Does parasitic infection impair the ability of bumblebees to learn flower-handling techniques?

Diversity of Nosema associated with bumblebees (Bombus spp.) from China

Bumblebees (Bombus spp.) are important pollinators of many economically important crops and microsporidia are among the most important infections of these hosts. Using molecular markers, we screened a large sample (n=1,009 bees) of workers of 27 different Bombus spp. from China (Sichuan, Qinghai, Inner Mongolia, and Gansu provinces). The results showed that 62 individuals representing 12 Bombus spp. were infected by microsporidia with an overall prevalence of 6.1%. Based on the haplotypes (ssrRNA sequences), we confirmed the presence of Nosema bombi, Nosema ceranae and (likely) Nosema thomsoni. In addition, four new putatively novel taxa were identified by phylogenetic reconstruction: Nosema A, Nosema B-complex, Nosema C-complex and Nosema D-complex. In many cases, hosts were infected by more than one Nosema taxon. Possible caveats of sequence analyses are discussed.

Bees brought to their knees: microbes affecting honey bee health

The biology and health of the honey bee Apis mellifera has been of interest to human societies for centuries. Research on honey bee health is surging, in part due to new tools and the arrival of colony-collapse disorder (CCD), an unsolved decline in bees from parts of the United States, Europe, and Asia. Although a clear understanding of what causes CCD has yet to emerge, these efforts have led to new microbial discoveries and avenues to improve our understanding of bees and the challenges they face. Here we review the known honey bee microbes and highlight areas of both active and lagging research. Detailed studies of honey bee-pathogen dynamics will help efforts to keep this important pollinator healthy and will give general insights into both beneficial and harmful microbes confronting insect colonies.

Food-environment mediates the outcome of specific interactions between a bumblebee and its trypanosome parasite

Specific host-parasite interactions, where the outcome of exposure to a parasite depends upon the genotypic identity of both parties, have implications for understanding host-parasite coevolution and patterns of genetic diversity. Thus, grasping the extent to which these interactions are mediated by environmental changes in a spatially and temporally heterogeneous world is vital. In this study, it is shown that the environment can influence specific host-parasite interactions in the well-studied system of the bumblebee Bombus terrestris and its trypanosome parasite Crithidia bombi. Naturally relevant variation in the quality of the food environment formed a three-way interaction with both host and parasite identity in determining the outcome of infection, with regard to the resistance of the host and the transmission of the parasite. The demonstration of such a host-genotype by parasite-genotype by environment interaction (G(H) x G(P) x E) shows the importance of considering environmental variation when investigating host-parasite interactions. Moreover, such interactions may to some extent explain levels of genetic diversity in natural host-parasite systems owing to the fact that they will create selection mosaics when environments are heterogeneous.

Consumption of a nectar alkaloid reduces pathogen load in bumble bees

Diet has a significant effect on pathogen infections in animals and the consumption of secondary metabolites can either enhance or mitigate infection intensity. Secondary metabolites, which are commonly associated with herbivore defense, are also frequently found in floral nectar. One hypothesized function of this so-called toxic nectar is that it has antimicrobial properties, which may benefit insect pollinators by reducing the intensity of pathogen infections. We tested whether gelsemine, a nectar alkaloid of the bee-pollinated plant Gelsemium sempervirens, could reduce pathogen loads in bumble bees infected with the gut protozoan Crithidia bombi. In our first laboratory experiment, artificially infected bees consumed a daily diet of gelsemine post-infection to simulate continuous ingestion of alkaloid-rich nectar. In the second experiment, bees were inoculated with C. bombi cells that were pre-exposed to gelsemine, simulating the direct effects of nectar alkaloids on pathogen cells that are transmitted at flowers. Gelsemine significantly reduced the fecal intensity of C. bombi 7 days after infection when it was consumed continuously by infected bees, whereas direct exposure of the pathogen to gelsemine showed a non-significant trend toward reduced infection. Lighter pathogen loads may relieve bees from the behavioral impairments associated with the infection, thereby improving their foraging efficiency. If the collection of nectar secondary metabolites by pollinators is done as a means of self-medication, pollinators may selectively maintain secondary metabolites in the nectar of plants in natural populations.

Does pathogen spillover from commercially reared bumble bees threaten wild pollinators?

The conservation of insect pollinators is drawing attention because of reported declines in bee species and the 'ecosystem services' they provide. This issue has been brought to a head by recent devastating losses of honey bees throughout North America (so called, 'Colony Collapse Disorder'); yet, we still have little understanding of the cause(s) of bee declines. Wild bumble bees (Bombus spp.) have also suffered serious declines and circumstantial evidence suggests that pathogen 'spillover' from commercially reared bumble bees, which are used extensively to pollinate greenhouse crops, is a possible cause. We constructed a spatially explicit model of pathogen spillover in bumble bees and, using laboratory experiments and the literature, estimated parameter values for the spillover of Crithidia bombi, a destructive pathogen commonly found in commercial Bombus. We also monitored wild bumble bee populations near greenhouses for evidence of pathogen spillover, and compared the fit of our model to patterns of C. bombi infection observed in the field. Our model predicts that, during the first three months of spillover, transmission from commercial hives would infect up to 20% of wild bumble bees within 2 km of the greenhouse. However, a travelling wave of disease is predicted to form suddenly, infecting up to 35-100% of wild Bombus, and spread away from the greenhouse at a rate of 2 km/wk. In the field, although we did not observe a large epizootic wave of infection, the prevalences of C. bombi near greenhouses were consistent with our model. Indeed, we found that spillover has allowed C. bombi to invade several wild bumble bee species near greenhouses. Given the available evidence, it is likely that pathogen spillover from commercial bees is contributing to the ongoing decline of wild Bombus in North America. Improved management of domestic bees, for example by reducing their parasite loads and their overlap with wild congeners, could diminish or even eliminate pathogen spillover.

Decline and conservation of bumble bees

Declines in bumble bee species in the past 60 years are well documented in Europe, where they are driven primarily by habitat loss and declines in floral abundance and diversity resulting from agricultural intensification. Impacts of habitat degradation and fragmentation are likely to be compounded by the social nature of bumble bees and their largely monogamous breeding system, which renders their effective population size low. Hence, populations are susceptible to stochastic extinction events and inbreeding. In North America, catastrophic declines of some bumble bee species since the 1990s are probably attributable to the accidental introduction of a nonnative parasite from Europe, a result of global trade in domesticated bumble bee colonies used for pollination of greenhouse crops. Given the importance of bumble bees as pollinators of crops and wildflowers, steps must be taken to prevent further declines. Suggested measures include tight regulation of commercial bumble bee use and targeted use of environmentally comparable schemes to enhance floristic diversity in agricultural landscapes.

Contact networks and transmission of an intestinal pathogen in bumble bee (Bombus impatiens) colonies

In socially living animals, individuals interact through complex networks of contact that may influence the spread of disease. Whereas traditional epidemiological models typically assume no social structure, network theory suggests that an individual's location in the network determines its risk of infection. Empirical, especially experimental, studies of disease spread on networks are lacking, however, largely due to a shortage of amenable study systems. We used automated video-tracking to quantify networks of physical contact among individuals within colonies of the social bumble bee Bombus impatiens. We explored the effects of network structure on pathogen transmission in naturally and artificially infected hives. We show for the first time that contact network structure determines the spread of a contagious pathogen (Crithidia bombi) in social insect colonies. Differences in rates of infection among colonies resulted largely from differences in network density among hives. Within colonies, a bee's rate of contact with infected nestmates emerged as the only significant predictor of infection risk. The activity of bees, in terms of their movement rates and division of labour (e.g., brood care, nest care, foraging), did not influence risk of infection. Our results suggest that contact networks may have an important influence on the transmission of pathogens in social insects and, possibly, other social animals.

Within-host dynamics of an intestinal pathogen of bumble bees

The success of a pathogen depends not only on its transmission to new hosts, but also on its ability to colonize and persist within its current host. Studies of within-host dynamics have focused on only a few diseases of humans, whereas little is known about the factors that influence pathogen populations as they develop inside non-human hosts. Here, we investigate pathogen dynamics occurring within bumble bees (Bombus impatiens) infected by the gut trypanosome Crithidia bombi. Infection by C. bombi showed several features characteristic of vertebrate diseases, including a rapid initial increase in infection intensity, marked oscillations in parasitaemia, and the stimulation of a systemic immune response in infected bees. Within-host dynamics generated substantial variation in the infectiousness and flower-visiting behaviour of bumble bees. Changes in bee foraging that arise from infection may influence the probability of C. bombi transmission between bees at flowers.

Bumble-bee foragers infected by a gut parasite have an impaired ability to utilize floral information

Parasitic infection can influence a variety of behavioural mechanisms in animals, but little is known about the effects of infection on the cognitive processes underlying ecologically relevant behaviours. Here, we examined whether parasitic infection alters cognitive aspects of foraging in a social insect, the bumble-bee (Bombus impatiens). In controlled experiments, we assessed the ability of foraging bees to discriminate rewarding from non-rewarding flowers on the basis of colour and odour. We found that natural and experimental infection by a protozoan parasite (Crithidia bombi, which lives exclusively within the gut tract), impaired the ability of foragers to learn the colour of rewarding flowers. Parasitic infection can thus disrupt central nervous system pathways that mediate cognitive processes in bumble-bees and as a consequence, can reduce their ability to monitor floral resources and make economic foraging decisions. It is postulated that this infection-induced change to cognitive function in bumble-bees is the result of communication between immune and nervous systems. Parasitized animals, including invertebrates, can therefore show subtle behavioural changes that are nonetheless ecologically significant and reflect complex mechanisms.

No trade-off between learning ability and parasitoid resistance in Drosophila melanogaster

Learning ability and immunity to parasites are linked at the physiological level in several insect species. The aim of this work was to investigate the relationship between learning and immunity at an evolutionary level. We tested whether selection for improved learning ability in Drosophila melanogaster led to changes in parasitoid resistance as a correlated response. Similarly, we assayed whether selection for better parasitoid resistance led to a change in learning ability. There was no significant difference between selected and control lines in either case; the estimated confidence intervals for the differences indicate that a trade-off relationship is unlikely.