Nonadditive effects of floral display and spur length on reproductive success in a deceptive orchid

Pollinators may mediate selection on traits affecting pollinator attraction and effectiveness, and while nonadditive effects of traits influencing the two components of pollination success are expected when seed production is pollen limited, they have been little studied. In a factorial design, we manipulated one putative attraction trait (number of flowers) and one putative efficiency trait (spur length) previously shown to be subject to pollinator-mediated selection in the deceptive orchid Dactylorhiza lapponica. Removal of half of the flowers reduced pollen removal, proportion of flowers receiving pollen, fruit set, and fruit mass compared to unmanipulated plants, while spur-tip removal increased fruit set and fruit mass but did not affect pollen removal or proportion of flowers receiving pollen. The effect of spur-tip removal on fruit mass was stronger among plants with intact number of flowers compared to plants with experimentally reduced number of flowers. The results demonstrate that number of flowers and spur length are direct targets of selection and may affect female fitness nonadditively. More generally, they show that the adaptive value of a given trait can depend on floral context and illustrate how experimental approaches can advance our understanding of the evolution of trait combinations.     

Work division of floral scent compounds in mediating pollinator behaviours

Floral scents are known as an olfactory signal for attracting pollinators, but why the flowers pollinated by highly specialised pollinators emit scents consisting of mixtures of many compounds and dominated by one or a few compounds is still poorly understood. We supposed that each (especially characteristic) chemical in floral scents may play a specific role in mediating pollinator behaviours and tested this supposition in a fig-fig wasp mutualism. Ficus curtipes is obligately pollinated by an undescribed Eupristina species. In the scent of F. curtipes receptive figs, over 50 compounds have been identified, and the scent is dominated by two compounds, 6-methyl-5-hepten-2-ol (OL) and 6-methyl-5-hepten-2-one (NE). We therefore tested the roles of the two major chemicals in mediating the pollinator behaviours. Our results show that OL and NE, respectively, act as a long-distance attractant and a fig-entry behaviour stimulant to the obligate pollinator wasp. Namely, OL attracts the wasps to the figs and NE guides the wasps into the figs. This finding on the work division of floral scent compounds partially explains the maintenance mechanism of the fig-fig wasp mutualism and the significance of the chemical diversity of floral scent in plant–pollinator interactions, especially in specialised pollination systems.     

Separating selection by diurnal and nocturnal pollinators on floral display and spur length in Gymnadenia conopsea

Most plants attract multiple flower visitors that may vary widely in their effectiveness as pollinators. Floral evolution is expected to reflect interactions with the most important pollinators, but few studies have quantified the contribution of different pollinators to current selection on floral traits. To compare selection mediated by diurnal and nocturnal pollinators on floral display and spur length in the rewarding orchid Gymnadenia conopsea, we manipulated the environment by conducting supplemental hand-pollinations and selective pollinator exclusions in two populations in central Norway. In both populations, the exclusion of diurnal pollinators significantly reduced seed production compared to open pollination, whereas the exclusion of nocturnal pollinators did not. There was significant selection on traits expected to influence pollinator attraction and pollination efficiency in both the diurnal and nocturnal pollination treatment. The relative strength of selection among plants exposed to diurnal and nocturnal visitors varied among traits and populations, but the direction of selection was consistent. The results suggest that diurnal pollinators are more important than nocturnal pollinators for seed production in the study populations, but that both categories contribute to selection on floral morphology. The study illustrates how experimental manipulations can link specific categories of pollinators to observed selection on floral traits, and thus improve our understanding of how species interactions shape patterns of selection.     

Herbivory-mediated pollinator limitation: negative impacts of induced volatiles on plant-pollinator interactions

Although induced plant responses to herbivory are well studied as mechanisms of resistance, how induction shapes community interactions and ultimately plant fitness is still relatively unknown. Using a wild tomato, Solanum peruvianum, native to the Peruvian Andes, we evaluated the disruption of pollination as a potential ecological cost of induced responses. More specifically, we tested the hypothesis that metabolic changes in herbivore-attacked plants, such as the herbivore-induced emission of volatile organic compounds (VOCs), alter pollinator behavior and consequentially affect plant fitness. We conducted a series of manipulative field experiments to evaluate the role of herbivore-induced vegetative and floral VOC emissions as mechanisms by which herbivory affects pollinator behavior. In field surveys and bioassays in the plants' native habitat, we found that real and simulated herbivory (methyl jasmonate application) reduced attractiveness of S. peruvianum flowers to their native pollinators. We show that reduced pollinator preference, not resource limitation due to leaf tissue removal, resulted in reduced seed set. Solitary bee pollinators use floral plant volatiles, emitted in response to herbivory or methyl jasmonate treatment, as cues to avoid inflorescences on damaged plants. This herbivory-induced pollinator limitation can be viewed as a general cost of induced plant responses as well as a specific cost of herbivory-induced volatile emission.     

Autecology of the tailed jay butterfly Graphium agamemnon (Lepidoptera : Rhopalocera : Papilionidae)

The Tailed Jay Graphium agamemnon is one of the attractive papilionid butterflies that enliven the environment of Visakhapatnam. It occurs throughout the year. It lays eggs singly on young leaves of the mast tree Polyalthia longifolia var. pendula (Annonaceae). The eggs take 3-4 days to hatch. The larvae go through 5 instars over a period of 15-16 days. The pupal period is 13-14 days. The total period from egg to adult emergence spans over 33-36 days. Based on this short life cycle, and larval and pupal development success studied every month, this butterfly can be multivoltine with a minimum of 7-8 broods in a year. Both CI and GR decreased with the age of larva, their average figures being 3.78 and 0.43 respectively. AD values are high (average 92%) and decreased through successive instars. Both ECD and ECI followed a similar pattern with an increase from instar I up to II, then a decrease up to IV and again an increase in instar V and the highest value is with fifth instar. Adults frequently visited flowers (12-35 flowers in a min) spending 1.0 to 3.2 seconds on a flower. The nectar concentration ranged between 16 and 58%. Peak foraging activity mostly fell between 0900-1000 h. The proboscis received pollen in most of the floral species visited, thus satisfying one of the characteristics of butterfly pollination. Being a fast and strong flier it is treated as "high energy" pollinator promoting cross-pollination.     

Mimics and magnets: the importance of color and ecological facilitation in floral deception

Plants that lack floral rewards can attract pollinators if they share attractive floral signals with rewarding plants. These deceptive plants should benefit from flowering in close proximity to such rewarding plants, because pollinators are locally conditioned on floral signals of the rewarding plants (mimic effect) and because pollinators are more abundant close to rewarding plants (magnet effect). We tested these ideas using the non-rewarding South African plant Eulophia zeyheriana (Orchidaceae) as a study system. Field observations revealed that E. zeyheriana is pollinated solely by solitary bees belonging to a single species of Lipotriches (Halictidae) that appears to be closely associated with the flowers of Wahlenbergia cuspidata (Campanulaceae), a rewarding plant with which the orchid is often sympatric. The pale blue color of the flowers of E. zeyheriana differs strongly from flowers of its congeners, but is very similar to that of flowers of W. cuspidata. Analysis of spectral reflectance patterns using a bee vision model showed that bees are unlikely to be able to distinguish the two species in terms of flower color. A UV-absorbing sunscreen was applied to the flowers of the orchid in order to alter their color, and this resulted in a significant decline in pollinator visits, thus indicating the importance of flower color for attraction of Lipotriches bees. Pollination success in the orchid was strongly affected by proximity to patches of W. cuspidata. This was evident from one of two surveys of natural populations of the orchid, as well as experiments in which we translocated inflorescences of the orchid either into patches of W. cuspidata or 40 m outside such patches. Flower color and location of E. zeyheriana plants relative to rewarding magnet patches are therefore key components of the exploitation by this orchid of the relationship between W. cuspidata and Lipotriches bee pollinators.     

Trait-mediated effects on flowers: artificial spiders deceive pollinators and decrease plant fitness

Although predators can affect foraging behaviors of floral visitors, rarely is it known if these top-down effects of predators may cascade to plant fitness through trait-mediated interactions. In this study we manipulated artificial crab spiders on flowers of Rubus rosifolius to test the effects of predation risk on flower-visiting insects and strength of trait-mediated indirect effects to plant fitness. In addition, we tested which predator traits (e.g., forelimbs, abdomen) are recognized and avoided by pollinators. Total visitation rate was higher for control flowers than for flowers with an artificial crab spider. In addition, flowers with a sphere (simulating a spider abdomen) were more frequently visited than those with forelimbs or the entire spider model. Furthermore, the presence of artificial spiders decreased individual seed set by 42% and fruit biomass by 50%. Our findings indicate that pollinators, mostly bees, recognize and avoid flowers with predation risk; forelimbs seem to be the predator trait recognized and avoided by hymenopterans. Additionally, predator avoidance by pollinators resulted in pollen limitation, thereby affecting some components of plant fitness (fruit biomass and seed number). Because most pollinator species that recognized predation risk visited many other plant species, trait-mediated indirect effects of spiders cascading down to plant fitness may be a common phenomenon in the Atlantic rainforest ecosystem.     

Comparison of the flower scent of the sexually deceptive orchid <Emphasis Type="Italic">Ophrys iricolor</Emphasis> and the female sex pheromone of its pollinator <Emphasis Type="Italic">Andrena morio</Emphasis>

Summary. Ophrys flowers mimic the female produced sex pheromone of their pollinator species to attract males for pollination. The males try to copulate with the putative female and thereby pollinate the flower. Using electrophysiological and chemical analyses, floral volatiles released by O. iricolor as well as the female sex pheromone of its pollinator species, Andrena morio are investigated. Overall, 38 peaks comprising 41 chemical compounds, were found to release reactions in the antennae of male A. morio bees. Analyses using coupled gas chromatography-mass spectrometry revealed the presence of alkanes and alkenes with 20 to 29 carbon atoms, aldehydes (C9 to C24) and two esters. Almost all of those compounds were found in similar proportions in both, the floral extracts of O. iricolor and cuticle surface extracts of A. morio females. The pattern of biologically active volatiles described here is very similar to that used by other Ophrys species pollinated by Andrena males.     

Flower color influences insect visitation in alpine New Zealand

Despite a long-standing belief that insect pollinators can select for certain flower colors, there are few experimental demonstrations that free-flying insects choose between natural flowers based on color. We investigated responses of insect visitors to experimental manipulations of flower color in the New Zealand alpine. Native syrphid flies (Allograpta and Platycheirus) and solitary bees (Hylaeus and Leioproctus) showed distinct preferences for visiting certain flower species. These responses were determined, in part, by flower color, as insects also responded to experimental manipulations of visible petal color in 7 out of 11 tests with different combinations of flower species and insect type. When preferences were detected, syrphid flies chose yellow over white petals regardless of flower species, whereas Hylaeus chose white over yellow Ourisia glandulosa. In some cases, the strength and direction of color preference depended on the context of other floral traits, in which case the response usually favored the familiar, normal combination of traits. Syrphid flies also visited in response to floral morphological traits but did not show preference based on UV reflectance. The unusually high preponderance of white flowers in the New Zealand alpine is not explained by complete generalization of flower color choice. Instead, the insect visitors show preferences based on color, including colors other than white, along with other floral traits. Furthermore, they can respond in complex ways to combinations of floral cues, suggesting that traits may act in nonadditive ways in determining pollinator visitation.     

Facilitation in an insect-pollinated herb with a floral display dimorphism

Population context should influence pollination success and selection on floral display in animal-pollinated plants because attraction of pollinators depends not only on the characteristics of individual plants, but also on the attractiveness of co-occurring conspecifics. The insect-pollinated herb Primula farinosa is polymorphic for inflorescence height. Natural populations may include both long-scaped plants, which present their flowers well above the soil surface, and short-scaped plants, with their flowers positioned close to the ground. We experimentally tested whether seed production in short-scaped P. farinosa varied with local morph frequency and surrounding vegetation height. In tall vegetation, short-scaped plants in polymorphic populations produced more fruit and tended to produce more seeds than short-scaped plants did in monomorphic populations. In low vegetation, population composition did not significantly affect fruit and seed output of short-scaped plants. The results suggest that long-scaped plants facilitate short-scaped plants in terms of pollinator attraction and that the facilitation effect is contingent on the height of the surrounding vegetation. The documented facilitation should contribute to the maintenance of the scape length polymorphism in ungrazed areas where litter accumulates and vegetation grows tall.     

Nectar secondary compounds affect self-pollen transfer: implications for female and male reproduction

Pollen movement within and among plants affects inbreeding, plant fitness, and the spatial scale of genetic differentiation. Although a number of studies have assessed how plant and floral traits influence pollen movement via changes in pollinator behavior, few have explored how nectar chemical composition affects pollen transfer. As many as 55% of plants produce secondary compounds in their nectar, which is surprising given that nectar is typically thought to attract pollinators. We tested the hypothesis that nectar with secondary compounds may benefit plants by encouraging pollinators to leave plants after visiting only a few flowers, thus reducing self-pollen transfer. We used Gelsemium sempervirens, a plant whose nectar contains the alkaloid gelsemine, which has been shown to be a deterrent to foraging bee pollinators. We found that high nectar alkaloids reduced the total and proportion of self-pollen received by one-half and one-third, respectively. However, nectar alkaloids did not affect female reproduction when we removed the potential for self-pollination (by emasculating all flowers on plants). We then tested the assumption that self-pollen in combination with outcrossed pollen depresses seed set. We found that plants were weakly self-compatible, but self-pollen with outcrossed pollen did not reduce seed set relative to solely outcrossed flowers. Finally, an exponential model of pollen carryover suggests that high nectar alkaloids could benefit plants via increased pollen export (an estimate of male function), but only when pollinators were efficient and abundant and plants had large floral displays. Results suggest that high nectar alkaloids may benefit plants via increased pollen export under a restricted set of ecological conditions, but in general, the costs of high nectar alkaloids in reducing pollination balanced or outweighed the benefits of reducing self-pollen transfer for estimates of female and male reproduction.     

Effects of pollinator density-dependent preferences on field margin visitations in the midst of agricultural monocultures: A modelling approach

Managed field margins offer a means of reducing the impact of agricultural monocultures within intensively managed environments. By providing refuge for wild plants and the pollinators associated with them, field margins can also contribute to enhancing the pollination services within the monoculture. However, the effects of the monoculture on pollinator behaviour need to be carefully considered. It is known that pollinators may show density-dependent preferences such as neophobia (an avoidance of unfamiliar items) when different types of flower are available within their environment, and the dominance of monoculture crops within the environment may consequently have adverse effects upon the preferences shown by pollinators living in the field margins within them. In order to examine how pollinator preferences for wild flowers are affected by monocultures, we modelled the effects of density-dependent preferences, flower densities, and the geometry of field margins within a monoculture landscape using numerical simulations. This was done by considering how the placement of pollinator nests within a simple, spatially explicit landscape consisting of fields of monoculture crops separated by margins containing wild flowers affected the ratio of wild and monoculture crops experienced by the pollinator, given that it could only forage within a limited distance from its nest. Increasing field margin width and decreasing monoculture field width both led to an increase in pollinators visiting wild flowers (which levelled off as width increased). The size of the monoculture fields had little additional effect once they had passed an intermediate width. Increasing wild flower density within the margins led to a shift away from preferring monocultures. When wild flowers were at low densities compared to the monoculture, even the addition of small amounts of extra wild flowers had a large effect in shifting foraging preferences away from the monoculture. The distance which pollinators normally forage over only has an effect upon preferences for wild flowers when the travel distance is small. This suggests that careful consideration of margin design might be extremely important for those species which do not travel far. Innate preferences for density-dependence and particular crop types may also have an effect on preference behaviour. We demonstrate that the way in which resources are presented to indigenous pollinators may be extremely important in influencing where they choose to forage within agricultural landscapes. Careful margin design, as well as increasing the density of wild flowers (such as by enhancing the wild seed bank within the margins), may lead to overall improvements in ecosystem function within intensively farmed monocultures.     

Anther-mimicking floral guides exploit a conflict between innate preference and learning in bumblebees (<Emphasis Type="Italic">Bombus terrestris</Emphasis>)

Bee-pollinated plants are frequently dichogamous: e.g. each flower has a discernable male and female phase, with only the male phase offering a pollen reward. Pollen-collecting bees should therefore discriminate against female-phase flowers to maximise their rate of pollen harvest, but this behaviour would reduce plant fitness due to inferior pollination. Here, we test the hypothesis that flowers use pollen-mimicking floral guides to prevent flower-phase discrimination. Such floral guides resemble pollen in spectral reflection properties and are widespread among angiosperm flowers. In an array of artificial flowers, bumblebees learned less well to discriminate between flower variants simulating different flowering phases when both flower variants carried an additional pollen-yellow guide mark. This effect depended crucially on the pollen-yellow colour of the guide mark and on its spatial position within the artificial flower. We suggest that floral guides evolved to inhibit flower-phase learning in bees by exploiting the innate colour preferences of their pollinators.     

Geographic structure of pollinator communities, reproductive assurance, and the evolution of self-pollination

Reproductive assurance is often invoked as an explanation for the evolution of self-fertilization in plants. However, key aspects of this hypothesis have received little empirical support. In this study, I use geographic surveys of pollinator communities along with functional studies of floral trait variation to examine the role of pollination ecology in mating system differentiation among populations and subspecies of the annual plant Clarkia xantiana. A greenhouse experiment involving 30 populations from throughout the species' range indicated that variation in two floral traits, herkogamy and protandry, was closely related to levels of autofertility and that trait variation was partitioned mainly among populations. Emasculation experiments in the field showed that autonomous selfing confers reproductive assurance by elevating fruit and seed production. Surveys of pollinator communities across the geographic range of the species revealed that bee pollinator abundance and community composition differed dramatically between populations of the outcrossing subspecies xantiana and the selfing subspecies parviflora despite their close proximity. Specialist bee pollinators of Clarkia were absent from selfing populations, but they were the most frequent visitors to outcrossing populations. Moreover, within the outcrossing subspecies xantiana, there was a close correspondence between specialist abundance and population differentiation in herkogamy, a key mating system trait. This spatial covariation arose, in part, because geographically peripheral populations had reduced herkogamy, higher autofertility, and lower pollinator abundance compared to central populations of xantiana. Finally, I detected strong spatial structure to bee communities both across the range of the species and within the outcrossing subspecies. In both cases, spatial structure was stronger for specialist bees compared to generalist bees, and pollinator communities varied in parallel with population variation in herkogamy. These results provide evidence that mating system differentiation parallels spatial variation in pollinator abundance and community composition at both broad and more restricted spatial scales, consistent with the hypothesis that pollinator abundance and reproductive assurance are important drivers of plant mating system evolution.     

Predispersal seed herbivores, not pollinators, exert selection on floral traits via female fitness

Herbivores that oviposit in flowers of animal-pollinated plants depend on pollinators for seed production and are therefore expected to choose flowers that attract pollinators. This provides a mechanism by which seed herbivores and pollinators could impose conflicting selection on floral traits. We measured phenotypic selection on floral traits of Lobelia siphilitica (Lobeliaceae) via female fitness to determine the relative strength of selection by pollinators and a specialist predispersal seed herbivore. We were able to attribute selection on flowering phenology to the herbivores. However, no selection could be attributed to pollinators, resulting in no conflicting selection on floral traits. Unlike pollinators, whose preference for certain floral traits does not always translate into higher fitness, any discrimination by seed herbivores is likely to decrease fitness of the preferred floral phenotype. Thus predispersal seed herbivores may be a significant agent of selection on floral traits.     

How habitat affects the benefits of communication in collectively foraging honey bees

Honey bees (Apis mellifera) use the dance language to symbolically convey information about the location of floral resources from within the nest. To figure out why this unique ability evolved, we need to understand the benefits it offers to the colony. Previous studies have shown that, in fact, the location information in the dance is not always beneficial. We ask, in which ecological habitats do honey bee colonies actually benefit from the dance language, and what is it about those habitats that makes communication useful? In this study, we examine the effects of floral distribution patterns on the benefits of dance communication across five different habitats. In each habitat, we manipulated colonies' ability to communicate and measured their foraging success, while simultaneously characterizing the naturally occurring floral distribution. We find that communication is most beneficial when floral species richness is high and patches contain many flowers. These are ecological features that could have helped shape the evolution of the honey bee dance language.     

Predation risk makes bees reject rewarding flowers and reduce foraging activity

In the absence of predators, pollinators can often maximize their foraging success by visiting the most rewarding flowers. However, if predators use those highly rewarding flowers to locate their prey, pollinators may benefit from changing their foraging preferences to accept less rewarding flowers. Previous studies have shown that some predators, such as crab spiders, indeed hunt preferentially on the most pollinator-attractive flowers. In order to determine whether predation risk can alter pollinator preferences, we conducted laboratory experiments on the foraging behavior of bumble bees (Bombus impatiens) when predation risk was associated with a particular reward level (measured here as sugar concentration). Bees foraged in arenas containing a choice of a high-reward and a low-reward artificial flower. On a bee’s first foraging trip, it was either lightly squeezed with forceps, to simulate a crab spider attack, or was allowed to forage safely. The foragers’ subsequent visits were recorded for between 1 and 4 h without any further simulated attacks. Compared to bees that foraged safely, bees that experienced a simulated attack on a low-reward artificial flower had reduced foraging activity. However, bees attacked on a high-reward artificial flower were more likely to visit low-reward artificial flowers on subsequent foraging trips. Forager body size, which is thought to affect vulnerability to capture by predators, did not have an effect on response to an attack. Predation risk can thus alter pollinator foraging behavior in ways that influence the number and reward level of flowers that are visited.     

Reduced pollinator service and elevated pollen limitation at the geographic range limit of an annual plant

Mutualisms are well known to influence individual fitness and the population dynamics of partner species, but little is known about whether they influence species distributions and the location of geographic range limits. Here, we examine the contribution of plant-pollinator interactions to the geographic range limit of the California endemic plant Clarkia xantiana ssp. xantiana. We show that pollinator availability declined from the center to the margin of the geographic range consistently across four years of study. This decline in pollinator availability was caused to a greater extent by variation in the abundance of generalist rather than specialist bee pollinators. Climate data suggest that patterns of precipitation in the current and previous year drove variation in bee abundance because of its effects on cues for bee emergence in the current year and the abundance of floral resources in the previous year. Experimental floral manipulations showed that marginal populations had greater outcross pollen limitation of reproduction, in parallel with the decline in pollinator abundance. Although plants are self-compatible, we found no evidence that autonomous selfing contributes to reproduction, and thus no evidence that it alleviates outcross pollen limitation in marginal populations. Furthermore, we found no association between the distance to the range edge and selfing rate, as estimated from sequence and microsatellite variation, indicating that the mating system has not evolved in response to the pollination environment at the range periphery. Overall, our results suggest that dependence on pollinators for reproduction may be an important constraint limiting range expansion in this system.     

外来植物大花老鸭嘴在迁入地的传粉机制和繁殖策略

外来植物在迁入地的生存和繁殖策略,是入侵生物学研究的热门话题之一。开展入侵植物的繁殖策略研究,有助于深入了解入侵生物在迁入地的生存繁衍机制,对入侵植物在可控范围内的适当使用具有重要的指导意义。通过野外传粉昆虫观测、光学和电镜扫描观测和人工控制授粉等方法,对外来植物大花老鸦嘴（Thunbergia grandiflora）在广州和深圳的4个居群的开花物候、花部结构、花粉活力、传粉昆虫及其行为等传粉生物学特征进行了研究。结果表明：大花老鸦嘴的单花花期为1 d,上午10：00时花粉活力最高,长雄蕊花粉活力可达73.92%,短雄蕊花粉活力可达71.54%,随后花粉活力逐渐下降。主要传粉者为扁柄木蜂（Xylocopa latipes）,开花当天访花高峰期的9：00—10：00,单花访花频率可达5 h-1以上,之后开始逐渐减少,14：00之后未观测到扁柄木蜂访花,18：00后仅剩蚂蚁等体型较小的昆虫长期栖息于花序或花蕾。控制授粉实验结果显示,无论是人工异株授粉、自花授粉,还是自然授粉,结果率均为0,4个居群均未见结实。光学显微镜下子房发育完整;电镜扫描观测结果显示其花粉在柱头上能萌发,但花粉管不能延伸到子房位置。目前在园林栽培的大花老鸭嘴均以块根或茎为繁殖体培育苗木,虽然这些植株均能开出鲜艳的花朵,花粉传递者也充足,但未见任何结果现象。因此,初步推测本次研究区域内的大花老鸭嘴种群可能来自相同或相近的无性系,故不同居群间异株授粉的不结果现象可能为“自交不亲和”,或“染色体多倍化”所致。     

Choosing rewarding flowers; perceptual limitations and innate preferences influence decision making in bumblebees and honeybees

Flowers exhibit great intra-specific variation in the rewards they offer. At any one time, a significant proportion of flowers often contain little or no reward. Hence, foraging profitably for floral rewards is problematic and any ability to discriminate between flowers and avoid those that are less rewarding will confer great advantages. In this study, we examine discrimination by foraging bees among flowers of nasturtium, Tropaeolum majus. Bee visitors included carpenter bees, Xylocopa violacea, which were primary nectar robbers; honeybees, Apis mellifera, which either acted as secondary nectar robbers or gathered pollen legitimately and bumblebees, Bombus hortorum, which were the only bees able to gather nectar legitimately. Many flowers were damaged by phytophagous insects. Nectar volume was markedly lower in flowers with damaged petals (which were also likely to be older) and in flowers that had nectar-robbing holes. We test whether bees exhibit selectivity with regards to the individual flowers, which they approach and enter, and whether this selectivity enhances foraging efficiency. The flowers approached (within 2 cm) by A. mellifera and B. hortorum were non-random when compared to the floral population; both species selectively approached un-blemished flowers. They both approached more yellow flowers than would be expected by chance, presumably a reflection of innate colour preferences, for nectar standing crop did not vary according to flower colour. Bees were also more likely to accept (land on) un-blemished flowers. A. mellifera gathering nectar exhibited selectivity with regards to the presence of robbing holes, being more likely to land on robbed flowers (they are not able to feed on un-robbed flowers). That they frequently approached un-robbed flowers suggests that they are not able to detect robbing holes at long-range, so that foraging efficiency may be limited by visual acuity. Nevertheless, by using a combination of long-range and short-range selectivity, nectar-gathering A. mellifera and B. hortorum greatly increased the average reward from the flowers on which they landed (by 68% and 48%, respectively) compared to the average standing crop in the flower population. Overall, our results demonstrate that bees use obvious floral cues (colour and petal blemishes) at long-range, but can switch to using more subtle cues (robbing holes) at close range. They also make many mistakes and some cues used do not correlate with floral rewards.