Impact of introduced honeybees, Apis mellifera, upon native bee communities in the Bonin (Ogasawara) Islands

The Bonin (Ogasawara) Islands are oceanic islands located in the northwest Pacific, and have ten native (nine endemic) bee species, all of which are nonsocial. The European honeybee (Apis mellifera), which was introduced to the islands for apiculture in the 1880s, became naturalized in a few islands shortly after introduction. To detect the impact of the honeybees upon native bee diversity, we analyzed pollen harvest by honeybees and surveyed the relative abundance of honeybees and native bees on flowers on several islands. Both hived and feral honeybee colonies were active throughout the year, harvesting pollen of both native and alien flowers and from both entomophilous and anemophilous flowers. Honeybees strongly depended on the alien plants, especially during winter to spring when native melittophilous flowers were rare. From June to November, honeybees exhaustively utilized native flowers, which had originally been utilized and pollinated by native bees. On Chichi and Haha Islands, where human disturbance of forests has been severe, both native and alien flowers were dominated by honeybees, and native bees were rare or extinct even in well-conserved forests. In contrast, on Ani Island and Haha's satellite islands where primary forests were well conserved and honeybees were still uncommon or absent, native bees remained dominant. These results suggest that competition for nectar and pollen of the native flowers between honeybees and native bees favors honeybees on the disturbed islands, which are thoroughly invaded by alien nectariferous, sometimes aggressive, weedy plants. Received: May 8, 1998 / Accepted: May 6, 1999     

Contrasting effects of plant invasion on pollination of two native species with similar morphologies

Invasive plants may decrease native plant density and disrupt interactions between native plants and their pollinators. We hypothesized that invasive Solidago canadensis (Asteraceae) competes for pollination services with two confamilial species, Ixeris chinensis and Sonchus arvensis. Breeding-system studies revealed that both native species are self-incompatible. In plots with all three species we found that Solidago received the highest visitation rates. To test the hypothesis of competition for pollination in the context of reduced native density, we established 3 plots for both native species with three Solidago densities (uninvaded, 50 and 75 % invaded) and corresponding decreases in native density. We investigated the effects of varying densities of Solidago on honeybee visitation rates, number of successive visits within individual ramets, pollen-load composition on bees, and seed set. For both native species, increasing Solidago density and decreasing native density resulted in bees carrying higher ratios of Solidago pollen and in bees visiting fewer capitula prior to departing from a plant. However, for other aspects of pollination, the native species responded very differently to Solidago. With increasing Solidago and decreasing native density, Ixeris received fewer honeybee visits and produced fewer seeds, demonstrating competition for pollination, but Sonchus attracted more honeybee visits and showed a non-significant trend toward setting more seeds, suggesting facilitation. These opposing effects occurred despite similarities in the native species’ floral morphology, suggesting that the effects of invasive plants are difficult to predict. In this case the different effects may relate to Sonchus being a taller plant with larger flowers.     

Pollinators and pollination of oilseed rape crops (Brassica napus L.) in Ireland: ecological and economic incentives for pollinator conservation

Pollinators are beneficial for many wild and crop plants. As a mass-flowering crop, oilseed rape has received much focus in terms of its pollination requirements but despite a threefold increase in area of cultivation of this crop in Ireland over the past 5 years, little is known about its pollination here. We surveyed the flower visiting insects found in commercial winter oilseed rape fields and evaluated the importance of different pollinator groups, investigated the contribution of insect pollination to oilseed rape seed production, and estimated the economic value of insect pollination to the crop at a national level. Our data showed that winter oilseed rape is visited by a wide variety of insect species, including the honeybee, bumblebees, solitary bees, and hoverflies. The honeybee, Eristalis hoverflies and bumblebees (especially Bombus sensu stricto and B. lapidarius) were the best pollinators of winter oilseed rape based on the number of pollen grains they carry, visitation rates per flower and their relative abundance per field. Exclusion of pollinators resulted in a 27 % decrease in the number of seeds produced, and a 30 % decrease in seed weight per pod in winter crops, with comparable values from a spring oilseed rape field also. The economic value of insect pollination to winter oilseed rape was estimated as €2.6 million per annum, while the contribution to spring oilseed rape was €1.3 million, resulting in an overall value of €3.9 million per annum. We can suggest the appropriate conservation and management of both honeybees and wild pollinators in agricultural areas to ensure continued provision of pollination services to oilseed rape, as a decrease in insect numbers has the potential to negatively influence crop yields.     

The most effective pollinator revisited: pollen dynamics in a spring-flowering herb

Most flowers are visited by a wide range of potential pollinators. However, their efficiency in pollen removal and deposition, and other behavioural factors affecting pollination effectiveness may greatly differ among taxa, and even individuals. Fritillary (Fritillaria meleagris L., Liliaceae) is a spring-flowering, critically endangered plant in the Polish flora, red-listed in most of the European countries of its range. Based on indirect evidence, that is, body pollen loads, visitation frequency and seasonal abundance, it is estimated that its key pollinators are queen bumblebees, but, as shown in the literature, the largest Fritillaria pollen loads are carried by solitary bees. To study pollinator effectiveness for floral visitors to F. meleagris, we performed a garden experiment, where we analysed pollen deposition and assessed pollen removal per single flower-visit in the plant. Similarly to field conditions reported in the literature, our experimental plants were serviced by nectar-seeking bumblebee queens and two taxa of solitary bees, small pollen-collecting Andrena and large, nectar-seeking Anthophora males. When “quality” component was addressed, despite the character of visits, insects from all groups deposited more pollen than was found on unvisited flowers, but they did not differ significantly from each other in pollen deposition on virgin stigmas. We also found some differences in pollen removal both within- and among-visitor species and control flowers, unfortunately due to extremely high variation of the results they were all statistically insignificant. However, when “quantity” component of insect performance was concerned, we observed that over 81 % of visits were by bumblebees. Bombus queens stayed on flowers significantly less time than small Andrena individuals (13 % of recorded visits) and equally long as Anthophora males (only 6 % of visits). We conclude that although all the visitor groups can pollinate the flowers of F. meleagris, bumblebee queens indeed proved to be the most effective pollinators of the plant, when both quality and quantity components of pollination are concerned.     

The pollination efficiency of a pollinator depends on its foraging strategy,flowering phenology,and the flower characteristics of a plant species

Honey bees and stingless bees are generalist visitors of several wild and cultivated plants. They forage with a high degree of floral fidelity and thereby help in the pollination services of those plants. We hypothesized that pollination efficiency might be influenced by flowering phenology, floral characteristics, and resource collection modes of the worker bees. In this paper, we surveyed the foraging strategies of honey bees (Apis cerana, Apis dorsata, and Apis florea) and stingless bees (Tetragonula iridipennis) concerning their pollination efficiencies. Bees showed different resource gathering strategies, including legitimate (helping in pollination as mixed foragers and specialized foragers) and illegitimate (serving as nectar robbers and pollen thieves) types of flower visitation patterns. Foraging strategies are influenced by the shape of flowers, the timing of the visitation, floral richness, and bee species. Honey bees and stingless bees mainly acted as legitimate visitors in most plants studied. Sometimes honey bees served as nectar robbers in tubular flowers and stingless bees as pollen thieves in large-sized flowers. Among the legitimate categories, mixed foragers have a comparatively lower flower visitation rate than the specialized nectar and pollen foragers. However, mixed foragers have greater abundance and higher values of the single-visit pollination efficiency index (PEi) than nectar and pollen foragers. The value of the combined parameter ‘importance in pollination (PI)’ was thus higher in mixed foragers than in nectar and pollen foragers.     

Alien pollinator promotes invasive mutualism in an insular pollination system

The alien predatory lizard, Anolis carolinensis, has reduced the insect fauna on the two main islands of the Ogasawara archipelago in Japan. As a result of this disturbance, introduced honeybees are now the dominant visitors to flowers instead of endemic bees on these islands. On the other hand, satellite islands not invaded by alien anoles have retained the native flower visitors. The effects of pollinator change on plant reproduction were surveyed on these contrasting island groups. The total visitation rates and the number of interacting visitor groups on main islands were 63% and 30% lower than that on satellite islands, respectively. On the main islands, the honeybees preferred to visit alien flowers, whereas the dominant endemic bees on satellite islands tended to visit native flowers more frequently than alien flowers. These results suggest that alien anoles destroy the endemic pollination system and caused shift to alien mutualism. On the main islands, the natural fruit set of alien plants was significantly higher than that of native plants. In addition, the natural fruit set was positively correlated with the visitation rate of honeybees. Pollen limitation was observed in 53.3% of endemic species but only 16.7% of alien species. These data suggest that reproduction of alien plants was facilitated by the floral preference of introduced honeybees.     

Pollinator effectiveness of native and non‐native flower visitors to an apparently generalist Andean shrub,Duranta mandonii (Verbenaceae)

Not all visitors to flowers are pollinators and pollinating taxa can vary greatly in their effectiveness. Using a combination of observations and experiments we compared the effectiveness of introduced honeybees with that of hummingbirds, native bees and moths on both the male and female components of fitness of the Andean shrub Duranta mandonii (Verbenaceae). Our results demonstrated significant variation among flower visitors in rates of visitation, pollen removal ability and contribution to fruit set. This variation was not always correlated; that is, taxa that regularly visited flowers did not remove the most pollen or contribute to fruit set. Despite the taxonomic diversity of visitors, the main natural pollinators of this shrub are large native bees, such as Bombus spp. Introduced honeybees were found to be as effective as native bees at pollinating this species. Duranta mandonii has high apparent generalization, but low realized generalization and can be considered to be a moderate ecological generalist (a number of species of large bees provide pollination services), but a functional specialist (most pollinators belong to a single functional group). The present study has highlighted the importance of measuring efficiency components when documenting plant–pollinator interactions, and has also demonstrated that visitation rates may give little insight into the relative importance of flower visitors.     

The relative importance of birds and bees in the pollination of Metrosideros excelsa (Myrtaceae)

Exclusion experiments were used to assess the effect of different pollinator groups on outcrossing and seed production in Metrosideros excelsa. The main study site was Little Barrier Island, New Zealand where indigenous bird and native solitary bees are the main flower visitors. Our results showed that native birds were more important pollinators of M. excelsa than native bees. Seed production was much higher in open pollination than in two exclusion experiments where either birds were excluded and native bees only had access to flowers, or where all pollinators had been excluded. The number of fertile seeds per capsule was 45% higher after open pollination than in treatments with bee visitation only and 28% higher than in treatments where all flower visitors were excluded. Estimated outcrossing rates were significantly higher (t_m = 0.71) for open pollination in the upper canopy (>4 m above‐ground level) where bird visitation is presumed to be more frequent than for a treatment with native bee access only (t_m = 0.40). Our results also suggest that a large proportion of seeds (66%) arise from autonomous self‐pollination when all pollinators are excluded. In four trees of a modified mainland population with predominantly introduced birds and a mixture of introduced and native bees there was no decrease in seed production for the treatment allowing bee access only, indicating that – in contrast to native bees – honeybees may be more efficient pollinators of M. excelsa. Observation of the foraging behaviour of both groups of bees showed that native bees contact the stigma of flowers less frequently than honeybees. This is likely to be a consequence of their smaller body size relative to honeybees.     

The presence of the invasive plant Solanum elaeagnifolium deters honeybees and increases pollen limitation in the native co-flowering species Glaucium flavum

Invasive plants can impact biodiversity and ecosystem functioning by displacing native plants and crop species due to competition for space, nutrients, water and light. The presence of co-flowering invasives has also been shown to affect some native plants through the reduction in pollinator visitation or through the deposition of heterospecific pollen on the native’s stigmas leading to stigma clogging. We examined the impact of the invasive plant Solanum elaeagnifolium Cavanilles (silver-leafed nightshade), native to South and Central America and South-western parts of North America, on the seed set of the native Glaucium flavum Crantz (yellow-horned poppy) on Lesvos Island, Greece. To do this we measured seed set and visitation rates to G. flavum before and after the placement of potted individuals of the invasive near the native plants. In addition, we hand-crossed G. flavum flowers with super-optimal amounts of conspecific pollen, bagged flowers to measure the rate of spontaneous selfing, and applied self-pollen to measure self-compatibility of G. flavum. The hand-selfing treatment resulted in very low seed set, which indicates that G. flavum is to a large degree self-incompatible and highlights the plant’s need for insect-mediated outcrossing. We show that the presence of the invasive significantly enhanced pollen limitation, although the overall visitation rates were not reduced and that this increase is due to a reduction in honeybee visitation in the presence of the invasive resulting in reduced pollination.     

Do exotic bumblebees and honeybees compete with native flower-visiting insects in Tasmania?

Honeybees, Apis mellifera, have been introduced by man throughout the globe. More recently, other bee species including various bumblebees (Bombus spp.) have been introduced to several new regions. Here we examine the impacts of honeybees and the bumblebee, Bombus terrestris, on native flower-visiting insects in Tasmania. To assess whether native insects have lower abundance or are excluded in areas that have been colonised by exotic bees, we quantified the abundance, diversity and floral preferences of flower-visiting insects at sites where bumblebees and honeybees were present, and compared them to sites where they were absent. This was achieved by hand searches at 67 sites, and by deploying sticky traps at 122 sites. Honeybees were by far the most abundant bee species overall, and dominated the bee fauna at most sites. There was considerable niche overlap between honeybees, bumblebees and native bees in terms of the flowers that they visited. Sites where bumblebees were established had similar species richness, diversity and abundance of native flower-visiting insects compared to sites where bumblebees were absent. In contrast, native bees were more than three times more abundant at the few sites where honeybees were absent, compared to those where they were present. Our results are suggestive of competition between honeybees and native bees, but exclusion experiments are needed to provide a definitive test.     

Initial floral visitor identity and foraging time strongly influence blueberry reproductive success

Priority effects occur when the order of species arrival affects subsequent ecological processes. The order that pollinator species visit flowers may affect pollination through a priority effect, whereby the first visitor reduces or modifies the contribution of subsequent visits. We observed floral visitation to blueberry flowers from honeybees, stingless bees or a mixture of both species and investigated how (i) initial visits differed in duration to later visits; and (ii) how visit sequences from different pollinator taxa influenced fruit weight. Stingless bees visited blueberry flowers for significantly longer than honeybees and maintained their floral visit duration, irrespective of the number of preceding visits. In contrast, honeybee visit duration declined significantly with an increasing number of preceding visits. Fruit weight was positively associated with longer floral visit duration by honeybees but not from stingless bee or mixed species visitation. Fruit from mixed species visits were heavier overall than single species visits, because of a strong priority effect. An initial visit by a stingless bee fully pollinated the flower, limiting the pollination contribution of future visitors. However, after an initial honeybee visit, flowers were not fully pollinated and additional visitation had an additive effect upon fruit weight. Blueberries from flowers visited first by stingless bees were 60% heavier than those visited first by honeybees when total floral visitation was short (∼1 min). However, when total visitation time was long (∼ 8 min), blueberry fruit were 24% heavier when initial visits were from honeybees. Our findings highlight that the initial floral visit can have a disproportionate effect on pollination outcomes. Considering priority effects alongside traditional measures of pollinator effectiveness will provide a greater mechanistic understanding of how pollinator communities influence plant reproductive success.     

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.     

Pollinator-mediated impacts of alien invasive plants on the pollination of native plants: the role of spatial scale and distinct behaviour among pollinator guilds

Alien invasive plant species can affect pollination, reproductive success and population dynamics of co-flowering native species via shared pollinators. Consequences may range from reproductive competition to facilitation, but the ecological drivers determining the type and magnitude of such indirect interactions remain poorly understood. Here, we examine the role of the spatial scale of invader presence and spatially contingent behavioural responses of different pollinator groups as potential key drivers, using the invasive Oxalis pes-caprae and the self-incompatible native annual Diplotaxis erucoides as a model system. Three treatments were assigned to native focal plants: (1) invader present at the landscape scale (hectares) but experimentally removed at the floral neighbourhood scale (pa); (2) invader present at both scales (pp); (3) invader absent at both scales (aa). Interestingly, we found pronounced spatially contingent differences in the responses of pollinators: honeybees and bumblebees were strongly attracted into invaded sites at the landscape scale, translating into native plant visitation facilitation through honeybees, while bumblebees almost exclusively visited Oxalis. Non-corbiculate wild bees, in contrast, showed less pronounced responses in foraging behavior, primarily at the floral neighborhood scale. Average heterospecific (Oxalis) pollen deposition onto stigmas of Diplotaxis was low (<1 %), but higher in the pp than in the pa treatment. Hand-pollination of Diplotaxis with Oxalis and conspecific pollen, however, reduced seed set by more than half when compared to hand-pollination with only conspecific pollen. Seed set of Diplotaxis, finally, was increased by 14 % (reproductive facilitation) in the pp treatment, while it was reduced by 27 % (reproductive competition) in the pa treatment compared to uninvaded populations. Our study highlights the crucial role of spatial scale and pollinator guild driving indirect effects of invasive on co-flowering native plant species.     

Pollinator visitation to mass-flowering courgette and co-flowering wild flowers: Implications for pollination and bee conservation on farms

Managing the complex relationship between pollinators and their habitat requirements is of particular concern to growers of pollinator-dependent crop species, such as courgette (Cucurbita pepo). Naturally occurring wild flowers (i.e. agricultural weeds) offer a free, sustainable, and often underappreciated resource for pollinators, however, they may compete with crop flowers for visits. To understand the extent to which floral resources mediate pollinator visitation to courgette flowers and courgette fields, plant community and pollinator visitation data were collected at two spatial scales: field scale (in margins, and in the cropped area) and farm scale (500 m and 2000 m radii) for nine courgette fields across the UK. Apis mellifera (honeybees) and Bombus spp. (bumblebees) were the only pollinators observed to visit courgette flowers. Bumblebees were significantly more abundant on courgette flowers in fields with a greater species richness of wild flowers in the crop, whilst honeybees were significantly more abundant on courgette flowers in areas with less semi-natural habitat. For both honeybees and bumblebees, their abundance in field margins did not significantly reduce their abundance on courgette flowers, suggesting that wild flowers were not competing with courgette flowers for pollinator visitation. Although solitary bees were not observed to visit courgette flowers, their abundance and species richness in courgette fields were significantly greater with more semi-natural habitat and a greater species richness of wild flowers. Therefore, allowing uncultivated areas around the crop to be colonised by species-rich wild flowers is an effective way of boosting the abundance of bumblebees, which are important visitors to courgette flowers, as well as the abundance and species richness of solitary bees, thereby benefitting pollinator conservation.     

Honeybees enhance reproduction without affecting the outcrossing rate in endemic Pedicularis densispica (Orobanchaceae)

There has been substantial debate in recent years surrounding the impact of introduced honeybees on native biota. This study reports on an investigation of Pedicularis densispica, a subalpine annual herb endemic to Southwest China, in an attempt to determine the impact of introduced domestic honeybees on pollen dispersal and thus on their reproductive success and mating system. Honeybees were introduced into the study site in 2004, and a sudden seasonal pollinator shift from bumblebees to honeybees was observed. Intra- and inter-plant visits by different pollinators were recorded in the field in 2003 and 2004. Fruit and seed sets prior to and after the pollinator shift were measured. Experimental pollinations were performed to characterize the breeding system. Outcrossing rates at the seed stage were estimated for both years using RAPD markers. Our results indicated that honeybees foraged between plants more frequently than bumblebees did. Our results also revealed that the introduction of honeybees significantly enhanced reproductive success. However, no significant difference was detected between the outcrossing rates due to bumblebee and honeybee pollination. P. densispica was almost completely outcrossing ( T(m) = 0.956 and 0.967, respectively in 2003 and 2004) but partially self-compatible. This study presents the first report of the outcrossing rate in the genus pedicularis and reveals a limited influence of pollination on the mating system in P. densispica. The pollinator shift did not reduce reproductive success of the plants and honeybees may be used to augment pollinator services for nectariferous P. densispica.     

The relationship between the abundances of bumblebees and honeybees in a native habitat

Abstract. 1. The western honeybee, Apis mellifera, has been introduced to many parts of the world and is sometimes purported to be detrimental to native bees because it reduces their food base. It is seldom viewed in this light in Europe; however, when beekeepers maintain very high bee densities, the species could also be displacing insects in its native European range by reducing the resource base. 2. In England, populations of bumblebees (Bombus Latr. Hym.) have been decreasing both in terms of diversity and abundance, mainly because of a loss of habitat resulting from agricultural intensification. The impact of competition from other flower feeders is largely unknown. 3. Nineteen dry lowland heaths in southern England were sampled once for honeybees and bumblebees. Honeybee abundance varied from 4 to 81 bees per 100 m² (mean = 30.89, median = 23), whereas bumblebees varied from 2 to 17 individuals per 100 m² (mean = 8.26, median = 7), belonging to between one and five species. There was a negative association between honeybee and bumblebee abundance but there was no apparent relationship between honeybee abundance and bumblebee diversity. 4. The Bray–Curtis coefficient was used to compare the similarity in honeybee and bumblebee floral host breadth at these 19 sites. The coefficient was negatively associated with honeybee abundance: thus where honeybees were most abundant, bumblebees were fewer and/or foraged on different flower species. 5. Foraging host breadth was also examined at four heathlands over a field season (April to September). No association between honeybee abundance and foraging host breadth was found for short‐tongued bees, although there was some evidence for a change in floral host breadth for long‐tongued bees. 6. It is concluded that the impact of honeybees on bumblebees is complex. Although competition between the two species cannot be ruled out, it is perhaps equally likely that bumblebees decline in response to other factors, and that honeybees move independently of this decline.     

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.     

Intersexual mimicry and flowering phenology facilitate pollination in a dioecious habitat specialist species, <Emphasis Type="Italic">Myristica fatua</Emphasis> (Myristicaceae)

Myristica fatua is a dioecious specialist species restricted to the endangered, freshwater Myristica swamp forests in the Western Ghats, India. Earlier studies have alluded to pollination by deception in members of the Myristica genus, and thus we examined the pollination ecology comprising floral biology, flower production, flower visitors, and reproductive success in M. fatua and inferred the potential strategies that could permit such deception in this habitat specialist tree. Male flowers provide pollen rewards for an extended period of time while female flowers are rewardless and both sexes are visited by generalist insects, mainly by honeybees and stingless bees. Bee visits were significantly more frequent and longer on male than on female flowers as bees collected pollen from male flowers. We found that flower production patterns create a preponderance of males compared to females in the swamp populations. Using a model of honeybee color vision, we found the distance between the color loci of male and female flowers and based on minimum visual angle subtended by these flowers, we suggest that the two floral sexes cannot be discriminated by bees. Bees are likely deceived by the perceptual similarity of rewardless female flowers to pollen-offering male flowers and pollination is the consequence of foraging errors made by pollinators that encounter largely male–rarely female flower mosaics as they forage among clump-distributed M. fatua trees in the swamp habitat.     

Does the invasive Lupinus polyphyllus increase pollinator visitation to a native herb through effects on pollinator population sizes?

Invasive plants may compete with native species for abiotic factors as light, space and nutrients, and have also been shown to affect native pollination interactions. Studies have mainly focused on how invasive plants affect pollinator behaviour, i.e. attraction of pollinators to or away from native flowers. However, when an invasive plant provides resources utilized by native pollinators this could increase pollinator population sizes and thereby pollination success in natives. Effects mediated through changes in pollinator population sizes have been largely ignored in previous studies, and the dominance of negative interactions suggested by meta-analyses may therefore be biased. We investigated the impact of the invasive Lupinus polyphyllus on pollination in the native Lotus corniculatus using a study design comparing invaded and uninvaded sites before and after the flowering period of the invasive. We monitored wild bee abundance in transects, and visit rate and seed production of potted Lotus plants. Bumblebee abundance increased 3.9 times in invaded sites during the study period, whereas it was unaltered in uninvaded sites. Total visit rate per Lotus plant increased 2.1 times in invaded sites and decreased 4.4 times in uninvaded sites. No corresponding change in seed production of Lotus was found. The increase in visit rate to Lotus was driven by an increase in solitary bee visitation, whereas mainly bumblebees were observed to visit the invasive Lupinus. The mechanism by which the invasive increases pollinator visit rates to Lotus could be increased availability of other flower resources for solitary bees when bumblebees forage on Lupinus.     

Modification of bumblebee behavior by floral color change and implications for pollen transfer in <Emphasis Type="Italic">Weigela middendorffiana</Emphasis>

Flowers of Weigela middendorffiana change the color from yellow to red. The previous study revealed that red-phase flowers no longer have sexual function and nectar, and bumblebees selectively visit yellow-phase flowers. The present study examined how retaining color-changed flowers can regulate the foraging behavior of bumblebees and pollen transport among flowers within (geitonogamous pollination) and between (outcrossing pollination) plants and how the behavior is influenced by display size (i.e., number of functional flowers) and visitation frequency. The visitation frequencies of bumblebees to plants and successive flower probes within plants were observed in the field using plants whose flower number and composition of the two color-phase flowers had been manipulated. To evaluate pollination efficiency over multiple pollinator visits, a pollen transport model was constructed based on the observed bumblebee behavior. In the simulation, three flowering patterns associated with display size and existence of color-changed flowers were postulated as follows: Type 1, large display (100 functional flowers) and no retention of color-changed flowers; Type 2, small display (50 functional flowers) and retention of color-changed flowers (50 old flowers), and; Type 3, large display (100 functional flowers) and retention of color-changed flowers (100 old flowers). Color-changed flowers did not contribute to increasing bumblebee attraction at a distance but reduced the number of successive flower probes within plants. Comparisons of pollen transfer between Types 1 and 3 revealed that the retention of color-changed flowers did not influence the total amount of pollen exported when pollinator visits were abundant (>100 visits) but decreased geitonogamous pollination. Comparisons between Types 2 and 3 revealed that the discouragement effect of floral color change on successive probes accelerated in plants with a large display size. Overall, the floral color change strategy contributed to reduce geitonogamous pollination, but its effectiveness was highly sensitive to display size and pollinator frequency.