Competition for bumblebee visitation between Melampyrum pratense and Viscaria vulgaris with healthy and Ustilago-infected flowers

Summary We studied bumblebee foraging on two sympatrically and simultaneously flowering species, Melampyrum pratense (Scrophulariaceae) and Viscaria vulgaris (Caryophyllaceae) during the flowering season of Viscaria in south-west Sweden. We distinguished between healthy and Ustilago-infected Viscaria plants. Both species shared the main insect visitor, queens of Bombus hortorum, which collected nectar from both species but pollen from Melampyrum only. The pattern of visitation changed over the season: bumblebees preferred Viscaria early on, but changed to Melampyrum later in the season, probably because of the higher sugar content of Melampyrum nectar and the possibility of collecting both nectar and pollen from the same flower. Pollen collecting is probably of increasing importance since the need of pollen for the developing larvae will increase with time. Flowers of Viscaria received fewer visits in plots with other species than in pure Viscaria plots during one year and received more visits early than late in the season during both years. Melampyrum flowers received similar amounts of visits in mixed and pure environments. They also received more visits early than late, although this was probably a result of pollinator satiation since Melampyrum became very abundant with time. Ustilago-infected plants received far fewer visits but because of its long flowering time the proportion of open flowers receiving visits was still high. Viscaria flowers received significantly more visits than flowers of other species when bumblebees made heterospecific flower visits from Ustilago-infected plants; thus Ustilago spores were probably effectively dispersed from infected to healthy plants by the pollinators. The mechanism behind competition for pollination in this system was competition through pollinator preference, since the visitation rate to Viscaria actually decreased, but also competition through improper pollen transfer (grains of both species were found on the bodies of bumblebees) since the incidence of switching between the two species increased, probably resulting in an increased misplacement of conspecific pollen grains with time.     

A trade-off between the amount and distance of pollen dispersal triggered by the mixed foraging behaviour of Sephanoides sephaniodes (Trochilidae) on Lapageria rosea (Philesiaceae)

Nectar thieves may increase or decrease pollinator-mediated pollen flow and thus may have positive or negative effects on plant reproductive success. In temperate rainforests of South America, the hummingbird Sephanoides sephaniodes acts as both a pollinator and non-destructive nectar thief on Lapageria rosea. Although pollinators that also act as nectar thieves have the potential to significantly modify plant reproductive success, no previous study has addressed this. To determine how the mixed behaviour of S. sephanoides affects pollen flow, we experimentally exposed some flowers to nectar theft and excluded nectar thieves from other flowers. We then assessed pollen dispersal into the floral neighbourhood. Thieved flowers exported less pollen, but the pollen exported was transferred farther into the neighbourhood. Our findings indicate a trade-off between distance and amount of pollen flow.     

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.     

Interaction between flowers, ants and pollinators: additional evidence for floral repellence against ants

It has been commonly suggested that ants negatively affect plant pollination, particularly in the tropics. We studied ant–flower–pollinator interactions in a lowland rainforest in Borneo. Frequency and duration of pollinator visits were compared between flowers attended by ants and flowers from which ants were excluded. In all four plant species studied, the activity of ants decreased the rate and/or duration of the pollinators’ floral visits. For this and other reasons it is expected that plants repel ants from flowers during anthesis. We tested this prediction for a different set of plant species in which we observed the behaviour of Dolichoderus thoracicus ants when encountering flowers. In eight out of 18 plant species studied, ants showed a significantly higher rejection rate when they encountered flowers than when they encountered controls. Our results are thus consistent with the hypothesis that ants may negatively affect plant fitness by reduced intensity of pollinator visits and that ants are repelled from flowers of many tropical plant species, although this repellence is clearly not ubiquitous.     

Consequences of nectar robbing for the fitness of a threatened plant species

The effect of nectar robbing on plant fitness is poorly understood and restricted to a few plant species. Furthermore, the available studies generally evaluate the effects of nectar robbing on female fitness, disregarding the male component. Here we measured the effects of the nectar-robbing bumblebees on male (measured as pollen analogue flow distance) and female (measured as seed production) reproductive success in the insect-dependent Polygala vayredae, a narrow endemic species from the pre-Pyrenees (Spain). Intense nectar robbing by bumblebees significantly reduced the nectar available to legitimate pollinators in the studied population, and this reduction affected both male and female fitness. Significant differences were observed in fluorescent dye dispersion between robbed and non-robbed flowers within the population. Fluorescent dyes from non-robbed flowers were dispersed to larger distances and over a larger number of flowers when compared with robbed ones. Moreover, significant differences were observed in both fruit set and seed ovule ratios between the two groups, with non-robbed flowers presenting higher reproductive outcomes. However, no effect on seed weight was detected among treatments. The data obtained suggest that in this species, nectar robbing has important indirect and negative effects on plant fecundity, through both male and female functions, due to a modification in the foraging behaviour of legitimate visitors.     

Food for insect pollinators on farmland: insect visits to flowers of annual seed mixtures

Intensification of agriculture in recent decades has impoverished farmland for insect pollinators by removing their food plants. Arable farmland can be enhanced as a habitat for these insects by growing annual nectar- and pollen-producing herbaceous plants for them in non-cropped areas such as set-aside and field margins. In 1996 and 1997, observations were made in Hertfordshire, UK, on the flowering phenology and flower-attractiveness to visiting Hymenoptera, Diptera and Lepidoptera of plots sown to mixtures of six annual flowering plant species: borage (Borago officinalis), buckwheat (Fagopyrum esculentum), cornflower (Centaurea cyanus), mallow (Malva sylvestris), marigold (Calendula officinalis) and phacelia (Phacelia tanacetifolia) in different proportions. The mixtures had good agronomic and biological properties. They established and flowered well from a range of seed rates and sowing-dates. They attracted a diversity of flower-visiting insects, including the honey bee and eightspecies of bumble bee amongst 16 species of aculeate Hymenoptera, 17 species of Diptera, mostly syrphids, and six species of Lepidoptera. Sequential sowings provided nectar and pollen from early summer to late autumn during the period after arable crops had finished flowering and food for pollinators was scarce. Different insect species were favoured by different sowing-dates and plant species     

Foraging behavior of three bee species in a natural mimicry system: female flowers which mimic male flowers in Ecballium elaterium (Cucurbitaceae)

Summary The behavior of Apis mellifera and two species of solitary bees which forage in the flowers of monoecious Ecballium elaterium (L.) A. Rich (Cucurbitaceae) were compared. The female flowers of E. elaterium resemble male flowers visually but are nectarless, and their number is relatively smaller. Apis mellifera was found to discriminate between the two genders and to pay relatively fewer visits to female flowers (mean of 30% relative to male flowers) from the beginning of their activity in the morning. The time spent by honeybees in female flowers is very short compared to that spent in male flowers. It is surmised that the bees remember the differences between the flowers where they foraged on the previous days. In contrast, the two species of solitary bees Lasioglossum politum (Morawitz) (Halictidae) and Ceratina mandibularis Fiese (Anthophoridae) visit the female flowers with nearly equal frequencies at the beginning of each foraging day and stay longer in these flowers. Over the day there is a decline in the relative frequency of visits to female flowers and also in the mean time spent in them. The study shows that bees can collect rewards at high efficiency from the flowers of Ecballium elaterium because of their partial discrimination ability and the scarcity of the mimic flowers. It is suggested that the memory pattern of some solitary bees may be different from that of Apis mellifera. It seems that the limited memory and discrimination ability of bees can lead to a high frequency of visits to the mimic flowers during a long flowering season.     

Bumblebee visits to Impatiens spp.: pattern and efficiency

Summary Three Japanese species of Impatiens, which secrete nectar continuously in long spurs, were visited by Bombus diversus workers consecutively throughout the day. B. diversus workers showed characteristic patterns of behavior in flower use, flower choice, and patch departure. (1) Bumblebees stayed longer on a flower which had been unvisited for a while than on a flower which had been visited recently. (2) Bumblebees preferred visiting flowers which had been unvisited for a while to visiting those which had been visited recently, and to visiting those which had been unvisited for a long period. (3) Bumblebees had a higher probability of leaving a patch after they had stayed on a flower for a short period than after they had stayed for a longer period. The bumblebees appeared to perceive both remotely and proximately chemical cues deposited by other foraging individuals, which indicated nectar rewards in a flower, and thus obtained a higher nectar intake than the mean amount of nectar left in a flower.     

Early reproductive failure increases nectar production and pollination success of late flowers in south Andean Alstroemeria aurea

Fertile ramets of bumblebee-pollinated Alstroemeria aurea, a clonal perennial native to the temperate forests of the southern Andes, produce single terminal inflorescences that may bear two or more temporally non-overlapping whorls of flowers. While fruit set is commonly high (>80%) among early-opening flowers, it is usually low (<20%) among late-opening flowers within ramets. Using flowering ramets with two whorls of flowers, we examined experimentally the following related hypotheses. First, late flowers act as a reserve of ovaries, increasing their likelihood of setting seed when early fruits abort due to either pollen or resource limitation. Second, where early fruit abortion has occurred, plants may actively ensure pollination of late flowers by increasing their attractants and rewards. In a natural population, we simulated (1) lack of pollen deposition in early flowers, by excising their stigmas just before receptivity, and (2) resource limitation, by removing all the leaves from an experimental flowering ramet. Treatments were applied to individual ramets according to a 2 × 2 factorial design. We found that when early flowers failed to set fruit due to stigma excision, nectar secretion and particularly pollen receipt strongly increased in late flowers. Higher pollen deposition contributed significantly to the observed five-fold increase in seed output of late flowers. Fruit and seed set from early flowers were more negatively affected by defoliation than that from late flowers. Defoliation did not interfere with a ramet's capacity to increase late reproductive output when early reproduction failed. These results support the assertion that late flowers act as a reserve of ovaries helping a plant to cope with an unpredictable environment. These results also suggest that plants may actively increase pollinator visitation by opportunistically increasing flower rewards. Received: 23 June 1998 / Accepted: 3 May 1999     

Variability in nectar production and standing crop, and their relation to pollinator visits in a Mediterranean shrub

Nectar standing crops in flowers within an individual plant are often highly variable. This variability may be a by-product of the foraging activity of insect pollinators. Alternatively, plants may be selected to produce highly variable rewards to reduce consecutive visitation by risk-averse pollinators, thus diminishing within-plant pollen transfer. This study evaluated the roles of pollinator control vs. plant control over nectar variability in the bee-pollinated shrub Rosmarinus officinalis L. (Lamiaceae). We sampled nectar production, standing crop and pollinator visits in three shrubs of one population over 17 days during one blooming season. Nectar production rates were highly variable (CV = 1.48), and increased after rainy days. Nectar standing crops were even more variable (CV = 2.16), decreased with increasing temperatures, and increased with time since the last rain. Pollinator visit rates decreased with variability in nectar standing crops, increased with flower number per shrub, and were unaffected by variability in nectar production rates. Repeated sampling of marked flowers revealed no correlation between their nectar standing crops and production rates. These findings support the role of reward variance in reducing pollinator visits, but suggest that plants are not in complete control of this variability. Rather, plant-generated variability can be modified by intensive foraging activity of pollinators. Such pollinator control over nectar variability is likely to reduce the selective advantage of plant-generated reward variation. Handling Editor: Neal Williams.     

The presence of aggressive ants is associated with fewer insect visits to and altered microbe communities in coffee flowers

The process of dispersal can shape ecological communities, but its influence is thought to be small compared to the effects of environmental variation or direct species interactions, particularly for microbial communities. Ants can influence movement patterns of insects and the microbes they vector, potentially affecting microbial establishment on plants, including in agroecosystems. Here, we examine how the presence of aggressive ants, which can influence floral visitation by bees and other pollinators, shapes the community composition of bacteria and fungi on coffee flowers in farms that differ in shade management intensity. We hypothesized that the presence of aggressive ants should reduce the frequency and diversity of floral visitors. Finally, we predicted that the effects of ants should be stronger in the low-shade farm, which has a less diverse community of floral visitors. We sampled microbial communities from nectar and pistils of coffee flowers near and far from nests of the aggressive ant Azteca sericeasur across two farms that vary in shade management and diversity of floral visitors. Bacterial and fungal community composition was characterized using Illumina sequencing of the 16S and ITS regions of the rRNA gene. Consistent with our expectation, Azteca presence was associated with a decrease in the number and diversity of visitors, visit duration and number of flowers visited. Azteca presence influenced microbial communities, but effects differed between farms. Azteca nests were associated with higher bacterial diversity in both farms, but the difference between flowers on trees with and without Azteca was greater in the high-shade farm. Azteca nests were associated with higher fungal diversity in the high-shade farm, but not the low-shade farm. In addition, the presence of ants was strongly associated with species composition of fungi and bacteria in flowers, but differentiation between ant and no-ant communities was greater in the low-shade farm. Specific operational taxonomic units (OTUs) were differentially associated with the presence of ants. We conclude that indirect interactions that influence dispersal may have large effects on microbial community composition, particularly in ephemeral microbial communities.     

Flexibility of Pollinator-Flower Relationship in Isodon umbrosus and I. effusus (Lamiaceae) and its Relation to Fruit-set and Seed-set

Isodon umbrosus was pollinated by two bumblebee pollinators, Bombus honshuensis and B. diversus. B. diversus was a secondary pollinator when both pollinators were present, but in the absence of B. honshuensis, B. diversus pollinated I. umbrosus, and the resulting seed production was comparable to when visited by the two bumblebee species together. Not only the high visitation frequency of B. diversus but also the low ratio of seed to ovules (= average seed set × fruit set) of I. umbrosus compensated for the low pollination efficiency of B. diversus. Since, the low pollination efficiency of B. diversus, based on the finding that the low average seed set of I. umbrosus did not require a definite morphological match between flowers and pollinators, and did not require very frequent visitations to each flower. In other words, Isodon umbrosus could briefly maintain constant seed production under the condition of the low ratio of seeds to ovules (the low seed set and the low fruit set) and a large number of flowers without B. honshuensis as legitimate pollinator. On the other hand, I. effusus was exclusively pollinated by B. diversus and produced almost the same volume of seeds as I. umbrosus. Isodon effusus had fewer flowers than I. umbrosus, but this smaller number of flowers was compensated for by the high ratio of seeds to ovules in I. effusus, which was maintained by frequent visitations of the morphologically matched pollinator, B. diversus. Received 17 September 1999/ Accepted in revised form 10 February 2000     

A test of the effect of floral color change on pollination effectiveness using artificial inflorescences visited by bumblebees

Floral color change has been recognized as a pollination strategy, but its relative effectiveness has been evaluated insufficiently with respect to other floral traits. In this study, effects of floral color change on the visitation pattern of bumblebees were empirically assessed using artificial flowers. Four inflorescence types were postulated as strategies of flowering behavior: type 1 has no retention of old flowers, resulting in a small display size; type 2 retains old flowers without nectar production; type 3 retains old flowers with nectar; and type 4 retains color-changed old flowers without nectar. Effects of these treatments varied depending on both the total display size (single versus multiple inflorescences) and the pattern of flower-opening. In the single inflorescence experiment, a large floral display due to the retention of old flowers (types 2–4) enhanced pollinator attraction, and the number of flower visits per stay decreased with color change (type 4), suggesting a decrease in geitonogamous pollination. Type-4 plants also reduced the foraging time of bees in comparison with type-2 plants. In the multiple inflorescence experiment, the retention of old flowers did not contribute to pollinator attraction. When flowering occurred sequentially within inflorescences, type-4 plants successfully decreased the number of visits and the foraging time in comparison with type-2 plants. In contrast, floral color change did not influence the number of visits, and it extended the foraging time when flowering occurred simultaneously within inflorescences but the opening of inflorescences progressed sequentially within a plant. Therefore, the effectiveness of floral color change is highly susceptible to the display size and flowering pattern within plants, and this may limit the versatility of the color change strategy in nature.     

Comparative effectiveness of wild bee pollination on the post-harvest characteristics of Grewia asiatica (Malvaceae)

Measuring the pollination potential of different native bees is helpful in identifying the most efficient species for their conservation and proper utilization. The current study was carried out at the Research Farm of The Islamia University of Bahawalpur to compare the pollination efficiency of four native bee species: Apis dorsata, Megachile bicolor, M. cephalotes and Megachile (Callomegachile) sp. Apis dorsata was the most abundant floral visitor of Grewia asiatica comprising 36.46% of floral visitors following by M. cephalotes and Megachile sp. The peak abundance of A. dorsata and M. bicolor was recorded at 12:00 and 08:00, respectively whereas for M. cephalotes and Megachile sp. at 16:00. Visitation frequency, stay time, body and proboscis length were significantly higher in A. dorsata whereas visitation rate, pollen load and pollen deposition were significantly higher in M. bicolor. The maximum fruit weight was recorded when M. bicolor and Megachile sp. had been the pollinators. The post-harvest fruit weight loss was lower in M. bicolor pollinated fruit -with more gradual wrinkling- than in fruit that had resulted from pollination by A. dorsata, M. cephalotes and Megachile sp. Our results suggest that M. bicolor was better pollinator of G. asiatica in terms of its reproductive success and post-harvest qualities. Further studies should focus on conservation of M. bicolor in the region.     

Nectar robbing of a carpenter bee and its effects on the reproductive fitness of Glechoma longituba (Lamiaceae)

The floral biology and pollination process of Glechoma longituba (Nakai) Kuprian, a clonal gynodioecious, perennial herb that is widely distributed in China was investigated in natural populations. During visits to the flowers of G. longituba, the carpenter bee—Xylocopa sinensis mainly displayed nectar-robbing behavior with minimal pollination. Nectar robbing behavior began at the onset of flowering and continued for about 3 weeks ending at about the middle of the flowering period. A total of 18.6% floral visits in this period were by nectar robbers, with about 90% of the flowers in the study populations being subjected to two or two nectar-robbing episodes. During controlled experiments, lower pollination efficiency was recorded for X. sinensis compared to legitimate pollinators. Pollination by the robber-like pollinator X. sinensis during the early-mid phase of the flowering season yielded seeds of 16.16%. Although nectar robbing by the carpenter bee seemed to have a slight enhancing effect on seed set in G. longituba, this effect was effectively masked by the more pronounced detrimental effect of nectar robbing. Experiments indicated that nectar robbing by the carpenter bee reduced seed production by more than 26%, largely owing to the consequent shortening of the life span of robbed flowers. Furthermore, 10.43% of the ovules and 13.04% of the nectaries in the robbed flowers were damaged, thus causing a decrease or entire loss of reproductive opportunity in the robbed flowers. In addition, a higher number of pollen grains remained on the androecia of robbed flowers indicating that nectar robbing may have a lowering effect on male fitness in G. longituba.     

A trade-off between diapause duration and fitness in female parasitoids

Abstract 1. Diapause allows insects to cope with adverse weather conditions but also poses substantial fitness costs, e.g. through reduced survival of the diapausing stage or sublethal effects in the following season.
2. The energetic costs of diapause in females of the parasitoid Asobara tabida Nees (Hymenoptera: Braconidae) were assessed using experimental manipulation of diapause duration.
3. An increase in diapause length not only led to higher mortality among diapausing pupae but also caused a significant decrease in egg load, fat reserves, and dry weight of the emerging adult females.
4. Only larvae with sufficient resources were able to survive the entire diapause period and there was a trade-off between the metabolic costs of diapause and adult fitness components.
5. In contrast, the size (tibia length) of emerging females increased with increasing diapause duration. It is proposed that this may be due to a genetic correlation between larval dry weight and adult size, so that selection for heavier larvae would result in larger adults even when resources are limited.
6. Ultimately, the evolution of diapause duration and its consequences for adult performance will depend on the costs of overwintering and the ecological conditions in the habitat.     

A meta-analysis of physiological and behavioral responses of parasitoid wasps to flowers of individual plant species

This meta-analysis summarizes published information on the use of various plant species by parasitoid wasps. Trials measured either a physiological response such as wasp longevity when supplied with flowers from a single plant species, or a behavioral response like attraction of wasps to field plantings. The log response ratio was the effect size used to standardize estimates and meta-analyses were conducted to make overall estimates for plant species included in multiple trials. Physiological response trials have been conducted on 126 different plant species and behavioral response trials tested 104 plant species. The log response ratio of different plant species ranged from 0 to 2.7 for longevity, and up to 4.1 for attraction. The longevity response ratio estimate is equivalent to a nearly 15-fold increase in the ratio of days longevity with the flower to days longevity in the starvation control. Wasp longevity increased the most with plants in the Polygonaceae, Apiaceae, Brassicaceae, Boraginaceae, Solanaceae, and Rosaceae. Plant species in the Onagraceae, Caryophyllaceae, Lamiaceae, Scrophulariaceae, Asteraceae, and Fabaceae tended to slightly increase wasp life spans, but their effect was lower and less consistent among species. There were a number of families which did not increase wasp longevity over the controls at all such as Chenopodiaceae. This review can help identify plant species which have been proven to supply nectar for parasitoids for potential use in a conservation biological control program, but plant selection should not be limited to the small list of species that were included in these studies.     

The foraging behaviour of a nectar feeding marsupial,Petaurus australis

Summary The foraging behaviour of non-flying nectar feeding mammals has been examined rarely. The exudivorous yellow-bellied glider (Petaurus australis) was observed to feed extensively (70% of the total feeding observation time) on the nectar of all species of Eucalyptus present at a site in southeastern Australia. Gliders harvested nectar, and presumably pollen also, whenever eucalypt flowers were available and selected trees with 2–3 times as many flowers as that on trees randomly selected along a transect. The abundance of flowering trees varied temporally and, at times when few flowering trees were present, gliders chose trees with fewer flowers than at times when flowering trees were abundant. When flowering trees were superabundant or scarce, there was no relationship between the number of flowers in a tree and the duration of visits by gliders. However, at intermediate levels of abundance, the amount of time a glider spent in a tree was related to the number of flowers in a tree. Gliders devoted 90% of the time outside their dens to foraging and the above relationship is suggested to reflect two foraging options which maximize net energy gain for different abundances of flowering trees. Although gliders spent considerable lengths of time in individual trees feeding, initial deposition of cross pollen when gliders first arrive in a tree may be substantial and thus, may provide significant amounts of outcrossing for these eucalypts.     

A test of pollinator specificity and morphological convergence between nectarivorous birds and rainforest tree flowers in New Guinea

Interactions between flowering trees in a representative sample of vegetation, and the birds that fed at their flowers, were studied for 2 years in lowland tropical hill forest in New Guinea. All 2,200 trees in a 3-ha plot were tagged, identified, mapped, and monitored monthly. Approximately 60% of all individual trees flowered during the study; all species that these flowering individuals belonged to were evaluated for bird visitation. Approximately 13% of the 164 resident species of New Guinea avifauna at the study site, especially honeyeaters and parrots, visited flowers. In the forest inventory plot, approximately 15–22% of all 86 tree species that flowered during the study were visited by birds; most of these tree species were canopy species. Results showed that there was no statistically significant correlation between bird species grouped by bill morphology and flower species grouped as morphotypes and ranked by nectar accessibility, although strong but unexpected bird/plant associations were evident. These associations may be related to variables such as body mass or perch size. These results are discussed in comparison with results from the Neotropics and Australia, and in terms of morphological convergence and pollinator specificity in pollination systems.