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.     

Bees use honest floral signals as indicators of reward when visiting flowers

Pollinators visit flowers for rewards and should therefore have a preference for floral signals that indicate reward status, so called ‘honest signals’. We investigated honest signalling in Brassica rapa L. and its relevance for the attraction of a generalised pollinator, the bumble bee Bombus terrestris (L.). We found a positive association between reward amount (nectar sugar and pollen) and the floral scent compound phenylacetaldehyde. Bumble bees developed a preference for phenylacetaldehyde over other scent compounds after foraging on B. rapa. When foraging on artificial flowers scented with synthetic volatiles, bumble bees developed a preference for those specific compounds that honestly indicated reward status. These results show that the honesty of floral signals can play a key role in their attractiveness to pollinators. In plants, a genetic constraint, resource limitation in reward and signal production, and sanctions against cheaters may contribute to the evolution and maintenance of honest signalling.     

Departure rules used by bees foraging for nectar: A field test

Summary Departure rules used by solitary long-tongued bees (Anthophora spp. andEucera spp.) collecting nectar from flowers ofAnchusa strigosa (Boraginaceae) were studied. The amount of nectar a bee receives from an individual flower was estimated by measuring the time elapsed since the previous bee visit to that flower. Measurements of nectar accumulation in experimentally emptied flowers indicated that this time interval is an accurate predictor of nectar volumes in flowers. We found that nectar rewards influence the probability of departure from individual plants, as well as distances of movements within plants. The probability of departure from individual plants was negatively related to the amount of reward received at the two lastvisited flowers. This result indicates that the bees used a probabllistic departure rule, rather than a simple threshold departure rule, and that rewards from both the current and the previously visited flower were important in determining departure points. Distances of inter-flower movements within plants were negatively related to the amount of reward received at the current flower. The overall results suggest that the pollinators ofA. strigosa make two types of departure decisions-departures from the whole plant and departures from the neighbourhood of individual flowers-and that they use different departure rules for each scale. Factors influencing the decision-making processes of the observed foraging behaviour are discussed.     

Evolution of honest reward signal in flowers

Some flowering plants signal the abundance of their rewards by changing their flower colour, scent or other floral traits as rewards are depleted. These floral trait changes can be regarded as honest signals of reward states for pollinators. Previous studies have hypothesized that these signals are used to maintain plant-level attractiveness to pollinators, but the evolutionary conditions leading to the development of honest signals have not been well investigated from a theoretical basis. We examined conditions leading to the evolution of honest reward signals in flowers by applying a theoretical model that included pollinator response and signal accuracy. We assumed that pollinators learn floral traits and plant locations in association with reward states and use this information to decide which flowers to visit. While manipulating the level of associative learning, we investigated optimal flower longevity, the proportion of reward and rewardless flowers, and honest- and dishonest-signalling strategies. We found that honest signals are evolutionarily stable only when flowers are visited by pollinators with both high and low learning abilities. These findings imply that behavioural variation in learning within a pollinator community can lead to the evolution of an honest signal even when there is no contribution of rewardless flowers to pollinator attractiveness.     

Honeybees mark with scent and reject recently visited flowers

Summary Experimental evidence is reported for flower-marking by honeybees (Apis mellifera ligustica) while they were foraging on an artificial patch of flowers yielding a continuous and equal flow of sucrose solution. Honeybees marked with scent and rejected all recently visited and nectar-depleted flowers. The short fade-out time of this scent allowed discrimination of flowers that temporarily provided no food. The repellent nature of this scent mark was demonstrated by the use of an air extractor connected to the patch; when the apparatus was turned on, the rejection behaviour disappeared. The movement pattern of foraging bees also contributed to foraging efficiency, as the probability of an immediate return to the flower just abandoned was very low. However, when a quick repeat visit took place, the presence of the repellent scent-mark promoted rapid rejection.     

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

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

Floral Temperature and Optimal Foraging: Is Heat a Feasible Floral Reward for Pollinators?

As well as nutritional rewards, some plants also reward ectothermic pollinators with warmth. Bumble bees have some control over their temperature, but have been shown to forage at warmer flowers when given a choice, suggesting that there is some advantage to them of foraging at warm flowers (such as reducing the energy required to raise their body to flight temperature before leaving the flower). We describe a model that considers how a heat reward affects the foraging behaviour in a thermogenic central-place forager (such as a bumble bee). We show that although the pollinator should spend a longer time on individual flowers if they are warm, the increase in total visit time is likely to be small. The pollinator's net rate of energy gain will be increased by landing on warmer flowers. Therefore, if a plant provides a heat reward, it could reduce the amount of nectar it produces, whilst still providing its pollinator with the same net rate of gain. We suggest how heat rewards may link with plant life history strategies.     

The repellent scent-mark of the honeybeeApis mellifera tigustica and its role as communication cue during foraging

Summary Experimental evidence for flower-marking in honeybees (Apis mellifera ligustica), using pairs of workers from the same colony foraging on an artificial patch of flowers, is reported. Workers marked artificial flowers with scent and strongly rejected all flowers they had recently visited. The same rejection behavior, in a lower although significant proportion, was observed when bees visited flowers just abandoned by the other individual of the pair. The repellent nature of this scent-mark was demonstrated with the use of an air extractor connected to the patch of artificial flowers. When the apparatus was turned on, the rejection behavior disappeared and bees accepted both flowers just abandoned by themselves and flowers just abandoned by the other bee. Differences in the response level of bees to their own marks or to the partner's marks suggest that the repellent scent-mark applied by a bee during foraging would basically be a self-use signal, although it certainly has value in communicating with other workers.     

Honeybee (Apis mellifera ligustica) Response to Differences in Handling Time, Rewards and Flower Colours

Free flying honeybees were tested outdoors on blue–white and blue–yellow dimorphic artificial flower patches to examine the influence of reward difference, flower handling‐time difference and flower colour choice on foraging decisions. We employed different flower‐well depths to vary handling times (costs), and differences in sucrose molarity to vary reward quality. Tests were performed with 2 and 6 μl rewards to vary quantity. We show that when handling time is correlated with flower‐colour morphs on a pedicellate artificial flower patch, a honeybee's foraging behaviour is dependent on the flower colours used in the choice tests. This supports a honeybee foraging model where constraints are a significant factor in decision making. Bees visiting blue–yellow flower patches exhibited flower constancy to colour, where they restricted most visits to a single flower colour, some bees to blue and others to yellow, irrespective of handing time differences. When offered a choice of equally rewarding blue or white flowers, bees were not constrained by flower colour and chose to visit flowers with a lower handling time. When reward molarity varied with well depth between blue and white flowers, foragers chose shallow‐well flowers (short‐handling time) with a smaller net harvest rate over deep‐well flowers (long‐handling time) with a greater net harvest rate. Results using the blue–white dimorphic flower patch suggest that when foraging options simultaneously involve reward and handling‐time choices, honeybee forager behaviour is inconsistent with an absolute method of evaluating profit.     

Ineffectiveness of nectar scent in generating bumblebee visits to flowers of Impatiens textori

To clarify if bumblebees can recognize nectar through its scent in Impatiens textori flowers, we examined the behavior of Bombus diversus on nectarless flowers in which the spurs had been artificially removed. Bumblebee visits to both natural flowers and spur‐cut flowers were captured using a long‐term video recording system. Visiting behavior and frequency were compared between the two flower types. Many bumblebees visited both types of flower, and their visit frequencies were not significantly different. However, the length of stay on each flower type did differ, with the bumblebees remaining on the spur‐cut flowers for a significantly shorter time than on the natural flowers. Our results suggest that bumblebees cannot detect the absence of nectar in I. textori flowers before probing them. Therefore, the nectar scent of I. textori does not serve to attract bumblebees although the presence of nectar will detain bumblebees on flowers for longer periods.     

Staminal hairs enhance fecundity in the pollen‐rewarding self‐incompatible lily Bulbine abyssinica

Flowering plants typically use floral rewards to attract animal pollinators. Unlike nectar, pollen rewards are usually visible and may thus function as a signal that influences landing decisions by pollen‐seeking insects. Here we artificially manipulate the presence of both pollen and staminal hairs (a putative false signal of pollen reward availability) in the hermaphroditic lily Bulbine abyssinica (Xanthorrhoeaceae) to investigate their effects on bee visitation and fecundity, and also test for trade‐offs between pollen production and seed production. Honeybees, the primary floral visitors, are probably not able to distinguish between colours of petals, staminal hairs and pollen of B. abyssinica, according to analysis of reflectance spectra in a bee vision model. Flowers with both pollen and hairs removed had the lowest levels of bee visitation, seed set and seed abortions. Flowers containing hairs had an ～50% increase in visitation rate and seed set compared with emasculated flowers, while intact controls had the highest seed abortion rate. Ovule discounting in intact flowers is probably due to ovarian self‐incompatibility (or strong early inbreeding depression) as ovules penetrated by tubes from self‐pollen uniformly failed to develop into seeds. These results show that staminal hairs can enhance plant fecundity by increasing attraction of pollen‐seeking insects to flowers without increasing the risk of ovule discounting through pollinator‐mediated self‐pollination. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 177 , 481–490.     

In a world of white,flower colour matters: A white–purple transition signals lack of reward in an alpine Euphrasia

The New Zealand alpine flora displays a range of unusual characteristics compared with other alpine floras, in particular the high frequency of species with small white flowers. The presence of both white and bright purple flowers on the same plant in the New Zealand alpine annual creeping eyebright (Euphrasia dyeri Wettst.) provides an ideal opportunity to investigate the significance of flower colour in an environment where coloured flowers are rare. The relationships among flower age, gender phase, reward availability and petal colour were assessed in natural populations of E. dyeri. The effect of pollination on flower colour was tested using hand pollination of bagged flowers. Direct observations and videos of flowers were used to assess patterns of flower visitation by native and introduced pollinators. Unpollinated white E. dyeri flowers changed from white to purple within 6 days. However, pollination of white flowers triggered a significantly faster colour change, typically within 1–2 days. White flowers had receptive stigmas, large amounts of lipid‐rich pollen and small amounts of nectar, whereas stigmas of purple flowers are not receptive and flowers did not provide pollen or nectar rewards. Flowers were mainly visited by native syrphid flies. Both native syrphids and introduced Bombus bees showed a marked avoidance of purple flowers, tending to preferentially visit white flowers. Our study suggests that flower colour change from white to bright purple in E. dyeri functions to direct pollinators to rewarding, receptive flowers. As many Euphrasia L. species are described as having variably coloured flowers, this mechanism may be more widespread in the genus. Furthermore, our results add to the growing evidence that the dominance of white flowers in the New Zealand alpine is not simply due to a lack of colour discrimination among pollinators.     

Changes in Florets’ Vertical Direction within Inflorescence Affects Pollinator Behavior,and Fitness in <i>Trifolium repens</i>

Ecological interactions between flowers and pollinators greatly affect the reproductive success. To facilitate these interactions, many flowers are known to display their attractive qualities, such as scent emission, flower rewards and floral vertical direction, in a rhythmic fashion. However, less is known about how plants regulate the relationship between these flower traits to adapt to pollinator visiting behavior and increase reproduction success. Here we investigated the adaptive significance of the flower bending from erect to downward in Trifolium repens. We observed the flowering dynamic characteristics (changes of vertical direction of florets, flowering number, pollen grain numbers, pollen viability and stigma receptivity over time after blossom) and the factors affecting the rate of flower bending in T. repens. Then we altered the vertical direction of florets in inflorescence of different types (upright and downward), and compared the pollinator behaviors and female reproductive success. Our results showed that florets opened sequentially in inflorescence, and then bend downwards slowly after flowering. The bending speed of florets was mainly influenced by pollination, and bending angle increased with the prolongation of flowering time, while the pollen germination rate, stigma receptivity and nectar secretion has a rhythm of “low-high-low” during the whole period with the time going. The visiting frequency of all the four species of pollinators on upward flowers was significantly higher than that of downward flowers, and they especially prefer to visit flowers with a bending angle of 30°–60°, when the flowers was exactly of the highest flower rewards (nectar secretion and number of pollen grains), stigma receptivity and pollen germination rate. The seed set ratio and fruit set ratio of upward flowers were significantly higher than downward flowers, but significantly lower than unmanipulated flowers. Our results indicated that the T. repens could increase female and male fitness by accurate pollination. The most suitable flower angle saves pollinators’ visiting energy and enables them to obtain the highest nectar rewards. This coordination between plants and pollinators maximizes the interests of them, which is a crucial factor in initiating specialized plant-pollinator relationships.     

Frequency-dependent selection by pollinators: mechanisms and consequences with regard to behaviour of bumblebees Bombus terrestris (L.) (Hymenoptera: Apidae)

Bombus terrestris , a typical pollinating insect species, was offered artificial flowers of two different corolla colours with the same sucrose solution reward in an array. Common colours were significantly preferred, and the strength of the frequency-dependent response increased as a result of learning. There were also frequency-independent biases towards blue flowers, probably because blue flowers appeared more conspicuous to bumblebees than yellow flowers, and the degree of preference for blue was greater when flowers had low nectar rewards. Flower-to-flower movements by individual bumblebees between flowers were non-random, were biased to movements within the same flower colour, and were also dependent on morph frequency. The mechanisms governing flower selection in bumblebees are discussed. Pollinators foraging similarly in a natural situation would induce positive frequency-dependent selection, assortative mating, and directional selection on different corolla colour morphs of the plant population being visited, resulting in stabilizing selection for a single flower colour.     

Individually recognizable scent marks on flowers made by a solitary bee

The marking of flowers with ephemeral scent is an underappreciated but vital element in the foraging behaviour of social bees. Using observational and experimental data, we tested whether a solitary bee (female Anthophora plumipes) uses scent marking while foraging on flowers of Cerinthe major in Portugal. Females used scent marks with at least two components that differed in their volatility and, furthermore, recognized the marks of different individuals. A very short-term component (<3 min) was attractive, resulting in the observed high level of immediate revisits: this component appeared to be adjusted according to the foraging needs of the moment. A longer-term component (<30 min) was initially repellent and matched the rate of nectar renewal; it, or the response to it, also appeared to be adjusted to the perceived level of nectar reward. There may be even longer-term effects associated with the specific foraging areas of individual bees. Observed differences in the way in which individuals responded to scent marks indicate that they may play a role as part of a dominance or exclusion mechanism among females. Copyright 2001 The Association for the Study of Animal Behaviour.     

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.     

The combined effect of color and odor on flower choice behavior of bumble bees in flower mimicry systems

Food-deceptive flowers are pollinated by animals that expecta reward but are cheated. Such plants profit from their similarityto rewarding plants and should develop signals that hinderdiscrimination. We use artificial rewarding model flowers andnonrewarding mimicking flowers that present similar visual cues. We test how additional scent cues change flower choiceof the mimic by bumble bees (Bombus terrestris) in two situations:(1) both flower types are simultaneously present and can becompared by the pollinator, and (2) both flower types are encounteredsuccessively in the absence of each other. We find that insituation 1, discrimination learning is greater if scents areused as cues for identifying the mimic, whether the mimic hasa different scent or if it is scentless while the model isscented. In situation 2, a generalization task, a scented mimicis avoided faster than a scentless one. Discrimination of themimic is poorest if it has the same scent as the model, thusdemonstrating a potential for scent mimicry, which has not yet been proved to exist among differently rewarding flowers. Thus,the best strategy for a mimic would be to have the same scentas the model, but this strategy may not be used due to evolutionaryconstraints. Alternatively, if there are several potentialmodels, then having no scent would be a better strategy thanmimicking just one of the models. In situation 1 flower discriminationby color cues is enhanced in the mere presence of scent, comparedto unscented controls, even if the scent does not provide a distinguishable cue itself. The results indicate that the presenceof scent may enhance color discrimination by improving attentiontowards visual cues and/or that combined color/odor cues maylead to better memory formation and retrieval.     

POLLINATOR EFFECTIVENESS AND SEED SET IN POPULATIONS OF AGALINIS STRICTIFOLIA (SCROPHULARIACEAE)

Examining variation in pollinator effectiveness and seed production resulting from single pollinator visits can provide a deeper understanding of how pollinators may influence reproduction in plant populations. When comparing populations, differences in the number of seeds produced from single pollinator visits to flowers may not always be attributable to differences in pollen deposition, but rather to differences in plant fecundity or resource availability. Pollinator effectiveness and seed production were studied for two populations over a 4-year period and were measured using single bee visit manipulations of flowers. No significant difference in pollinator effectiveness (pollen deposited on stigmas) was observed between the two populations. However, a significant difference between the two populations was observed in the number of seeds produced per flower. The Wellhouse population produced approximately three times as many seeds/flower from a single pollinator visit as did the Firefly Meadow population. Within each population, pollinators (Bombus pennsylvanicus and Apis mellifera) did not differ in the number of pollen grains deposited on stigmas or seeds produced per flower from single visit experiments. Differences in plant density, pollen viability, and ovules per flower also could not account for a significant amount of the variation. A resource augmentation experiment (water and fertilizer application) resulted in a decrease in seeds per flower per bee visit for the water treatment at the Wellhouse population only. For both populations, pollen deposition, pollen viability, and ovules per flower were unaffected by the resource augmentation. Alternative possibilities for the observed differences in seeds per flower per bee visit are discussed.     

Floral preferences of bumblebees (Bombus edwardsii) in relation to intermittent versus continuous rewards

Worker bumblebees, Bombus edwardsii, preferably feed from artificial flowers yielding the same (continuous) reward on each visit rather than from flowers yielding variable (intermittent) rewards, even though the long-term expectation of reward is the same at each type of flower. However, variation in degree of preference among individual bees is high. Preferences after long foraging experience correspond closely to early preferences. Rate of flower visitation increases as mean reward increases, and may accelerate preference formation. Preferences are discussed in light of processes thought to control learning in honeybees. From these findings we propose that reward variance and expected time between reinforcements be considered as constraints in models of optimal foraging behaviour.     

Does the Flower Constancy of Bumble Bees Reflect Foraging Economics?

We examined the effects of floral reward level and spatial arrangement on the propensity of bumble bees to exhibit flower constancy. In three separate experiments, we compared the flower constancy of bees on dimorphic arrays of blue and yellow flowers that differed either in reward concentration, reward volume, or inter‐flower distance. Overall, flower choice patterns varied among bees, ranging from random selection to complete constancy. When flowers contained greater reward volumes and were spaced farther apart, bees showed less flower constancy and more moves to closely neighbouring flowers. Changes in reward concentration had no effect on flower constancy; however, more dilute rewards produced shorter flight times between flowers. In addition, there was a strong positive relationship between degree of flower constancy and net rate of energy gain when flowers were spaced farther apart, indicating that constant bees were more economic foragers than inconstant bees. Together, these results support the view that the flower constancy of pollinators reflects an economic foraging decision.