Terrestris Workers Research Articles

Bumblebee foraging rhythms under the midnight sun measured with radiofrequency identification.

BackgroundIn the permanent daylight conditions north of the Arctic circle, there is a unique opportunity for bumblebee foragers to maximise intake, and therefore colony growth, by remaining active during the entire available 24-h period. We tested the foraging rhythms of bumblebee (Bombus terrestris and B. pascuorum) colonies in northern Finland during the summer, when the sun stays above the horizon for weeks. We used fully automatic radio-frequency identification to monitor the foraging activity of more than 1,000 workers and analysed their circadian foraging rhythms.ResultsForagers did not use the available 24-h foraging period but exhibited robust diurnal rhythms instead. A mean of 95.2% of the tested B. terrestris workers showed robust diurnal rhythms with a mean period of 23.8 h. Foraging activity took place mainly between 08:00 and 23:00, with only low or almost no activity during the rest of the day. Activity levels increased steadily during the morning, reached a maximum around midday and decreased again during late afternoon and early evening. Foraging patterns of native B. pascuorum followed the same temporal organisation, with the foraging activity being restricted to the period between 06:00 and 22:00.ConclusionsThe results of the present study indicate that the circadian clock of the foragers must have been entrained by some external cue, the most prominent being daily cycles in light intensity and temperature. Daily fluctuations in the spectral composition of light, especially in the UV range, could also be responsible for synchronising the circadian clock of the foragers under continuous daylight conditions.

The appeasement effect of sterility signaling in dominance contests among Bombus terrestris workers

The establishment of dominance hierarchies through aggressive interactions is very common in insect societies. In many cases, it is also mediated through pheromone emissions that enable individuals to evaluate the reproductive quality and level of aggressiveness of the dominant individual, thereby reducing the number and intensity of costly fights. Here, we studied these processes in the primitively eusocial bee Bombus terrestris, using a paired bee system. Specifically, we investigated the behavioral, reproductive, and pheromonal correlates of dominance establishment. Workers were shown to establish dominance hierarchies using overt aggression within 3–4 days. Thereafter, the aggression drastically decreased, and dominance was maintained mostly by ritualized agonistic behavior. The behaviorally dominant bee lost the ester compounds that workers produce in their Dufour's gland (the so-called “sterility signal”) concomitantly with the development of her ovaries. The other bee announced as subordinate by continuously producing high amounts of those esters. The hypothesis that sterility signaling serves as an appeasement signal to pacify the dominant bees is supported by the negative correlation found between the proportion of these esters and the level of aggression that the subordinate received from the dominant worker. Physical interactions, and presumably also the ensuing overt aggression between the bees, were essential for the above pheromonal change to take place and enabled the dominant workers to develop their ovaries and to lay eggs. The subordinate bee’s signaling of non-reproductive status may minimize energy expenditure in costly fights and help stabilize the reproductive division of labor among workers.

Infection with the trypanosome Crithidia bombi and expression of immune-related genes in the bumblebee Bombus terrestris

Social bees and other insects are frequently parasitized by a large range of different microorganisms. Among these is Crithidia bombi (Kinetoplastida: Trypanosomatidae), a common gut parasite of bumblebees, Bombus spp. (Insecta: Apidae). Bumblebees are important pollinators in commercial and natural environments. There are clear detrimental effects of C. bombi infections on the fitness of bumblebees. However, little has been known about how the bee's immune system responds to infections with trypanosome parasites. Here, we study the immune response of Bombus terrestris on infection by C. bombi. We measured the expression of four immune-related genes ( Hemomucin, MyD88, Relish, and TEP7) using RT-qPCR in adult B. terrestris workers that were either healthy or infected with the trypanosome parasite C. bombi. The potential recognition gene Hemomucin was significantly upregulated in the infected bees. Further, there was substantial and significant variation in all four genes among different bumblebee colonies irrespective of infection status.

Analysis of a normalised expressed sequence tag (EST) library from a key pollinator, the bumblebee Bombus terrestris

BackgroundThe bumblebee, Bombus terrestris (Order Hymenoptera), is of widespread importance. This species is extensively used for commercial pollination in Europe, and along with other Bombus spp. is a key member of natural pollinator assemblages. Furthermore, the species is studied in a wide variety of biological fields. The objective of this project was to create a B. terrestris EST resource that will prove to be valuable in obtaining a deeper understanding of this significant social insect.ResultsA normalised cDNA library was constructed from the thorax and abdomen of B. terrestris workers in order to enhance the discovery of rare genes. A total of 29'428 ESTs were sequenced. Subsequent clustering resulted in 13'333 unique sequences. Of these, 58.8 percent had significant similarities to known proteins, with 54.5 percent having a "best-hit" to existing Hymenoptera sequences. Comparisons with the honeybee and other insects allowed the identification of potential candidates for gene loss, pseudogene evolution, and possible incomplete annotation in the honeybee genome. Further, given the focus of much basic research and the perceived threat of disease to natural and commercial populations, the immune system of bumblebees is a particularly relevant component. Although the library is derived from unchallenged bees, we still uncover transcription of a number of immune genes spanning the principally described insect immune pathways. Additionally, the EST library provides a resource for the discovery of genetic markers that can be used in population level studies. Indeed, initial screens identified 589 simple sequence repeats and 854 potential single nucleotide polymorphisms.ConclusionThe resource that these B. terrestris ESTs represent is valuable for ongoing work. The ESTs provide direct evidence of transcriptionally active regions, but they will also facilitate further functional genomics, gene discovery and future genome annotation. These are important aspects in obtaining a greater understanding of this key pollinator species.

Is Bombus terrestris (Hymenoptera: Apidae) a stronger competitor against native Japanese species? A comparison of foraging efficiency

The introduced Bombus terrestris has recently become naturalized in Japan and it may be responsible for the observed decline of native bumblebee populations. We compared the foraging ability of B. terrestris and a Japanese native bumblebee, B. ignitus, in an experimental cage. The 6-day experiment showed no significant difference in mean foraging load between them even though B. terrestris was significantly smaller than B. ignitus. However, B. terrestris was significantly more efficient, with a higher mean foraging load per unit time, and also brought back more forage per unit body mass than B. ignitus. The proboscis length of B. terrestris workers was more suited to the flower size of Salvia farinacea and the species may thus be more efficient in foraging. Once it has invaded a new region, B. terrestris can become the dominant bumblebee. When resources are low, B. terrestris, with a large number of superior foragers, can disadvantage native bees through resource depletion.

Hydrocarbon Footprints as a Record of Bumblebee Flower Visitation

Bumblebees leave traces of cuticular hydrocarbons on flowers they visit, with the amount deposited being positively related to the number of visits. We asked whether such footprint hydrocarbons are retained on flowers for sufficiently long periods of time so as to reflect bee visitation in pollination studies. In laboratory experiments, flower corollae (Primula veris, Digitalis grandiflora) visited by Bombus terrestris workers retained bee-derived nonacosenes (C(29)H(58)) in near-unchanged quantities for 24 hours, both at 15 and 25 degrees C. Additionally, synthetic (Z)-9-tricosene applied to flower corollae of the deadnettle Lamium maculatum was retained for 48 hours in an unchanged quantity. In a field survey, the amount of footprint alkenes on flowers of comfrey (Symphytum officinale) plants was positively correlated with the number of bumblebee visits that those plants had received during the day. Together, these data suggest that flowers retain a long-term quantitative record of bumblebee visitation. The analysis of petal extracts by gas chromatography could provide a cheap and reliable way of quantifying bumblebee visits in landscape scale studies of pollination.

Foraging distance inBombus terrestrisL. (Hymenoptera: Apidae)

A major determinant of bumblebees pollination efficiency is the distance of pollen dispersal, which depends on the foraging distance of workers. We employ a transect setting, controlling for both forage and nest location, to assess the foraging distance of Bombus terrestris workers and the influence of environmental factors on foraging frequency over distance. The mean foraging distance of B. terrestris workers was 267.2 m ± 180.3 m (max. 800 m). Nearly 40% of the workers foraged within 100 m around the nest. B. terrestris workers have thus rather moderate foraging ranges if rewarding forage is available within vicinity of the nests. We found the spatial distribution and the quality of forage plots to be the major determinants for the bees foraging decision-making, explaining over 80% of the foraging frequency. This low foraging range has implications for using B. terrestris colonies as pollinators in agriculture.

Nutritive value of 15 single pollens and pollen mixes tested on larvae produced by bumblebee workers (Bombus terrestris, Hymenoptera: Apidae)

Laboratory trials were performed at 27 °C on micro-colonies comprised of three Bombus terrestris workers. They were fed with syrup and pollen paste ad libitum. The micro-colonies were terminated on the 14th day after egg-laying. Two bioassays tested the nutritive value of 6 pure pollens and 9 commercial pollen mixes on bumblebee larvae. Among 10 criteria tested, the most sensitive to the different pollens were the mean weight of larvae and the rate of discarded larvae. Differentiation between treatments was easier with single pollens that ranged from 14.4% to 24.9% crude protein for Helianthus and Castanea respectively, than with mixes that ranged from 12.9% to 17.6%. The best performance ranking was assigned to Castanea, Papaver and Rubus, which produced larvae weighing 110–150 mg, the lowest to Helianthus and Cistus which resulted in larvae of 20–50 mg. The largest larvae (240 mg) were produced with the mix at 96% Brassica and 15.9% protein and the smallest with the mix at the lowest protein content (12.9%) assembling Picris, Hedera, Amaranthus, Solanum, Helianthus and graminaceae.

Impact of insecticidal proteins expressed in transgenic plants on bumblebee microcolonies

AbstractIn order to assess the risk that insecticidal transgenic plants may pose for bumblebees, we tested whether Bombus terrestris (L.) (Hymenoptera: Apidae) workers are able to detect insecticidal proteins dissolved in sucrose solution and whether consumption of these proteins will affect survival and offspring production. Feeders containing either Bacillus thuringiensis toxin (Cry1Ab), Kunitz soybean trypsin inhibitor (SBTI), or Galanthus nivalis agglutinin (GNA) were offered to bumblebee colonies at low (0.01% wt/vol for SBTI and GNA, 0.001% for Cry1Ab) and high concentrations (0.1% for SBTI and GNA, 0.01% for Cry1Ab) together with a control (pure sucrose solution) in a glasshouse chamber. No difference was found in the number of visits and the duration of visits among the different concentrations for each of the insecticidal proteins, indicating that bumblebees do not discriminate the compounds. To investigate the impact of the different insecticidal proteins on B. terrestris, microcolonies were established by placing three newly emerged bumblebee workers in wooden boxes. Within a few days, a hierarchy in each microcolony was established and the dominant worker developed its ovaries and laid haploid eggs. Bumblebees were fed with Cry1Ab (0.01%), SBTI, or GNA (both at 0.01 and 0.1%) dissolved in sucrose solution and also fed mixed floral pollen for a maximum period of 80 days. Additionally, microcolonies with three drones each were established to measure individual bee longevity. While the Cry1Ab did not affect microcolony performance, the consumption of SBTI and especially GNA affected survival of B. terrestris workers and drones and caused a significant reduction in the number of offspring. The use of microcolonies appears to be well‐suited to measure lethal and sublethal effects of insecticidal proteins expressed in transgenic plants on bumblebees.

Effects of different stimulation methods on colony initiation and development of Bombus terrestris L. (Hymenoptera: Apidae) queens

The effects of different stimulation methods on colony development of Bombus terrestris queens were examined to increase rearing efficiency and reduce production costs. A total of 350 hibernated queens obtained from a commercial company were used. Each queen was placed separately in the starting box and four starting methods were tested: only the queen (q), one queen stimulated with one B. terrestris worker (qtw), one honeybee worker (qhw), and one B. terrestris pupa (qtp). There were significant differences among the treatments in terms of egg laying, colony production and saleable production ratio, the colony initiation time, the time of the emergence of the first worker, the number of egg cells in the first brood. Starting methods had no significant effect on the other colony characteristics. The highest egg laying ratio, colony production ratio and saleable colony production ratio were found in the qtw group (82.8, 53.3, 39.0%) followed by qtp (72.8, 42.8, 28.5%), qhw (57.1, 28.6, 16.2%) and q (35.7, 17.1, 14.3%) groups, respectively. The queens with qtw treatment developed more egg cells (4.39±0.21) in the first brood and started egg laying (11.75±0.83 days) earlier than in the other treatments. The results showed that adding a B. terrestris worker is the best starting method for the mass rearing of B. terrestris. Qtp treatment was found to be the second most successful starting method.

Plasticity of worker reproductive strategies in Bombus terrestris: lessons from artificial mixed-species colonies

We used the experimental paradigm of artificial mixed colonies of two phylogenetically related bumblebee species to analyse the dynamics of the reproductive skew in societies of Bombus terrestris. Artificial mixed-species colonies were set up by introducing callow B. terrestris workers either into a queenright (QR) or a queenless (QL) colony of B. lapidarius. The introduced B. terrestris workers were well integrated into their host B. lapidarius colony and displayed nesting activities that did not differ from those of the resident B. lapidarius workers. However, the introduced B. terrestris workers did show a different reproductive behaviour. While B. lapidarius workers did not develop ovaries in a B. lapidarius QR colony but did so in a B. lapidarius QL group, adopted B. terrestris workers in a B. lapidarius QR colony developed ovaries as if they were under QL conditions. These results indicate that, in mixed-species colonies, B. terrestris workers are irresponsive to the queen's inhibitive action on ovary development. In QL homospecific and heterospecific predominately B. terrestris mixed-worker colonies (1Bl + 5Bt), reproduction was dominated by a single B. terrestris worker, whereas in QR B. lapidarius or QL equally mixed-worker colonies (3Bl + 3Bt), almost all B. terrestris workers developed ovaries. We suggest that in the presence of enough heterospecific workers, B. terrestris workers behave as parasites. This last finding suggests that worker reproduction in B. terrestris is highly plastic and that the experimental paradigm of artificial mixed colonies may provide new insights into the evolution of social parasitism in this taxon.

Bumblebee inquilinism in Bombus�(Fernaldaepsithyrus) sylvestris (Hymenoptera, Apidae): behavioural and chemical analyses of host-parasite interactions

The bumblebee Bombus sylvestris is an obligate social inquiline of B. pratorum and nest- invading females are known to integrate themselves into host colonies, usually without any aggression. We investigated whether cuticular compounds could be involved in discrimination processes in common bumblebees, and whether they play a role in the social integration of inquilines in host nests. We tested nestmate recognition in B. terrestris workers, and analysed cuticular chemical profiles of hairs taken from allocolonial individuals. Bumblebees belonging to the same colony shared a common odour. Then, we investigated how B. sylvestris females integrated into colonies of natural and non-natural hosts, B. pratorum and B. terrestris, respectively. Inquiline females apparently succeeded in entering a host colony by expressing non-aggressive behaviours and had no chemical signature; subsequently, acquisition of a chemical signature similar to the host colony might facilitate their integration into the host nest. Bombus sylvestris / inquilinism / discrimination process / chemical signature / non-aggressive strategy

Nesting in Bombus terrestris (Linnaeus) and foraging by B. terrestris workers from a B. hypocrita sapporensis (Cockerell) nest in Mukawa-cho, Yufutsu-gun, Hokkaido, Japan

Nesting in Bombus terrestris (Linnaeus) and foraging by B. terrestris workers from a B. hypocrita sapporensis (Cockerell) nest in Mukawa-cho, Yufutsu-gun, Hokkaido, Japan

No evidence that reproductive bumblebee workers reduce the production of new queens

Kin selection theory predicts potential conflict between queen and workers over male parentage in hymenopteran societies headed by one, singly mated queen, because each party is more closely related to its own male offspring. In ‘late-switching’ colonies of the bumblebee Bombus terrestris, i.e. colonies whose queens lay haploid eggs relatively late in the colony cycle, workers start to lay male eggs shortly after the queen lays the female eggs that will develop into new queens. It has been hypothesized that this occurs because workers recognize, via a signal given by the queen instructing female larvae to commence development as queens, that egg laying is now in their kin-selected interest. This hypothesis assumes that aggressive behaviour in egg-laying workers does not substantially reduce the production of new queens, which would decrease the workers' fitness payoff from producing males. We tested the hypothesis that reproductive activity in B. terrestris workers does not reduce the production of new queens. We used microsatellite genotyping to sex eggs and hence to select eight size-matched pairs of ‘late-switching’ colonies from a set of commercial colonies. From one colony of each pair we removed every egg-laying or aggressive worker observed. From the other colony, we simultaneously removed a nonegg-laying, nonaggressive worker. Removed workers were replaced with young workers from separate colonies at equal frequencies within the pair. There was no significant difference in queen productivity between colonies with reduced or normal levels of egg-laying or aggressive workers. Therefore, as predicted, reproductive B. terrestris workers did not significantly reduce the production of new queens.

Homing ability of the bumblebee Bombus terrestris (Hymenoptera: Apidae)

The ability of bumblebees to locate their nest site following artificial displacement was investigated. Bombus terrestris(Hymenoptera: Apidae) workers were marked with numbered tags and displaced up to 15 km from their nest. Bees returned from distances of up to 9.8 km, with the proportion of bees returning declining with distance of the release site from the nest. Bees were slow to return to their nests, often taking several days when even the most distant sites were all within 1 h direct flight time. These findings concur with those of earlier studies on various Hymenoptera, and they are consistent with previous suggestions that the most probable homing mechanism is a systematic search until familiar landmarks are encountered. Our results also suggest that the natural foraging range of B. terrestris may span several km. Bombus terrestris / central place foraging / navigation / home range

Foraging habitats and foraging distances of bumblebees, Bombus spp. (Hym., Apidae), in an agricultural landscape

Abstract: In selected foraging habitats of an agricultural landscape flower visits of bumblebees and community structure of foraging bumblebees were studied, with special regard to the role of crops as super‐abundant resources. Most crops represent temporal foraging habitats with high abundance of bumblebees but mainly with low diversity in the bumblebee forage community, in contrast to permanent foraging habitats such as, for example, a hedgerow. The high numbers of bumblebees in the monoculture of crop plantations consisted mainly of short‐tongued bumblebee species. The role of foraging distances for the visitation rate of foraging habitats was studied by performing capture–recapture experiments with natural nests of Bombus terrestris, Bombus lapidarius and Bombus muscorum. Differences were found on the species as well as the individual level. The foraging distances of B. muscorum were more restricted to the neighbourhood of the nesting habitat than the foraging activity of B. terrestris and B. lapidarius. High percentages of B. terrestris workers were recaptured while foraging on super‐abundant resources in distances up to 1750 m from the nest. Isolated patches of highly rewarding forage crops, in agricultural landscapes, are probably only accessed by bumblebee species with large mean foraging distances, such as the short‐tongued B. terrestris. Species like the rare, long‐tongued B. muscorum depend on a close connection between nesting and foraging habitat. A restricted foraging radius might be one important factor of bumblebee species loss and potential pollinator limitation in modern agricultural landscapes. Furthermore, long‐distance flights of bumblebee pollinators have to be considered in the present discussion on gene flow from transgenic plant species on a landscape scale.

Repellent scent-marking of flowers by a guild of foraging bumblebees ( Bombus spp.)

We have found that foraging bumblebees (Bombus hortorum, B. pascuorum, B. pratorum and B. terrestris) not only avoid flowers of Symphytum officinale that have recently been visited by conspecifics but also those that have been recently visited by heterospecifics. We propose that the decision whether to reject or accept a flower is influenced by a chemical odour that is left on the corolla by a forager, which temporarily repels subsequent foragers. Honeybees and carpenter bees have previously been shown to use similar repellent forage-marking scents. We found that flowers were repellent to other bumblebee foragers for approximately 20 min and also that after this time nectar levels in S. officinale flowers had largely replenished. Thus bumblebees could forage more efficiently by avoiding flowers with low rewards. Flowers to which extracts of tarsal components were applied were more often rejected by wild B. terrestris workers than flowers that had head extracts applied, which in turn were more often rejected than flowers that had body extracts applied. Extracts from four Bombus species were equally repellent to foragers. The sites of production of the repellent scent and its evolutionary origins are discussed.

Effects of social conditions on Juvenile Hormone mediated reproductive development in Bombus terrestris workers

Abstract. During the annual life cycle of the bumble bee Bombus terrestris (L.) colony, there is a stage characterized by worker reproduction in the presence of the queen. It has been proposed that this is a result of a decrease in queen inhibition. This hypothesis was examined by studying the effects of queens taken from colonies at different stages of development on several aspects of worker physiology and behaviour: rates of Juvenile Hormone (JH) release in vitro, ovary development, and behaviour associated with reproduction. After optimizing and validating the radiochemical assay for JH release for bumble bee workers, we found that queenless workers had significantly more developed ovaries and higher rates of release of JH than did queenright workers, confirming and extending previous findings that suggest that bumblebee ovarian development is under JH control. Mated queens, separated from their colony and brood, can have the same inhibitory effect on the reproductive development of callow workers. In contrast, workers confined with virgin queens or in queenless groups demonstrated a significantly higher rate of release of JH, overt aggression and threatening behaviours. However, there were no differences in rates of release of JH between workers confined in groups in the laboratory with queens taken from colonies either before or after the onset of worker reproduction. Furthermore, overt aggression and threatening behaviours were similar and low in both types of groups. These results gave no support to the hypothesis that a decrease in queen inhibition is associated with the onset of worker reproduction. We also show that young workers reared in colonies either before or after worker reproduction occurs, or in queenless colonies, all demonstrated similar, low rates of release of JH. These results suggest that older workers may inhibit the corpora allata of younger workers in queenless colonies.

Effects of sublethal doses of deltamethrin (Decis Ce) onBombus terrestris

SUMMARYExperimental plants of oilseed rape (Brassica napus cv. oleifera) were sprayed with deltamethrin at a rate of 12.5 g/ha, which is regarded as producing sublethal effects for Bombus terrestris workers foraging on the flowers. Contamination levels were assessed before and after treatment in small samples: 1.0 g of anthers, 0.7–2.0 g of nectar, 0.4–3.5 g of honey, 1.0 g of workers. Weights of residues ranged from 0.002 to 0.006 mg/kg in honey, from 0.012 to 0.019 mg/kg in nectar, from 0.047 to 0.605 mg/kg in anthers and from 0.149 to 0.932 mg/kg in workers. Six days after spraying no residues could be detected in anthers and nectar (detection thresholds = 0.0012–0.0062 mg/kg). In laboratory tests, contamination of worker bees by a topical application of 0.08–0.16 mg/kg of deltamethrin resulted in an increased uptake of sugar solution from a feeder by 40–100%. Contamination of the sugar solution by 0.1–0.2 mg/kg of deltamethrin resulted in a reduction of food uptake by 47–59%. No negative effect on lifespan was observed in either case of contamination in laboratory conditions. Queens were fed with a sugar solution contaminated by 0.01–0.2 mg/l of deltamethrin during either a 5-day period or continuously. None of these treatments affected the size of the first batch of workers produced by the queen or the period from egg to emergence. It is suggested that some deltamethrin detoxification might be due to intestinal enzymes. The effectiveness of this detoxification would be enough to prevent the bees from oral intoxication after collecting nectar from rape flowers sprayed with deltamethrin at rates much greater than the registered one.

Buzz foraging mechanism of bumble bees

SUMMARYThe buzz foraging vibrations of Bombus terrestris and B. hortorum workers, measured by an accelerometer attached to the flower stem, reached relatively high acceleration magnitudes of 212 m/s2 at frequencies of 374 Hz. B. hortorum workers were more effective than B. terrestris workers at vibrating the anthers of Symphytum officinale (comfrey) because they inserted their heads into the corolla for nectar. The escape buzz produced by B. terrestris during capture was similar to that produced during pollen foraging on Actinidia deliciosa (kiwifruit) and required 0.3 W to generate.Both species of bumble bees created the high magnitude vibrations by triggering resonance frequency vibrations in their ventral and dorsal plates (sternites and tergites). This caused amplification of a random vibration input, at the resonance frequency, by a factor of 2.3.B. terrestris workers that had not previously foraged on kiwifruit flowers showed buzz foraging, indicating that it was either an innate foraging technique, or that floral cues exist which stimulate buzzing.