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.     

Mixed bumblebee and blowfly pollination of Cypripedium flavum (Orchidaceae) in Sichuan,China

Most Cypripedium species are specialized orchids pollinated by, in a broad sense, bees or flies. Here we present the first evidence that a slipper orchid, Cypripedium flavum, is pollinated by both bees and flies, i.e., bumblebees and blowflies. Artificial pollination experiments demonstrated that the flowers of C. flavum are self-compatible, but need pollen vectors for successful reproduction. Field observations detected 25 insects visiting the flowers, and 14 of these insects entered into the labellum of the flowers, but only female bumblebees, Bombus hypnorum, B. remotus, and the blowfly Calliphora vomitoria exited of the labellum with pollen smears of C. flavum. The floral functional morphology of C. flavum appears to be more suited to bumblebees than to blowflies. The bumblebees are more efficient pollinators of the orchid, but blowflies are more frequent visitors, so they pollinated more flowers despite being less efficient.     

Pollination Biology of Hosta sieboldiana (Lodd.) Engler and H. sieboldii (Paxton) J. Ingram (Liliaceae)

Abstract The pollination biology of Hosta sieboldiana and H. sieboldii is investigated comparatively in Central Japan. Both species have homogamous, one-day flowers pollinated by bumblebees. The abdomens of the bees touch the stigma on the extended style when they land on the anthers inside the herkogamous flower, and autogamy is effectively prevented. However, the flowers are fairly self-compatible, and geitonogamy may occur rather frequently because two or more flowers on a scape very often bloom at the same time and many ramets are contiguous. The pollen/ovule ratios suggest that these species are facultative outbreeders. The flower of H. sieboldii seems completely suited to bumblebee pollination. In H. sieboldiana the stigma of the flower, whose style strongly protrudes, is not always touched by bumblebees, but frequent visitation of bumblebees results in pollination of almost all the flowers. Both species have similar pollination systems but seem reproductively isolated by blooming times and habitats. Their common pollinators, however, may sometimes cause introgressive hybridization in contiguous populations.     

POLLINATION ECOLOGY OF MONARDA DIDYMA,M. CLINOPODIA,AND HYBRIDS (LAMIACEAE) IN THE SOUTHERN APPALACHIAN MOUNTAINS

Monarda didyma L. and M. clinopodia L. occasionally form introgressive populations in the mountains of eastern Tennessee and western North Carolina. Monarda didyma produces large red scentless nectar-rich flowers and is pollinated primarily by the Ruby-throated Hummingbird (Archilochus colubris [Linnaeus]). The smaller, white, scented flowers of Monarda clinopodia produce small amounts of nectar and are pollinated mainly by bumblebees (Bombus spp.). The two species are highly interfertile and have strongly overlapping flowering periods. Differences in pollinator sets and pollinator constancy appear to be only partial barriers to hybridization. Lack of suitable habitats for the establishment of hybrid progeny might be the most important isolating mechanism between the two species.     

Association of flower color with pollen reward may explain increased bumblebee visitation to the Scotch broom yellow morph

Given that pollinators usually visit flowers for hidden rewards, they need to rely on floral traits that indicate reward status (“honest signals”). However, the relationship between pollination, honest signals, and floral rewards is little documented in natural conditions. The Scotch broom (Cytisus scoparius) is an invasive shrub with polymorphism in the color of its flowers that can be yellow, orange, or red. In three areas dominated by the Scotch broom, we described the abundance of the floral morphs and estimated bumblebee (Bombus terrestris) visitation rate. We examined whether bumblebee visitation to the floral morphs was related to pollen reward. We collected flowers and classified their stamens according to their function: reward or pollen export. Then, we measured anther size and estimated pollen quantity. The yellow morph was more abundant and more visited by bumblebees than the orange and red morphs. The yellow flowers did indeed offer more pollen than the other morphs and this occurred only for rewarding anthers, suggesting that bumblebees could use yellow color as an honest signal to visit the most rewarding flowers. We discuss whether innate and/or learned preferences of bumblebees can explain why the yellow morph is more visited, pollinated, and abundant, while the other morphs are maintained at a lower frequency. This is one of the few field works that shows that variation in intra-specific floral traits is associated with variation in floral reward and pollinator visitation rate, helping to understand the foraging preferences of pollinators and the coexistence of floral morphs in nature.

Clinical trials registration: Not applicable.

     

Highly efficient pollination by bumblebees ensures seed production in Pedicularis lachnoglossa (Orobanchaceae), an early‐flowering Himalayan plant

Abstract Pedicularis (Orobranchaceae) is a common high altitude genus of the Himalayas that may be affected by pollination limitation. Using Pedicularis lachnoglossa from Yulong (Jade Dragon) Snow Mountain in Lijiang (Yunnan Province, southwest China), we investigated the effects of high altitude habitats on the process of pollination and seed production. Floral biology, pollinator foraging behavior, breeding system, and pollination efficiency were examined using observation and exclusionary techniques. Pedicularis lachnoglossa was found to be entomophilous and exclusively pollinated by Bombus friseanus and B. yunnanicola. Our results indicated that pollination limitation in P. lachnoglossa was not significant. Under open pollination, approximately 80% of flowers were successfully pollinated and developed to fruits, and about 38% of ovules developed to mature seeds. Bumblebee pollination is highly precise and efficient in P. lachnoglossa, because its flowering phenology and floral characters enhance the foraging of bumblebees on flowers. This study supports that animal pollination plays a crucial role in the outbreeding of the early flowering Pedicularis. The evolution of floral specification in Pedicularis has the advantages of adaptation to bumblebee pollination in adverse high altitude habitats.     

ON THE DARWINIAN HYPOTHESIS OF THE ADAPTIVE SIGNIFICANCE OF TRISTYLY

Darwin proposed that the function of the stamen-style polymorphism in heterostylous plants is to increase the probability of legitimate (compatible) pollinations among the floral morphs. Conspicuous pollen trimorphism in tristylous Pontederia cordata enables a test of the hypothesis. Comparison of the composition of pollen loads in naturally pollinated stigmas of intact and emasculated flowers were made at a population in Paugh Lake, Ontario, which was visited primarily by bumblebees. The magnitude of legitimate pollination was analyzed by ANOVA. In intact flowers, significant legitimate pollination was detected in the long-styled morph only. Following emasculation legitimate pollination was evident in the long- and short-styled morphs, with the mid-styled morph just short of displaying significant legitimate pollination. Similar results were obtained by chi-square analysis. It has been suggested that heterostyly may reduce mutual interference between maternal and paternal reproductive function. Two aspects of pollen-stigma interference were investigated in P. cordata. The potential importance of stigmatic or stylar clogging by incompatible pollen was examined by controlled field pollinations and measurements of seed set. The results indicate that prior application of large amounts of incompatible pollen has no significant effect on the seed set of open-pollinated inflorescences. Comparison of legitimate pollen capture in intact and emasculated flowers provided no evidence that the presence of stamens within flowers of the floral morphs interferes with the receipt of legitimate pollen. Pollen-stigma interference remains to be demonstrated in heterostylous plants.     

Mechanisms that limit pollinator range in Ericaceae

Studies were conducted in 2001-2003 at Valdai National Park (Novgorod region) and at the Zvenigorod biological station of Moscow State University. The morphology of flowers, flowering dynamics and composition of insect visiting flowers of Ericaceae species: Andromeda polifolia, Chamaedaphne calyculata, Ledum palustre, Oxycoccus palustris, Vaccinium myrtillus, V. uliginosum, and V. vitis-idaea L. were studied. Some species of insects visiting flowers were excluded from the list of pollinators on the basis of observation on their behavior. L. palustre was visited mainly by flies where as other investigated species were visited mainly by bumblebees. In some cases bumblebees were the only visitors of the investigated plants. Mechanisms that protect flowers from flies and short-tongued solitary bees visits and ensure a best pollination by bumblebees are various among different species of Ericaceae. Efficiency of nectary protection also differs among different plant species and is defined by particularities of their habitats and flowering phenology. As far as all species of this family during the flowering are dominants in typical habitats, a competition for the pollination with species of other families in most cases is megligible. Flowering periods of V. vitis-idaea and V. myrtillus in forest ecosystems overlapped weakly. Moreover, V. myrtillus is pollinated mainly by bumblebee queens where as pollinators of V. vitis-idaea are bumblebee workers, solitary bees and horse flies. The other investigated plant species inhabit only oligotrophic peat bogs. Thery are pollinated by bumblebees but periods of flowering are not overlapped and consequently follow one after another. L. palustre and V. uliginosum flower simultaneosly but they are pollinated by different pollinators.     

Pollinator Restriction in the Narrow-tube Flower Type of Mertensia ciliata (James) G. Don (Boraginaceae)

Abstract Mertensia ciliata (Boraginaceae) includes two flower types with different corolla tube widths, wide and narrow. The former is pollinated by both queens and worker bumblebees, while the latter type is pollinated exclusively by bumblebee workers. Morphological comparisons between the flowers and the bumblebees showed that the relationship between the width of the corolla tube mouth and the head width of the bumblebee is a primary factor restricting pollinators to workers in the narrow-tube type. Length of the proboscides and corolla tube are of secondary importance for this restriction, since some queens are not able to reach the bottom of a narrow corolla tube even if their proboscis is extended fully.     

The pollination biology of Calypso bulbosa var. Americana (Orchidaceae): Initial deception of bumblebee visitors

Summary The orchid Calypso bulbosa var. americana has deceptive flowers that provide no rewards for visitors. Near Banff, Alberta, the flowering period of this species is synchronized with the emergence of its pollinators, large bumblebee queens, in late spring. Calypso flowers appear to rely on the initial attraction and deception of newly-emerged naive bumblebees for pollination. Indirect evidence suggests that individual bees subsequently learn to avoid these flowers and that avoidance is learned quite rapidly. Avoidance behavior by pollinators is obviously detrimental to sexual reproduction in Calypso. This negative effect appears to be offset by the large number of seeds produced in plants which are effectively pollinated. A test of the hypothesis that Calypso flowers mimic flowers of the shooting star, Dodecatheon radicatum (Primulaceae) failed to provide evidence for mimicry.     

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.     

Insect and wind pollination of an alpine biennial Aconitum gymnandrum (Ranunculaceae)

Wind pollination can provide reproductive insurance for animal‐pollinated dioecious plants in the absence of available pollinators, but combinations of insect and wind pollination (ambophily) have rarely been studied in hermaphrodite herbs. We examined the stable occurrence of insect pollination and wind pollination over 4 years in a population of a biennial Aconitum species (A. gymnandrum) with actinomorphic and degenerate sepals. The total frequency of visits of two bumblebee species showed no distinct fluctuations in the studied population among the 4 years. However, seed production of netted flowers after emasculation indicated wind pollination had occurred. The seed number of bagged flowers with one visit by bumblebees was significantly less than that of netted flowers after one visit, or in control flowers. Both seed number and fruit set of netted flowers were significantly lower than in control flowers. These results suggest that wind pollination provides supplementary pollen to unvisited and/or once‐visited flowers, but accounts for only a small amount of seed production compared to bumblebee pollination in natural conditions. Such a combination of insect and wind pollination might play an important role in maintaining sexual reproduction of this biennial herb, allowing it to persist in arid habitats on the Qinghai‐Tibetan Plateau, especially during Quaternary glacial periods when pollinator populations oscillated extensively.     

THE POLLINATION ECOLOGY OF AQUILEGIA ELEGANTULA AND A. CAERULEA (RANUNCULACEAE) IN COLORADO

Aquilegia elegantula Greene and A. caerulea James occur in montane and subalpine habitats in the southern Rocky Mountains of western North America. The red and yellow flowers of A. elegantula are nodding, odorless, protogynous, and secrete a concentrated (44%) sucrose nectar in the floral spurs. Seed set in flowers under pollinator exclosures was 12% while seed set in open-pollinated flowers was 65%. The flowers of A. elegantula are pollinated primarily by the Broad-tailed Hummingbird (Selasphorus platycercus [Swainson]) and by at least three species of pollen-foraging bumblebees, of which Bombus occidentalis Greene is the most common. The blue and white flowers of A. caerulea are erect, mildly fragrant, protandrous, and secrete a 26% sucrose nectar. Seed set in caged flowers in the field averaged 39%. in uncaged flowers 54%. The most important pollinators of A. caerulea are the crepuscular hawkmoth, Hyles (=Celerio) lineata (Fabricius) and ten species of pollen-foraging Bombus. The most abundant bumblebee species, B. occidentalis, is also a frequent nectar thief. Differences in pollination systems alone probably do not constitute an effective anti-hydridization mechanism between A. elegantula and A. caerulea, but do serve to reinforce differences in habitat and flowering time that distinguish the two species.     

FREQUENCY-DEPENDENT VARIATION FOR OUTCROSSING RATE AMONG FLOWER-COLOR MORPHS OF IPOMOEA PURPUREA

In a series of previous studies, it has been shown that populations of the common morning glory Ipomoea purpurea are typically polymorphic for flower-color genes that bias pollinator service and, hence, the rate of outcrossing. In this study, we show that the rate of outcrossing for the white flower-color morph depends on its frequency in experimental populations of the morning glory. White flowers are visited less often by the primary pollinator, bumblebees, and have lower outcrossing rates than colored flowers when they are in the minority. In contrast, blue or pink flower morphs are not undervisited and do not have lowered outcrossing rates when they are rare. In plant populations where genes that increase selling are selectively favored due to their transmission bias, undervisitation of rare morphs may act to stabilize genetic variation for outcrossing rates.     

The pollination ecology of Pedicularis rex subsp. lipkyana and P. rex subsp. rex (Orobanchaceae) from Sichuan, southwestern China

Pedicularis is one of few genera for which pollination ecology has been studied extensively. Although over half of the species of Pedicularis are found in the mountains of southwestern China, pollination ecology has been studied there on a few species only. The present paper reports pollination ecology of Pedicularis rex subsp. lipskyana and P. rex subsp. rex from Sichuan, southwestern China. The two subspecies are nectariferous, with sugar content 28% in P. rex lipskyana and 22% in P. rex, respectively. The flowers of the two subspecies are adapted to pollination by bumblebee workers. Bumblebees are the primary and effective pollinators although honeybees were also observed pollinating P. rex subsp. lipskyana at one studied population. Usually bumblebees entered the corolla tube from the right side in an upright or a nearly half-inverted position to imbibe nectar located at the base of the corolla tube. The stigma usually contacts the side region of the thorax and abdomen or occasionally directly contacted pollen loads, leading to pollination of flowers. Much less commonly, bumblebees foraged on flowers of P. rex subsp. rex in an inverted position for pollen, which pollinated flowers sternotribically. We suggest that pollination mechanism is closely associated with floral morphology. Some important differences were found from a previous study on P. rex in floral morphology and the primary pollination mechanism.     

Habitat dependent pollination success in a Cape orchid

Summary We examined the pollination success of Disa uniflora (Orchidaceae) in two habitats. Plants occurring in a rocky gorge were far more successful than plants occurring in an adjacent open valley. More than 55% of flowers in the gorge were pollinated and set fruit compared with less than 25% of flowers in the valley. These differences are explained by the preference of Meneris tulbaghia (Satyridae), the exclusive pollinator of the orchid, for rocky, sheltered habitats. Fruit set of hand-pollinated flowers did not differ significantly between the two habitats, indicating that resources did not account for the variation in fruiting success.     

Thermal insulation and accumulation of heat in the downy inflorescences of Saussurea medusa (Asteraceae) at high elevation in Yunnan,China

Himalayan snowball plants, which are considered to be an extreme form of downy plants, have very dense trichomes on well-developed bracts that surround the inflorescences. It has been postulated that the downy inflorescences of these plants might serve to keep the interior of inflorescences warmer than the outside and, thus, to protect reproductive cells from low temperatures in their Himalayan habitat. In the present study, we examined the downy inflorescences of Saussurea medusa Maxim. in native habitats in the high alpine zone of the Henduan Mountains in Yunnan, China, and we analyzed the temperature within inflorescences after absorbance of light energy. S. medusa is pollinated by bumblebees and we found that its inflorescences accumulated heat not on the inside, but, rather, on the upper surfaces. The thick hollow stems and the overlapping bracts with obvious epinasty might serve not only to retain heat, but also as an insulator to protect the inside against overheating, with apparent local warming of flowers that are located at the tops of plants, which are cone-shaped. We made a model that mimicked the warming of inflorescences, providing support for the hypothesis that the downy bracts of S. medusa have two functions: thermal insulation to protect the inside of flowers and the accumulation of heat on the upper surfaces of the inflorescence. Such a system might be effective in attracting pollinators and also in protecting tissues from extreme variations in temperature. Electronic Publication     

Deceptive pollination of the Lady's Slipper Cypripedium tibeticum (Orchidaceae)

To test whether the nectarless flowers of Cypripedium tibeticum attract pollinators through mimicry like the allied species C. macranthos var. rebunense, pollination biology of C. tibeticum was investigated in western China. Although C. tibeticum was also pollinated by bumble bee queens, i.e. Bombus lepidus , B. lucorum and B. hypnorum , no special, rewarding model plants were found in the habitat. Field experimentation confirmed that the flowers were self-compatible but insects were required to transfer orchid pollen to the stigma. Both Bombus queens and workers were visitors, but queens were much more frequent than workers and only queens were effective pollinators. Floral functional morphology analysis showed that it was large queens rather than small workers that fitted well with the flowers of C. tibeticum. With the faint sweet-fruity scent, the minor floral fragrance compound, ethyl acetate, probably plays a role in attracting bumble bees by food deception. The dark flowers with the inflated, trap-like labellum are hypothesized to mimic the nest site of queens. Therefore, bumble bee queens tend to be attracted by C. tibeticum through nest site mimic combined with food deception. Considering that the co-blooming flowers of C. flavum are pollinated by the Bombus workers, and C. smithii pollinated by a queen, we suggest that using the same bumblebees with different body sizes as the pollinators is the main reproductive isolation between interfertile C. tibeticum and C. flavum, while C. tibeticum and C. smithii tend to hybridize naturally.     

Multi-cycle synchronous protandry in raceme-like inflorescences of a bumblebee-pollinated herb Aconitum grossedentatum

Multi-cycle synchronous dichogamy is expected to be a mechanism for reducing self-pollination and sexual interference. It is often found in plants with umbellate inflorescences where pollinator movement is unpredictable, but not in plants with raceme inflorescences that are pollinated by bumblebees. Plants with raceme inflorescences often acropetally open flowers, resulting in an arrangement of females at lower level and males at upper level. This is good enough to preclude geitonogamy because bees tend to move upwardly within the inflorescences. Furthermore, although the degree of segregation of sexes varies among species, their intraspecific variations within a population have rarely been examined. Here, we present a synchronous protandry in bee-pollinated Aconitum grossedentatum, which has a raceme-like inflorescence and opens flowers basipetally. To evaluate the functional significance of synchronous dichogamy in mating, we firstly observed the distribution of sex phases of open flowers. Then, we assessed the effect of each phase flower on foraging behavior by pollinators and seed-set success. The inflorescences tended to exhibit either male- or female-phase flowers at any moment early in the flowering season, but the degree of segregation of sexes declined over time within a population. The degree of the segregation did not affect bumblebee visits to flowers, but it decreased seed-set success of female-phase flowers at that time. Our results demonstrated that synchronous protandry was beneficial for pollination success in A. grossedentatum by avoiding geitonogamy. Nevertheless, we also found asynchronous protandry late in the season, suggesting that the benefits by synchronous protandry decreased over the season.

     

NEGATIVE FREQUENCY-DEPENDENT SELECTION BY POLLINATORS ON ARTIFICIAL FLOWERS WITHOUT REWARDS

Many species of nonmodel deceptively pollinated orchids are polymorphic for corolla color. These species are pollinated by naive insects searching for nectar, and are not mimics. It has been suggested that the foraging behavior of insect pollinators during the avoidance learning process results in these stable corolla color polymorphisms; for this to occur pollinators must induce negative frequency-dependent selection on corolla color. Therefore the hypothesis that pollinator behavior results in a preference for rare color morphs of deceptive species was tested experimentally. Bumblebees (Bombus terrestris) foraged in the laboratory on arrays of artificial flowers with different corolla color morphs. Morphs were varied in frequency, and bumblebee preferences were recorded on arrays where morphs did and did not contain sucrose solution rewards. Bumblebees preferred the most common color morph when flowers contained sucrose solution rewards, but overvisited rare morphs when sampling flowers that contained no rewards. Bumblebees also tended to move between unlike color morphs when these were unrewarding, suggesting that a probabilistic sampling strategy was adopted. Thus experiments demonstrated that pollinator behavior could result in a selective advantage for rare color morphs of plant species that are pollinated by deception without mimicry, which would induce negative frequency-dependent selection on corolla color. The observed pollinator behavior could allow stable corolla color polymorphisms to be maintained by selection in nonmodel deceptively pollinated species.