Size variation and foraging rate in bumblebees (Bombus terrestris)

Summary: Size polymorphism is an important life history trait in bumblebees with strong impact on individual behavior and colony organization. Within a colony larger workers tend to serve as foragers, while smaller workers fulfill in-hive tasks. It is often assumed that size-dependent division of labor relates to differences in task performance. In this study we examined size-dependent interindividual variability in foraging, i.e. whether foraging behavior and foraging capability of bumblebee workers are affected by their size. We observed two freely foraging Bombus terrestris colonies and measured i) trip number, ii) trip time, iii) proportion of nectar trips, and iv) nectar foraging rate of different sized foragers. In all observation periods large foragers exhibited a significantly higher foraging rate than small foragers. None of the other three foraging parameters was affected by worker size. Thus, large foragers contributed disproportionately more to the current nectar influx of their colony. We provide a detailed discussion of the possible proximate mechanisms underlying the differences in foraging rate.     

The degree of parasitism of the bumblebee (Bombus terrestris) by cuckoo bumblebees (Bombus (Psithyrus) vestalis)

Host–parasite systems are characterised by coevolutionary arms races between host and parasite. Parasites are often the driving force, as they replicate much faster than their hosts and have shorter generation times and larger population sizes, resulting in higher mutation rates per time interval. This scenario does not fit all host–parasite systems. Socially parasitic cuckoo bumblebees (Bombus (Psithyrus) vestalis) parasitise colonies of Bombus terrestris share most life history characteristics with their hosts. As they parasitise only a subset of all available colonies, their population size should be lower than that of their hosts. This might have strong negative effects on the genetic diversity of B. vestalis and their adaptability. Here, we study for the first time the population structure of a Bombus/Bombus (Psithyrus) system. Highly polymorphic DNA markers were used to reconstruct sibships from individuals collected in the wild. The analysis of the host and parasite populations revealed a rate of parasitism of about 42% (range 33–50%). The population size of B. vestalis was lower compared to their hosts, which was also reflected in low within-group genetic distance. An analysis of the reconstructed queen genotypes revealed more supersisters amongst the B. vestalis queens when compared to the B. terrestris host. The data suggest that B. vestalis females and males do not disperse over long distances. This shows a potential for local adaptation to their hosts.     

Density-dependent and frequency-dependent selection by bumblebees Bombus terrestris (L.) (Hymenoptera: Apidae)

The behaviour of bumblebee workers foraging on arrays of artificial flowers of two colour morphs was observed. Experiments were conducted on arrays of varying morph frequencies and at three different total flower densities. Bumblebees consistently showed a preference for the commonest colour morph, and this behaviour was not significantly affected by changing density. In contrast, frequency-independent preferences changed significantly with density. At low densities, there was a strong bias towards the more conspicuous colour, whereas at higher densities there was no overall colour bias. Flight distances between flowers decreased significantly at high density. Bumblebees also visited flowers of similar colours sequentially, but this behaviour was not density-dependent. It is suggested that as densities increase, there is an increased probability that bumblebees detect yellow flowers, which were probably less conspicuous compared with blue flowers, and that this might be caused by changes in flight speed with flight distance. Where there is a positive relationship between pollinator visitation and the relative fitness of a floral morph, the observed behaviour would induce positive frequency-dependent selection on a plant population with two corolla colour morphs on which the bumblebees were foraging, which would result in stabilizing selection for a single corolla colour, irrespective of density. There was no indication that rare colour morphs would be preferred at high density. The probability of different corolla colour morphs going to fixation would, however, be affected by density.     

Foraging activity of bumblebees (Bombus terrestris L.) on Bt-expressing eggplants

A greenhouse experiment was setup to study foraging behavior of the bumblebee Bombus terrestris L. on Cry3Bb-expressing genetically modified (GM) eggplants and their near-isogenic control. Commonly, more bumblebees visited GM eggplants compared to near-isogenic control, but this difference was only marginally significant. The mean length of feeding bouts was similar on the two treatments. Neither the number of flowers produced nor their size could explain bumblebees?? tendency to prefer GM eggplants. Volatile compounds were extracted from five plants per genotype and separated using gas chromatography. Thirteen compounds were identified and five of them appeared significantly more abundant in GM eggplants. Six of the identified compounds [(+)-limonene, Z-jasmone, p-cymene, ??-pinene, methyl-salicilate, and (?)-limonene] were tested in electrophysiological bioassays with antennas detached from young bumblebees, and a response was recorded in all six cases. Experimental results indicate that pollination activity of bumblebees is compatible with this GM eggplant event as a food source and that chemical cues may have an important role in plant identification. The implications for environmental risk assessment of GM plants are discussed.     

Frequency-dependent selection by pollinators: mechanisms and consequences with regard to behaviour of bumblebees Bombus terrestris (L.) (Hymenoptera: Apidae)

Bombus terrestris , a typical pollinating insect species, was offered artificial flowers of two different corolla colours with the same sucrose solution reward in an array. Common colours were significantly preferred, and the strength of the frequency-dependent response increased as a result of learning. There were also frequency-independent biases towards blue flowers, probably because blue flowers appeared more conspicuous to bumblebees than yellow flowers, and the degree of preference for blue was greater when flowers had low nectar rewards. Flower-to-flower movements by individual bumblebees between flowers were non-random, were biased to movements within the same flower colour, and were also dependent on morph frequency. The mechanisms governing flower selection in bumblebees are discussed. Pollinators foraging similarly in a natural situation would induce positive frequency-dependent selection, assortative mating, and directional selection on different corolla colour morphs of the plant population being visited, resulting in stabilizing selection for a single flower colour.     

Isolation and characterization of chitin from bumblebee (Bombus terrestris)

Insect chitin possessing shell-like structure was prepared from the bumblebee corpses by a consequent treatment with 1M HCl and 1M NaOH. The bumblebee chitin was compared with crustacean (shrimp) chitin by using elemental analysis, Fourier-transform infrared (FT-IR) and solid-state (13)C cross-polarization magic angle spinning nuclear magnetic resonance (CP/MAS)-NMR spectroscopy and confocal microscopy. Both chitins (bumblebee and shrimp) exhibited identical spectra, while the bumblebee chitin had a 5% lower degree of acetylation and was characterized by a fine membrane texture.     

Contextual modulation of visuomotor associations in bumble-bees (Bombus terrestris)

We performed the following experiment to investigate whether contextual cues can prevent interference during the acquisition of potentially competing visuomotor associations. In the bumble-bee (Bombus terrestris). Bees discriminated between horizontally and vertically orientated gratings of black and white stripes in order to reach a feeder and between different diagonally orientated gratings to gain access to their nest. Once bees were well trained on these two tasks, the discrimination task at the nest was changed so that bees had to distinguish between horizontal and vertical gratings at both sites. Whereas they still approached the horizontal grating to reach food, they now had to approach a vertical grating in order to return to their nest. The new task was learnt rapidly. Errors at the feeder did not increase during or after the acquisition of this potentially competing behaviour at the nest entrance. For a brief period during acquisition, bees showed some hesitation at the feeder and hovered for slightly longer before choosing between horizontals and verticals. After acquisition was complete, bees showed a slight increase in their preference for verticals over a more ambiguous stimulus of an array of dots. These findings are consistent with the hypothesis that different contextual signals are associated with approaching the nest or approaching the feeder, and that these contextual signals facilitate learnt associations between orientation detectors and motor commands.     

Regulation of queen-worker conflict in bumble-bee (Bombus terrestris) colonies

In annual colonies of bumble-bees overt queen-worker conflict is limited to a distinct 'competition phase' (CPh). In unmanipulated Bombus terrestris colonies, the queen's switch to male production (the 'switch point', SP) accounted for only-22% of the variation in the onset of the CPh. In some colonies, the CPh even began before the SP. The CPh was more strongly correlated with the transition in queen production (r=0.79). Replacing the queen eggs with male eggs or doubling the number of workers in young colonies resulted in a significantly earlier onset of the CPh and a significantly earlier transition to queen production. Replacing queen eggs with female eggs did not have this effect. These manipulations did not affect the timing of the queen's switch from female to male production. These findings show that the mechanism underlying the queen-worker conflict in insect societies is more complex than previously appreciated. The onset of queen-worker conflict cannot be attributed simply to a single factor such as the queen's switch to male production or a decrease in queen inhibition. Rather, multiple cues are important.     

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.     

Reproductive responses to photoperiod and temperature by artificially hibernated bumblebee (Bombus terrestris) queens

Post‐hibernated bumblebee (Bombus terrestris) queens were kept for 1 week under photoperiodic conditions of 8 h light : 16 h dark, and at four different temperatures (24, 28, 32 and 36°C). The reproductive performance of the queens was then observed. It was found that exposure temperature and hibernation duration did not affect the oviposition rate. The pre‐oviposition period was found to be shortest (3.8 ± 0.7 days) for queens that had hibernated for 4.0 months and had been activated at 28°C. Timing of the initiation of the switch‐point was not affected by exposure temperature and hibernation duration. Significantly higher numbers of workers (268.0 ± 31.4) and sexual queens (119.3 ± 16.8) were produced by the queens that had hibernated for 3.0 months and had been activated at 28 and 36°C, respectively. The queens that had hibernated for 4.0 months and had been activated at 36°C produced the highest number of males (296.2 ± 32.3).     

Bumble bees (Bombus terrestris) store both food and information in honeypots

Social insect foragers often transmit information about foodsources to nest mates. In bumble bees (Bombus terrestris), forexample, successful foragers use excited motor displays anda pheromone as communication signals. In addition, bees couldmake use of an indirect pathway of information flow, via thehoney stores. We show here that, indeed, bees in the nest continuouslymonitor honeypots and sample their contents, thus obtaininginformation on supply and demand of nectar. When there is aninflux of nectar into the nest, the colony deploys more workersfor foraging. The number of new foragers depends on sugar concentration.Foragers returning with high-quality sugar solution displaymore "excited runs" on the nest structure. The recruits' response,however, does not depend on modulated behavior by foragers:more workers start to forage with high quality of incoming nectar,even when this nectar is brought by a pipette. Moreover, weshow that the readiness of bees to respond to recruitment signalsor incoming nectar also depends on colony demand. When colonynectar stores are full, the response of bees to equal amountsof nectar influx is smaller than when stores are empty. Whencolony nectar stores are depleted, foragers spend more timerunning excitedly and less time probing pots in the nest andrun with higher average speed, possibly to disperse the alertingpheromone more efficiently. However, more bees respond to nectarinflux to empty stores, whether or not this is accompanied byforager signals. Thus, honeypots serve to store informationas well as food.     

Novel microsatellite DNA loci for Bombus terrestris (Linnaeus, 1758)

We present details and characteristics of 123 novel polymorphic microsatellite DNA loci for Bombus terrestris. Thirty-four of these loci have been tested in nine other Bombus species and 25 of them showed polymorphisms in at least one species. These microsatellite DNA loci together with the already established 60 loci will be useful for characterizing wild and managed populations of B. terrestris and other Bombus species as well as for detailed genetic studies in including mapping studies and genome annotations.