Juliet L. Osborne

Bumblebee vulnerability and conservation world-wide

We review evidence from around the world for bumblebee declines and review management to mitigate threats. We find that there is evidence that some bumblebee species are declining in Europe, North America, and Asia. People believe that land-use changes may be having a negative effect through reductions in food plants in many parts of the world, but that other factors such as pathogens may be having a stronger effect for a few species in some regions (especially for Bombus s. str. in North America). Evidence so far is that greater susceptibility to land-use change is associated world-wide with small climatic ranges, range edges, and late-starting colony-development cycles. More evidence is needed on the roles of pollen specialization, nest sites, hibernation sites, and pesticides. It is still too early to assess the success of schemes aimed at improving forage in agricultural and conservation areas. However, schemes aimed at raising public awareness have been very successful. Until proven safe, we recommend that live bumblebees should not be moved across continents or oceans for commercial pollination.ZusammenfassungWir geben eine weltweite Übersicht für Hummeln über den Rückgang der Vorkommenshäufigkeit oder der Verbreitungsgebiete bestimmter Arten, über den Zeitverlauf dieser Änderungen, über Gefährdungsfaktoren und Charakteristika der besonders gefährdungsanfälligen Arten und über die weltweit daher am meisten gefährdeten Arten, sowie über Maßnahmen zur Abschwächung der Gefährdung. Hiernach finden wir Belege, dass einige Hummelarten in Europa (Abb. 1, 2), Nordamerika und Asien auf dem Rückgang sind. Es wird allgemein angenommen, dass die Verminderung des Futterpflanzenangebots durch die Änderungen in der Landnutzung in vielen Teilen der Welt einen negativen Einfluss ausübt, allerdings könnten andere Faktoren wie Pathogene bei einigen Arten in einigen Regionen (insbesondere bei Bombus s. str. in Nordamerika) einen stärkeren und kurzfristiger wirksamen Effekt haben. Die bisherigen Anhaltspunkte weisen darauf hin, dass die weltweit anfälligsten Arten tendentiell in spezialisierten klimatischen Bereichen und nahe an den Grenzen dieser Bereiche vorkommen und spät beginnende aber lange Kolonieentwicklungszyklen haben. Zur Rolle von Pollenspezialisierung, Nistplätzen, Überwinterungsplätzen und Pestiziden werden noch mehr Angaben benötigt. Aus der derzeitig verfügbaren begrenzten Information aus neueren Sammlungen um die Welt erstellen wir eine vorläufige Einschätzung, welche Arten wahrscheinlich am stärksten gefährdet sind (Tab. I) und wo diese vorkommen (Abb. 4). Es ist noch zu früh, um den Erfolg von Maßnahmen zur Verbesserung des Futterangebots in landwirtschaftlichen oder Erhaltungsgebieten zu beurteilen. Allerdings waren Maßnahmen zur Verbesserung der öffentlichen Wahrnehmung sehr erfolgreich.

Bees and the Pollination of Crops and Wild Flowers in the European Community

(1991). Bees and the Pollination of Crops and Wild Flowers in the European Community. Bee World: Vol. 72, No. 2, pp. 47-59.

An interspecific comparison of foraging range and nest density of four bumblebee (Bombus) species

Bumblebees are major pollinators of crops and wildflowers in northern temperate regions. Knowledge of their ecology is vital for the design of effective management and conservation strategies but key aspects remain poorly understood. Here we employed microsatellite markers to estimate and compare foraging range and nest density among four UK species: Bombus terrestris, Bombus pascuorum, Bombus lapidarius, and Bombus pratorum. Workers were sampled along a 1.5‐km linear transect across arable farmland. Eight or nine polymorphic microsatellite markers were then used to identify putative sisters. In accordance with previous studies, minimum estimated maximum foraging range was greatest for B. terrestris (758 m) and least for B. pascuorum (449 m). The estimate for B. lapidarius was similar to B. pascuorum (450 m), while that of B. pratorum was intermediate (674 m). Since the area of forage available to bees increases as the square of foraging range, these differences correspond to a threefold variation in the area used by bumblebee nests of different species. Possible explanations for these differences are discussed. Estimates for nest density at the times of sampling were 29, 68, 117, and 26/km2 for B. terrestris, B. pascuorum, B. lapidarius and B. pratorum, respectively. These data suggest that even among the most common British bumblebee species, significant differences in fundamental aspects of their ecology exist, a finding that should be reflected in management and conservation strategies.

Disease associations between honeybees and bumblebees as a threat to wild pollinators.

Emerging infectious diseases (EIDs) pose a risk to human welfare, both directly and indirectly, by affecting managed livestock and wildlife that provide valuable resources and ecosystem services, such as the pollination of crops. Honeybees (Apis mellifera), the prevailing managed insect crop pollinator, suffer from a range of emerging and exotic high-impact pathogens, and population maintenance requires active management by beekeepers to control them. Wild pollinators such as bumblebees (Bombus spp.) are in global decline, one cause of which may be pathogen spillover from managed pollinators like honeybees or commercial colonies of bumblebees. Here we use a combination of infection experiments and landscape-scale field data to show that honeybee EIDs are indeed widespread infectious agents within the pollinator assemblage. The prevalence of deformed wing virus (DWV) and the exotic parasite Nosema ceranae in honeybees and bumblebees is linked; as honeybees have higher DWV prevalence, and sympatric bumblebees and honeybees are infected by the same DWV strains, Apis is the likely source of at least one major EID in wild pollinators. Lessons learned from vertebrates highlight the need for increased pathogen control in managed bee species to maintain wild pollinators, as declines in native pollinators may be caused by interspecies pathogen transmission originating from managed pollinators.

Ontogeny of orientation flight in the honeybee revealed by harmonic radar.

Cognitive ethology focuses on the study of animals under natural conditions to reveal ecologically adapted modes of learning. But biologists can more easily study what an animal learns than how it learns. For example, honeybees take repeated ‘orientation’ flights before becoming foragers at about three weeks of age. These flights are a prerequisite for successful homing. Little is known about these flights because orienting bees rapidly fly out of the range of human observation. Using harmonic radar, we show for the first time a striking ontogeny to honeybee orientation flights. With increased experience, bees hold trip duration constant but fly faster, so later trips cover a larger area than earlier trips. In addition, each flight is typically restricted to a narrow sector around the hive. Orientation flights provide honeybees with repeated opportunities to view the hive and landscape features from different viewpoints, suggesting that bees learn the local landscape in a progressive fashion. We also show that these changes in orientation flight are related to the number of previous flights taken instead of chronological age, suggesting a learning process adapted to changes in weather conditions, flower availability and the needs of bee colonies.

Flight metabolic rate and Pgi genotype influence butterfly dispersal rate in the field.

Dispersal is a key life-history trait, especially in species inhabiting fragmented landscapes. The process of dispersal is affected by a suite of morphological, physiological, and behavioral traits, all of which have a more or less complex genetic basis and are affected by the prevailing environmental conditions. To be able to identify genetic and phenotypic effects on dispersal, movements have to be recorded over relevant spatial and temporal scales. We used harmonic radar to track free-flying Glanville fritillary butterflies (Melitaea cinxia) released in the field and reconstructed their flight tracks for several hours. Flight track lengths for individual butterflies ranged from tens of meters to several kilometers. Butterflies were most mobile at midday and in intermediate temperatures. Flight metabolic rate (MR), measured prior to the tracking, explained variation in mobility at all scales studied. One-third of the variation in the distance moved in one hour could be attributed to variation in flight MR. Heterozygous individuals at a single nucleotide polymorphism in the phosphoglucose isomerase (Pgi) gene moved longer distances in the morning and at lower ambient temperatures than homozygous individuals. A similar genotype x temperature interaction was found to affect the metabolic rate. Our results establish connections from molecular variation in a single gene to flight physiology and movement behavior at the landscape level. These results indicate a fitness advantage to the heterozygous genotype in low temperatures and suggest a mechanism by which varying environmental conditions maintain genetic polymorphism in populations.

An economic model of the limits to foraging range in central place foragers with numerical solutions for bumblebees

1. A model is described that evaluates the maximum economic foraging range in central place foragers by using optimality criteria to discriminate between foraging sites at different distances from the foragers central place.

Landscape context not patch size determines bumble-bee density on flower mixtures sown for agri-environment schemes

Bumble-bee declines across Europe have been linked to loss of habitat and forage availability due to agricultural intensification. These declines may have severe ecological and commercial consequences since bumble-bees pollinate a range of wildflowers and crops. In England, attempts are being made to reintroduce forage resources through agri-environment schemes, yet there are few data on how the area of forage, or the landscape context in which it is provided, affects their success. We investigated the effects of sown forage patches on bumble-bees across sites varying in landscape characteristics. Bumble-bee densities were higher on sown patches compared with control habitats but did not vary with patch size, i.e. total forager numbers were proportional to patch area. Importantly, the relative response to sown forage patches varied widely across a landscape gradient such that their impact in terms of attracting foraging bumble-bees was greatest where the proportion of arable land was highest.

Estimation of bumblebee queen dispersal distances using sibship reconstruction method

Dispersal ability is a key determinant of the propensity of an organism to cope with habitat fragmentation and climate change. Here we quantify queen dispersal in two common bumblebee species in an arable landscape. Dispersal was measured by taking DNA samples from workers in the spring and summer, and from queens in the following spring, at 14 sites across a landscape. The queens captured in the spring must be full sisters of workers that were foraging in the previous year. A range of sibship reconstruction methods were compared using simulated data sets including or no genotyping errors. The program Colony gave the most accurate reconstruction and was used for our analysis of queen dispersal. Comparison of queen dispersion with worker foraging distances was used to take into account an expected low level of false identification of sister pairs which might otherwise lead to overestimates of dispersal. Our data show that Bombus pascuorum and B. lapidarius queens can disperse by at least 3 and 5 km, respectively. These estimates are consistent with inferences drawn from studies of population structuring in common and rare bumblebee species, and suggest that regular gene flow over several kilometres due to queen dispersal are likely to be sufficient to maintain genetic cohesion of ubiquitous species over large spatial scales whereas rare bumblebee species appear unable to regularly disperse over distances greater than 10 km. Our results have clear implications for conservation strategies for this important pollinator group, particularly when attempting to conserve fragmented populations.

Tracking butterfly movements with harmonic radar reveals an effect of population age on movement distance

We used harmonic radar to track freely flying Glanville fritillary butterfly (Melitaea cinxia) females within an area of 30 ha. Butterflies originated from large and continuous populations in China and Estonia, and from newly established or old (> 5 years) small local populations in a highly fragmented landscape in Finland. Caterpillars were raised under common garden conditions and unmated females were tested soon after eclosion. The reconstructed flight paths for 66 individuals comprised a total distance of 51 km with high spatial resolution. Butterflies originating from large continuous populations and from old local populations in Finland exhibited similar movement behaviors, whereas butterflies originating from newly established local populations in the fragmented landscape in Finland moved significantly more than the others. There was no difference in the lengths of individual flight bouts, but the new-population females flew more frequently, resulting in longer daily movement tracks. The flight activity of all individuals was affected by environmental conditions, peaking at 19–23°C (depending on population type), in the early afternoon, and during calm weather. Butterflies from all population types showed a strong tendency to follow habitat edges between the open study area and the neighboring woodlands.