Flower Constancy Research Articles

Extensive loss of forage diversity in social bees owing to flower constancy in simulated environments.

Many bees visit just one flower species during a foraging trip, i.e. they show flower constancy. Flower constancy is important for plant reproduction but it could lead to an unbalanced diet, especially in biodiversity-depleted landscapes. It is assumed that flower constancy does not reduce dietary diversity in social bees, such as honeybees or bumblebees, but this has not yet been tested. We used computer simulations to investigate the effects of flower constancy on colony diet in plant species-rich and species-poor landscapes. We also explored if communication about food sources, which is used by many social bees, further reduces forage diversity. Our simulations reveal an extensive loss of forage diversity owing to flower constancy in both species-rich and species-poor environments. Small flower-constant colonies often discovered only 30-50% of all available plant species, thereby increasing the risk of nutritional deficiencies. Communication often interacted with flower constancy to reduce forage diversity further. Finally, we found that food source clustering, but not habitat fragmentation impaired dietary diversity. These findings highlight the nutritional challenges flower-constant bees face in different landscapes and they can aid in the design of measures to increase forage diversity and improve bee nutrition in human-modified landscapes.

Learning-based oviposition constancy in insects

The search for resources occupies a major part of the time and energy budget of many insects. In this context, many insects display constancy behavior, in which they learn to return to the same, predictably rewarding resource repeatedly. Flower constancy is one such behavior, where nectar-feeding insects restrict foraging visits to a few plant species, sometimes overlooking potentially better-rewarding plants. This phenomenon is well documented in multiple species and is believed to be an optimal strategy to maximize benefits, while minimizing energy and time expenditure. Oviposition constancy is a similar behavior, where insects, dependent on previous experience, restrict their visits to a few types of potential oviposition sites. In contrast to flower constancy, the prevalence of oviposition constancy and the rationale behind this behavior are relatively unknown. An improved understanding of oviposition constancy can act as a lens into the evolutionary history of local insect adaptations, it can help gauge the impact of climate change on insect plant interactions, and it can aid the design of crop-pest management strategies. In this review, we discuss the potential benefits of oviposition constancy in insects and the extent of plasticity occurring in host-plant choice for oviposition.

Flowering phenology of species with similar flower colours in species-rich communities

Within a community, co-occurring plant species are expected to diverge in floral display or flowering phenology to decrease interspecific competition and thus increase intraspecific pollination. However, co-occurring species can also benefit from floral signal standardisation (similar colour signals among flowers of different species) because it facilitates pollinator attraction. Considering the interaction of flower colour display and flowering phenology, we investigated the visual similarity of rewarding flowers among species from highly diverse tropical and temperate vegetation types. For six groups of co-occurring, closely related bee-pollinated species with similar floral displays from Brazilian campo rupestre (51 species) and Spanish Mediterranean vegetation (30 species), we first investigated whether flower colours can be discriminated by bees based on colour locus distance in the bee vision hexagon. We then tested whether flowering phenology overlapped or was segregated. We found that within both vegetation regions, flower colour was generally not distinguishable within groups by bees. The small perceptual distance of colour loci in the bee visual space did not enable discrimination. The flowering periods of the Mediterranean species overlapped, while the Brazilian campo rupestre species tended to have segregated phenologies. Mediterranean species may benefit from the increased standardisation of signals displayed during the short flowering season, while the sequential flowering phenology of campo rupestre species may decrease interspecific competition and help maintain a recognizable signal for bees over time, favouring flower constancy. We concluded that the standardisation of the floral colour signal within these two species-rich plant communities is advantageous for most of the species studied, despite having different flowering phenologies.

Sexual dimorphism in a dioecious species with complex, specialist-pollinated flowers.

Pollinators with flower constancy and long nectar-feeding organs should favor less or no sexual dimorphism in the individual flowers of dioecious plants. This hypothesis is deduced because such pollinators can discriminate between intersexual flower size differences, and morphological differences between male and female flowers often diminish pollen transfer. We compared floral traits and pollinator behavior between male and female flowers in the hawkmoth-pollinated species, Trichosanthes cucumeroides. In field studies, we removed petal fringes on both sexes and observed pollinators to assess the role of elaborate petal fringes in pollinator attraction and pollination success for each flower sex. Female flowers had a similar front flower size and fringe extension as male flowers, supporting our hypothesis. In contrast, females allocated fewer resources to floral biomass. Additionally, they had smaller and narrower petal lobes, lower fringe density, shorter tubes with inferior nectar rewards, and lower display size than males, which is inconsistent with the hypothesis. Nocturnal hawkmoths prefer flowers with long fringe extensions. Fringe removal significantly decreased hawkmoth visitations to both female and male flowers but reduced success only in females. A literature survey indicated that female flowers of specialist-pollinated species are similar in size or larger than the males and thus tend to attract more pollinators compared with female flowers of generalist-pollinated species. Female flowers have evolved fringe extensions that are similar to those of male flowers, likely increasing pollinator attraction even slightly, and had less biomass in other floral parts and produced less nectar compared with male flowers. Our findings imply that female-biased resource limitation and flower-size sensitivity of pollinators together exert sex-specific selection of floral traits in T. cucumeroides.

Bumblebee flower constancy and pollen diversity over time.

Bees often focus their foraging effort on a few or even a single flower species, even if other equally rewarding flower species are present. Although this phenomenon-called flower constancy-has been widely documented during single foraging trips, it is largely unknown if the behavior persists over longer time periods, especially under field conditions with large temporal variations of resources. We studied the pollen diet of individuals from nine different Bombus terrestris colonies for up to 6 weeks, to investigate flower constancy and pollen diversity of individuals and colonies, and how these change over time. We expected high degrees of flower constancy and foraging consistency over time, based on foraging theory and previous studies. Instead, we found that only 23% of the pollen foraging trips were flower constant. The fraction of constant pollen samples did not change over the study period, although repeatedly sampled individuals that were flower constant once often showed different preferences at other sampling occasions. The similarity of pollen composition in samples collected by the same individuals at different occasions dropped with time. This suggests that the flower preferences change in response to shifting floral resources. The average diversity of pollen from single foraging trips was around 2.5 pollen types, while the colony-level pollen diversity was about three times higher. How rapidly preferences change in response to shifting resources, and if this differs between and within bee species depending on factors such as size, should be the focus of future research.

Intraspecific floral colour variation in three Pedicularis species

Flower constancy describes the phenomenon that pollinators tend to successively visit flowers of a single species during foraging, reducing reproductive interference in natural communities. The extent of flower constancy is largely determined by the floral traits of co-flowering species. Both higher inter-specific and lower intraspecific differences of floral traits should contribute to a higher level of flower constancy. However, previous studies mainly focused on interspecific difference, and the intraspecific variation (consistency) of floral traits received much less attention. We hypothesise that selection may favour lower intraspecific floral trait variation in communities composed of multiple co-flowering congeners. We investigated the floral colour variation of three focal Pedicularis species that share pollinators in 19 communities composed of either single or multiple Pedicularis species. Colour was quantified using image-based colour analysis as perceived by pollinators. We found that most of the intrapopulation floral colour variation was below the colour discrimination threshold of bumblebees, implying strongly constrained by the visual selection by pollinators. Contrary to the hypothesis, there is no significant difference in intraspecific floral colour variation between different community contexts. It may be due to the relatively large interspecific floral colour differences of most co-flowering species. The influence of community context on intraspecific variation may be reflected in floral traits other than colours.

The Movement Ecology of Mutualism (CSEE/ESA 2022, OOS17)

The Movement Ecology of Mutualism (<scp>CSEE</scp>/<scp>ESA</scp> 2022, <scp>OOS17</scp>)

When should bees be flower constant? An agent-based model highlights the importance of social information and foraging conditions.

Many bee species show flower constancy, that is, a tendency to visit flowers of one type during a foraging trip. Flower constancy is important for plant reproduction, but the benefits of constancy to bees is unclear. Social bees, which often use communication about food sources, show particularly strong flower constancy. We aimed to better understand the benefits of flower constancy in social bees and how these benefits depend on foraging conditions. We hypothesised that sharing social information increases the benefits of flower constancy because social foragers share information selectively about high-quality food sources, thereby reducing the need to sample alternatives. We developed an agent-based model that allowed us to simulate bee colonies with and without communication and flower constancy in different foraging environments. By varying key environmental parameters, such as food source numbers and reward size, we explored how the costs and benefits of flower constancy depend on the foraging landscape. Flower constancy alone performed poorly in all environments, while indiscriminate flower choice was often the most successful strategy. However, communication improved the performance of flower constant colonies considerably in most environments. This combination was particularly successful when high-quality food sources were abundant and competition was weak. Our findings help explain why social bees tend to be more flower constant than solitary bees and suggest that flower constancy can be an adaptive strategy in social bees. Simulations suggest that anthropogenic changes of foraging landscapes will have different effects on the foraging performance of bees that vary in flower constancy.

Pollinator conservation: Where will bees go in the Anthropocene future?

For pollinator conservation in human-transformed landscapes, it is crucial to know whether species can overcome gaps between fragments of natural habitat. A new study reveals why colony size, recruitment communication, and flower constancy increase the foraging ranges in social bees.

Sociality is a key driver of foraging ranges in bees

Bees are important pollinators of wild and agricultural plants1,2,3,4,5 and there is increasing evidence that many bee populations decline due to a combination of habitat loss, climate change, pesticides, and other anthropogenic effects.6,7,8,9,10,11 One trait that shapes both their role in plant reproduction12,13 and their exposure to anthropogenic stressors is the distance at which bees forage. It has been suggested that bee sociality14 and diet15 affect bee foraging ranges, but how these traits and their potential interactions drive foraging ranges remains unclear. We analyzed flight distance data from 90 bee species and developed an agent-based model to test how social, dietary, and environmental factors affect foraging ranges. We confirm that bee sociality is positively associated with foraging range, with average-sized social bees foraging up to 3 times farther from the nest than size-matched solitary bees. A comparative analysis of social bees and computer simulations shows that foraging distances increase with colony size, supporting the hypothesis that greater foraging distances are an emergent property of increasing colony sizes in a food-limited environment. Flower constancy and communication, two traits often found in social bees, synergistically increase foraging distances further in many simulated environments. Diet breadth (oligolectic versus polylectic diet), on the other hand, does not appear to affect foraging ranges in solitary bees. Our findings suggest that multiple traits linked to bee sociality explain why social bees have greater foraging ranges. This has implications for predicting pollination services and for developing effective conservation strategies for bees and isolated plant populations.15,16,17,18,19.

Microhabitat and Pollinator Differentiation Drive Reproductive Isolation between Two Sympatric Salvia Species (Lamiaceae).

Evaluation of multiple barriers contributing to reproductive isolation between sympatric plant species is key to understanding the mechanism of their coexistence; however, such investigations in biodiversity hotspots are still rare. In this study, we investigated and compared geography, microhabitat, phenology, flora, and pollinators, in addition to pollen–pistil interactions, seed production, and seed germination of the closely related sympatric Salvia digitaloides and S. flava on Yulong Snow Mountain, Southwestern Yunnan, China. The geographic distribution of these species overlapped, but their adaptation to physical and chemical properties of soil microhabitats differed. They shared the same flowering time but differed in flower size, style length, nectar volume, sugar concentration, and flower longevity. Both species shared bumblebees as effective pollinators, but flower constancy for the two species was relatively strong. Pollen tube growth, seed production, and seed germination were lower in interspecific than in intraspecific crosses. Our study suggested that microhabitat and pollinator isolation acted as the most important isolating barriers in maintaining the coexistence of the two Salvia species. Our study also highlighted that post-pollination barriers play an important role in preventing the gene flow between these two Salvia species.

Goldilocks’ quarter-hectare urban farm: An agent-based model for improved pollination of community gardens and small-holder farms

Industrial agriculture’s expansive monocultures contrast against community farms of diverse crops in small allotments constrained by urban regulations. These human-controlled differences in scale, arrangement and crop diversity, may differently impact insect foraging and pollination. The relationship between human management and insect pollination is under-explored, especially regarding the implications of insect flower constancy–the tendency of insects to favour visits to a single rewarding flower species during a foraging bout. Although high flower-constancy is associated with reliable pollen transport, its impact on pollination depends also on planting regimes, the vicinity of neighbouring crops, weeds and wildflowers. This study explores the potential pollination impacts of insect flower constancy, heterospecific pollen transfer, and human-dictated crop patch size, using an agent-based model of insect pollinators foraging from two flowering plant species. Highly constant pollinators were most effective in the smallest patches when heterospecific pollen transfer was an issue. As patch size increased, pollination rates improved overall, but less constant insects produced better pollination rates for intermediate sized areas because they rapidly switched preferences between flower species. As patch size increased further, the influence of flower constancy on pollination rates was reduced. Study results suggest that typical community farms containing small single-crop patches operated by independent growers within an allotment, may be better pollinated if operated collaboratively to increase single-crop patch size. Crop patches needn’t be large homogeneous agricultural monocultures, but neither should they be so small and heterogeneous as to inhibit pollination. We found a “Goldilocks zone” around 11m x 11m to be a good compromise for pollination, regardless of the level of flower constancy in local insects. As climate and human land use increasingly impact insect populations, the relationships between pollination systems, growers and policy makers must be continually examined to safeguard food supplies and native ecosystems.

The contribution of plant spatial arrangement to bumble bee flower constancy.

Floral constancy of foraging bees influences plant reproduction. Constancy as observed in nature arises from at least four distinct mechanisms frequently confounded in the literature: context-independent preferences for particular plant species, preferential visitation to the same species as the previous plant visited (simple constancy), the spatial arrangement of plants, and the relative abundances of co-flowering species. To disentangle these mechanisms, we followed individual bee flight paths within patches where all flowering plants were mapped, and we used step selection models to estimate how each mechanism influences the probability of selecting any particular plant given the available plants in a multi-species community. We found that simple constancy was positive: bees preferred to visit the same species sequentially. In addition, bees preferred to travel short distances and maintain their direction of travel between plants. After accounting for distance, we found no significant effect of site-level plant relative abundances on bee foraging choices. To explore the importance of the spatial arrangement of plants for bee foraging choices, we compared our full model containing all parameters to one with spatial arrangement removed. Due to bees' tendency to select nearby plants, combined with strong intraspecific plant clumping, spatial arrangement was responsible for about 50% of the total observed constancy. Our results suggest that floral constancy may be overestimated in studies that do not account for the spatial arrangement of plants, especially in systems with intraspecific plant clumping. Plant spatial patterns at within-site scales are important for pollinator foraging behavior and pollination success.

Host Plant Constancy in Ovipositing Manduca sexta

Many pollinating insects exhibit flower constancy, i.e. they target flower species they have already experienced and fed from. While the insects might profit from reduced handling costs when revisiting similar flowers, flower constancy, in addition, is of benefit for the plants as it guarantees pollen transfer to conspecifics. Here we investigate whether the previous experience of an insect can also result in oviposition constancy, i.e. whether ovipositing on a given plant species will drive future oviposition preference in a female insect. We show that female hawkmoths (Manduca sexta), after having oviposited on a given plant species only once, indeed will prefer this plant in future oviposition choices. As oviposition preference is even affected 24 h after the moth has oviposited on a given plant, long term memory seems to be involved in this oviposition constancy. Our data furthermore suggest that, as shown for flower constancy, ovipositing moths increase their handling efficiency by targeting those host plants they have already experienced.

Novel pollen analogue technique shows bumblebees display low floral constancy and prefer sites with high floral diversity

ContextThere have been dramatic global declines in pollinating insects. A common land management intervention to support wild pollinators is to plant non-crop flowering plants (‘pollinator planting’). However, there are limited data on which species or spatial arrangement of planting provide maximum benefit to wild pollinators.ObjectivesHere we investigate which flowering species and locations are visited by free-foraging Bombus terrestris (buff-tailed bumblebees) in species-rich semi-natural grassland and woodland.MethodsTwo study nests of buff-tailed bumblebees were established in Wytham Woods, UK. Pollen analogue pigments were sprayed on open flowers in the study area over a period of two months, with unique colours used to identify separate sections of the study area. Pollen load analysis was used to identify forage species and foraging locations.ResultsBumblebees showed low flower constancy, visiting five flower species per trip on average, and as a group the sampled bumblebees visited 36 of the 49 plant species identified in study area surveys. Many individuals foraged in multiple, spatially-discrete locations during single trips.ConclusionsThe positive relationship between floral diversity and pollen load species diversity, and the positive relationship between site floral diversity and frequency of visitation, suggest behavioural strategies that maximize the diversity of flower species visited, in line with the energetic costs and benefits hypothesis. This supports recommendations for pollinator plantings with high species diversity, potentially spread across many small forage areas across the landscape.

Conical flower cells reduce surface gloss and improve colour signal integrity for free-flying bumblebees

Colour signals of flowers facilitate detection, spontaneous preference, discrimination and flower constancy by important bee pollinators. At short distances bees orient to floral colour patterns to find a landing platform and collect nutrition, potentially improving the plants’ reproductive success when multiple flowers are visited sequentially. In addition to pigments and backscattering structures within the petals’ internal layers, the epidermal micro-structure of the petals’ surface may also influence petal reflectance properties and thus influence overall colour patterns via optical effects. Gloss, i.e., shine caused by specular reflections of incident light from smooth surfaces, may for example alter the visual appearance of surfaces including flowers. We classify the epidermal surface properties of petals from 39 species of flowering plants from 19 families by means of a cell shape index, and measure the respective surface spectral reflectance from different angles. The spontaneous behavioural preferences of free flying bumblebees (Bombus terrestris) for surfaces with different micro-textures was then tested using specially prepared casts of selected flower petals. We specifically tested how the petal colour as function of the angle of incident light, surface structure and bee approach angle influences bumblebees’ spontaneous choices for artificial flowers. We observe that bumblebees spontaneously prefer artificial flowers with conical-papillate micro-structures under both multidirectional illumination and under spotlight conditions if approaching against the direction of spotlight, suggesting conical cells help promote constant signals by removing gloss that may confound the integrity of colour signalling.

Nocturnal Bees as Crop Pollinators

Bees are typically diurnal but around 1% of described species have nocturnal activity. Nocturnal bees are still poorly studied due to bias towards studying diurnal insects. However, knowledge concerning their biology and role as crop pollinators has increased. We review the literature on nocturnal bees’ traits and their host plants, and assess the crop pollination effectiveness of this neglected group. Nocturnal bees have visual adaptations to cope with low light intensities, and floral scents are a key sensory cue used to find their host flowers. Nocturnal bees generally show high flower constancy, the ability to vibrate flowers, and high transfer rates of pollen grains to stigmas. The flowers visited by nocturnal bees range from small radial and zygomorphic flowers to large brush blossoms; moreover, they visit plants with different flowering strategies. Nocturnal bees are effective pollinators of regional fruit crops in Brazil, such as cambuci (Campomanesia phaea), guaraná (Paullinia cupana), cajá (Spondias mombin), and in North America of cultivated pumpkins (Cucurbita species). However, they most likely are pollinators of several other crops. Strategies to host high numbers of nocturnal bees around cropping areas should be taken, such as preserving adjacent native forests, restricting soil management, providing food resources beyond crop flowers, and avoiding light pollution.

How do insects choose flowers? A review of multi‐attribute flower choice and decoy effects in flower‐visiting insects

Understanding why animals (including humans) choose one thing over another is one of the key questions underlying the fields of behavioural ecology, behavioural economics and psychology. Most traditional studies of food choice in animals focus on simple, single-attribute decision tasks. However, animals in the wild are often faced with multi-attribute choice tasks where options in the choice set vary across multiple dimensions. Multi-attribute decision-making is particularly relevant for flower-visiting insects faced with deciding between flowers that may differ in reward attributes such as sugar concentration, nectar volume and pollen composition as well as non-rewarding attributes such as colour, symmetry and odour. How do flower-visiting insects deal with complex multi-attribute decision tasks? Here we review and synthesise research on the decision strategies used by flower-visiting insects when making multi-attribute decisions. In particular, we review how different types of foraging frameworks (classic optimal foraging theory, nutritional ecology, heuristics) conceptualise multi-attribute choice and we discuss how phenomena such as innate preferences, flower constancy and context dependence influence our understanding of flower choice. We find that multi-attribute decision-making is a complex process that can be influenced by innate preferences, flower constancy, the composition of the choice set and economic reward value. We argue that to understand and predict flower choice in flower-visiting insects, we need to move beyond simplified choice sets towards a view of multi-attribute choice which integrates the role of non-rewarding attributes and which includes flower constancy, innate preferences and context dependence. We further caution that behavioural experiments need to consider the possibility of context dependence in the design and interpretation of preference experiments. We conclude with a discussion of outstanding questions for future research. We also present a conceptual framework that incorporates the multiple dimensions of choice behaviour.

Comparative study of the insect community visiting flowers of invasive goldenrods (Solidago canadensis and S. gigantea)

Giant and Canadian goldenrods (Solidago gigantea and S. canadensis, respectively) are two closely related insect-pollinated plants of the family Asteraceae, native to North America and invasive in the Palearctic region. Here we study the structure of the flower-visitor community associated with these species in seven sites from three geographic locations in Central Russia throughout two flowering seasons, using qualitative (visitor set) and quantitative (visitation rate and relative frequency of different functional groups of flower-visiting insects) approaches. These species do not have a specific visitor set and are visited by a wide range of insects. Most differences in pollinator sets are in rare occasional visitors rather than specific ones preferring one of the two plant species. Hoverflies, honey bees, muscoid flies, and wasps are the most frequent visitors of goldenrods, though this generalization can be done only after obtaining large samples: brief observations at one site can bring about misleading results due to high spatial and temporal variation. Though intra-species spatial and temporal variation exceeds inter-species variation, all else equal, insect visitation rate to S. gigantea is higher. This is especially true of pronounced anthophilous insects. These results, together with the shown flower constancy of flower visitors, suggest that insects can distinguish between these two species.

Flower constancy of the Cape honey bee pollinator of two co-flowering Erica species from the Cape Floristic Region (South Africa)

Erica is the largest genus of the Cape Floristic Region (CFR) of South Africa, and multiple species often coexist. Species that co-flower and share a pollinator may suffer from reproductive interference through heterospecific pollen deposition, especially when such species are closely related. However, little is known about the mechanisms that enable coexistence of co-flowering Erica species that share pollinators. Here we focus on a site where two co-flowering Erica species which are both predominantly pollinated by Cape honey bees grow intermingled, to ask whether this is associated with flower constancy and/or differences in pollen deposition sites. Despite minor differences in sites of pollen deposition on honey bees, most pollen of both Erica species was found on the head and ventral thorax regions. Subsequent visits of honey bees were almost always to the same Erica species from which they departed, and were significantly more constant than expected from random foraging in two consecutive years. Based on differences in pollen types between the two species, we established that pollen loads carried by honey bees were between 67% and 99% species-specific, whereas pollen loads on stigmas were on average over 93% pure. We conclude that despite the superficial similarities of small-flowered Erica species, honeybees can use species-specific cues for constant foraging, which is associated with almost complete absence of heterospecific pollen transfer. Future analyses of pollen carryover rates could help to quantify the benefit of pollinator behaviour for plant fecundity, and inclusion of more study sites sampled throughout the Erica flowering season could shed further light on the importance of flower constancy for facilitating coexistence of Erica species in the CFR.