Bumblebee (Hymenoptera: Apidae) Visitation Frequency Drives Seed Yields and Interacts with Site-Level Species Richness to Drive Pollination Services in Sunflower

Understanding whether pollinator behaviors and species richness drive crop yields is a key area of investigation in pollination ecology. Using sunflower as a study species we describe variation in mean floral visitation times among bee taxa and test how interactions between bee richness and the proportion of bumblebees in localized communities impact seed yield. Seven bee genera commonly visited sunflower including Agapostemon, Bombus, Halictus, Lasioglossum, Megachile, Melissodes, and Svastra. Mean visitation times to sunflower varied across genera and Bombus and Halictus spp. spent the most time foraging on inflorescences, but the number of visits by Bombus spp. was the only parameter associated with increased yields. Experimental pollination deficit reduced seed development and yields, and these effects were stronger in stands of wild-type sunflower in the field compared to a confection variety grown in the greenhouse. Relationships between bee richness and pollination services differed for potted and wild sunflower: when bees had short-term access to potted sunflower, bee richness and relative Bombus abundances were not associated with pollination quotients. When bees had long-term access to wild sunflower, relative Bombus abundances predicted pollination services but were modified by site-level bee richness: as richness increased, the effects of Bombus abundance decreased. Our studies demonstrate that bee species richness is not always a clear predictor of pollination services; instead, our results underscore the importance of specific taxa when species richness is low (here, bumblebees), and show that the effects of bee functional groups important for pollination may be modified by changes in site-level species richness.

Beeporter: Tools for high-throughput analyses of pollinator-virus infections

Pollinators are in decline thanks to the combined stresses of disease, pesticides, habitat loss, and climate. Honey bees face numerous pests and pathogens but arguably none are as devastating as Deformed wing virus (DWV). Understanding host-pathogen interactions and virulence of DWV in honey bees is slowed by the lack of cost-effective high-throughput screening methods for viral infection. Currently, analysis of virus infection in bees and their colonies is tedious, requiring a well-equipped molecular biology laboratory and the use of hazardous chemicals. Here we describe virus clones tagged with green fluorescent protein (GFP) or nanoluciferase (nLuc) that provide high-throughput detection and quantification of virus infections. GFP fluorescence is measured noninvasively in living bees via commonly available long-wave UV light sources and a smartphone camera, or a standard ultraviolet transilluminator gel imaging system. Nonlethal monitoring with GFP allows continuous screening of virus growth and serves as a direct breeding tool for identifying honey bee parents with increased antiviral resistance. Expression using the nLuc reporter strongly correlates with virus infection levels and is especially sensitive. Using multiple reporters, it is also possible to visualize competition, differential virulence, and host tissue targeting by co-occuring pathogens. Finally, it is possible to directly assess the risk of cross-species "spillover" from honey bees to other pollinators and vice versa.

Novel nectar robbing negatively affects reproduction in Digitalis purpurea

With many plant-pollinator interactions undergoing change as species' distributions shift, we require a better understanding of how the addition of new interacting partners can affect plant reproduction. One such group of floral visitors, nectar robbers, can deplete plants of nectar rewards without contributing to pollination. The addition of nectar robbing to the floral visitor assemblage could therefore have costs to the plant´s reproductive output. We focus on a recent plant colonist, Digitalis purpurea, a plant that in its native range is rarely robbed, but experiences intense nectar robbing in areas it has been introduced to. Here, we test the costs to reproduction following experimental nectar robbing. To identify any changes in the behavior of the principal pollinators in response to nectar robbing, we measured visitation rates, visit duration, proportion of flowers visited, and rate of rejection of inflorescences. To find the effects of robbing on fitness, we used proxies for female and male components of reproductive output, by measuring the seeds produced per fruit and the pollen export, respectively. Nectar robbing significantly reduced the rate of visitation and lengths of visits by bumblebees. Additionally, bumblebees visited a lower proportion of flowers on an inflorescence that had robbed flowers. We found that flowers in the robbed treatment produced significantly fewer seeds per fruit on average but did not export fewer pollen grains. Our finding that robbing leads to reduced seed production could be due to fewer and shorter visits to flowers leading to less effective pollination. We discuss the potential consequences of new pollinator environments, such as exposure to nectar robbing, for plant reproduction.

Next-Generation Sequencing and Its Impacts on Entomological Research in Ecology and Evolution

The advent of NGS-based methods has been profoundly transforming entomological research. Through continual development and improvement of different methods and sequencing platforms, NGS has promoted mass elucidation of partial or whole genetic materials associated with beneficial insects, pests (of agriculture, forestry and animal, and human health), and species of conservation concern, helping to unravel ecological and evolutionary mechanisms and characterizing survival, trophic interactions, and dispersal. It is shifting the scale of biodiversity and environmental analyses from individuals and biodiversity indicator species to the large-scale study of communities and ecosystems using bulk samples of species or a mixed "soup" of environmental DNA. As the NGS-based methods have become more affordable, complexity demystified, and specificity and sensitivity proven, their use in entomological research has spread widely. This article presents several examples on how NGS-based methods have been used in entomology to provide incentives to apply them when appropriate and to open our minds to the expected advances in entomology that are yet to come.

Bumble bees show an induced preference for flowers when primed with caffeinated nectar and a target floral odor

Caffeine is a widely occurring plant defense chemical^1,2 that occurs in the nectar of some plants, e.g., Coffea or Citrus spp., where it may influence pollinator behavior to enhance pollination.^3,4 Honey bees fed caffeine form longer lasting olfactory memory associations,⁵ which could give plants with caffeinated nectar an adaptive advantage by inducing more visits to flowers. Caffeinated free-flying bees show enhanced learning performance⁶ and are more likely to revisit a caffeinated target feeder or artificial flower,^7-9 although it is not clear whether improved memory of the target cues or the perception of caffeine as a reward is the cause. Here, we show that inexperienced bumble bees (Bombus terrestris) locate new food sources emitting a learned floral odor more consistently if they have been fed caffeine. In laboratory arena tests, we fed bees a caffeinated food alongside a floral odor blend (priming) and then used robotic experimental flowers¹⁰ to disentangle the effects of caffeine improving memory for learned food-associated cues versus caffeine as a reward. Inexperienced bees primed with caffeine made more initial visits to target robotic flowers emitting the target odor compared to control bees or those primed with odor alone. Caffeine-primed bees tended to improve their floral handling time faster. Although the effects of caffeine were short lived, we show that food-locating behaviors in free-flying bumble bees can be enhanced by caffeine provided in the nest. Consequently, there is potential to redesign commercial colonies to enhance bees' forage focus or even bias bees to forage on a specific crop.

The role of soils on pollination and seed dispersal

Ongoing environmental changes are affecting physical, chemical and biological soil components. Evidence of impacts of soil changes on pollinators' and seed dispersers' behaviour, fitness and density is scarce, but growing. Here, we reviewed information on such impacts and on a number of mechanisms that may explain its propagation, taking into account the full range of resources required by the large and diverse number of species of these two important functional groups. We show that while there is substantial evidence on the effects of soil nitrogen enrichment and changes in soil water content on the quality and quantity of floral and fruit resources, little is known on the effects of changes of other soil properties (e.g. soil pH, soil structure, other nutrients). Also, the few studies showing correlations between soil changes and pollinator and seed disperser foraging behaviour or fitness do not clearly identify the mechanisms that explain such correlation. Finally, most studies (including those with nitrogen and water) are local and limited to a small number of species, and it remains unclear how variable such effects are across time and geographical regions, and the strength of interactive effects between soil properties. Increasing research on this topic, taking into consideration how impacts propagate through species interaction networks, will provide essential information to predict impacts of ongoing environmental changes and help guide conservation plans that aim to minimize impacts on ecosystem functioning. This article is part of the theme issue 'The role of soils in delivering Nature's Contributions to People'.

Fluorescence from abnormally sterile pollen of the Japanese apricot

We observed trees of the Japanese apricot, Prunus mume 'Nanko' (Rosaceae), bearing two types of flowers: 34% had blue fluorescent pollen under UV irradiation, and 66% had non-fluorescent pollen. The fluorescent pollen grains were abnormally crushed, sterile, and devoid of intine and pollenkitt. The development of microspores within anthers was investigated: in the abnormally developed anthers, tapetal cells were vacuolated at the unicellular microspore stage, and fluorescent pollen was produced. Compounds responsible for the blue fluorescence of pollen were identified as chlorogenic acid and 1-O-feruloyl-β-D-glucose. The anthers with fluorescent pollen contained 6.7-fold higher and 3.8-fold lower amounts of chlorogenic acid and N ¹,N ⁵,N ¹⁰-tri-p-coumaroylspermidine, respectively, compared to those with non-fluorescent pollen. The tapetal vacuolization, highly accumulated chlorogenic acid, and deficiency of N ¹,N ⁵,N ¹⁰-tri-p-coumaroylspermidine imply that low-temperature stress during the early unicellular microspore stage caused a failure in microsporogenesis. Furthermore, potential effects of the visual difference on the bee behavior were also discussed through the colorimetry. The sterility, likely induced by low-temperature stress, and the preference of honeybees for fluorescence may reduce the pollination efficiency of P. mume.

Pollination in the Rainforest: Scarce Visitors and Low Effective Pollinators Limit the Fruiting Success of Tropical Orchids

A single plant might be visited by many flower visitors but not all might act as pollinators. Legitimate pollinators might also differ considerably in their efficiency, limiting pollination success. Unsuitable climatic conditions such as rain also affect pollinator activity. However, in the evergreen rainforest there is no prolonged dry season and flowering occurs usually under rain. Here, we explore the dependence on pollinators and the efficiency of flower visitors for the fruiting success of 10 Andean rainforest orchids. All species were self-compatible but strictly pollinator-dependent. Overall, we found low levels of fruit set in control flowers while experimental geitonogamous and cross-pollinations increased fruit set, revealing extensive pollination limitation in all populations. Seed viability dropped considerably after self and geitonogamous pollinations suggesting the possibility of early-acting inbreeding depression. Even though we monitored flower visitors on an extensive survey, few visitors were seen in these species and even fewer acted as legitimate pollinators. Thus, even though orchid pollination might be extremely diversified, these results show that few visitors are pollinating these species, explaining the low levels of fruit set recorded in the area studied.

The evolution of flower longevity in unpredictable pollination environments

Pollination requires a flower to remain open for long enough to allow for the arrival of pollinators. However, maintaining flowers costs energy and resources. Therefore, flower longevity, the length of time a flower remains viable, is critical for the outcome of plant reproduction. Although previous studies showed that the evolution of flower longevity depends on the rates of pollen deposition and removal, whether plants should increase or decrease flower life span when the pollination environment is unpredictable has not been explored. Moreover, the common hypothesis that an unpredictable pollination environment should select for increased flower longevity may be too simplistic since there is no distinction drawn between the effects of spatial and temporal variation. Adopting evolutionary game theory, we investigate the evolution of flower longevity under three types of variation: spatial heterogeneity, daily fluctuations within a flowering season and yearly fluctuations between flowering seasons. We find that spatial heterogeneity often selects for a shorter flower lifespan, while temporal fluctuations of fitness accrual rates at both daily and yearly time scales tends to favour greater longevity, although daily and yearly fluctuations have somewhat different effects. However, the presence of correlation between female and male fitness accrual rates seems to have no effect on flower longevity. Our work suggests that explicit measurements of spatial and temporal variation in both female and male functions may provide a better understanding of the evolution of flower longevity and reproduction.

Spatial variation in the intensity of interactions via heterospecific pollen transfer may contribute to local and global patterns of plant diversity

BACKGROUND

Studies that aim to understand the processes that generate and organize plant diversity in nature have a long history in ecology. Among these, the study of plant-plant interactions that take place indirectly via pollinator choice and floral visitation has been paramount. Current evidence, however, indicates that plants can interact more directly via heterospecific pollen (HP) transfer and that these interactions are ubiquitous and can have strong fitness effects. The intensity of HP interactions can also vary spatially, with important implications for floral evolution and community assembly.

SCOPE

Interest in understanding the role of heterospecific pollen transfer in the diversification and organization of plant communities is rapidly rising. The existence of spatial variation in the intensity of species interactions and their role in shaping patterns of diversity is also well recognized. However, after 40 years of research, the importance of spatial variation in HP transfer intensity and effects remains poorly known, and thus we have ignored its potential in shaping patterns of diversity at local and global scales. Here, I develop a conceptual framework and summarize existing evidence for the ecological and evolutionary consequences of spatial variation in HP transfer interactions and outline future directions in this field.

CONCLUSIONS

The drivers of variation in HP transfer discussed here illustrate the high potential for geographic variation in HP intensity and its effects, as well as in the evolutionary responses to HP receipt. So far, the study of pollinator-mediated plant-plant interactions has been almost entirely dominated by studies of pre-pollination interactions even though their outcomes can be influenced by plant-plant interactions that take place on the stigma. It is hence critical that we fully evaluate the consequences and context-dependency of HP transfer interactions in order to gain a more complete understanding of the role that plant-pollinator interactions play in generating and organizing plant biodiversity.

Causes and consequences of variation in heterospecific pollen receipt in Oenothera fruticosa

PREMISE

Heterospecific pollen transfer, the transfer of pollen between species, is common among co-flowering plants, yet the amount of pollen received is extremely variable among species. Intraspecific variation in heterospecific pollen receipt can be even greater, but we lack an understanding of its causes and fitness consequences in wild populations.

METHODS

We examined potential drivers of variation in heterospecific pollen receipt in Oenothera fruticosa. We evaluated the relationship between heterospecific and conspecific pollen receipt and considered how visitation by different pollinator groups, local floral neighborhood composition, and flowering phenology affect the total amount and proportion of heterospecific pollen received. Finally, we tested whether variation in heterospecific pollen receipt translated into lower seed production.

RESULTS

Heterospecific pollen was ubiquitous on O. fruticosa stigmas, but the amount received was highly variable and unrelated to conspecific pollen receipt. Heterospecific pollen receipt depended on pollinator type, the proportion of nearby conspecific flowers, and flowering date. Significant interactions revealed that the effects of pollinator type and neighborhood were not independent, further contributing to variation in heterospecific pollen. Naturally occurring levels of heterospecific pollen were sufficient to negatively impact seed set, but large amounts of conspecific pollen counteracted this detrimental effect.

CONCLUSIONS

Although selection could act on floral traits that attract quality pollinators and promote synchronous flowering in O. fruticosa, the risk of heterospecific pollen is equally dependent on local floral context. This work highlights how extrinsic and intrinsic factors contribute to intraspecific variation in heterospecific pollen receipt in wild plants, with significant fitness consequences.

The importance of small natural features in forests-How the overgrowth of forest gaps affects indigenous flower supply and flower-visiting insects and seed sets of six Campanula species

The abandonment of historical land-use forms within forests, such as grazing or coppicing, and atmospheric nitrogen deposition, has led to an increasing overgrowth of forest gaps and canopy closure in forest ecosystems of Central Europe. From 1945 to 2015, 81% of the forest gaps greater than 150 m2 within the study area transitioned into a closed forest.This study investigated how the overgrowth process affects flower supply, flower visitors, and reproduction of Campanula species. Six native Campanula species with different light requirements were used as phytometers.The forest gaps in the studied area are a feature of the historical European cultural landscape. We compared large gaps caused by human activities, small gaps caused by habitat conditions, and closed forests. In eight blocked replicates, each with the three habitat categories, we recorded the flower cover and number of indigenous flowering species in the immediate surroundings, and, of six Campanula species, flower visitors and seed production.Forest gaps and their size positively affected the number of flowering plant species in the surrounding area, the number of all flower visitor groups, and the number of seeds produced by all six Campanula species. Flower cover in the surrounding area was higher in large gaps, but there was no difference between small gaps and closed forests. Among flower visitors, small bees varied the most between the three habitat categories, and flies varied the least. The effect on the number of seeds produced was particularly strong for three light-demanding Campanula species.The overgrowth of forest gaps negatively affected flower supply, flower-visiting insects, and seed sets of six Campanula species. Forest gaps should be managed to maintain the reproduction of open forest plants and their pollinators.

The role of functional diversity and facilitation in small-scale pollinator habitat

People in urban and rural areas are planting habitat patches for pollinators in response to growing public awareness of the risks of pollinator declines; yet research rarely has been undertaken to inform the composition of such patches. Determining which key functional plant traits to prioritize and how plant-pollinator interaction dynamics operate in these small-scale, fragmented patches is critical to ensuring the efficacy of pollinator restoration efforts across landscapes. We established small-scale (2.5 m diameter) experimental patches and manipulated plant diversity and resource level (nectar) to determine the effects on pollinator abundance, pollinator diversity, and plant-pollinator facilitation-competition dynamics. Our results showed that in small-scale habitat, plant diversity and resource availability significantly affected the abundance and diversity of pollinating insects. Specifically, the treatments that contained high-resource plant species increased pollinator abundance and diversity the most. Plant diversity increased pollinator diversity and abundance only in the absence of high-resource plants. Pollination facilitation was observed in high-resource treatments, but varied among plant species. Competition for pollinators was observed in high-diversity treatments but did not affect seed set for high-resource plants in any of the treatments. Our results suggest that managers or landowners planting small-scale pollinator habitat should prioritize including species with high nectar production, and secondarily, a diverse mix of species if space and resources allow. The protocols we used to monitor pollinators can be used by community science observers with limited training, expanding the potential for assessment of future pollinator habitat restoration projects. Shared research identifying features critical to effective restoration will help conserve plant-pollinator mutualisms across landscapes.

Causes and consequences of variation in heterospecific pollen receipt in Oenothera fruticosa

PREMISE

Heterospecific pollen transfer, the transfer of pollen between species, is common among co-flowering plants, yet the amount of pollen received is extremely variable among species. Intraspecific variation in heterospecific pollen receipt can be even greater, but we lack an understanding of its causes and fitness consequences in wild populations.

METHODS

We examined potential drivers of variation in heterospecific pollen receipt in Oenothera fruticosa. We evaluated the relationship between heterospecific and conspecific pollen receipt and considered how visitation by different pollinator groups, local floral neighborhood composition, and flowering phenology affect the total amount and proportion of heterospecific pollen received. Finally, we tested whether variation in heterospecific pollen receipt translated into lower seed production.

RESULTS

Heterospecific pollen was ubiquitous on O. fruticosa stigmas, but the amount received was highly variable and unrelated to conspecific pollen receipt. Heterospecific pollen receipt depended on pollinator type, the proportion of nearby conspecific flowers, and flowering date. Significant interactions revealed that the effects of pollinator type and neighborhood were not independent, further contributing to variation in heterospecific pollen. Naturally occurring levels of heterospecific pollen were sufficient to negatively impact seed set, but large amounts of conspecific pollen counteracted this detrimental effect.

CONCLUSIONS

Although selection could act on floral traits that attract quality pollinators and promote synchronous flowering in O. fruticosa, the risk of heterospecific pollen is equally dependent on local floral context. This work highlights how extrinsic and intrinsic factors contribute to intraspecific variation in heterospecific pollen receipt in wild plants, with significant fitness consequences.

Pollination by nectar-foraging pompilid wasps: a new specialized pollination strategy for the Australian flora

The Pompilidae is a cosmopolitan and diverse group of wasps, which commonly feed on nectar. However, pollination systems specialized on pompilids have not been documented in detail outside of southern Africa. Here, we studied Caladenia drummondii (Orchidaceae) where, based on floral traits and preliminary field observations, we predicted pollination by sexual deception of male pompilid wasps. Detailed pollinator observations were undertaken using floral baiting experiments at sites spanning 375 km. Following evidence for nectar on some flowers of C. drummondii, the sugar content on the labellum was analysed by GC-MS. Floral spectral reflectance was measured and compared with Caladenia using other pollination strategies. Males of a single species of pompilid wasp (Calopompilus sp.) were the only visitors capable of pollinating C. drummondii. Attempts to feed from the surface of the labellum were frequent and were associated with removal and deposition of pollinia. GC-MS analysis revealed larger quantities of sugar on the labellum than reported in other Caladenia species. While no sexual or courtship behaviour was observed, the zig-zag and circling flight on approach to the flower is suggestive of odour-based attraction. Floral spectral reflectance was similar to sexually deceptive Caladenia. This study represents the first confirmation of a specialized pompilid pollination system outside of Africa. Although pollination occurs during nectar-foraging, long-distance sexual attraction cannot be ruled out as an explanation for the exclusive male visitation. The similarity in floral spectral reflectance to other Caladenia indicates colour may not impose a constraint on the evolution of pollination by pompilids.

Pollination of the strongly scented Sarcoglottis acaulis (Orchidaceae) by male orchid bees: nectar as resource instead of perfume

The Neotropical orchid genus Sarcoglottis comprises ~40 species, which emit strong floral scents, presumably involved in pollinator attraction. Information on basic aspects of its natural history is scant, with the few studies available so far pointing to nectar-seeking orchid bees as pollinators. Here, we investigated the reproductive biology of Sarcoglottis acaulis, addressing the ecological meaning of its floral scent. In Atlantic Forest fragments of NE Brazil, we described the floral biology, determined the breeding system and recorded the pollinators of S. acaulis. Additionally, we chemically characterized its floral scent and assessed its role on pollinator attraction. Although self-compatible, S. acaulis depends on pollinators to boost fruit set. Male orchid bees of Eulaema atleticana and E. niveofasciata were the only recorded pollinators. They foraged for nectar only, in spite of the strong scent emitted by S. acaulis flowers. The floral scent is composed of six compounds, of which geraniol and nerol elicited electroantennographic responses in Eulaema bees. A synthetic mixture of these compounds attracted Eulaema bees in field assays but did not trigger the stereotyped scent-gathering behaviour. The floral scent of S. acaulis acts in signalling and nectar is the sole reward for pollinators. Despite the low pollinator frequency, S. acaulis shows a high fruit set (77%), particularly when compared to other orchids. Attributes such as pollinia that release small pollen loads, allowing a single pollinia-carrying bee to pollinate several flowers, low amount of nectar, steady-state flowering and traplining behaviour of pollinators, might act together to assure this outstanding fruit set.

Ecosystem functions in degraded riparian forests of southeastern Kenya

Species community structures shape ecosystem functions, which are mostly stronger pronounced in intact than in degraded environments. Riparian forests in semiarid Africa provide important habitats for endangered plant and animal species and provide various ecosystem functions, that is, services to people settling along these streams. Most of these riparian forests are severely disturbed by human activities and dominated by invasive exotic plant species in the meanwhile. Thus, ecosystem functions are negatively influenced. While most studies have analyzed a specific metric to measure the degree of ecosystem function, little is known about how strongly different ecosystem functions respond to anthropogenic disturbances in parallel. In this study, we analyzed a set of four proxies of ecosystem functions, ground-dwelling arthropod abundances, pollination, seed dispersal, and predation, along a highly disturbed riparian forest in southeastern Kenya. We assessed the land cover and land use manually and with an Unmanned Aerial Vehicle. Our data show that ecosystem functions respond differently to vegetation cover, human disturbances, and the availability of the invasive exotic shrub Lantana camara. The occurrence of representatives from the groups Saltatoria and Formicidae profits from heterogeneous habitat structures and natural riparian forest, while representatives of the Araneae profit from high proportion of agricultural fields. In general, predation is higher in mixed land use and natural riparian forest, while pollination and seed dispersal showed no significant trend in regard on land coverage. Along with this, predation also increased with rising proportion of natural riparian forest, while the proportion of agricultural land negatively affects predation, but in parallel showed a slightly significant positive trend with seed dispersal. Human disturbances and the occurrence of the invasive exotic L. camara shrub did not significantly affect our metrics of ecosystem functioning, except of the negative impact of human disturbances on pollinators. In conclusion, our results underpin that ecosystem functions respond highly variable and individually to environmental changes.

Biomechanics of pollen pellet removal by the honey bee

Honey bees (Apis mellifera) carry pollen back to their hive by mixing it with nectar and forming it into a pellet. The pellet must be firmly attached to their legs during flight, but also easily removable when deposited in the hive. How does the honey bee achieve these contrary aims? In this experimental study, we film honey bees removing pollen pellets and find they peel them off at speeds 2-10 times slower than their typical grooming speeds. Using a self-built pollen scraper, we find that slow removal speeds reduce the force and work required to remove the pellet under shear stress. Creep tests on individual pollen pellets revealed that pollen pellets are viscoelastic materials characterized by a Maxwell model with long relaxation times. The relaxation time enables the pellet to remain a solid during both transport and removal. We hope that this work inspires further research into viscoelastic materials in nature.

Blueberry Yields Increase With Bee Visitation Rates, but Bee Visitation Rates are not Consistently Predicted by Colony Stocking Densities

Pollinator-dependent crops rely on the activity of managed and wild pollinators. While farm management and surrounding landscape can influence wild pollinator contributions, managed pollinator contributions may be primarily driven by their stocking densities, though this is not well studied across crops. We selected 20 southern highbush blueberry farms along two independent gradients of honey bee Apis mellifera L. (Hymenoptera: Apidae) stocking density (~1-11 hives/acre) and bumble bee Bombus impatiens Cresson (Hymenoptera: Apidae) stocking density (0 - 3 colonies/acre) ensuring that stocking densities were not correlated with farm or landscape attributes. Across farms, we observed managed and wild bee visitation rates, and measured yield estimates. Farms with greater bumble bee stocking densities had higher bumble bee visitation rates and yield estimates, but farms with higher honey bee stocking densities only received higher honey bee visitation rates at the end of bloom and did not have higher yield estimates. The main wild pollinator, the southeastern blueberry bee Habropoda laboriosa (Fabricius) (Hymenoptera: Apidae), showed higher visitation rates on organic farms and in late bloom. In general, higher visitation rates by honey bees, bumble bees, and H. laboriosa were correlated with higher yields. Our results suggest that yields are limited by bee visitation rates, and that within the stocking density ranges studied, increasing managed bumble bees, but not honey bees, increases their visitation rates. While H. laboriosa had the greatest effect on yield estimates, its activity appears to be limited by both a phenological mismatch with crop bloom and farm management.

A sit-and-wait predator, but not an active-pursuit predator, alters pollinator-mediated selection on floral traits

Indirect species interactions are ubiquitous in nature, often outnumbering direct species interactions. Yet despite evidence that indirect interactions have strong ecological effects, relatively little is known about whether they can shape adaptive evolution by altering the strength and/or direction of natural selection. We tested whether indirect interactions affect the strength and direction of pollinator-mediated selection on floral traits of the bumble-bee pollinated wildflower Lobelia siphilitica. We estimated the indirect effects of two pollinator predators with contrasting hunting modes: dragonflies (Aeshnidae and Corduliidae) and ambush bugs (Phymata americana, Reduviidae). Because dragonflies are active pursuit predators, we hypothesized that they would strengthen pollinator-mediated selection by weakening plant-pollinator interactions (i.e., a density-mediated indirect effect). In contrast, because ambush bugs are sit-and-wait predators, we hypothesized that they would weaken or reverse the direction of pollinator-mediated selection by altering pollinator foraging behavior (i.e., a trait-mediated indirect effect). Specifically, if ambush bugs hunt from plants with traits that attract pollinators (i.e., prey), then pollinators will spend less time visiting those plants, weakening or reversing the direction of selection on attractive floral traits. We did not find evidence that high dragonfly abundance strengthened selection on floral traits via a density-mediated indirect effect: neither pollen limitation (a proxy for the strength of plant-pollinator interactions) nor directional selection on floral traits of L. siphilitica differed significantly between high- and low-dragonfly abundance treatments. In contrast, we did find evidence that ambush bug presence affected selection on floral traits via a trait-mediated indirect effect: ambush bugs hunted from L. siphilitica plants with larger daily floral displays, reversing the direction of pollinator-mediated selection on daily display size. These results suggest that indirect species interactions have the potential to shape adaptive evolution by altering natural selection.

Bumblebees (Bombus terrestris) Improve 'Hass' Avocado (Persea americana) Pollination

Pollination is limiting for avocado production. We examined whether adding bumblebees (BBs; ca. 10 hives/ha) to conventional honeybees (HB; 5 hives/ha) would improve 'Hass' avocado pollination and yields. A preliminary trial (2017/18) in an avocado orchard with four consecutive rows of 'Hass' followed by one row of 'Ettinger' serving as a pollenizer (20% 'Ettinger') showed a considerable increase in 'Hass' yield in rows adjacent to (up to 80 m from) the BB hives vs. distant rows (=controls). In 2018/19, the trials were extended to three additional orchards. A significant yield increase was obtained in the BB hive-adjacent trees compared to BB hive-distant ones. Similar results were obtained in 2019/20, in experiments conducted throughout the country. The SNP analysis, to determine the parents of 'Hass' fruit at varying distances from the BB hives, showed no differences in the cross-pollination rate ('Hass' × 'Ettinger'). However, pollination rates and the number of germinating pollen grains per stigma decreased with distance from the hives, and correlated to the negative gradient in yield. Taken together, our data suggest that adding BB hives to 'Hass' avocado orchards, at ca. 10 hives/ha resulting in 0.5-1.0 BB visits/tree per min, increases pollination and, accordingly, total yield.

A meta-analysis of responses in floral traits and flower-visitor interactions to water deficit

Alterations in water availability and drought events as predicted by climate change scenarios will increasingly impact natural communities with effects already emerging at present. Water deficit leads to increasing physiological stress in plants, likely affecting floral development and causing changes in floral morphology, nectar and pollen production or scent. Understanding how these floral traits are altered by water deficit is necessary to predict changes in plant-pollinator interactions and how communities are impacted in the future. Here we employ a meta-analysis approach to synthesize the current evidence of experimental water deficit on floral traits and plant-pollinator interactions. Furthermore, we explore experimental factors potentially increasing heterogeneity between studies and provide ideas how to enhance comparability between studies. In the end, we highlight future directions and knowledge gaps for floral traits and plant-pollinator interactions under water deficit. Our analysis showed consistent decreases in floral size, number of flowers and nectar volume to reduced water availability. Other floral traits such as the start of flowering or herkogamy showed no consistent pattern. This indicates that effects of reduced water availability differ between specific traits that are potentially involved in different functions such as pollinator attraction or efficiency. We found no general decreasing visitation rates with water deficit for flower-visitor interactions. Furthermore, the comparison of available studies suggests that increased reporting of plant stress severity and including more hydraulic and physiological measurements will improve the comparability across experiments and aid a more mechanistic understanding of plant-pollinator interactions under altered environmental conditions. Overall, our results show that water deficit has the potential to strongly affect plant-pollinator interactions via changes in specific floral traits. Linking these changes to pollination services and pollinator performance is one crucial step for understanding how changing water availability and drought events under climate change will alter plant and pollinator communities.

The neglected other half - role of the pistil in plant heat stress responses

Heat stress coinciding with reproductive stage leads to a significant loss in reproductive organs viability, resulting in lower seed-set and crop productivity. Successful fertilization and seed formation are determined by the viability of male and female reproductive organs. The impact of heat stress on the male reproductive organ (pollen) is studied more often compared to the female reproductive organ (pistil). This is attributed to easier accessibility of the pollen coupled with the notion that the pistil's role in fertilization and seed-set under heat stress is negligible. However, depending on species and developmental stages, recent studies reveal varying degrees of sensitivity of the pistil to heat stress. Remarkably, in some cases, the vulnerability of the pistil is even greater than the pollen. This article summarizes the current knowledge of the impact of heat stress on three critical stages of pistil for successful seed-set, that is, female reproductive organ development (gametogenesis), pollen-pistil interactions including pollen capture on stigma and pollen tube growth in style, as well as fertilization and early embryogenesis. Further, future research directions are suggested to unravel molecular basis of heat stress tolerance in pistil, which is critical for sustaining crop yields under predicted warming scenarios.

The impacts of flowering phenology on the reproductive success of the narrow endemic Nouelia insignis Franch. (Asteraceae)

Nouelia insignis Franch. (Asteraceae) is a short, narrow endemic and endangered tree, growing with a natural population in the dry and hot valley of the Jinsha River in the southwest area of China. In this work, flowering phenology (time and duration), floral biology, visit frequency and behavior of pollinators, and pollination characteristics were studied based on investigation in the field and analysis in the laboratory with the help of a stereomicroscope, and the relationship between seed setting rate and reproductive traits, as well as the relationship between flowering time and rainfall before flowering, was tested using the method of general linear regression model. The results showed that natural population of N. insignis exhibited high flowering synchrony with relatively stable flowering duration, and the flowering time fluctuated greatly depending on the rainfall 5 months before flowering. The pollination of N. insignis required pollinators, and insect activities played a very important role in the pollination process. However, lack of the pollinators was not a limitation for reproductive fitness in N. insignis, although the number of pollinators was small and the frequency of visits was low. In addition, no pollen limitation was found during pollination. The average seed setting rate of N. insignis in the natural condition was only 1.52%-3.73%, and it was generally affected by changes in flowering phenology between years and had a higher seed set in early flowering year. The annual variation of seed set might be related to the annual variations of stamen and pistil functions, such as changes of pollen viability and stigma receptivity, which were closely related to flowering time. The results of this study are of value for further conservation actions on natural population of this threatened endemic plant.

Time to Integrate Pollinator Science into Soybean Production

Soybeans cover 129 million hectares globally. Soybean productivity can increase with pollinator management, but soybean cultivation practices commonly ignore biotic pollination. If pollinator habitats are created within soybean landscapes and policies to limit agricultural expansion are implemented, millions of hectares could be restored for biodiversity without loss of soybean production.

Transmission of bee-like vibrations in buzz-pollinated plants with different stamen architectures

In buzz-pollinated plants, bees apply thoracic vibrations to the flower, causing pollen release from anthers, often through apical pores. Bees grasp one or more anthers with their mandibles, and vibrations are transmitted to this focal anther(s), adjacent anthers, and the whole flower. Pollen release depends on anther vibration, and thus it should be affected by vibration transmission through flowers with distinct morphologies, as found among buzz-pollinated taxa. We compare vibration transmission between focal and non-focal anthers in four species with contrasting stamen architectures: Cyclamen persicum, Exacum affine, Solanum dulcamara and S. houstonii. We used a mechanical transducer to apply bee-like vibrations to focal anthers, measuring the vibration frequency and displacement amplitude at focal and non-focal anther tips simultaneously using high-speed video analysis (6000 frames per second). In flowers in which anthers are tightly arranged (C. persicum and S. dulcamara), vibrations in focal and non-focal anthers are indistinguishable in both frequency and displacement amplitude. In contrast, flowers with loosely arranged anthers (E. affine) including those with differentiated stamens (heterantherous S. houstonii), show the same frequency but higher displacement amplitude in non-focal anthers compared to focal anthers. We suggest that stamen architecture modulates vibration transmission, potentially affecting pollen release and bee behaviour.

Environmental and Management Effects on Demographic Processes in the U.S. Threatened Platanthera leucophaea (Nutt.) Lindl. (Orchidaceae)

Populations of the U.S. threatened orchid, Platanthera leucophaea, are restricted to fragmented grassland and wetland habitats. We address the long-term (1998–2020) interactive effects of habitat (upland prairie vs. wetland), fire management (burned vs. unburned) and climatic variation, as well as pollination crossing effects, on population demography in 42 populations. Our analysis revealed the consistent interactive effects of habitat, dormant season burning, and climatic variation on flowering, reproduction, and survival. Burning increased flowering and population size under normal or greater than normal precipitation but may have a negative effect during drought years apparently if soil moisture stress reduces flowering and increases mortality. Trends in the number of flowering plants in populations also correspond to precipitation cycles. As with flowering and fecundity, survival is significantly affected by the interactive effects of habitat, fire, and climate. This study supports previous studies finding that P. leucophaea relies on a facultative outcrossing breeding system. Demographic modeling indicated that fire, normal precipitation, and outcrossing yielded greater population growth, and that greater fire frequency increased population persistence. It also revealed an ecologically driven demographic switch, with wetlands more dependent upon survivorship than fecundity, and uplands more dependent on fecundity than survivorship. Our results facilitate an understanding of environmental and management effects on the population demography of P. leucophaea in the prairie region of its distribution. Parallel studies are needed in the other habitats such as wetlands, especially in the eastern part of the range of the species, to provide a more complete picture.

Do annual and perennial populations of an insect-pollinated plant species differ in mating system?

BACKGROUND AND AIMS

Theory predicts that outcrossing should be more prevalent among perennials than annuals, a pattern confirmed by comparative evidence from diverse angiosperm families. However, intraspecific comparisons between annual and perennial populations are few because such variation is uncommon among flowering plants. Here, we test the hypothesis that perennial populations outcross more than annual populations by investigating Incarvillea sinensis, a wide-ranging insect-pollinated herb native to China. The occurrence of both allopatric and sympatric populations allows us to examine the stability of mating system differences between life histories under varying ecological conditions.

METHODS

We estimated outcrossing rates and biparental inbreeding in 16 allopatric and five sympatric populations in which both life histories coexisted using 20 microsatellite loci. In each population we measured height, branch number, corolla size, tube length and herkogamy for ~30 individuals. In a sympatric population, we recorded daily flower number, pollinator visitation and the fruit and seed set of annual and perennial plants.

KEY RESULTS

As predicted, outcrossing rates (t) were considerably higher in perennial (mean = 0.76) than annual (mean = 0.09) populations. This difference in mating system was also maintained at sympatric sites where plants grew intermixed. In both allopatric and sympatric populations the degree of herkogamy was consistently larger in outcrossing than selfing plants. Perennials were more branched, with more and larger flowers than in annuals. In a sympatric population, annuals had a significantly higher fruit and seed set than perennials.

CONCLUSIONS

Genetically based differences in herkogamy between annuals and perennials appear to play a key role in governing outcrossing rates in populations, regardless of variation in local ecological conditions. The maintenance of mating system and life history trait differentiation between perennial and annual populations of I. sinensis probably results from correlated evolution in response to local environmental conditions.

Florivory of Early Cretaceous flowers by functionally diverse insects: implications for early angiosperm pollination

Florivory (flower consumption) occurs worldwide in modern angiosperms, associated with pollen and nectar consumption. However, florivory remains unrecorded from fossil flowers since their Early Cretaceous appearance. We test hypotheses that earliest angiosperms were pollinated by a diverse insect fauna by evaluating 7858 plants from eight localities of the latest Albian Dakota Formation from midcontinental North America, in which 645 specimens (8.2%) were flowers or inflorescences. Well-preserved specimens were categorized into 32 morphotypes, nine of which displayed 207 instances of damage from 11 insect damage types (DTs) by four functional-feeding groups of hole feeding, margin feeding, surface feeding and piercing-and-sucking. We assessed the same DTs inflicted by known florivores on modern flowers that also are their pollinators, and associated insect mouthpart types causing such damage. The diverse, Dakota florivore-pollinator community showed a local pattern at Braun's Ranch of flower morphotypes 4 and 5 having piercing-and-sucking as dominant and margin feeding as minor interactions, whereas Dakotanthus cordiformis at Rose Creek I and II had an opposite pattern. We found no evidence for nectar robbing. These data support the rapid emergence of early angiosperms of florivore and associated pollinator guilds expressed at both the local and regional community levels.

Let's move out together: a framework for the intersections between movement and mutualism

Movement is a widespread behavior across organisms and is driven in part by interspecific interactions. Generally, negative interspecific interactions (such as competition and natural enemies) are more often studied in the context of movement than positive interactions (mutualism). Mutualistic relationships are incredibly common, yet only a subset are studied in the context of movement (transportation mutualisms). Overall, the costs and benefits that an individual experiences are shaped both by their movement behavior and their mutualistic relationships, as well as the intersection between these. Here we argue that the intersection between movement behavior and mutualistic relationships is understudied, and we present a conceptual framework to synthesize the links between movement and mutualisms and give examples of species that exhibit each. Our framework serves both to highlight the ways that mutualism can shape movement (and vice versa) and to draw parallels across different organisms (enabling a more abstract perspective of these biological systems, complementing the system-focused perspective). Finally, we show how considering movement in light of mutualisms (and vice versa) presents a number of new research questions to be answered by each empirical and theoretical approach going forward.

Comparison of two criteria on the essential number calculation of Andrena camellia

Andrena camellia Wu is one of the primary pollinators of Camellia oleifera A. in China. In this paper, the essential number of individuals for efficient pollination by this species was calculated via two criteria, based on various indicators including counts of pollen grains in provisions, from single visits, and from single foraging trips overall; single flower visit duration; single flight period duration; number of eggs laid by a single female over their lifetime; and the average number of flowers per plant. Based on the number of pollen grains collected per flower visit, the essential number of females necessary is 2107 in a 1-ha Camellia oleifera garden with 1800 plants, while only 1998 female individuals are essentially needed when estimated based on the mean number of pollen grains collected in a single flight period. We argue that the essential number estimated by the former method is more reasonable and accurate for practical applications.

Aphidophagous hoverflies reduce foxglove aphid infestations and improve seed set and fruit yield in sweet pepper

BACKGROUND

Larvae of many hoverfly species prey upon aphids, whereas the adults, by relying on nectar and pollen, contribute to the pollination of many plant species. Despite their great potential for pest control and pollination, important gaps still exist regarding the efficacy of hoverflies in regulating infestations of major aphid pests in augmentative biological control programs. Here, we tested the potential of the commercially available hoverflies Eupeodes corollae and Sphaerophoria rueppellii to regulate populations of the foxglove aphid Aulacorthum solani in sweet pepper.

RESULTS

In a semi-field experiment, aphid numbers were 93.2% and 78.4% lower in the E. corollae and S. rueppellii treatments, respectively, compared to the control. Fruit yield was increased by 390% and 361% and seed set by 395% and 399% for E. corollae and S. rueppellii. In a separate laboratory trial, we found that under conditions of limited prey, hoverfly larvae did not complete development, but that larvae of S. rueppellii survived significantly longer than larvae of E. corollae.

CONCLUSION

We have shown for the first time that E. corollae and S. rueppellii can reduce infestations of foxglove aphid in sweet pepper. The limited amount of prey, related to the small size of the A. solani colonies, means that hoverfly larvae were often not able to complete development. In practice, repeated releases of hoverflies, possibly in combination with other natural enemies, might be used to achieve effective suppression of A. solani infestations. © 2021 Society of Chemical Industry.

Single-nucleotide polymorphisms that uniquely identify cultivars of avocado (Persea americana)

PREMISE

Progeny of avocado (Persea americana) are highly variable due to high levels of heterozygosity. Breeding programs need molecular resources to allow the assessment of genetic differences and the selection of genotypes. Polymorphisms that uniquely identify different avocado cultivars provide a valuable tool to accelerate avocado research and development, including, for example, genotype selection.

METHODS

A double-digest restriction site-associated DNA sequencing (ddRADseq) approach was used to screen 10 avocado cultivars for single-nucleotide polymorphisms (SNPs). The fragments were size selected with Blue Pippin and PCR using universal Illumina primers, and catalog tags were then created with de novo alignment using Stacks software. Catalog tags were tabulated and filtered to identify alleles unique to each cultivar.

RESULTS

A total of 104 million sequences were collected, and 52 homozygous SNPs were identified that uniquely distinguished nine avocado cultivars. The cultivars Carmen Hass and Hass have a strong genetic similarity and no homozygous SNPs distinguishing these cultivars could be identified; therefore, both cultivars were grouped together.

DISCUSSION

The resource described here for cultivars of P. americana presents a new and significant molecular resource that can enable targeted genotype selection, paternity analysis, germplasm genotyping, pollination dynamics investigation, and crop improvement.

Elevated CO2 Impacts on Plant-Pollinator Interactions: A Systematic Review and Free Air Carbon Enrichment Field Study

Simple Summary

Climate change is having a profound impact on pollination systems, yet we still do not know to what extent increasing concentrations of carbon dioxide (CO₂) will directly affect the interactions between plants and their pollinators. We review all the existing published literature on the effect of elevated CO₂ (eCO₂) on flowering time, nectar and pollen production and plant–pollinator interactions. We also conduct a field experiment to test the effect of eCO₂ on bluebells and their pollinators. We found that few studies have assessed the impact of eCO₂ on pollination, and our field data found that bluebells flowered on average 6 days earlier under eCO₂ conditions. Hoverflies and bumble bees were the main visitors to bluebell flowers, but insect activity was low early in the flowing period. Although we did not find a difference in the number of visits made by insects to bluebell flowers under eCO₂, or the amount of seeds those flowers produced, the change in the timing of flowering could mean that a mismatch could develop between bluebells and their pollinators in the future, which would affect pollination success.

Abstract

The impact of elevated CO₂ (eCO₂) on plant–pollinator interactions is poorly understood. This study provides the first systematic review of this topic and identifies important knowledge gaps. In addition, we present field data assessing the impact of eCO₂ (150 ppm above ambient) on bluebell (Hyacinthoides non-scripta)–pollinator interactions within a mature, deciduous woodland system. Since 1956, only 71 primary papers have investigated eCO₂ effects on flowering time, floral traits and pollination, with a mere 3 studies measuring the impact on pollination interactions. Our field experiment documented flowering phenology, flower visitation and seed production, as well as the abundance and phenology of dominant insect pollinators. We show that first and mid-point flowering occurred 6 days earlier under eCO₂, but with no change in flowering duration. Syrphid flies and bumble bees were the dominant flower visitors, with peak activity recorded during mid- and late-flowering periods. Whilst no significant difference was recorded in total visitation or seed set between eCO₂ and ambient treatments, there were clear patterns of earlier flowering under eCO₂ accompanied by lower pollinator activity during this period. This has implications for potential loss of synchrony in pollination systems under future climate scenarios, with associated long-term impacts on abundance and diversity.

Biosafety of bee pollinators in genetically modified agro-ecosystems: Current approach and further development in the EU

Bee pollinators are an important guild delivering a fundamental input to European agriculture due to the ecological service they provide to crops in addition to the direct economic revenues from apiculture. Bee populations are declining in Europe as a result of the effects of several environmental stressors, both natural and of anthropic origin. Efforts are ongoing in the European Union (EU) to improve monitoring and management of pollinator populations to arrest further declines. Genetically modified (GM) crops are currently cultivated in a limited area in Europe, and an environmental risk assessment (ERA) is required prior to their authorization for cultivation. The possible impacts of GM crops on pollinators are deemed relevant for the ERA. Existing ecotoxicological studies indicate that traits currently expressed in insect-resistant GM plants are unlikely to represent a risk for pollinators. However, new mechanisms of insect resistance are being introduced into GM plants, including novel combinations of Cry toxins and double strand RNA (dsRNA), and an ERA is required to consider lethal and sublethal effects of these new products on nontarget species, including insect pollinators. The evaluation of indirect effects linked to the changes in management practices (e.g. for herbicide-tolerant GM crops) is an important component of EU regulations and a requirement for ERA. This paper reviews current approaches used to test the sensitivity of pollinators to GM plants and their products to determine whether sufficient data are being provided on novel GM plants to satisfy EU risk assessment requirements. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Pollinator Efficacy in Yellow Passion Fruit (Passiflora edulis f. flavicarpa Deg., Passifloraceae)

A pollinator's efficacy is the result obtained from a single visit by a species and is often used to measure the importance of different species of floral visitors as pollinators. Therefore, this study aimed to measure the efficacy of fruit and seed set in Passiflora edulis f. flavicarpa Deg. (Passifloraceae) by manual cross-pollination, natural pollination and by Xylocopa (Neoxylocopa) frontalis (Olivier 1789), performed through a single and two visits, to evaluate if there is occurrence of pollen limitation and pollinators' limitation in the studied crop fields. Four different treatments were performed: (1) manual cross-pollination (n = 37 flowers); (2) natural pollination (n = 52 flowers); (3) pollination by a single visit of X. frontalis (n = 41 flowers); and (4) pollination by two visits of X. frontalis (n = 31 flowers). We verified that fruit set occurred in all the treatments performed. Fruit set rate by manual cross-pollination was significantly higher than that obtained under natural conditions indicating the occurrence of pollen limitation and pollinators' limitation under natural pollination. Regarding the number of seeds, analysis of variance (ANOVA) did not indicate a significant difference between the mean seed set for the different types of pollination evaluated. The management of occupied nests, the introduction of trap-nests, and the conservation of native vegetation in areas close to the yellow passion fruit crops can increase pollinators' population. The greater richness and abundance of pollinators are essential to promote a greater number of fruit and seed set in the yellow passion fruit crops.

Making the most of your pollinators: An epiphytic fig tree encourages its pollinators to roam between figs

Ficus species are characterized by their unusual enclosed inflorescences (figs) and their relationship with obligate pollinator fig wasps (Agaonidae). Fig trees have a variety of growth forms, but true epiphytes are rare, and one example is Ficus deltoidea of Southeast Asia. Presumably as an adaptation to epiphytism, inflorescence design in this species is exceptional, with very few flowers in female (seed-producing) figs and unusually large seeds. Figs on male (pollinator offspring-generating) trees have many more flowers. Many fig wasps pollinate one fig each, but because of the low number of flowers per fig, efficient utilization by F. deltoidea's pollinators depends on pollinators entering several female figs. We hypothesized that it is in the interest of the plants to allow pollinators to re-emerge from figs on both male and female trees and that selection favors pollinator roaming because it increases their own reproductive success. Our manipulations of Blastophaga sp. pollinators in a Malaysian oil palm plantation confirmed that individual pollinators do routinely enter several figs of both sexes. Entering additional figs generated more seeds per pollinator on female trees and more pollinator offspring on male trees. Offspring sex ratios in subsequently entered figs were often less female-biased than in the first figs they entered, which reduced their immediate value to male trees because only female offspring carry their pollen. Small numbers of large seeds in female figs of epiphytic F. deltoidea may reflect constraints on overall female fig size, because pollinator exploitation depends on mutual mimicry between male and female figs.

Large herbivores transform plant-pollinator networks in an African savanna

Pollination by animals is a key ecosystem service^1,2 and interactions between plants and their pollinators are a model system for studying ecological networks,^3,4 yet plant-pollinator networks are typically studied in isolation from the broader ecosystems in which they are embedded. The plants visited by pollinators also interact with other consumer guilds that eat stems, leaves, fruits, or seeds. One such guild, large mammalian herbivores, are well-known ecosystem engineers^5-7 and may have substantial impacts on plant-pollinator networks. Although moderate herbivory can sometimes promote plant diversity,⁸ potentially benefiting pollinators, large herbivores might alternatively reduce resource availability for pollinators by consuming flowers,⁹ reducing plant density,¹⁰ and promoting somatic regrowth over reproduction.¹¹ The direction and magnitude of such effects may hinge on abiotic context-in particular, rainfall, which modulates the effects of ungulates on vegetation.¹² Using a long-term, large-scale experiment replicated across a rainfall gradient in central Kenya, we show that a diverse assemblage of native large herbivores, ranging from 5-kg antelopes to 4,000-kg African elephants, limited resource availability for pollinators by reducing flower abundance and diversity; this in turn resulted in fewer pollinator visits and lower pollinator diversity. Exclusion of large herbivores increased floral-resource abundance and pollinator-assemblage diversity, rendering plant-pollinator networks larger, more functionally redundant, and less vulnerable to pollinator extinction. Our results show that species extrinsic to plant-pollinator interactions can indirectly and strongly alter network structure. Forecasting the effects of environmental change on pollination services and interaction webs more broadly will require accounting for the effects of extrinsic keystone species.

Plant-pollinator interactions between generalists persist over time and space

Generalist species are the linchpins of networks, as they are important for maintaining network structure and function. Previous studies have shown that interactions between generalists tend to occur consistently across years and sites. However, the link between temporal and spatial interaction persistence across scales remains unclear. To address this gap, we collected data on plant-pollinator interactions throughout the flowering period for 5 yr across six plots in a subalpine meadow in the Rocky Mountains. We found that interactions between generalists tended to persist more in time and space such that interactions near the network core were more frequently recorded across years, within seasons, and among plots. We posit that species' tolerance of environmental variation across time and space plays a key role in generalization by regulating spatiotemporal overlap with interaction partners. Our results imply a role of spatiotemporal environmental variation in organizing species interactions, marrying niche concepts that emphasize species environmental constraints and their community role.

The role of ontogenetic allometry and nonallometric flower shape variation in species-level adaptive diversification - Calceolaria polyrhiza (Calceolariaceae) as a case study

Organism shape changes predictably during ontogeny, resulting in specific patterns of ontogenetic allometry. In several plant and animal lineages, among-species variation in the shape of mature organisms mirrors variation along their growth trajectories. Hence, ontogenetic allometry is an important bias in evolution. This bias should be stronger at reduced evolutionary time scales, in which among-trait correlations had less time to evolve. Nevertheless, it was shown that adaptation of organism shape frequently involved departures from the ancestral ontogenetic allometry. Moreover, only a moderate fraction of shape variation is correlated with size during ontogeny. Hence, nonallometric variation in shape (NAVSh) is likely to contribute to adaptation, even at reduced evolutionary time scales. We explored the contributions of allometric variation in shape (AVSh), NAVSh, and size variation to adaptive evolution in the angiosperm species Calceolaria polyrhiza. This strongly relies on oil-collecting bees for pollination and experienced transitions in the size of pollinators during the last 2 Ma. Using geometric morphometrics, we described corolla morphology in several populations across its distribution range. Variation in corolla shape was decomposed into an allometric and a nonallometric component, and corolla size was estimated. We then looked for the correlation between these aspects of morphology and the pollinator. Our results suggest that adaptation to pollinators with different sizes relied on NAVSh, which resulted from shifts in the allometric slope and from shape changes that occurred early in flower development. We conclude that NAVSh can contribute to adaptation in flowering plants, even at the species-level.

Color signals of bee-pollinated flowers: the significance of natural leaf background

PREMISE

Flower color is a primary pollinator attractant and generally adjusted to the cognitive system of the pollinators. The perception of flower color depends on the visual system of pollinators and also on environmental factors such as light conditions and the background against which flowers are displayed.

METHODS

Using bee-pollinated Fabaceae species as a model, we analyzed flower color diversity and compared flower color signals considering both the standard green and the natural leaf background of two tropical seasonally dry vegetations-a mountain rupestrian grassland (campo rupestre) and a woody savanna (cerrado)-compared to a nontropical Mediterranean shrubland.

RESULTS

By using natural background, bees discriminated color for 58% of the flowers in the campo rupestre and for only 43% in cerrado. Both vegetations were surpassed by 75% of bee color discrimination in Mediterranean vegetation. Chromatic contrast and purity were similar among the three vegetation types. Green contrast and brightness were similar between the tropical vegetations but differed from the Mediterranean shrubland. Green contrast differences were lost when using a standard green background, and most variables (purity, green contrast, and brightness) differed according to the background (natural or standard green) in all vegetations.

CONCLUSIONS

The natural background influenced bee perception of flower color regardless of vegetation. The background of the campo rupestre promoted green contrast for flowers, ensuring flower detection by pollinators and, along with bees, may also act as a selective pressure driving the diversity of flower colors in Fabaceae species. We highlight the importance of considering the natural background coloration when analyzing flower color signals.

Ecosystem services provided by aculeate wasps

The aculeate wasps are one of the most diverse and speciose insect taxa; they are omnipresent across ecosystems and exhibit diverse co-evolutionary and exploitative associations with other organisms. There is widespread conjecture that aculeate wasps are likely to perform essential ecological and economic services of importance to the health, well-being and nutritional needs of our planet. However, the scope and nature of the ecosystem services they provide are not well understood relative to other insect groups (e.g. bees, butterflies, beetles); an appreciation of their value is further tarnished by their public reputation as pointless pests. Here, we conduct the first comprehensive review of how aculeate wasps contribute to the four main areas of ecosystem services: regulatory, provisioning, supporting and cultural services. Uniting data from a large but previously disconnected literature on solitary and social aculeate wasps, we provide a synthesis on how these insects perform important ecosystem services as parasites, predators, biological indicators, pollinators, decomposers and seed dispersers; and their additional services as a sustainable alternative to meat for human consumption, and medicinal potential as sources of research leads for anti-microbials and cancer treatments. We highlight how aculeate wasps offer substantial, but largely overlooked, economic benefits through their roles in natural pest management and biological control programs. Accordingly, we provide data-driven arguments for reasons to consider the ecosystem service value of aculeate wasps on a par with other 'useful' insects (e.g. bees). Finally, we provide a research roadmap identifying the key areas of research required to capitalise better on the services provided by these important insects.

Phloem-Feeding Herbivores Affect Floral Development and Reproduction in the Etruscan Honeysuckle (Lonicera etrusca Santi)

Floral development depends on multifactor processes related to genetic, physiological, and ecological pathways. Plants respond to herbivores by activating mechanisms aimed at tolerating, compensating, or avoiding loss of biomass and nutrients, and thereby survive in a complex landscape of interactions. Thus, plants need to overcome trade-offs between development, growth, and reproduction vs. the initiation of anti-herbivore defences. This study aims to assess the frequency of phloem-feeding herbivores in wild populations of the Etruscan honeysuckle (Lonicera etrusca Santi) and study their effects on floral development and reproduction. The incidence of herbivory by the honeysuckle aphid (Hyadaphis passerinii del Guercio) was assessed in three wild populations of the Iberian Peninsula. The effect of herbivory on floral morphology, micromorphology of stigmas and pollen, floral rewards, pollination, and fruit and seed set were studied. The herbivory by aphids reduces the size of flowers and pollen. Additionally, it stops nectar synthesis and causes malformation in pollen and microstructures of stigmas, affecting pollination. As a consequence, fruit set and seed weight are reduced. This work provides evidence of the changes induced by phloem-feeding herbivores in floral development and functioning that affect the ecological processes necessary to maintain the reproductive success of plants.

Phosphorus fertilization induces nectar secretion for honeybee visitation and cross-pollination of almond trees

Precise phosphorus (P) application requires a mechanistic understanding of mineral effects on crop biology and physiology. Photosynthate assimilation, metabolism, and transport require phosphorylation, and we postulated that P is critical for the bloom and fruit-set of almond trees that rely on stored carbohydrate reserves. Hence, we studied the growth, physiology and carbohydrate dynamics in 2-year-old almond trees irrigated with P concentrations between 1 mg l-1 and 20 mg l-1. Almond trees attained maximal photosynthesis, transpiration, and growth by 6 mg P l-1 irrigation. Nevertheless, almond trees continued to extract P in 10 mg P l-1 and 15 mg P l-1 irrigations, which corresponded to larger yields. We attributed the augmented productivity to increased fruit-set (59% between 6 mg P l-1 and 15 mg P l-1), caused by more frequent (29%) honeybee visits. High P improved pollinator visitation by enabling almond trees to utilize more of their starch reserves for nectar secretion (which increased by ~140% between 6 mg P l-1 and 15 mg P l-1). This work elucidates the benefits of P fertilization to plant-pollinator mutualism, critical to almond productivity, and reveals novel indices for optimal P application in almond orchards.

Corridors through time: Does resource continuity impact pollinator communities, populations, and individuals?

Spatial aspects of connectivity have received considerable attention from ecologists and conservationists, yet temporal connectivity, the periodic linking of habitats, plays an equally important, but largely overlooked role. Different biological and biophysical attributes of ecosystems underpin temporal connectivity, but here we focus on resource continuity, the uninterrupted availability of foraging sites. We test the response of pollinators to resource continuity at community, population, and individual levels using a novel natural experiment consisting of farms with either single or sequential cropping systems. We found significant effects at the population level; colony density of an important crop pollinator (Bombus impatiens L.) was greater when crop floral resources were continuously available. However, we did not find significant effects at the community or individual level; wild bee abundance, diversity and body size did not respond to resource continuity. Raspberry farms with greater early season resources provided by blueberry had greater bumble bee populations, suggesting beneficial effects on resource availability due to crop diversity. Better understanding the impact of resource continuity via crop diversity on broader patterns of biodiversity is essential for the co-management of biodiversity and ecosystem services.

Intraspecific relationships between floral signals and rewards with implications for plant fitness

Within-species variation in traits such as petal size or colour often provides reliable information to pollinators about the rewards offered to them by flowers. In spite of potential disadvantages of allowing pollinators to discriminate against less-rewarding flowers, examples of informative floral signals are diverse in form and widely distributed across plant taxa, apparently having evolved repeatedly in different lineages. Although hypotheses about the adaptive value of providing reward information have been proposed and tested in a few cases, a unified effort to understand the evolutionary mechanisms favouring informative floral signals has yet to emerge. This review describes the diversity of ways in which floral signals can be linked with floral rewards within plant species and discusses the constraints and selective pressures on floral signal-reward relationships. It focuses particularly on how information about floral rewards can influence pollinator behaviour and how those behavioural changes may, in turn, affect plant fitness, selecting either for providing or withholding reward information. Most of the hypotheses about the evolution of floral signal-reward relationships are, as yet, untested, and the review identifies promising research directions for addressing these considerable gaps in knowledge. The advantages and disadvantages of sharing floral reward information with pollinators likely play an important role in floral trait evolution, and opportunities abound to further our understanding of this neglected aspect of floral signalling.

A geographical perspective on the relationship between Impatiens spur lengths and bill lengths of sunbirds in Afrotropical mountains

Trait matching-a correlation between the morphology of plants and their pollinators-has been frequently observed in pollination interactions. Different intensities of natural selection in individual regions should cause such correlations to be observable across different local assemblages. In this study, we focused on matching between spur lengths of the genus Impatiens and bill lengths of sunbirds in tropical Africa. For 25 mountain and island locations, we compiled information about the composition and traits of local Impatiens and sunbird assemblages. We found that assemblage mean and maximum values of bill lengths were positively correlated with mean and maximum spur lengths across locations. Moreover, our results suggest that the positive correlations hold only for forest sunbird assemblages sharing the same habitat with Impatiens species. We further show that long-billed sunbirds seem to locally match the morphology of multiple Impatiens plant species, not vice versa. Our observation implies that trait matching significantly contributes to structuring of Impatiens-sunbird pollination systems. We suggest that special habitat preferences together with spatial isolation of mountain environment might play a role in this case.