Test of the invasive pathogen hypothesis of bumble bee decline in North America

Reproductive Interference in an Introduced Bumblebee: Polyandry may Mitigate Negative Reproductive Impact

As a signature of reproductive interference (RI), we reviewed hybrid production in eusocial bumblebees in Japan, by comparing introduced Bombus terrestris with native B. ignitus in Honshu (main island of Japan) and with native B. hypocrita sapporoensis in Hokkaido (northern island of Japan). In this review, we present additional new data showing hybrid production between introduced B. terrestris and native B. ignitus in Honshu. Interspecific mating with introduced B. terrestris disrupts the reproduction of native B. h. sapporoensis and B. ignitus, which belong to the same subgenus of Bombus, through inviable egg production. This interference appears to facilitate species replacement on Hokkaido. Simultaneously, the mating frequencies for queens of B. terrestris have increased, suggesting that polyandry might evolve in response to the extent of RI between B. terrestris and B. h. sapporoensis. To suppress the population size of B. terrestris in Hokkaido, two methods have been proposed: the mass release of B. h. sapporoensis males to induce RI between the two species and the spraying of insecticides against foraging workers so that the workers will carry the insecticides back to their colonies, killing the immature bees within the colonies. A candidate insecticide type is insect growth regulator, which may disrupt larval development without any apparent effect on foraging workers.

Pathogen shifts in a honeybee predator following the arrival of the Varroa mite

Emerging infectious diseases (EIDs) are a global threat to honeybees, and spillover from managed bees threaten wider insect populations. Deformed wing virus (DWV), a widespread virus that has become emergent in conjunction with the spread of the mite Varroa destructor, is thought to be partly responsible for global colony losses. The arrival of Varroa in honeybee populations causes a dramatic loss of viral genotypic diversity, favouring a few virulent strains. Here, we investigate DWV spillover in an invasive Hawaiian population of the wasp, Vespula pensylvanica, a honeybee predator and honey-raider. We show that Vespula underwent a parallel loss in DWV variant diversity upon the arrival of Varroa, despite the mite being a honeybee specialist. The observed shift in Vespula DWV and the variant-sharing between Vespula and Apis suggest that these wasps can acquire DWV directly or indirectly from honeybees. Apis prey items collected from Vespula foragers were positive for DWV, indicating predation is a possible route of transmission. We also sought cascading effects of DWV shifts in a broader Vespula pathogen community. We identified concurrent changes in a suite of additional pathogens, as well as shifts in the associations between these pathogens in Vespula. These findings reveal how hidden effects of the Varroa mite can, via spillover, transform the composition of pathogens in interacting species, with potential knock-on effects for entire pathogen communities.

Invasion Success and Management Strategies for Social Vespula Wasps

Three species of Vespula have become invasive in Australia, Hawai'i, New Zealand, and North and South America and continue to spread. These social wasp species can achieve high nest densities, and their behavioral plasticity has led to substantial impacts on recipient communities. Ecologically, they affect all trophic levels, restructuring communities and altering resource flows. Economically, their main negative effect is associated with pollination and the apicultural industry. Climate change is likely to exacerbate their impacts in many regions. Introduced Vespula spp. likely experience some degree of enemy release from predators or parasites, although they are exposed to a wide range of microbial pathogens in both their native and introduced range. Toxic baits have been significantly improved over the last decade, enabling effective landscape-level control. Although investigated extensively, no effective biological control agents have yet been found. Emerging technologies such as gene drives are under consideration.

Phylogeny and population genetic analyses reveals cryptic speciation in the Bombus fervidus species complex (Hymenoptera: Apidae)

Bumble bees (Bombus Latrielle) are significant pollinators of flowering plants due to their large body size, abundant setae, and generalist foraging strategies. However, shared setal coloration patterns among closely and distantly related bumble bee species makes identification notoriously difficult. The advent of molecular genetic techniques has increased our understanding of bumble bee evolution and taxonomy, and enables effective conservation policy and management. Individuals belonging to the North American Bombus fervidus species-complex (SC) are homogenous in body structure but exhibit significant body color phenotype variation across their geographic distribution. Given the uncertainty of the genealogical boundaries within the SC, some authors have synonymized all members of the B. fervidus SC within a single taxon, while others propose an alternative two taxa hypothesis. Operating under the phylogenetic species concept, our analysis supports the hypothesis that there are two independent lineages of bumble bees within the B. fervidus SC. With the current evidence, however, it is not possible to assign valid names to either of them, because both lineages include the color phenotypes found in the original species descriptions of B. fervidus and B. californicus. Cryptic speciation does not seem to be the product of Müllerian mimicry between the clades, because diverging coloration patterns are observed when the distribution of the clades overlaps. Furthermore, within each lineage there is evidence for strong population differentiation that is correlated with geographic distribution rather than color phenotype. In our study, we demonstrate the importance of obtaining a broad sample of multiple populations when conducting lower-level phylogenetic analyses. In addition to improving our knowledge of bumble bee diversification patterns, characterizing the evolutionary history of these pollinators provides the foundation needed to guide contemporary conservation assessments and management strategies.

Landscape predictors of pathogen prevalence and range contractions in US bumblebees

Several species of bumblebees have recently experienced range contractions and possible extinctions. While threats to bees are numerous, few analyses have attempted to understand the relative importance of multiple stressors. Such analyses are critical for prioritizing conservation strategies. Here, we describe a landscape analysis of factors predicted to cause bumblebee declines in the USA. We quantified 24 habitat, land-use and pesticide usage variables across 284 sampling locations, assessing which variables predicted pathogen prevalence and range contractions via machine learning model selection techniques. We found that greater usage of the fungicide chlorothalonil was the best predictor of pathogen (Nosema bombi) prevalence in four declining species of bumblebees. Nosema bombi has previously been found in greater prevalence in some declining US bumblebee species compared to stable species. Greater usage of total fungicides was the strongest predictor of range contractions in declining species, with bumblebees in the northern USA experiencing greater likelihood of loss from previously occupied areas. These results extend several recent laboratory and semi-field studies that have found surprising links between fungicide exposure and bee health. Specifically, our data suggest landscape-scale connections between fungicide usage, pathogen prevalence and declines of threatened and endangered bumblebees.

Immune-cognitive system connectivity reduces bumblebee foraging success in complex multisensory floral environments

Bumblebees are declining at alarming rate worldwide, posing a significant threat to the function and diversity of temperate ecosystems. These declines have been attributed, in part, to the direct effect of specific pathogens on bumblebee survival. However, pathogens may also have a negative impact on host populations indirectly through immune-induced cognitive deficits in infected individuals. To gain greater insight into mechanisms and potential conservation implications of such 'immune-brain crosstalk' in bumblebees, we non-pathogenetically activated humoral and cellular immune pathways in individuals and then tested for long-term reductions in cognitive performance and foraging proficiency. We show that chronic activation of humoral, but not a cellular, immune pathways and effectors in foragers significantly reduces their ability to flexibly and efficiently harvest resources in multi-sensory floral environments for at least 7 days post-treatment. Humoral defense responses thus have the potential to confer significant foraging costs to bumblebee foragers over timeframes that would negatively impact colony growth and reproductive output under natural conditions. Our findings indicate that fitness effects of immune-brain crosstalk should be considered before attributing wild bumblebee decline to a particular pathogen species.

Bumblebee olfactory learning affected by task allocation but not by a trypanosome parasite

Parasites can induce behavioural changes in their host organisms. Several parasite species are known to infect bumblebees, an important group of pollinators. Task allocation within bumblebee colonies can also cause differences in behaviour. Thus, task allocation may lead to context-dependent impacts of parasites on host behaviour. This study uses Bombus terrestris and its gut trypanosome Crithidia bombi, to investigate the effects of parasitism, task allocation (foraging or nest-work) and their interactions, on olfactory learning. Prior to undergoing the olfactory learning task, bees were orally infected with a field-realistic dose of C. bombi, and observed to determine task allocation. Parasitism did not significantly affect olfactory learning, but task allocation did, with foragers being significantly more likely to learn than nest bees. There was no significant interaction between parasitism and task. These results suggest that C. bombi is unlikely to affect pollination services via changes in olfactory learning of its host if bees are under no environmental or nutritional stress. However, wild and commercial colonies are likely to face such stressors. Future studies in the field are needed to extrapolate our results to real world effects.

Do managed bees have negative effects on wild bees?: A systematic review of the literature

Managed bees are critical for crop pollination worldwide. As the demand for pollinator-dependent crops increases, so does the use of managed bees. Concern has arisen that managed bees may have unintended negative impacts on native wild bees, which are important pollinators in both agricultural and natural ecosystems. The goal of this study was to synthesize the literature documenting the effects of managed honey bees and bumble bees on wild bees in three areas: (1) competition for floral and nesting resources, (2) indirect effects via changes in plant communities, including the spread of exotic plants and decline of native plants, and (3) transmission of pathogens. The majority of reviewed studies reported negative effects of managed bees, but trends differed across topical areas. Of studies examining competition, results were highly variable with 53% reporting negative effects on wild bees, while 28% reported no effects and 19% reported mixed effects (varying with the bee species or variables examined). Equal numbers of studies examining plant communities reported positive (36%) and negative (36%) effects, with the remainder reporting no or mixed effects. Finally, the majority of studies on pathogen transmission (70%) reported potential negative effects of managed bees on wild bees. However, most studies across all topical areas documented the potential for impact (e.g. reporting the occurrence of competition or pathogens), but did not measure direct effects on wild bee fitness, abundance, or diversity. Furthermore, we found that results varied depending on whether managed bees were in their native or non-native range; managed bees within their native range had lesser competitive effects, but potentially greater effects on wild bees via pathogen transmission. We conclude that while this field has expanded considerably in recent decades, additional research measuring direct, long-term, and population-level effects of managed bees is needed to understand their potential impact on wild bees.

Larvae act as a transient transmission hub for the prevalent bumblebee parasite Crithidia bombi

Disease transmission networks are key for understanding parasite epidemiology. Within the social insects, structured contact networks have been suggested to limit the spread of diseases to vulnerable members of their society, such as the queen or brood. However, even these complex social structures do not provide complete protection, as some diseases, which are transmitted by workers during brood care, can still infect the brood. Given the high rate of feeding interactions that occur in a social insect colony, larvae may act as disease transmission hubs. Here we use the bumblebee Bombus terrestris and its parasite Crithidia bombi to determine the role of brood in bumblebee disease transmission networks. Larvae that were artificially inoculated with C. bombi showed no signs of infection seven days after inoculation. However, larvae that received either an artificial inoculation or a contaminated feed from brood-caring workers were able to transmit the parasite to naive workers. These results suggest that the developing brood is a potential route of intracolonial disease transmission and should be included when considering social insect disease transmission networks.

Bumble bee ecophysiology: integrating the changing environment and the organism

Bumble bees are among the most ecologically and economically important pollinators worldwide, yet many of their populations are being threatened by a suite of interrelated, human-mediated environmental changes. Here, I discuss recent progress in our understanding of bumble bee ecophysiology, including advances related to thermal biology in light of global warming; nutritional biology in the context of declining food resources; and the capacity for bumble bees to exhibit physiological plasticity or adaptations to novel or extreme environments, with reference to their evolutionary history and current biogeography.

Microsporidia: An Emerging Threat to Bumblebees?

Microsporidia may cause emerging infectious diseases (EIDs) in bumblebees. Two drivers - commercial bumblebees and managed honey bees - have been identified as possible sources of pathogen spillover. In addition, declines in bumblebee populations may have led to lower genetic diversity and subsequent higher susceptibility to infection, enabling microsporidia to increase in prevalence. There is strong evidence for relatively recent increases in the prevalence of Nosema bombi in North America. However, the lack of definitive data on spillover by microsporidia, in North America or elsewhere, makes it difficult to identify the causes of such increases. Phylogenomic studies are urgently needed to identify the global population structure of microsporidia in bumblebees, and thus identify the source of current and future epidemics.

Long-term prevalence of the protists Crithidia bombi and Apicystis bombi and detection of the microsporidium Nosema bombi in invasive bumble bees

An initial survey in 2009 carried out at a site in northwestern Patagonia region, Argentina, revealed for the first time in South America the presence of the flagellate Crithidia bombi and the neogregarine Apicystis bombi, two pathogens associated with the Palaearctic invasive bumble bee Bombus terrestris. In order to determine the long-term persistence and dynamics of this microparasite complex, four additional collections at the same site (San Carlos de Bariloche) were conducted along the following seven years. Both protists were detected in all collections: prevalence was 2%-21.6% for C. bombi and 1.2%-14% for A. bombi. In addition, the microsporidium Nosema bombi was recorded for the first time in the country in the last two collections, at prevalences of 12.4% and 2.4% and unusually high infection intensities (Average = 6.56 × 10⁷ spores per individual). Due to the exceptional dispersal ability of the exotic B. terrestris, these three multihost pathogens should be considered as potential threats to South American native bumble bees.

Absence of Leishmaniinae and Nosematidae in stingless bees

Bee pollination is an indispensable component of global food production and plays a crucial role in sustainable agriculture. The worldwide decline of bee populations, including wild pollinators, poses a threat to this system. However, most studies to date are situated in temperate regions where Apini and Bombini are very abundant pollinators. Tropical and subtropical regions where stingless bees (Apidae: Meliponini) are generally very common, are often overlooked. These bees also face pressure due to deforestation and agricultural intensification as well as the growing use and spread of exotic pollinators as Apis mellifera and Bombus species. The loss or decline of this important bee tribe would have a large impact on their provided ecosystem services, in both wild and agricultural landscapes. The importance of pollinator diseases, which can contribute to decline, has not been investigated so far in this bee tribe. Here we report on the first large pathogen screening of Meliponini species in southern Brazil. Remarkably we observed that there was an absence of Leishmaniinae and Nosematidae, and a very low occurrence of Apicystis bombi. Our data on disease prevalence in both understudied areas and species, can greatly improve our knowledge on the distribution of pathogens among bee species.