apart from coriander, flower presentation method and wasp gender generally did not affect parasitoid longevity for the set of species tested. As there was little evidence that parasitoid wasp longevity would be altered on excised flowers, and because of reasons pertaining to improved logistical and experimental requirements, the use of excised flowers is cautiously recommended to researchers for further laboratory evaluations of the effects of nectar provision on parasitoid fitness.     

Bumblebees Do Not Respond to Variance in Nectar Concentration

Worker bumblebees (Bombus fervidus) were given repeated binary choices between two colors of artificial flowers with the same associated mean nectar concentration (X? = 20%), but with different variances in nectar concentration. Flowers of one color, yellow or blue, rewarded a bee with 1 μl of 20% sucrose solution (low-variance flower type) on each visit (p = 1) and flowers of the other color rewarded a bee on each visit with 1 μl of either 10% or 30% sucrose (p = 0.5; high-variance flower type). Of the 10 bees tested, nine showed no preference for either the high- or low-variance flowers (indifferent or risk-insensitive). This result is similar to honeybee responses to variation in nectar concentration, despite differences in foraging ecology between bumblebees and honeybees. Flower-choice behaviour in the presence of variance in nectar concentration is a response to the expected concentration of the alternative flower types. Possible mechanisms of risk-sensitive foraging behaviour in bees are discussed.     

Crab spiders (Thomisidae) attract insect flower‐visitors without UV signalling

1. Crab spiders (Thomisidae) indirectly affect insect flower‐visitor and flowering plant interactions by consuming and altering the behaviour of insects. 2. Although one expects insect flower‐visitors to avoid crab spiders actively, some crab spider species are known to attract flower‐visitors. Crab spiders may use UV signalling to lure potential prey to the flowers they occupy. 3. In the present study, a field experiment was conducted to examine the effects of crab spiders occupying three prairie plant species for the insect flower‐visitor community. Pollinating insects were significantly attracted to inflorescences with crab spiders compared to inflorescences without crab spiders for two plant species, and herbivorous insects were attracted to inflorescences with crab spiders for one of these plant species. The two flowering plant species with increased pollinator visitation showed increased seed weights for plants with crab spiders, indicating crab spider presence indirectly increased pollination. 4. To test the UV signalling hypothesis, inflorescences with crab spiders of one plant species were observed under both a UV‐blocking plastic and a clear plastic control. Contrary to our prediction, flower‐visitors were not more likely to land on inflorescences under the clear plastic; the UV signalling hypothesis was not supported. Other unknown explanations underlie prey attraction to inflorescences with crab spiders.     

Geitonogamy in rewarding and unrewarding inflorescences: modelling pollen transfer on actual foraging sequences

Many orchid species are unusual in that they provide no nectar or pollen rewards for their pollinators. Absence of reward is expected to have a fundamental effect on pollinator visitation patterns. In particular the number of flowers visited per inflorescence is expected to be affected in both unrewarding and co-flowering rewarding species. We used arrays of artificial inflorescences, which could be either rewarding or unrewarding and were differentiated by their colour, to test how many flowers bumblebees visit in each type of inflorescence. The frequency of the two colours was varied, thus modelling the case where different frequencies of both an unrewarding and rewarding species were present in a patch. We found that bumblebees visited more flowers per rewarding inflorescence after they have experienced unrewarding or partially emptied rewarding inflorescences. We used these results to simulate pollen transfer and thus predict selfing rates on rewarding inflorescences. We found these increased when nectar depleted or when there was a greater proportion of unrewarding inflorescences in the patch. Conversely, we found that the number of flowers bumblebees visited on each unrewarding inflorescence did not significantly change through experiments. Selfing rates for unrewarding inflorescences were predicted to depend principally on the number of these inflorescences bumblebees visited rather than on the number of flowers they visit per inflorescence. This was because most visitors to orchids are supposed to be naive, and pollinators that commence foraging carrying no pollen will necessarily self any flower they pollinate on the first inflorescence they visit. Thus the average selfing rate is expected to increase as the sequence of inflorescences visited decreases in length.     

Status of self-pollen in bee pollination efficiency of white clover (Trifolium repens L.)

Flowers of white clover (Trifolium repens L.) are hermaphrodite and self-incompatible; their cross-pollination depends entirely on insect visitors, mainly bees (Apoidea). Because self-pollination of white clover occurs before flower anthesis, we determined whether selfing affected the pollination efficiency of a honeybee visit. We compared pollen deposition in emasculated and intact flowers following (1) a single honeybee visit, (2) open-pollination for a day and (3) enclosure in a cloth bag to prevent insect visits. In emasculated flowers, open-pollination resulted in more pollen deposited than after one visit (+30%) which is consistent with flowers being visited more than once by pollinators during the course of a day. On intact flowers, saturation of the stigma was achieved after the first visit of a honeybee (near 280 grains) because of self-pollination. Additional visits did not increase pollen deposits, but they improved pollen efficiency in terms of numbers of pollen tubes reaching the ovules. In such a context of easily saturated stigmas, self-pollen does not inhibit cross-pollen activity, but represents a constraint for pollination which demands multiple bee visits to each flower to achieve maximum fertilization. Received: 20 May 1997 / Accepted: 25 October 1997     

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.     

Are pollination syndromes useful predictors of floral visitors in Tasmania?

Abstract Diurnal visitors to the flowers of many native plant species were identified in a wide range of Tasmanian sclerophyllous vegetation between September 1996 and April 1997. These foraging profiles were analysed to determine whether they were characteristic of various floral morphologies in predictable ways. It was found that although visitor profiles were sometimes consistent with classic pollination syndromes, these syndromes were unreliable predictors of floral visitors. Very few flowers were exclusively bird‐pollinated, and none were strictly fly‐, beetle‐, wasp‐, or butterfly‐pollinated. The majority of flowering plants were unspecialized in their morphology, and consequently hosted a diverse array of visitors. In addition, visitor profiles to congeners with similar floral morphologies, and even to conspecifics, differed between habitats. Altitude was a major factor in determining visitors, with flies being the most abundant visitors above 700 m. However, congeners in several genera of Epacridaceae, as well as the genus Correa, which differed in floral morphology also differed in visitor profiles. Tubular flowers were associated with birds, while flowers with more accessible nectar were visited by insects. The only taxa exhibiting a bee‐pollination syndrome that were largely visited by bees were the Fabaceae and Goodenia ovata Sm. Several species with purple or pink flowers were also predominantly visited by bees, but did not strictly conform to the melittophilous syndrome. In contrast, other flowers exhibiting an ostensibly mellitophilous syndrome hosted very few bees. Of these, species that occurred at high altitude were mainly visited by flies, while others received very few potential pollen vectors.     

火把花的繁殖生物学研究

火把花（Colquhounia coccinea?）表现出典型的鸟媒综合征,因此具有作为引鸟景观植物的开发潜力。以自然、人工生境的火把花居群为研究对象,通过观察和试验对其开花物候、花部综合特征、访花动物及其行为、繁育系统、种子萌发特性进行研究,以明确火把花的繁殖特性及对不同访花动物的吸引潜力。结果显示,火把花的整体花期持续约3个月,单花花期为9.6±0.6 d;花蜜较丰富且稀薄,具有较短的花冠管和己糖为主的花蜜糖组成;访花动物主要是中华蜜蜂和多种食蜜鸟类,尤其是短喙的泛化鸟类,且在非自然生境中仍然能吸引鸟类访花;完全自交亲和但需要传粉者才能完成授粉,不存在花粉限制;不同授粉处理种子的发芽能力无显著差异;中华蜜蜂能有效传粉,鸟类的传粉作用需进一步验证。综上所述,火把花可供观赏的时间很长,具有明显的吸引鸟类访花的能力,容易通过有性繁殖途径快速获得大量幼苗。     

Nested structure is dependent on visitor sex in the flower‒visitor networks in Kyoto,Japan

The characteristics of flower‒visitor networks, comprised of multiple species interacting with each other, predict ecological and evolutionary processes. Intraspecific and interspecific variations in interaction patterns should affect network structures. Because female and male visitors usually differ in flower‐visiting patterns due to mating strategy, visitor sex should affect nestedness, in which specialist species interact with a subset of species that interact with generalist species. I hypothesized that a network of male visitors and flowering plants would be more nested than a female network because males are less picky about which flowers they visit. To examine the effect of visitor sex on nestedness, I used museum specimens of insects and built 11 flower–visitor species networks, each composed of female and male subnetworks, and compared the strength of nestedness and related network metrics between the subnetworks. I found that male subnetworks were significantly more nested than female ones, and species networks were less nested than male or female subnetworks. The result may be attributable to the by‐chance selection of flowers by males. Because a nested structure is predicted to promote community stability in mutualistic flower–visitor networks, the greater nestedness of male subnetworks may suggest a positive effect of male visitors on pollination community stability.     

Insights from measuring pollen deposition: quantifying the pre-eminence of bees as flower visitors and effective pollinators

Using our accumulated datasets from Kenyan savanna, Mediterranean garigue, UK gardens and heathland, involving 76 plants from 30 families, we present detailed data to quantify the superiority of bees as pollinators of most flowering plants when compared with other flower visitors. Bees provided the majority of visits to study species at all sites, and 33 of the 76 plants received more than 90% of their visits from bees. Furthermore, pollen deposition onto stigmas from single-visit events (SVD, a measure of pollination effectiveness) was significantly higher for bees than non-bees at all the four sites where a major proportion of the flora was sampled. Solitary bees, and also bumblebees in temperate habitats, were the best potential pollinators for most plants in this respect, and significantly out-performed honeybees. Only a few plants were well served by bombyliid flies, and fewer again by larger hoverflies, butterflies, or solitary wasps. Bees also achieved better matches of their visit timing to peak pollen availability (measured indirectly as peak SVD), and made much shorter visits to flowers than did non-bees, permitting a substantially greater visit frequency. Additionally, they deposited significantly lower levels of potentially deleterious heterospecific pollen on stigmas in heathland and Mediterranean garigue, though not in the UK garden with densely clustered high-diversity flowering, or in the Kenyan savanna site with particularly dispersed flowering patches and some specialist non-bee flowers. Our data provide a novel and quantified characterisation of the specific advantages of bees as flower visitors, and underline the need to conserve diverse bee communities.