Food in a row: urban trees offer valuable floral resources to pollinating insects

Target and non-target effects of insecticide use during ornamental milkweed production

There are widespread public efforts to conserve wildlife in urbanized landscapes via the installation of nursery-grown plants that support Lepidoptera taxa. Insecticides are commonly used during nursery production to suppress key plant pests, and many products have extended periods of toxicity and affect a wide range of herbivore taxa. While there are plentiful toxicological data on bee species, predominantly the Western honey bee (Apis mellifera L.), little is known about how insecticides affect nonpest lepidopterans. Lepidoptera has different modes of exposure (e.g., leaf-feeding) and differences in susceptibility to insecticide target sites compared to bees. Consequently, many products compatible with bee conservation pose an uncertain risk to nonpest lepidopterans and thus may represent an under-recognized conflict with conservation efforts. Using the monarch butterfly (Danaus plexippus, L.), tropical milkweed (Asclepias curassavica, L.), and oleander aphid (Aphis nerii, Fonscolombe, 1841) system, we conducted leaf and whole-plant feeding assays to evaluate effects of acute and chronic monarch exposure to industry standard and alternative reduced-risk insecticides used during nursery production. We also evaluated the efficacy of these insecticides against their target pest, the oleander aphid. Our results indicate that insecticides used to control pests on ornamental milkweed can cause monarch larval mortality up to 4 wk after treatment application. Furthermore, the duration of aphid suppression is often shorter than the duration of adverse effects on monarchs. This study demonstrates a conflict between insect pest management and Lepidoptera conservation during ornamental plant production and has implications for the conservation value of ornamentals after retail sale.

Bee pollen as a food and feed supplement and a therapeutic remedy: recent trends in nanotechnology

Pollen grains are the male reproductive part of the flowering plants. It is collected by forager honey bees and mixed with their salivary secretions, enzymes, and nectar, which form fermented pollen or "bee bread" which is stored in cells of wax honeycombs. Bee pollen (BP) is a valuable apitherapeutic product and is considered a nutritional healthy food appreciated by natural medicine from ancient times. Recently, BP has been considered a beneficial food supplement and a value-added product that contains approximately 250 different bioactive components. It contains numerous beneficial elements such as Mg, Ca, Mn, K, and phenolic compounds. BP possesses strong antioxidant, anti-inflammatory, antimicrobial, antiviral, analgesic, immunostimulant, neuroprotective, anti-cancer, and hepatoprotective properties. It is used for different purposes for the welfare of mankind. Additionally, there is a growing interest in honey bee products harvesting and utilizing for many purposes as a natural remedy and nutritive function. In this review, the impacts of BP on different organisms in different ways by highlighting its apitherapeutic efficacy are described.

Novel indices reveal that pollinator exposure to pesticides varies across biological compartments and crop surroundings

Declines in insect pollinators have been linked to a range of causative factors such as disease, loss of habitats, the quality and availability of food, and exposure to pesticides. Here, we analysed an extensive dataset generated from pesticide screening of foraging insects, pollen-nectar stores/beebread, pollen and ingested nectar across three species of bees collected at 128 European sites set in two types of crop. In this paper, we aimed to (i) derive a new index to summarise key aspects of complex pesticide exposure data and (ii) understand the links between pesticide exposures depicted by the different matrices, bee species and apple orchards versus oilseed rape crops. We found that summary indices were highly correlated with the number of pesticides detected in the related matrix but not with which pesticides were present. Matrices collected from apple orchards generally contained a higher number of pesticides (7.6 pesticides per site) than matrices from sites collected from oilseed rape crops (3.5 pesticides), with fungicides being highly represented in apple crops. A greater number of pesticides were found in pollen-nectar stores/beebread and pollen matrices compared with nectar and bee body matrices. Our results show that for a complete assessment of pollinator pesticide exposure, it is necessary to consider several different exposure routes and multiple species of bees across different agricultural systems.

Aphid infestations reduce monarch butterfly colonization, herbivory, and growth on ornamental milkweed

Anthropogenic disturbance is driving global biodiversity loss, including the monarch butterfly (Danaus plexippus), a dietary specialist of milkweed. In response, ornamental milkweed plantings are increasingly common in urbanized landscapes, and recent evidence indicates they have conservation value for monarch butterflies. Unfortunately, sap-feeding insect herbivores, including the oleander aphid (Aphis nerii), frequently reach high densities on plants in nursery settings and urbanized landscapes. Aphid-infested milkweed may inhibit monarch conservation efforts by reducing host plant quality and inducing plant defenses. To test this, we evaluated the effects of oleander aphid infestation on monarch oviposition, larval performance, and plant traits using tropical milkweed (Asclepias curassavica), the most common commercially available milkweed species in the southern U.S. We quantified monarch oviposition preference, larval herbivory, larval weight, and plant characteristics on aphid-free and aphid-infested milkweed. Monarch butterflies deposited three times more eggs on aphid-free versus aphid-infested milkweed. Similarly, larvae fed aphid-free milkweed consumed and weighed twice as much as larvae fed aphid-infested milkweed. Aphid-free milkweed had higher total dry leaf biomass and nitrogen content than aphid-infested milkweed. Our results indicate that oleander aphid infestations can have indirect negative impacts on urban monarch conservation efforts and highlight the need for effective Lepidoptera-friendly integrated pest management tactics for ornamental plants.

Diversity of greenspace design and management impacts pollinator communities in a densely urbanized landscape: the city of Paris, France

The response of insect pollinator communities to increasing urbanization is shaped by landscape and local factors. But what about habitats that are already highly artificial? We investigated the drivers of pollinator diversity in a dense urban matrix, the city of Paris. We monitored insect pollinator communities monthly (March-October) for two consecutive years in 12 green spaces that differed in their management practices, focusing on four insect orders (Hymenoptera, Diptera, Lepidoptera, Coleoptera). Pollinator abundance and species richness were both positively tied to green space size and flowering plant species richness, but negatively linked to surrounding impervious surfaces. In addition, environmental features at both the local and landscape scales influenced the composition and functional diversity of wild bee communities. Indeed, small and large bees responded differently, with the occurrence of large-bodied species being impaired by the proportion of impervious surfaces but strongly enhanced by plant species richness. Also, sites with a majority of spontaneous plant species had more functionally diverse bee communities, with oligolectic species more likely to be found.

These results, consistent with the literature, can guide the design and management practices of urban green spaces to promote pollinator diversity and pollination function, even in dense urban environments.

Variation in the Concentration of Tilia spp. Pollen in the Aeroplankton of Lublin and Szczecin, Poland

Although lime trees have numerous benefits, they can pose a threat to allergy sufferers during the flowering period, as their pollen exhibits allergenic properties. This paper presents the results of 3 years of aerobiological research (2020-2022) carried out with the volumetric method in Lublin and Szczecin. A comparison of the pollen seasons in both cities revealed substantially higher concentrations of lime pollen in the air of Lublin than of Szczecin. In the individual years of the study, the maximum pollen concentrations were approximately 3-fold higher, and the annual pollen sum was about 2-3 times higher in Lublin than in Szczecin. Considerably higher lime pollen concentrations were recorded in both cities in 2020 than in the other years, which was probably associated with the 1.7-2.5 °C increase in the average temperature in April compared to the other two years. The maximum lime pollen concentrations were recorded during the last ten days of June or at the beginning of July in both Lublin and Szczecin. This period was associated with the greatest risk of pollen allergy development in sensitive subjects. The increased production of lime pollen in 2020 and in 2018-2019 with the increase in the mean temperature in April, reported in our previous study, may indicate a response of lime trees to the global warming phenomenon. Cumulative temperatures calculated for Tilia may serve as a basis for forecasting the beginning of the pollen season.

Anthropogenic effects on the body size of two neotropical orchid bees

To accommodate an ever-increasing human population, agriculture is rapidly intensifying at the expense of natural habitat, with negative and widely reported effects on biodiversity in general and on wild bee abundance and diversity in particular. Cities are similarly increasing in area, though the impact of urbanisation on wild bees is more equivocal and potentially positive in northern temperate regions. Yet agriculture and urbanisation both lead to the loss and alteration of natural habitat, its fragmentation, a potential reduction in floral availability, and warmer temperatures, factors thought to be drivers of wild bee decline. They have also been shown to be factors to which wild bee populations respond through morphological change. Body size is one such trait that, because of its relation to individual fitness, has received growing attention as a morphological feature that responds to human induced modification in land use. Here, we investigated the change in body size of two sympatric orchid bee species on the Yucatan Peninsula of Mexico in response to urbanization and agricultural intensification. By measuring 540 male individuals sampled from overall 24 sites, we found that Euglossa dilemma and Euglossa viridissima were on average smaller in urban and agricultural habitats than in natural ones. We discuss the potential role of reduced availability of resources in driving the observed body size shifts. Agricultural and urban land management in tropical regions might benefit wild bees if it encompassed the planting of flowering herbs and trees to enhance their conservation.

Some bee-pollinated plants provide nutritionally incomplete pollen amino acid resources to their pollinators

For pollinators such as bees, nectar mainly provides carbohydrates and pollen provides proteins, amino acids, and lipids to cover their nutritional needs. Here, to examine differences in pollinator resources, we compared the amino acid profiles and total amino acid contents of pollen from 32 common entomophilous plants in seven families. Our results showed that the amino acid profiles and contents in pollen samples differed according to the plant family and the chromatography method used, i.e., high-performance liquid chromatography (HPLC) versus ion exchange chromatography (IEX). Pollen from Boraginaceae species had the highest total amino acid contents (361.2–504 μg/mg) whereas pollen from the Malvaceae family had the lowest total amino acid contents (136–243.1 μg/mg). Calculating an amino acid score (AAS) that reflects pollen nutritional quality showed that slightly less than half of the species (19 out of 32) had the maximum nutritional score (AAS = 1) and offered high nutritional quality pollen amino acids for bee pollinators. Though they had high total amino acid contents, the amino acid composition of the studied Boraginaceae species and several members of the Fabaceae was not optimal, as their pollen was deficient in some essential amino acids, resulting in suboptimal amino acid scores (AAS < 0.7). Except for cysteine, the measured amino acid contents were higher using IEX chromatography than using HPLC. IEX chromatography is more robust and is to be preferred over HPLC in future amino acid analyses. Moreover, our observations show that some bee-pollinated species fail to provide complete amino acid resources for their pollinators. Although the implications for pollinator behavior remain to be studied, these deficiencies may force pollinators to forage from different species to obtain all nutritionial requirements.

Host and gut microbiome modulate the antiparasitic activity of nectar metabolites in a bumblebee pollinator

Antimicrobial nectar secondary metabolites can support pollinator health by preventing or reducing parasite infections. To better understand the outcome of nectar metabolite-parasite interactions in pollinators, we determined whether the antiparasitic activity was altered through chemical modification by the host or resident microbiome during gut passage. We investigated this interaction with linden (Tilia spp.) and strawberry tree (Arbutus unedo) nectar compounds. Unedone from A. unedo nectar inhibited the common bumblebee gut parasite Crithidia bombi in vitro and in Bombus terrestris gynes. A compound in Tilia nectar, 1-[4-(1-hydroxy-1-methylethyl)-1,3-cyclohexadiene-1-carboxylate]-6-O-β-d-glucopyranosyl-β-d-glucopyranose (tiliaside), showed no inhibition in vitro at naturally occurring concentrations but reduced C. bombi infections of B. terrestris workers. Independent of microbiome status, tiliaside was deglycosylated during gut passage, thereby increasing its antiparasitic activity in the hindgut, the site of C. bombi infections. Conversely, unedone was first glycosylated in the midgut without influence of the microbiome to unedone-8-O-β-d-glucoside, rendering it inactive against C. bombi, but subsequently deglycosylated by the microbiome in the hindgut, restoring its activity. We therefore show that conversion of nectar metabolites by either the host or the microbiome modulates antiparasitic activity of nectar metabolites. This article is part of the theme issue 'Natural processes influencing pollinator health: from chemistry to landscapes'.

Non-Native Non-Apis Bees Are More Abundant on Non-Native Versus Native Flowering Woody Landscape Plants

Simple Summary

Bees and other pollinators play a vital role in food production and natural ecosystems. Native bee populations are declining due in part to habitat loss. Individuals can help bees by landscaping with plants that provide pollen and nectar. Most information on bee-friendly plants concerns herbaceous ornamentals, but flowering trees and shrubs, too, can provide food for urban bees. Conservation organizations recommend landscaping mainly with native plants to support native bees, but some studies suggest that including some non-invasive non-native plants that bloom earlier or later than native plants can help support bees when resources from native plants are scarce. That strategy might backfire, however, if such plants disproportionately host invasive bee species. This study tested that hypothesis by identifying all non-native bees among 11,275 bees previously collected from 45 species of flowering woody plants across hundreds of urban sites. Besides the ubiquitous honey bee, six other non-native bee species comprised 2.9% of the total collection. Two alien species considered to have invasive tendencies by outcompeting native bees were more abundant on non-native plants. Planting their favored hosts might facilitate those bees’ spread in urban areas. Pros and cons of non-native woody landscape plants for urban bee conservation warrant further study.

Abstract

Urban ecosystems can support diverse communities of wild native bees. Because bloom times are conserved by geographic origin, incorporating some non-invasive non-native plants in urban landscapes can extend the flowering season and help support bees and other pollinators during periods when floral resources from native plants are limiting. A caveat, though, is the possibility that non-native plants might disproportionately host non-native, potentially invasive bee species. We tested that hypothesis by identifying all non-native bees among 11,275 total bees previously collected from 45 species of flowering woody landscape plants across 213 urban sites. Honey bees, Apis mellifera L., accounted for 22% of the total bees and 88.6% of the non-native bees in the collections. Six other non-native bee species, accounting for 2.86% of the total, were found on 16 non-native and 11 native woody plant species. Non-Apis non-native bees in total, and Osmia taurus Smith and Megachile sculpturalis (Smith), the two most abundant species, were significantly more abundant on non-native versus native plants. Planting of favored non-native hosts could potentially facilitate establishment and spread of non-Apis non-native bees in urban areas. Our host records may be useful for tracking those bees’ distribution in their introduced geographical ranges.

Neonicotinoid Pesticides Cause Mass Fatalities of Native Bumble Bees: A Case Study From Wilsonville, Oregon, United States

In June of 2013 an application of dinotefuran on an ornamental planting of European linden trees (Tilia cordata Mill. [Malvales: Malvalceae]) in a shopping mall parking lot in Wilsonville, Oregon provoked the largest documented pesticide kill of bumble bees in North America. Based on geographic information systems and population genetic analysis, we estimate that between 45,830 and 107,470 bumble bees originating from between 289 and 596 colonies were killed during this event. Dinotefuran is a neonicotinoid that is highly effective in exterminating and/or harming target pest insects and non-target beneficial insects. Analysis to detect the concentration of pesticides in flowers that received foliar application revealed that the minimum reported dinotefuran concentration of a sampled T. cordata flower was 7.4 ppm, or in excess of 737% above the LC50 of the beneficial pollinator, the honey bee (Apis mellifera Linnaeus, 1758 [Hymenoptera: Apidae]). Furthermore, sampled Vosnesensky bumble bees (Bombus vosnesenskii Radoskowski, 1862 [Hymenoptera: Apidae]) were found to have an average dinotefuran concentration of 0.92 ppm at the time of death, which exceeds the maximum LC50 of A. mellifera (0.884 ppm). Our study underscores the lethal impact of the neonicotinoid pesticide dinotefuran on pollinating insect populations in a suburban environment. To our knowledge, the documentation and impact of pesticide kills on wild populations of beneficial insects has not been widely reported in the scientific literature. It is likely that the vast majority of mass pesticide kills of beneficial insects across other environments go unnoticed and unreported.

Seasonal Changes and the Interaction between the Horse Chestnut Leaf Miner Cameraria ohridella and Horse Chestnut Leaf Blotch Disease Caused by Guignardia aesculi

The horse chestnut leaf miner Cameraria ohridella (Lepidoptera: Gracillariidae) is an invasive pest of horse chestnut Aesculus hippocastanum (Sapindales: Sapindaceae) and has spread through Europe since 1985. Horse chestnut leaf blotch is a fungal disease caused by Guignardia aesculi (Botryosphaeriales: Botryosphaeriaceae) that also seriously damages horse chestnut trees in Europe. The interaction between the leaf miner and the fungus has not yet been sufficiently described. Therefore, the aim of the present study was to assess leaf damage inflicted to horse chestnut by both C. ohridella and G. aesculi during the vegetation season and to model their interaction. The damage to leaf area was measured monthly from May to September 2013 in České Budějovice, the Czech Republic using digital image analysis of sampled leaves. A simple phenomenological model describing the expected dynamics of the two species was developed. The study revealed that the damage caused by both the pests and the fungus varied significantly among sampling sites within the city. The overall leaf damage exceeded 50% in no-raking sites in August. The mathematical model indicates that infestation by C. ohridella is more affected by G. aesculi than vice versa. Guignardia aesculi is thus the superior competitor of the two species. Our findings highlight the delicate interplay between insect pests and fungal pathogens and the spatiotemporal dynamics influencing them, calling for more research in this understudied area.

Climate Change-Induced Stress Reduce Quantity and Alter Composition of Nectar and Pollen From a Bee-Pollinated Species (Borago officinalis, Boraginaceae)

In temperate ecosystems, elevated temperatures, and drought occur especially during spring and summer, which are crucial periods for flowering, pollination, and reproduction of a majority of temperate plants. While many mechanisms may underlie pollinator decline in the wake of climate change, the interactive effects of temperature and water stress on the quantity and quality of floral nectar and pollen resources remain poorly studied. We investigated the impact of temperature rise (+3 and +6°C) and water stress (soil humidity lower than 15%) on the floral resources produced by the bee-pollinated species Borago officinalis. Nectar volume decreased with both temperature rise and water stress (6.1 ± 0.5 μl per flower under control conditions, 0.8 ± 0.1 μl per flower under high temperature and water stress conditions), resulting in a 60% decrease in the total quantity of nectar sugars (mg) produced per flower. Temperature rise but not water stress also induced a 50% decrease in pollen weight per flower but a 65% increase in pollen polypeptide concentration. Both temperature rise and water stress increased the total amino acid concentration and the essential amino acid percentage in nectar but not in pollen. In both pollen and nectar, the relative percentage of the different amino acids were modified under stresses. We discuss these modifications in floral resources in regards to plant-pollinator interactions and consequences on plant pollination success and on insect nutritional needs.

A "plan bee" for cities: Pollinator diversity and plant-pollinator interactions in urban green spaces

Green infrastructure in cities is considered to serve as a refuge for insect pollinators, especially in the light of an ongoing global decline of insects in agricultural landscapes. The design and maintenance of urban green spaces as key components of green infrastructure play a crucial role in case of nesting opportunities and for foraging insects. However, only few research has explored the impact of urban green space design on flower visitor communities, plant-pollinator interaction and the provision of the ecosystem service of pollination in cities. We investigated the abundance and diversity of pollinator communities in different urban park types in designed, standardized vegetation units, linked the visitation rates to the structural composition of the park types and derived indices for implemented pollination performances. The study was performed in two different structural park elements, flower beds and insect-pollinating trees. To gain a comprehensive understanding of the interaction between plants and pollinators, we calculated a plant-pollinator network of the recorded community in the investigation area. Visitation rates at different park types clearly showed, that the urban community gardens in comparison to other urban park types had a significantly higher abundance of pollinator groups, comparable to results found on a rural reference site. Tilia trees contributed significantly to the ecosystem service of pollination in investigated green spaces with a high supply of nectar and pollen during their flowering period. Calculations of pollination performances showed that recreational parks had comparably low visitation rates of pollinators and a high potential to improve conditions for the ecosystem service of pollination. The results indicated the strong potential of cities to provide a habitat for different groups of pollinators. In order to access this refuge, it is necessary to rely on near-natural concepts in design and maintenance, to create a wide range of flower diversity and to use even small green patches. Based on the findings, we encourage an integrated management of urban free spaces to consider parks as key habitats for pollinators in anthropogenic dominated, urban environments.

Wild Bee Conservation within Urban Gardens and Nurseries: Effects of Local and Landscape Management

Across urban environments, vegetated habitats provide refuge for biodiversity. Gardens (designed for food crop production) and nurseries (designed for ornamental plant production) are both urban agricultural habitats characterized by high plant species richness but may vary in their ability to support wild pollinators, particularly bees. In gardens, pollinators are valued for crop production. In nurseries, ornamental plants rarely require pollination; thus, the potential of nurseries to support pollinators has not been examined. We asked how these habitats vary in their ability to support wild bees, and what habitat features relate to this variability. In 19 gardens and 11 nurseries in California, USA, we compared how local habitat and landscape features affected wild bee species abundance and richness. To assess local features, we estimated floral richness and measured ground cover as proxies for food and nesting resources, respectively. To assess landscape features, we measured impervious land cover surrounding each site. Our analyses showed that differences in floral richness, local habitat size, and the amount of urban land cover impacted garden wild bee species richness. In nurseries, floral richness and the proportion of native plant species impacted wild bee abundance and richness. We suggest management guidelines for supporting wild pollinators in both habitats.

Temperate Agroforestry Systems and Insect Pollinators: A Review

Agroforestry can provide ecosystem services and benefits such as soil erosion control, microclimate modification for yield enhancement, economic diversification, livestock production and well-being, and water quality protection. Through increased structural and functional diversity in agricultural landscapes, agroforestry practices can also affect ecosystem services provided by insect pollinators. A literature review was conducted to synthesize information on how temperate agroforestry systems influence insect pollinators and their pollination services with particular focus on the role of trees and shrubs. Our review indicates that agroforestry practices can provide three overarching benefits for pollinators: (1) providing habitat including foraging resources and nesting or egg-laying sites, (2) enhancing site and landscape connectivity, and (3) mitigating pesticide exposure. In some cases, agroforestry practices may contribute to unintended consequences such as becoming a sink for pollinators, where they may have increased exposure to pesticide residue that can accumulate in agroforestry practices. Although there is some scientific evidence suggesting that agroforestry practices can enhance crop pollination and yield, more research needs to be conducted on a variety of crops to verify this ecosystem service. Through a more comprehensive understanding of the effects of agroforestry practices on pollinators and their key services, we can better design agroforestry systems to provide these benefits in addition to other desired ecosystem services.

Challenges for Monitoring the Extent and Land Use/Cover Changes in Monarch Butterflies’ Migratory Habitat across the United States and Mexico

This paper presents a synopsis of the challenges and limitations presented by existing and emerging land use/land cover (LULC) digital data sets when used to analyze the extent, habitat quality, and LULC changes of the monarch (Danaus plexippus) migratory habitat across the United States of America (US) and Mexico. First, the characteristics, state of the knowledge, and issues related to this habitat are presented. Then, the characteristics of the existing and emerging LULC digital data sets with global or cross-border coverage are listed, followed by the data sets that cover only the US or Mexico. Later, we discuss the challenges for determining the extent, habitat quality, and LULC changes in the monarchs’ migratory habitat when using these LULC data sets in conjunction with the current state of the knowledge of the monarchs’ ecology, behavior, and foraging/roosting plants used during their migration. We point to approaches to address some of these challenges, which can be categorized into: (a) LULC data set characteristics and availability; (b) availability of ancillary land management information; (c) ability to construct accurate forage suitability indices for their migration habitat; and (d) level of knowledge of the ecological and behavioral patterns of the monarchs during their journey.

Urban bumblebees are smaller and more phenotypically diverse than their rural counterparts

With urbanization identified as being one of the key drivers of change in global land use, and the rapid expansion of urban areas world-wide, it is relevant to evaluate how novel ecological conditions in cities shape species functional traits, which are essential for how species interact with their environments and with each other. Despite the many comparative studies on organisms living in urban and non-urban areas, our knowledge on species responses to urban environments remains limited. For one, much of the ecological research has assumed that the environment changes in a linear fashion from the city core to the city edges, whereas in reality the environments within the cities are highly heterogeneous. Furthermore, studies on species responses to these highly variable ecosystems are often based on interspecific mean trait values, which ignore the potential for high levels of intraspecific variation among individuals in key functional traits. The current study investigated intraspecific functional trait differences for four functional traits associated with body size, mobility and resource selection among rural and urban populations of two common bumblebee species, Bombus pascuorum and Bombus lapidarius, in urban centres and adjacent rural areas in Switzerland. We document shifts in functional traits towards smaller individuals and higher multidimensional trait variation in urban populations compared to rural conspecifics of both species. This shows that urban individuals for both species are on average smaller sized but populations are distinctively different from rural population by increasing their trait richness and diversifying their trait combinations. In addition, we found bimodality in tongue length within urban B. pascuorum populations. Our results suggest that urban and rural populations possibly experience differential selection pressures resulting in trait differences across and among populations. We argue that variations in the respective foraging landscapes in cities leads to smaller sized but phenotypically more diverse populations, and drive functional trait divergence.

Linden (Tilia cordata) associated bumble bee mortality: Metabolomic analysis of nectar and bee muscle

Linden (Tilia spp.), a profusely flowering temperate tree that provides bees with vital pollen and nectar, has been associated with bumble bee (Bombus spp.) mortality in Europe and North America. Bee deaths have been attributed, with inadequate evidence, to toxicity from mannose in nectar or starvation due to low nectar in late blooming linden. Here, we investigated both factors via untargeted metabolomic analyses of nectar from five T. cordata trees beneath which crawling/dead bumble bees (B. vosnesenskii) were observed, and of thoracic muscle of 28 healthy foraging and 29 crawling bees collected from linden trees on cool mornings (< 30°C). Nectar contained the pyridine alkaloid trigonelline, a weak acetylcholinesterase inhibitor, but no mannose. Principal component analysis of muscle metabolites produced distinct clustering of healthy and crawling bees, with significant differences (P<0.05) in 34 of 123 identified metabolites. Of these, TCA (Krebs) cycle intermediates were strongly represented (pathway analysis; P<0.01), suggesting that the central metabolism is affected in crawling bees. Hence, we propose the following explanation: when ambient temperature is low, bees with energy deficit are unable to maintain the thoracic temperature required for flight, and consequently fall, crawl, and ultimately, die. Energy deficit could occur when bees continue to forage on linden despite limited nectar availability either due to loyalty to a previously energy-rich source or trigonelline-triggered memory/learning impairment, documented earlier with other alkaloids. Thus, the combination of low temperature and nectar volume, resource fidelity, and alkaloids in nectar could explain the unique phenomenon of bumble bee mortality associated with linden.

Seasonal variation of pollen collected by honey bees (Apis mellifera) in developed areas across four regions in the United States

For honey bees (Apis mellifera), colony maintenance and growth are highly dependent on worker foragers obtaining sufficient resources from flowering plants year round. Despite the importance of floral diversity for proper bee nutrition, urban development has drastically altered resource availability and diversity for these important pollinators. Therefore, understanding the floral resources foraged by bees in urbanized areas is key to identifying and promoting plants that enhance colony health in those environments. In this study, we identified the pollen foraged by bees in four developed areas of the U.S., and explored whether there were spatial or temporal differences in the types of floral sources of pollen used by honey bees in these landscapes. To do this, pollen was collected every month for up to one year from colonies located in developed (urban and suburban) sites in California, Texas, Florida, and Michigan, except during months of pollen dearth or winter. Homogenized pollen samples were acetolyzed and identified microscopically to the lowest taxonomic level possible. Once identified, each pollen type was classified into a frequency category based on its overall relative abundance. Species richness and diversity indices were also calculated and compared across states and seasons. We identified up to 64 pollen types belonging to 39 plant families in one season (California). Species richness was highest in CA and lowest in TX, and was highest during spring in every state. In particular, “predominant” and “secondary” pollen types belonged to the families Arecaceae, Sapindaceae, Anacardiaceae, Apiaceae, Asteraceae, Brassicaceae, Fabaceae, Fagaceae, Lythraceae, Myrtaceae, Rhamnaceae, Rosaceae, Rutaceae, Saliaceae, and Ulmaceae. This study will help broaden our understanding of honey bee foraging ecology and nutrition in urban environments, and will help promote the use of plants that serve the dual purpose of providing aesthetic value and nutritious forage for honey bee colonies placed in developed landscapes.

Foraging choices balanced between resource abundance and handling concerns: how the honeybee, Apis mellifera, select the flowers of Robinia pseudoacacia

Nectar is a main resource harvested by foraging honeybees: their ability in selecting among flowers is the key to optimize resource collection. This ability is expected to be the result of co-evolutionary traits between the plant and the pollinator visiting it; notwithstanding, novel interactions may occur between native and invasive species. Analysing foraging efforts, flexibility and individual constrains has to be taken into account. The foraging pattern of the ubiquitous honeybee on Robinia pseudoacacia, a North-American species widely naturalized in European countries, grounds a perfect case study. The plant shows papilionate flowers especially reach in nectar, but their tripping mechanism is difficult for the small/light-weight honeybee. Yet Apis mellifera is known to pay frequent and constant visits to them: in fact, one of the most appreciated unifloral honey is produced out of R. pseudoacacia. The aim of this study was to understand when and how the bees overcome physical constraints to succeed in flower visits, and to what extent this flexibility extend from the individual to the species. Data were collected in Italy, through focal observations of foraging individuals, nectar content measurements and experiments with manipulated inflorescences. Results clearly indicate that nectar content changes accordingly to the state of flowers (visited or unvisited), which also show slight changes in appearance. Foraging individuals, able to detect these differences, perform active choices preferentially selecting already-visited flowers: lower in nectar content but easier to manipulate. Even if the choice is primarily driven by handling constraints, individual experience and strength of stimuli are prompting visits also to unvisited flowers, notwithstanding a higher risk of failure in resource collection. Behavioural plasticity matching a satisfactory compromise grounds the decision that maximizes the intake of resource balanced with the effort to gain it.

Urbanisation modulates plant-pollinator interactions in invasive vs. native plant species

Pollination is a key ecological process, and invasive alien plant species have been shown to significantly affect plant-pollinator interactions. Yet, the role of the environmental context in modulating such processes is understudied. As urbanisation is a major component of global change, being associated with a range of stressors (e.g. heat, pollution, habitat isolation), we tested whether the attractiveness of a common invasive alien plant (Robinia pseudoacacia, black locust) vs. a common native plant (Cytisus scoparius, common broom) for pollinators changes with increasing urbanisation. We exposed blossoms of both species along an urbanisation gradient and quantified different types of pollinator interaction with the flowers. Both species attracted a broad range of pollinators, with significantly more visits for R. pseudoacacia, but without significant differences in numbers of insects that immediately accessed the flowers. However, compared to native Cytisus, more pollinators only hovered in front of flowers of invasive Robinia without visiting those subsequently. The decision rate to enter flowers of the invasive species decreased with increasing urbanisation. This suggests that while invasive Robinia still attracts many pollinators in urban settings attractiveness may decrease with increasing urban stressors. Results indicated future directions to deconstruct the role of different stressors in modulating plant-pollinator interactions, and they have implications for urban development since Robinia can be still considered as a "pollinator-friendly" tree for certain urban settings.

Improvements in the Rearing of the Tachinid Parasitoid Exorista larvarum (Diptera: Tachinidae): Influence of Adult Food on Female Longevity and Reproduction Capacity

Exorista larvarum (L.), a polyphagous gregarious larval parasitoid of lepidopterans, can be mass produced both in vivo, using the greater wax moth Galleria mellonella (L.) (Lepidoptera: Pyralidae) as a factitious host, and in vitro, on artificial media composed of crude components. The present study was focused on another aspect of E. larvarum rearing, namely the influence of adult food on parasitoid performance. The standard food, consisting of lump sucrose and cotton balls soaked in a honey and water solution (1), was compared with other foods or food combinations, namely lump sucrose alone (2), honey and water solution (3), sucrose and water solution either alone (4) or combined with bee-collected pollen (5), and, finally, pollen alone (6). All foods were provided together with distilled water supplied in drinking troughs. Based on the parameters considered (i.e., female longevity, number of eggs laid on host larvae, puparia obtained from eggs, and adults emerged from puparia), pollen alone was deemed to be the most suitable food for adult females of E. larvarum. In particular, the pollen showed a longevity-promoting effect, increasing the number of eggs laid on host larvae throughout the female lifespan. The use of this adult food may also result in a higher flexibility of the management of E. larvarum colonies because it can be replaced weekly, as no desiccation or mold infections were ever found to occur.

Quantifying bee assemblages and attractiveness of flowering woody landscape plants for urban pollinator conservation

Urban and suburban landscapes can be refuges for biodiversity of bees and other pollinators. Public awareness of declining pollinator populations has increased interest in growing plants that provide floral resources for bees. Various publications and websites list "bee-friendly" plants, but such lists are rarely based on empirical data, nor do they emphasize flowering trees and shrubs, which are a major component of urban landscapes. We quantified bee visitation to 72 species of flowering woody landscape plants across 373 urban and suburban sites in Kentucky and southern Ohio, USA, sampling and identifying the bee assemblages associated with 45 of the most bee-attractive species. We found strong plant species effects and variation in seasonal activity of particular bee taxa, but no overall differences in extent of bee visitation or bee genus diversity between native and non-native species, trees and shrubs, or early-, mid-, and late-season blooming plants. Horticulturally-modified varieties of Hydrangea, Prunus, and Rosa with double petals or clusters of showy sterile sepals attracted few bees compared to related plants with more accessible floral rewards. Some of the non-native woody plant species bloomed when floral resources from native plants were scarce and were highly bee-attractive, so their use in landscapes could help extend the flowering season for bees. These data will help city foresters, landscape managers, and the public make informed decisions to create bee-friendly urban and suburban landscapes.

Uptake and dissipation of neonicotinoid residues in nectar and foliage of systemically treated woody landscape plants

Systemic neonicotinoid insecticides used in urban arboriculture could pose a risk to bees and other pollinators foraging on treated plants. We measured uptake and dissipation of soil-applied imidacloprid and dinotefuran in nectar and leaves of 2 woody plant species, a broadleaf evergreen tree (Ilex × attenuata) and a deciduous shrub (Clethra alnifolia), to assess concentrations to which pollinators and pests might be exposed in landscape settings. Three application timings, autumn (postbloom), spring (prebloom), and summer (early postbloom), were evaluated to see if taking advantage of differences in the neonicotinoids' systemic mobility and persistence might enable pest control while minimizing transference into nectar. Nectar and tissue samples were collected from in-ground plants and analyzed for residues by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) in 2 successive years. Concentrations found in nectar following autumn or spring applications ranged from 166 to 515 ng/g for imidacloprid and from 70 to 1235 ng/gg for dinotefuran, depending on plant and timing. These residues exceed concentrations shown to adversely affect individual- and colony-level traits of bees. Summer application mitigated concentrations of imidacloprid (8-31 ng/g), but not dinotefuran (235-1191 ng/g), in nectar. Our data suggest that dinotefuran may be more persistent than is generally believed. Implications for integrated pest and pollinator management in urban landscapes are discussed. Environ Toxicol Chem 2018;37:860-870. © 2017 SETAC.

Temperature and water stress affect plant-pollinator interactions in Borago officinalis (Boraginaceae)

Climate change alters the abiotic constraints faced by plants, including increasing temperature and water stress. These changes may affect flower development and production of flower rewards, thus altering plant–pollinator interactions. Here, we investigated the consequences of increased temperature and water stress on plant growth, floral biology, flower‐reward production, and insect visitation of a widespread bee‐visited species, Borago officinalis. Plants were grown for 5 weeks under three temperature regimes (21, 24, and 27°C) and two watering regimes (well‐watered and water‐stressed). Plant growth was more affected by temperature rise than water stress, and the reproductive growth was affected by both stresses. Vegetative traits were stimulated at 24°C, but impaired at 27°C. Flower development was mainly affected by water stress, which decreased flower number (15 ± 2 flowers/plant in well‐watered plants vs. 8 ± 1 flowers/plant under water stress). Flowers had a reduced corolla surface under temperature rise and water stress (3.8 ± 0.5 cm² in well‐watered plants at 21°C vs. 2.2 ± 0.1 cm² in water‐stressed plants at 27°C). Both constraints reduced flower‐reward production. Nectar sugar content decreased from 3.9 ± 0.3 mg/flower in the well‐watered plants at 21°C to 1.3 ± 0.4 mg/flower in the water‐stressed plants at 27°C. Total pollen quantity was not affected, but pollen viability decreased from 79 ± 4% in the well‐watered plants at 21°C to 25 ± 9% in the water‐stressed plants at 27°C. Flowers in the well‐watered plants at 21°C received at least twice as many bumblebee visits compared with the other treatments. In conclusion, floral modifications induced by abiotic stresses related to climate change affect insect behavior and alter plant–pollinator interactions.

Do linden trees kill bees? Reviewing the causes of bee deaths on silver linden (Tilia tomentosa)

For decades, linden trees (basswoods or lime trees), and particularly silver linden (Tilia tomentosa), have been linked to mass bee deaths. This phenomenon is often attributed to the purported occurrence of the carbohydrate mannose, which is toxic to bees, in Tilia nectar. In this review, however, we conclude that from existing literature there is no experimental evidence for toxicity to bees in linden nectar. Bee deaths on Tilia probably result from starvation, owing to insufficient nectar resources late in the tree's flowering period. We recommend ensuring sufficient alternative food sources in cities during late summer to reduce bee deaths on silver linden. Silver linden metabolites such as floral volatiles, pollen chemistry and nectar secondary compounds remain underexplored, particularly their toxic or behavioural effects on bees. Some evidence for the presence of caffeine in linden nectar may mean that linden trees can chemically deceive foraging bees to make sub-optimal foraging decisions, in some cases leading to their starvation.

Nutritional composition of honey bee food stores vary with floral composition

Sufficiently diverse and abundant resources are essential for generalist consumers, and form an important part of a suite of conservation strategies for pollinators. Honey bees are generalist foragers and are dependent on diverse forage to adequately meet their nutritional needs. Through analysis of stored pollen (bee bread) samples obtained from 26 honey bee (Apis mellifera L.) hives across NW-England, we quantified bee bread nutritional content and the plant species that produced these stores from pollen. Protein was the most abundant nutrient by mass (63%), followed by carbohydrates (26%). Protein and lipid content (but not carbohydrate) contributed significantly to ordinations of floral diversity, linking dietary quality with forage composition. DNA sequencing of the ITS2 region of the nuclear ribosomal DNA gene identified pollen from 89 distinct plant genera, with each bee bread sample containing between 6 and 35 pollen types. Dominant genera included dandelion (Taraxacum), which was positively correlated with bee bread protein content, and cherry (Prunus), which was negatively correlated with the amount of protein. In addition, proportions of amino acids (e.g. histidine and valine) varied as a function of floral species composition. These results also quantify the effects of individual plant genera on the nutrition of honey bees. We conclude that pollens of different plants act synergistically to influence host nutrition; the pollen diversity of bee bread is linked to its nutrient content. Diverse environments compensate for the loss of individual forage plants, and diversity loss may, therefore, destabilize consumer communities due to restricted access to alternative resources.