Cryptic dioecy and insect pollination in Rosa setigera Michx. (Rosaceae), a rare plant of Carolinian Canada

Rosa setigera is unique among known roses because it is truly dioecious, yet the plants and flowers are almost impossible to sex. Subtle differences in the inflorescences have been detected, for example, there are more flowers per inflorescence on male plants than on female plants and petal expansion from the day of opening to the next day is greater in females than in males; in addition, pollen from female plants appears somewhat collapsed and does not germinate. Pollinators ( Apis mellifera , other bees (Apidae), Eristalis tenax and other Syrphidae: Diptera) visit the blossoms mostly in the morning to collect pollen (there is no nectar) and in choice experiments do not discriminate between male flowers and female flowers on landing, but do spend significantly more time on the former. They do discriminate between freshly opened flowers and day-old flowers, and show marked preference for bouquets of five flowers vs . single flowers. The female plants, with smaller inflorescences and lesser interest to pollen foraging insects, seem to encourage them to forage at more flowers than do their male counterparts. This may be biologically significant for effective pollen flow in a dioecious pollenonly plant with pollenivorous pollinators.     

Honey bees and wild pollinators differ in their preference for and use of introduced floral resources

Introduced plants may be important foraging resources for honey bees and wild pollinators, but how often and why pollinators visit introduced plants across an entire plant community is not well understood. Understanding the importance of introduced plants for pollinators could help guide management of these plants and conservation of pollinator habitat. We assessed how floral abundance and pollinator preference influence pollinator visitation rate and diversity on 30 introduced versus 24 native plants in central New York. Honey bees visited introduced and native plants at similar rates regardless of floral abundance. In contrast, as floral abundance increased, wild pollinator visitation rate decreased more strongly for introduced plants than native plants. Introduced plants as a group and native plants as a group did not differ in bee diversity or preference, but honey bees and wild pollinators preferred different plant species. As a case study, we then focused on knapweed (Centaurea spp.), an introduced plant that was the most preferred plant by honey bees, and that beekeepers value as a late‐summer foraging resource. We compared the extent to which honey bees versus wild pollinators visited knapweed relative to coflowering plants, and we quantified knapweed pollen and nectar collection by honey bees across 22 New York apiaries. Honey bees visited knapweed more frequently than coflowering plants and at a similar rate as all wild pollinators combined. All apiaries contained knapweed pollen in nectar, 86% of apiaries contained knapweed pollen in bee bread, and knapweed was sometimes a main pollen or nectar source for honey bees in late summer. Our results suggest that because of diverging responses to floral abundance and preferences for different plants, honey bees and wild pollinators differ in their use of introduced plants. Depending on the plant and its abundance, removing an introduced plant may impact honey bees more than wild pollinators.     

So many visitors and so few pollinators: variation in insect frequency and effectiveness governs the reproductive success of an endemic milkwort

Plant–pollinator interactions are one of the most important and variable mutualisms having major implications for plant fitness. The present study evaluates the interactions between an endemic milkwort, Polygala vayredae, and its floral visitors by studying the temporal variability, foraging behaviour and effectiveness of floral visitors in three populations during three consecutive years. The flowers were visited by a diverse array of insects, totalling 24 different species. However, only four species were effective pollinators, depositing pollen on stigmas after one visit, while the remaining species behaved as nectar robbers, secondary nectar robbers or nectar thieves and were completely ineffective for pollination. Among the effective pollinators, two groups with distinct foraging behaviours were observed: the nectar collecting long-tongued bees Bombus pascuorum and Anthophora sp. and the pollen collectors Eucera longicornis and Halictus sp. No significant differences were observed among pollinators in their efficiency in pollen deposition on stigmas, but significant differences were observed in the foraging behaviour between nectarivorous and pollen collectors. Variation in the abundance and assemblage of floral visitors was observed at the temporal scale and among populations, with the effective pollinators being generally scarce. Consequently, the reproductive outcome in this species was low and significantly variable among populations and years. The results highlight the importance of studying floral visitor effectiveness when determining pollinator assemblages.     

Pollination efficiency and foraging behaviour of honey bees and non‐Apis bees to sweet cherry

1. Crop pollination generally increases with pollinator diversity and wild pollinator visitation. To optimize crop pollination, it is necessary to investigate the pollination contribution of different pollinator species. In the present study, we examined this contribution of honey bees and non‐Apis bees (bumble bees, mason bees and other solitary bees) in sweet cherry.
2. We assessed the pollination efficiency (fruit set of flowers receiving only one visit) and foraging behaviour (flower visitation rate, probability of tree change, probability of row change and contact with the stigma) of honey bees and different types of non‐Apis bees.
3. Single visit pollination efficiency on sweet cherry was higher for both mason bees and solitary bees compared with bumble bees and honey bees. The different measures of foraging behaviour were variable among non‐Apis bees and honey bees. Adding to their high single visit efficiency, mason bees also visited significantly more flower per minute, and they had a high probability of tree change and a high probability to contact the stigma.
4. The results of the present study highlight the higher pollination performance of solitary bees and especially mason bees compared with bumble bees and honey bees. Management to support species with high pollination efficiency and effective foraging behaviour will promote crop pollination.

     

Foraging behavior affects pollen removal and deposition in Impatiens capensis (Balsaminaceae)

Flowers of most plant species are visited by a variety of animals. Some of these visitors are effective pollinators while others remove resources without transferring pollen. Studies comparing the effectiveness of different visitors as pollinators often compare taxa without considering variation in behavior within a taxon. Wilson and Thomson (Ecology 72: 1503-1507, 1991) documented the effects of honey bees and bumble bees on the pollination dynamics of Impatiens capensis. They found that pollen-collecting honey bees removed large numbers of pollen grains from anthers but deposited little of it on stigmas; bumble bees, which sought nectar, removed less pollen but deposited more of it on stigmas. It is unclear whether the low pollen transfer efficiencies of honey bees are explained by their morphology or by their pollen-collecting behavior. We repeated the work of Wilson and Thomson at a site where honey bees were foraging for nectar, not pollen. We measured the quantity of pollen remaining in anthers, the number of pollen grains deposited on stigmas, and seed production after single visits by honey bees and bumble bees. The differences between the taxa disappeared when they were foraging in a similar manner. Our results clearly demonstrate the importance of foraging behavior on the pollination effectiveness of floral visitors.     

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

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

     

Pollinator genetics and pollination: do honey bee colonies selected for pollen‐hoarding field better pollinators of cranberry Vaccinium macrocarpon?

1. Genetic polymorphisms of flowering plants can influence pollinator foraging but it is not known whether heritable foraging polymorphisms of pollinators influence their pollination efficacies. Honey bees Apis mellifera L. visit cranberry flowers for nectar but rarely for pollen when alternative preferred flowers grow nearby. 2. Cranberry flowers visited once by pollen‐foraging honey bees received four‐fold more stigmatic pollen than flowers visited by mere nectar‐foragers (excluding nectar thieves). Manual greenhouse pollinations with fixed numbers of pollen tetrads (0, 2, 4, 8, 16, 32) achieved maximal fruit set with just eight pollen tetrads. Pollen‐foraging honey bees yielded a calculated 63% more berries than equal numbers of non‐thieving nectar‐foragers, even though both classes of forager made stigmatic contact. 3. Colonies headed by queens of a pollen‐hoarding genotype fielded significantly more pollen‐foraging trips than standard commercial genotypes, as did hives fitted with permanently engaged pollen traps or colonies containing more larvae. Pollen‐hoarding colonies together brought back twice as many cranberry pollen loads as control colonies, which was marginally significant despite marked daily variation in the proportion of collected pollen that was cranberry. 4. Caloric supplementation of matched, paired colonies failed to enhance pollen foraging despite the meagre nectar yields of individual cranberry flowers. 5. Heritable behavioural polymorphisms of the honey bee, such as pollen‐hoarding, can enhance fruit and seed set by a floral host (e.g. cranberry), but only if more preferred pollen hosts are absent or rare. Otherwise, honey bees' broad polylecty, flight range, and daily idiosyncrasies in floral fidelity will obscure specific pollen‐foraging differences at a given floral host, even among paired colonies in a seemingly uniform agricultural setting.     

Adult pollen diet essential for egg maturation by a solitary Osmia bee

Reproduction is a nutritionally costly activity for many insects, as their eggs are rich in lipids and proteins. That cost seems especially acute for non-social bees, which for their size, lay enormous eggs. All adult female bees visit flowers, most of them to collect pollen and nectar, or sometimes oils, to feed their progeny. For adult bees, the need for pollen feeding has only been detailed for the honey bee, Apis mellifera. To experimentally test for the reproductive value of adult pollen feeding by a non-social bee, Osmia californica (Hymenoptera: Apiformes: Megachilidae), young female bees plus males were released into large glasshouse cages provided with either a male-fertile sunflower cultivar or a pollen-less one. Females regularly visited and drank nectar from flowers of both cultivars. Abundant orange pollen was seen regularly in guts of females confined with the male-fertile sunflowers, indicative of active pollen ingestion. All females’ terminal oocytes (next egg to be laid) were small at emergence. Oocytes of females confined with the pollen-less sunflowers remained small, despite frequent nectaring and exposure to other floral stimuli. In contrast, the basal oocytes of female O. californica with access to pollen had swelled to full size within ten days following emergence, enabling them to lay eggs in provided nest tubes. Adult females of this solitary bee required dietary pollen to reproduce; nitrogen stores acquired as larvae were inadequate. Early and regular pollen feeding in part paces the onset and maximum tempo of solitary bees’ lifetime reproductive output.     

The endangered Iris atropurpurea (Iridaceae) in Israel: honey-bees,night-sheltering male bees and female solitary bees as pollinators

Background and Aims

The coastal plain of Israel hosts the last few remaining populations of the endemic Iris atropurpurea (Iridaceae), a Red List species of high conservation priority. The flowers offer no nectar reward. Here the role of night-sheltering male solitary bees, honey-bees and female solitary bees as pollinators of I. atropurpurea is documented.

Methods

Breeding system, floral longevity, stigma receptivity, visitation rates, pollen loads, pollen deposition and removal and fruit- and seed-set were investigated.

Key Results

The main wild pollinators of this plant are male eucerine bees, and to a lesser extent, but with the potential to transfer pollen, female solitary bees. Honey-bees were found to be frequent diurnal visitors; they removed large quantities of pollen and were as effective as male sheltering bees at pollinating this species. The low density of pollen carried by male solitary bees was attributed to grooming activities, pollen displacement when bees aggregated together in flowers and pollen depletion by honey-bees. In the population free of honey-bee hives, male bees carried significantly more pollen grains on their bodies. Results from pollen analysis and pollen deposited on stigmas suggest that inadequate pollination may be an important factor limiting fruit-set. In the presence of honey-bees, eucerine bees were low removal–low deposition pollinators, whereas honey-bees were high removal–low deposition pollinators, because they removed large amounts into corbiculae and deposited relatively little onto receptive stigmas.

Conclusions

Even though overall, both bee taxa were equally effective pollinators, we suggest that honey-bees have the potential to reduce the amount of pollen available for plant reproduction, and to reduce the amount of resources available to solitary bee communities. The results of this study have potential implications for the conservation of this highly endangered plant species if hives are permitted inside reserves, where the bulk of Oncocyclus iris species are protected.     

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

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

In search of the best pollinators for canola (<Emphasis Type="Italic">Brassica napus</Emphasis> L.) production in Pakistan

To discover the pollinator community of canola (Brassica napus L.) and the best pollinators for canola production, an experiment was performed at the research farm of Bahauddin Zakariya University, Multan, Pakistan. The insect pollinator community was composed of 35 species in 3 orders and 14 families. Most of the bees (Hymenoptera) and a butterfly species (Lepidoptera) foraged for nectar, whereas all the flies (Diptera) foraged either for pollen or both nectar and pollen. Eight major pollinators were tested for their pollination efficiency. The nectar-robbing behavior of many species made it difficult to judge the efficiency of an insect on the basis of visitation rate and stay time; therefore, the amounts of pollen deposited and pollen harvested per visit were also measured. The single visit efficiency in terms of the number of seeds per pod revealed that Apis dorsata, A. florea, and Halictus sp. were superior for canola pollination, having Spears’ values of 1.62, 1.55 and 1.73, respectively. With the increase in the number of seeds per pod, seed weight per pod also increased, confirming the importance of these three pollinator species in canola production.     

Intra‐floral resource partitioning between endemic and invasive flower visitors: consequences for pollinator effectiveness

1. Sympatric flower visitor species often partition nectar and pollen and thus affect each other's foraging pattern. Consequently, their pollination service may also be influenced by the presence of other flower visiting species. Ants are solely interested in nectar and frequent flower visitors of some plant species but usually provide no pollination service. Obligate flower visitors such as bees depend on both nectar and pollen and are often more effective pollinators. 2. In Hawaii, we studied the complex interactions between flowers of the endemic tree Metrosideros polymorpha (Myrtaceae) and both, endemic and introduced flower‐visiting insects. The former main‐pollinators of M. polymorpha were birds, which, however, became rare. We evaluated the pollinator effectiveness of endemic and invasive bees and whether it is affected by the type of resource collected and the presence of ants on flowers. 3. Ants were dominant nectar‐consumers that mostly depleted the nectar of visited inflorescences. Accordingly, the visitation frequency, duration, and consequently the pollinator effectiveness of nectar‐foraging honeybees (Apis mellifera) strongly decreased on ant‐visited flowers, whereas pollen‐collecting bees remained largely unaffected by ants. Overall, endemic bees (Hylaeus spp.) were ineffective pollinators. 4. The average net effect of ants on pollination of M. polymorpha was neutral, corresponding to a similar fruit set of ant‐visited and ant‐free inflorescences. 5. Our results suggest that invasive social hymenopterans that often have negative impacts on the Hawaiian flora and fauna may occasionally provide neutral (ants) or even beneficial net effects (honeybees), especially in the absence of native birds.     

BUMBLE BEE POLLINATION AND FLORAL MORPHOLOGY: FACTORS INFLUENCING POLLEN DISPERSAL IN THE ALPINE SKY PILOT,POLEMONIUM VISCOSUM (POLEMONIACEAE)

Pollen dispersal success in entomophilous plants is influenced by the amount of pollen produced per flower, the fraction of pollen that is exported to other flowers during a pollinator visit, visitation frequency, and the complementarity between pollen donor and recipients. For bumble bee-pollinated Polemonium viscosum the first three determinants of male function are correlated with morphometric floral traits. Pollen production is positively related to corolla and style length, whereas pollen removal per visit by bumble bee pollinators is a positive function of corolla flare. Larger-flowered plants receive more bumble bee visits than small-flowered individuals. We found no evidence of tradeoffs between pollen export efficiency and per visit accumulation of outcross pollen; each was influenced by unique aspects of flower morphology. Individual queen bumble bees of the principal pollinator species, Bombus kirbyellus, were similar in male, female, and absolute measures of pollination effectiveness. An estimated 2.9% of the pollen that bumble bees removed from flowers during a foraging bout was, on average, deposited on stigmas of compatible recipients. Significant plant-to-plant differences in pollen production, pollen export per visit, and outcross pollen receipt were found for co-occurring individuals of P. viscosum indicating that variation in these fitness related traits can be seen by pollinator-mediated selection.     

Morning floral heat as a reward to the pollinators of the Oncocyclus irises

Relationships between flowering plants and their pollinators are usually affected by the amount of reward, mainly pollen or nectar, offered to pollinators by flowers, with these amounts usually positively correlated with floral display. The large Oncocyclus iris flowers, despite being the largest flowers in the East Mediterranean flora, are nectarless and have hidden pollen. No pollinators visit the flowers during daytime, and these flowers are pollinated only by night-sheltering solitary male bees. These iris flowers are partially or fully dark-colored, suggesting that they gather heat by absorbing solar radiation. Here we test the hypothesis that the dark-colored flowers of the Oncocyclus irises offer heat reward to their male solitary bee pollinators. Floral temperature was higher by 2.5°C than ambient air after sunrise. Solitary male bees emerged earlier after sheltering in Oncocyclus flowers than from other experimental shelter types. Pollination tunnels facing east towards the rising sun hosted more male bees than other aspects. We suggest that floral heat reward can explain the evolution of dark floral colors in Oncocyclus irises, mediated by the pollinators’ behavior.     

Flower choice by honey bees (Apis mellifera L.): sex-phase of flowers and preferences among nectar and pollen foragers

Bees foraging for nectar should choose different inflorescences from those foraging for both pollen and nectar, if inflorescences consist of differing proportions of male and female flowers, particularly if the sex phases of the flowers differ in nectar content as well as the occurrence of pollen. This study tested this prediction using worker honey bees (Apis mellifera L.) foraging on inflorescences of Lavandula stoechas. Female flowers contained about twice the volume of nectar of male flowers. As one would predict, bees foraging for nectar only chose inflorescences with disproportionately more female flowers: time spent on the inflorescence was correlated with the number of female flowers, but not with the number of male flowers. Inflorescence size was inversely correlated with the number of female flowers, and could be used as a morphological cue by these bees. Also as predicted, workers foraging for both pollen and nectar chose inflorescences with relatively greater numbers of both male and female flowers: time spent on these inflorescences was correlated with the number of male flowers, but not with the number of females flowers. A morphological cue inversely associated with such inflorescences is the size of the bract display. Choice of flowers within inflorescences was also influenced predictably, but preferences appeared to be based upon corolla size rather than directly on sex phase.     

Risk‐averse inflorescence departure in hummingbirds and bumble bees: could plants benefit from variable nectar volumes?

Most hermaphroditic, many-flowered plants should suffer reduced fitness from within-plant selfing (geitonogamy). Large inflorescences are most attractive to pollinators, but also promote many flower visits during a single plant visit, which may increase selfing and decrease pollen export. A plant might avoid the negative consequences of attractiveness through modification of the floral display to promote fewer flower visits, while retaining attractiveness. This report shows that increasing only the variance in nectar volume per flower results in fewer flower visits per inflorescence by wild hummingbirds ( Selasphorus rufus ) and captive bumble bees ( Bombus flavifrons ) foraging on artificial inflorescences. Inflorescences were either constant (all flowers contained the same nectar volume) or variable (half the flowers were empty, the other half contained twice as much nectar as in the constant flowers). Both types of inflorescence were simultaneously available to foragers. Risk-averse foraging behaviour was expressed as a patch departure preference: birds and bees visited fewer flowers on variable inflorescences, and this preference was expressed when resource variability could be determined only by concurrent sampling. When variance treatments were clearly labelled with colour and offered to hummingbirds, the departure effect was maintained; however, when preference was measured by inflorescence choice, birds did not consistently prefer to visit constant inflorescences. The reduced visitation lengths on variable inflorescences by both birds and bees documented in this study imply that variance in nectar production rates within inflorescences may represent an adaptive trait to avoid the costs of geitonogamy.     

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

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

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

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

Trapline foraging by bumble bees: IV. Optimization of route geometry in the absence of competition

Foraging on resources that are fixed in space but that replenishover time, such as floral nectar and pollen, presents animalswith the problem of selecting a foraging route. What can flowervisitors such as bees do to optimize their foraging routes,that is, reduce return time or route distance? Some repeatedlyvisit a set of plants in a significantly predictable sequence(so-called "trapline foraging"), which may also enhance theirforaging efficiency. A moderate level of optimization and repetitionof foraging routes can be reached by following simple movementrules for choosing the distances and turning angles of successiveflights, without the use of spatial memory. If pollinators canlearn the locations of patches and choose among possible foragingroutes or paths, however, even better performance may be achieved.We tested whether and how bumble bees can optimize and repeattheir foraging routes in laboratory experiments with artificialflowers that secreted nectar at a constant rate. With increasingexperience, foraging routes of bees became more repeatable andefficient than expected from a combination of simple movementrules between successive flowers. We suggest that trapline foragingis a more sophisticated pattern of spatial use than searchingand is based on memory. On the other hand, certain spatial configurationsof flowers hampered optimization by the bees; bees preferredto choose short distances over straight moves and showed littleplasticity in this regard. Developing an efficient trapline,therefore, may require prior selection of a set of plants withan appropriate spatial configuration.     

Differences in pollinator effectiveness of birds and insects visiting Banksia menziesii (Proteaceae)

Summary The effectiveness of nectarivorous birds, introduced honey bees and staphylined beetles as pollinators of Banksia menziesii was assessed. Staphylinids removed substantial amounts of pollen but did not deposit any onto stigmata. Abundance of beetles on inflorescences was related to the mean number of florets opening per day. Honey bees collecting pollen were more likely to effect pollination than those collecting nectar which only contacted stigmata when arriving or leaving an inflorescence. Nectar-foraging birds probed between florets 10.2±0.8 (±SE) times, contacting 8–16 stigmata during each probe. Bees visited inflorescences ten times more frequently than birds although they deposited only 25% of the pollen that birds did on stigmata. Fruit set was ten times greater on inflorescences visited by birds than on inflorescences visited by bees. Bees were capable of removing as much pollen as birds but, because of direct pollen transfer to birds when florets opened during foraging, actual removal was probably much less. Selection for floret opening during nectar foraging by birds may have resulted from pollen removal by non-pollinating animals, such as staphylinids.