Bee Hosts Research Articles

Bee species-specific nesting material attracts a generalist parasitoid: implications for co-occurring bees in nest box enhancements.

Artificial nests (e.g., nest boxes) for bees are increasingly being used to contribute to nesting habitat enhancement for bees that use preexisting cavities to provision brood. They usually incorporate additional nesting materials that vary by species. Cavity-nesting bees are susceptible to brood parasitoids that recognize their host(s) using visual and chemical cues. Understanding the range of cues that attract parasitoids to bee nests, including human-made analogues, is important if we wish to control parasitism and increase the potential value of artificial nests as habitat-enhancement strategies. In this study, we investigated the cues associated with the orientation of the generalist brood parasitoid Monodontomerus obscurus Westwood (Hymenoptera: Torymidae) to the nests of a common cavity-nesting resin bee Megachile campanulae (Robertson) (Megachilidae). The parasitoids were reared from previously infested M. campanulae brood cells and placed into choice trials where they were presented with pairs of different nest material cues. Among different materials tested, we found that Mo. obscurus was most attracted to fresh resin collected directly from Pinus strobus trees followed by previously used resin collected from the bee nest. The parasitoid also attacked other bee species in the same nest boxes, including those that do not use resin for nesting. Our findings suggest that M. campanulae could act as a magnet, drawing parasites away from other bee hosts co-occurring in nest boxes, or, as an attractant of Mo. obscurus to nest boxes, increasing attacks on co-occurring host bee species, potentially undermining bee diversity enhancement initiatives.

Genomics and host specialization of honey bee and bumble bee gut symbionts.

Gilliamella apicola and Snodgrassella alvi are dominant members of the honey bee (Apis spp.) and bumble bee (Bombus spp.) gut microbiota. We generated complete genomes of the type strains G. apicola wkB1(T) and S. alvi wkB2(T) (isolated from Apis), as well as draft genomes for four other strains from Bombus. G. apicola and S. alvi were found to occupy very different metabolic niches: The former is a saccharolytic fermenter, whereas the latter is an oxidizer of carboxylic acids. Together, they may form a syntrophic network for partitioning of metabolic resources. Both species possessed numerous genes [type 6 secretion systems, repeats in toxin (RTX) toxins, RHS proteins, adhesins, and type IV pili] that likely mediate cell-cell interactions and gut colonization. Variation in these genes could account for the host fidelity of strains observed in previous phylogenetic studies. Here, we also show the first experimental evidence, to our knowledge, for this specificity in vivo: Strains of S. alvi were able to colonize their native bee host but not bees of another genus. Consistent with specific, long-term host association, comparative genomic analysis revealed a deep divergence and little or no gene flow between Apis and Bombus gut symbionts. However, within a host type (Apis or Bombus), we detected signs of horizontal gene transfer between G. apicola and S. alvi, demonstrating the importance of the broader gut community in shaping the evolution of any one member. Our results show that host specificity is likely driven by multiple factors, including direct host-microbe interactions, microbe-microbe interactions, and social transmission.

Infra-population and -community dynamics of the parasites Nosema apis and Nosema ceranae, and consequences for honey bee (Apis mellifera) hosts.

Nosema spp. fungal gut parasites are among myriad possible explanations for contemporary increased mortality of western honey bees (Apis mellifera, hereafter honey bee) in many regions of the world. Invasive Nosema ceranae is particularly worrisome because some evidence suggests it has greater virulence than its congener N. apis. N. ceranae appears to have recently switched hosts from Asian honey bees (Apis cerana) and now has a nearly global distribution in honey bees, apparently displacing N. apis. We examined parasite reproduction and effects of N. apis, N. ceranae, and mixed Nosema infections on honey bee hosts in laboratory experiments. Both infection intensity and honey bee mortality were significantly greater for N. ceranae than for N. apis or mixed infections; mixed infection resulted in mortality similar to N. apis parasitism and reduced spore intensity, possibly due to inter-specific competition. This is the first long-term laboratory study to demonstrate lethal consequences of N. apis and N. ceranae and mixed Nosema parasitism in honey bees, and suggests that differences in reproduction and intra-host competition may explain apparent heterogeneous exclusion of the historic parasite by the invasive species.

Immunogene and viral transcript dynamics during parasitic Varroa destructor mite infection of developing honey bee (Apis mellifera) pupae.

The ectoparasitic Varroa destructor mite is a major contributor to the ongoing honey bee health crisis. Varroa interacts with honey bee viruses, exacerbating their pathogenicity. In addition to vectoring viruses, immunosuppression of the developing honey bee hosts by Varroa has been proposed to explain the synergy between viruses and mites. However, the evidence for honey bee immune suppression by V. destructor is contentious. We systematically studied the quantitative effects of experimentally introduced V. destructor mites on immune gene expression at five specific time points during the development of the honey bee hosts. Mites reproduced normally and were associated with increased titers of deformed wing virus in the developing bees. Our data on different immune genes show little evidence for immunosuppression of honey bees by V. destructor. Experimental wounding of developing bees increases relative immune gene expression and deformed wing virus titers. Combined, these results suggest that mite feeding activity itself and not immunosuppression may contribute to the synergy between viruses and mites. However, our results also suggest that increased expression of honey bee immune genes decreases mite reproductive success, which may be explored to enhance mite control strategies. Finally, our expression data for multiple immune genes across developmental time and different experimental treatments indicates co-regulation of several of these genes and thus improves our understanding of the understudied honey bee immune system.

The ability to cause infection in a pathogenic fungus uncovers a new biological feature of honey bee viruses

We demonstrated that honey bee viruses including Deformed wing virus (DWV), Black queen cell virus (BQCV) and Israeli acute paralysis virus (IAPV) could infect and replicate in the fungal pathogen Ascosphaera apis that causes honey bee chalkbrood disease, revealing a novel biological feature of honey bee viruses. The phylogenetic analysis show that viruses of fungal and honey bee origins form two clusters in the phylogenetic trees distinctly and that host range of honey bee viruses is dynamic. Further studies are warranted to investigate the impact of the viruses on the fitness of their fungal host and phenotypic effects the virus–fungus combination has on honey bee hosts.

Life cycle of Huarpea fallax (Hymenoptera: Sapygidae) in a xeric forest in Argentina

This study describes the life cycle of Huarpea fallax (Hymenoptera: Sapygidae) in a xeric forest in La Pampa province, Argentina. This cleptoparasitic wasp attacks the nests of two species of leaf-cutter bees: Megachile catamarcensis and Anthidium vigintipunctatum, both belonging to the family Megachilidae. Nests of these bee species were obtained during a trap-nesting programme. Adult emergence showed a unimodal pattern indicating a univoltine life cycle. The period from egg-laying to adult emergence lasted for 10–13 months; however, one female took about 2 years to emerge, suggesting parsivoltinism. Most females attack one cell per host nest, the outermost cells being the ones most parasitized. However, the position of the attacked cells was variable. In this paper, although there were insufficient data to prove a correlation, the data suggest a positive trend between body size of sapygid wasps and their host bees.

Tracing horizontal Wolbachia movements among bees (Anthophila): a combined approach using multilocus sequence typing data and host phylogeny

The endosymbiotic bacterium Wolbachia enhances its spread via vertical transmission by generating reproductive effects in its hosts, most notably cytoplasmic incompatibility (CI). Additionally, frequent interspecific horizontal transfer is evident from a lack of phylogenetic congruence between Wolbachia and its hosts. The mechanisms of this lateral transfer are largely unclear. To identify potential pathways of Wolbachia movements, we performed multilocus sequence typing of Wolbachia strains from bees (Anthophila). Using a host phylogeny and ecological data, we tested various models of horizontal endosymbiont transmission. In general, Wolbachia strains seem to be randomly distributed among bee hosts. Kleptoparasite-host associations among bees as well as other ecological links could not be supported as sole basis for the spread of Wolbachia. However, cophylogenetic analyses and divergence time estimations suggest that Wolbachia may persist within a host lineage over considerable timescales and that strictly vertical transmission and subsequent random loss of infections across lineages may have had a greater impact on Wolbachia strain distribution than previously estimated. Although general conclusions about Wolbachia movements among arthropod hosts cannot be made, we present a framework by which precise assumptions about shared evolutionary histories of Wolbachia and a host taxon can be modelled and tested.

Ascosphaera callicarpa, a New Species of Bee-Loving Fungus, with a Key to the Genus for Europe

We studied the bee specialist fungus Ascosphaera in wild solitary bees to investigate the diversity of the genus in nature and the ecology of these fungi with their bee hosts. A new morphologically distinctive species was discovered which also has a unique nrITS sequence. This new species, here named Ascosphaera callicarpa, is common on the larval feces of the solitary bee Chelostoma florisomne which nests in the Phragmites reeds of thatched roofs in Europe. Because collections of Ascosphaera from wild bees are scarce and because little is known about the ecology and distribution of the majority of the species in the genus, a key to the species thus far reported for Europe is included.

Total Synthesis, Proof of Absolute Configuration, and Biosynthetic Origin of Stylopsal, the First Isolated Sex Pheromone of Strepsiptera

The asymmetric total synthesis of the diastereomers of stylopsal establishes the absolute configuration of the first reported sex pheromone of the twisted-wing parasite Stylops muelleri as (3R,5R,9R)-trimethyldodecanal. The key steps for the diastereo- and enantiodivergent introduction of the methyl groups are two different types of asymmetric conjugate addition reactions of organocopper reagents to α,β-unsaturated esters, whereas the dodecanal skeleton is assembled by Wittig reactions. The structure of the natural product was confirmed by chiral gas chromatography (GC) techniques, GC/MS and GC/electroantennography (EAD) as well as field tests. An investigation into the biosynthesis of the pheromone revealed that it is likely to be produced by decarboxylation of a 4,6,10-trimethyltridecanoic acid derivative, which was found in substantial amounts in the fat body of the female, but not in the host bee Andrena vaga. This triple-branched fatty acid precursor thus seems to be biosynthesized de novo through a polyketide pathway with two consecutive propionate-propionate-acetate assemblies to form the complete skeleton. The simplified, motionless and fully host-dependent female exploits a remarkable strategy to maximize its reproductive success by employing a relatively complex and potent sex pheromone.

New Species of Exomalopsis and Its Associated Cleptoparasite Nomada from Colombia with Description of the Nest (Hymenoptera: Apoidea: Anthophila: Apidae)

We describe two new species of bees from Colombia; one is a species of Exomalopsis found nesting in the city of Medellin, Colombia, (the host) and its cleptoparasitic bee, a species of the genus Nomada. In addition, we provide information on the nest architecture of the new species and provide data on occupancy by both the host bee and its cleptoparasite. We present an updated list of the species of Exomalopsis and Nomada of Colombia and taxonomic keys to the species of Exomalopsis.

Does sensory deception matter in eusocial obligate food robber systems? A study of Lestrimelitta and stingless bee hosts

Social parasites can break into their host colonies using sensory deception, force, or both. To evaluate the role of sensory deception in eusocial obligate food robbers, we studied the Mesoamerican stingless bee Lestrimelitta niitkib –host species system, including preferred and nonpreferred host species. The use of citral as a propaganda substance is documented in L. niitkib , but possible mechanisms used by individual scouts to overcome host species recognition have not been studied. We analysed the cuticular profiles of L. niitkib and host species, coupled with bioassays of time to aggression (latency) and included data on host species raid frequency. We found that L. niitkib has a simple, but not insignificant, cuticular profile. Generally, L. niitkib cuticular profiles were similar to (but did not mimic) profiles of its preferred host species and differed from profiles of nonpreferred hosts. As expected, latency generally fitted a recognition system based on the degree of similarity between the cleptobiont's cuticular label and the host species template, with chemically similar species reacting slower and chemically distant species reacting rapidly to L. niitkib . There was a positive correlation between raid ratio and latency, indicating that the speed of detection and aggression towards L. niitkib scouts could influence host species selection. Cuticular profile similarity of individual L. niitkib scouts to host species may help L. niitkib scouts evade recognition and attacks from guards. In a further step, unnoticed L. niitkib scouts could successfully recruit nestmates to mass-raid host species colonies. The fact that L. niitkib can also plunder aggressive species, suggests that obligate cleptobiosis within its narrow biological niche could be characterized by flexibility in invasion strategies to allow exploiting a broad range of host species and be successful over evolutionary times. ► We studied the role of sensory deception in an obligate cleptobiont bee system. ► The cuticular profiles of Lestrimelitta and preferred hosts were similar. ► Similarity in cuticular profiles was related to time to aggression and raid ratio. ► Host selection may depend on speed of response to cleptobionts. ► Sensory deception may help Lestrimelitta scouts evade recognition and attacks from chemically similar host species.

Activation and interruption of the reproduction of Varroa destructor is triggered by host signals (Apis mellifera)

The reproductive cycle of the parasitic mite Varroa destructor is closely linked to the development of the honey bee host larvae. Using a within colony approach we introduced phoretic Varroa females into brood cells of different age in order to analyze the capacity of certain stages of the honey bee larva to either activate or interrupt the reproduction of Varroa females. Only larvae within 18h (worker) and 36h (drones), respectively, after cell capping were able to stimulate the mite’s oogenesis. Therewith we could specify for the first time the short time window where honey bee larvae provide the signals for the activation of the Varroa reproduction. Stage specific volatiles of the larval cuticle are at least part of these activation signals. This is confirmed by the successful stimulation of presumably non-reproducing mites to oviposition by the application of a larval extract into the sealed brood cells. According to preliminary quantitative GC–MS analysis we suggest certain fatty acid ethyl esters as candidate compounds.If Varroa females that have just started with egg formation are transferred to brood cells containing host larvae of an elder stage two-thirds of these mites stopped their oogenesis. This confirms the presence of an additional signal in the host larvae allowing the reproducing mites to adjust their own reproductive cycle to the ontogenetic development of the host. From an adaptive point of view that sort of a stop signal enables the female mite to save resources for a next reproductive cycle if the own egg development is not sufficiently synchronized with the development of the host.The results presented here offer the opportunity to analyze exactly those host stages that have the capacity to activate or interrupt the Varroa reproduction in order to identify the crucial host signals.

Diversity and evolutionary patterns of bacterial gut associates of corbiculate bees

The animal gut is a habitat for diverse communities of microorganisms (microbiota). Honeybees and bumblebees have recently been shown to harbour a distinct and species poor microbiota, which may confer protection against parasites. Here, we investigate diversity, host specificity and transmission mode of two of the most common, yet poorly known, gut bacteria of honeybees and bumblebees: Snodgrassella alvi (Betaproteobacteria) and Gilliamella apicola (Gammaproteobacteria). We analysed 16S rRNA gene sequences of these bacteria from diverse bee host species across most of the honeybee and bumblebee phylogenetic diversity from North America, Europe and Asia. These focal bacteria were present in 92% of bumblebee species and all honeybee species but were found to be absent in the two related corbiculate bee tribes, the stingless bees (Meliponini) and orchid bees (Euglossini). Both Snodgrassella alvi and Gilliamella apicola phylogenies show significant topological congruence with the phylogeny of their bee hosts, albeit with a considerable degree of putative host switches. Furthermore, we found that phylogenetic distances between Gilliamella apicola samples correlated with the geographical distance between sampling locations. This tentatively suggests that the environmental transmission rate, as set by geographical distance, affects the distribution of G.apicola infections. We show experimentally that both bacterial taxa can be vertically transmitted from the mother colony to daughter queens, and social contact with nest mates after emergence from the pupa greatly facilitates this transmission. Therefore, sociality may play an important role in vertical transmission and opens up the potential for co-evolution or at least a close association of gut bacteria with their hosts.

Variable virulence among isolates of Ascosphaera apis: testing the parasite–pathogen hypothesis for the evolution of polyandry in social insects

The queens of many eusocial insect species are polyandrous. The evolution of polyandry from ancestral monoandry is intriguing because polyandry undermines the kin-selected benefits of high intracolonial relatedness that are understood to have been central to the evolution of eusociality. An accumulating body of evidence suggests that polyandry evolved from monoandry in part because genetically diverse colonies better resist infection by pathogens. However, a core assumption of the "parasite-pathogen hypothesis", that there is variation in virulence among strains of pathogens, remains largely untested in vivo. Here, we demonstrate variation in virulence among isolates of Ascosphaera apis, the causative organism of chalkbrood disease in its honey bee (Apis mellifera) host. More importantly, we show a pathogen-host genotypic interaction for resistance and pathogenicity. Our findings therefore support the parasite-parasite hypothesis as a factor in the evolution of polyandry among eusocial insects.

Functional and evolutionary insights into the simple yet specific gut microbiota of the honey bee from metagenomic analysis

The honey bee, Apis mellifera, harbors a characteristic gut microbiota composed of only a few species which seem to be specific to social bees. The maintenance of this stable and distinct microbial community depends on the social lifestyle of these insects. As in other animals, the bacteria in the gut of honey bees probably govern important functions critical to host health. We recently sequenced a metagenome of the gut microbiota of A. mellifera, assigned gene contents to bins corresponding to the major species present in the honey bee gut, and compared functional gene categories between these species, and between the complete metagenome and those of other animals. Gene contents could be linked to different symbiotic functions with the host. Further, we found a high degree of genetic diversity within each of these species. In the case of the gammaproteobacterial species Gilliamella apicola, we experimentally showed a link between genetic variation of isolates and functional differences suggesting that niche partitioning within this species has emerged during evolution with its bee hosts. The consistent presence of only a few species, combined with strain variation within each of these species, makes the gut microbiota of social bees an ideal model for studying functional, structural, and evolutionary aspects of host-associated microbial communities: many characteristics resemble the gut microbiota of humans and other mammals, but the complexity is considerably reduced. In this addendum, we summarize and discuss our major findings and provide a detailed perspective on future research.

Efficacy of Apilife Var® and Thymovar® against Varroa destructor as an autumn treatment in a cool climate

SummaryThe relationship between Varroa destructor and its European honey bee host, Apis mellifera is such that colony mortality is inevitable if mite populations are not controlled. Although in the past, synthetic pyrethroids and organophosphates were used effectively, mite resistance has developed, and today beekeepers are relying more on thymol-based products such as Apilife Var® and Thymovar®. These products, although toxic to the mite are known to show high variability in efficacy under field conditions, as ambient temperatures > 15°C are essential for the evaporation of the active ingredient thymol. To assess their efficacy as an autumn treatment, in cool temperate climates such as Ireland, a field trial was carried out in 2009. A total of 26 colonies were standardised for brood area, and treatments were administered on 21 August following the manufacturers' instructions. To estimate the percentage efficacy of the test treatments, oxalic acid was administered as a winter treatment on 11 December. Thymovar®, with a mean percentage efficacy of 84.7% was more effective than Apilife Var® (53.8%). Natural mite mortality during the test period was approximately 17.8%. An apparent negative impact of the treatments on colony development is discussed with special reference to bee population, brood area and bee mortality.

Host range evolution in a selected group of osmiine bees (Hymenoptera: Megachilidae): the Boraginaceae-Fabaceae paradox

Bees are extraordinarily diverse with respect to host plant choice and adaptation. Recent findings suggest that bee host range might be largely governed by evolutionary constraints related to pollen digestion or flower recognition and handling. In the present study, we applied phylogenetic inference to investigate whether such constraints underlie host plant choice in bees of the Annosmia-Hoplitis group (Megachilidae) and to what extent these bees have evolved specialized adaptations for pollen collection. We demonstrate that most pollen specialist species exclusively exploit either Boraginaceae or Fabaceae, whereas all pollen generalists harvest pollen from both Boraginaceae and Fabaceae. The counterintuitive affinity towards these two plant families, which are neither closely related nor share similar flower morphologies, demonstrates that pollen host choice is considerably constrained in this group of bees. We hypothesize that this Boraginaceae-Fabaceae paradox might be the result of (1) similar secondary metabolites in the pollen of both families; (2) metabolites that can be detoxified by the same physiological tools; or (3) similar pollen nutrient composition. Contrary to the widely held belief that specialized adaptations for pollen collection are rare among bees, such adaptations are common in the Annosmia-Hoplitis bees, where they have evolved several times independently to exploit flowers of widely different morphologies.

Host Location by Ichneumonid Parasitoids is Associated with Nest Dimensions of the Host Bee Species

Parasitoid fitness depends on the ability of females to locate a host. In some species of Ichneumonoidea, female parasitoids detect potential hosts through vibratory cues emanating from them or through vibrational sounding produced by antennal tapping on the substrate. In this study, we (1) describe host location behaviors in Grotea gayi Spinola (Hymenoptera: Ichneumonidae) and Labena sp. on nests of Manuelia postica Spinola (Hymenoptera: Apidae), (2) compare nest dimensions between parasitized and unparasitized nests, (3) correlate the length of M. postica nests with the number of immature individuals developing, and (4) establish the relative proportion of parasitized nests along the breeding period of M. postica. Based on our results, we propose that these parasitoids use vibrational sounding as a host location mechanism and that they are able to assess host nest dimensions and choose those which may provide them with a higher fitness. Finally, we discuss an ancestral host-parasitoid relationship between Manuelia and ichneumonid species.

Host adaptations reduce the reproductive success ofVarroa destructorin two distinct European honey bee populations

Honey bee societies (Apis mellifera), the ectoparasitic mite Varroa destructor, and honey bee viruses that are vectored by the mite, form a complex system of host–parasite interactions. Coevolution by natural selection in this system has been hindered for European honey bee hosts since apicultural practices remove the mite and consequently the selective pressures required for such a process. An increasing mite population means increasing transmission opportunities for viruses that can quickly develop into severe infections, killing a bee colony. Remarkably, a few subpopulations in Europe have survived mite infestation for extended periods of over 10 years without management by beekeepers and offer the possibility to study their natural host–parasite coevolution. Our study shows that two of these “natural” honey bee populations, in Avignon, France and Gotland, Sweden, have in fact evolved resistant traits that reduce the fitness of the mite (measured as the reproductive success), thereby reducing the parasitic load within the colony to evade the development of overt viral infections. Mite reproductive success was reduced by about 30% in both populations. Detailed examinations of mite reproductive parameters suggest these geographically and genetically distinct populations favor different mechanisms of resistance, even though they have experienced similar selection pressures of mite infestation. Compared to unrelated control colonies in the same location, mites in the Avignon population had high levels of infertility while in Gotland there was a higher proportions of mites that delayed initiation of egg-laying. Possible explanations for the observed rapid coevolution are discussed.

Characterization of the active microbiotas associated with honey bees reveals healthier and broader communities when colonies are genetically diverse.

Recent losses of honey bee colonies have led to increased interest in the microbial communities that are associated with these important pollinators. A critical function that bacteria perform for their honey bee hosts, but one that is poorly understood, is the transformation of worker-collected pollen into bee bread, a nutritious food product that can be stored for long periods in colonies. We used 16S rRNA pyrosequencing to comprehensively characterize in genetically diverse and genetically uniform colonies the active bacterial communities that are found on honey bees, in their digestive tracts, and in bee bread. This method provided insights that have not been revealed by past studies into the content and benefits of honey bee-associated microbial communities. Colony microbiotas differed substantially between sampling environments and were dominated by several anaerobic bacterial genera never before associated with honey bees, but renowned for their use by humans to ferment food. Colonies with genetically diverse populations of workers, a result of the highly promiscuous mating behavior of queens, benefited from greater microbial diversity, reduced pathogen loads, and increased abundance of putatively helpful bacteria, particularly species from the potentially probiotic genus Bifidobacterium. Across all colonies, Bifidobacterium activity was negatively correlated with the activity of genera that include pathogenic microbes; this relationship suggests a possible target for understanding whether microbes provide protective benefits to honey bees. Within-colony diversity shapes microbiotas associated with honey bees in ways that may have important repercussions for colony function and health. Our findings illuminate the importance of honey bee-bacteria symbioses and examine their intersection with nutrition, pathogen load, and genetic diversity, factors that are considered key to understanding honey bee decline.