Regulation of reproduction by dominant workers in bumblebee (Bombus terrestris) queenright colonies

The mechanisms of regulating worker reproduction in bumblebees were studied by direct behavioral observations and by measuring ovarian development and juvenile hormone (JH) biosynthesis rates in workers under different social conditions. Workers in the last stage of Bombus terrestris colony development (the competition phase) had the lowest ovarian development and JH biosynthesis rates. Callows introduced into colonies immediately after queen removal (dequeened colonies) demonstrated a significant increase in ovarian development before, but not during, the competition phase. These findings differ from the higher ovarian development in colonies during the competition phase predicted by the prevailing hypothesis that worker reproduction starts in response to a decrease in queen inhibition. Reproduction of callows housed with dominant workers in small queenless groups was inhibited as in queenright colonies. This suggests that the reduced ovarian development and JH biosynthesis rates observed in dequeened and normally developing colonies during the competition phase also reflect inhibition by dominant workers. Thus, two distinct stages of inhibition of reproduction seem to exist: (1) before the competition phase, when the queen slows down worker ovarian development and prevents oviposition; (2) during the competition phase, when dominant workers inhibit ovarian development of other workers. Between these stages there seems to be a temporal “window” of enhanced worker reproductive development. The queen's typical switch to haploid egg production was not associated with changes in worker ovarian development or JH biosynthesis rates. These findings suggest that regulation of worker reproduction in B. terrestris is not determined by simple changes in the queen's inhibition capacity or by the sex of offspring and that the worker's role is more important than previously believed. Received: 18 March 1998 / Accepted after revision: 18 July 1998     

Worker discrimination among queens in newly founded colonies of the fire ant Solenopsis invicta

Although colonies of the fire ant Solenopsis invicta are often founded by small groups of queens, all but one of the queens are soon eliminated due to worker attacks and queen fighting. The elimination of supernumerary queens provides an important context for tests of discrimination by the workers, since the outcome of these interactions strongly affects the workers' inclusive fitness. To test whether workers in newly founded colonies discriminate among nestmate queens, paired cofoundresses were narrowly separated by metal screens that prevented direct fighting, but through which the workers could easily pass. Soon after the first workers completed development, they often attacked one of the queens; these attacks were strongly associated with queen mortality. When one queen's brood was discarded, so that the adult workers were all the daughters of just one queen, the workers were significantly less likely to bite their mother than the unrelated queen; however, this tendency was comparatively weak. Queens kept temporarily at a higher temperature to increase their rate of investment in brood-rearing lost weight more rapidly than paired queens and were subsequently more likely to be attacked and killed by workers. Workers were more likely to bite queens that had been temporarily isolated than queens that remained close to brood and workers. When queens were not separated by screens, the presence of workers stimulated queen fights. These results show that workers discriminate strongly among equally familiar queens and that discrimination is based more on the queens' condition and recent social environment than on kinship. Received: 9 June 1998 / Accepted after revision: 10 October 1998     

Social influences on body size and developmental time in the bumblebee Bombus terrestris

In many social insects, including bumblebees, the division of labor between workers relates to body size, but little is known about the factors influencing larval development and final size. We confirmed and extend the evidence that in the bumblebee Bombus terrestris the adult bee body size is positively correlated with colony age. We next performed cross-fostering experiments in which eggs were switched between incipient (before worker emergence) and later stage colonies with workers. The introduced eggs developed into adults similar in size to their unrelated nestmates and not to their same-age full sisters developing in their mother colony. Detailed observations revealed that brood tending by the queen decreases, but does not cease, in young colonies with workers. We next showed that both worker number and the queen presence influenced the final size of the developing brood, but only the queen influence was mediated by shortening developmental time. In colonies separated by a queen excluder, brood developmental time was shorter in the queenright compartment. These findings suggest that differences in body size are regulated by the brood interactions with the queen and workers, and not by factors inside the eggs that could vary along with colony development. Finally, we developed a model showing that the typical increase in worker number and the decrease in brood contact with the queen can account for the typical increase in body size. Similar self-organized social regulation of brood development may contribute to the optimization of growth and reproduction in additional social insects.     

A comparison of visual and olfactory learning performance in the bumblebee Bombus terrestris

Animals use cues from a range of sensory modalities to discriminate stimuli and as predictors of reward. Whilst there is appreciable variation in the cognitive performance of animals, we know surprisingly little about the extent to which learning varies among individuals across different sensory modalities. Do individuals that are good at learning in one sensory modality also perform well in another (performance is correlated between modalities), or do individuals demonstrate specialisation in learning performance in one modality (trading-off performance between modalities)? We tested these hypotheses by examining the performance of 76 Bombus terrestris workers, from four colonies, in both an odour-and visual learning task. Olfactory learning was assessed using proboscis extension reflex (PER) conditioning and visual (colour) learning was examined using a well-established free-flying paradigm. Our results showed neither a correlation, nor a trade-off, in individual performance for learning tasks using different sensory modalities. However, there was considerable variation among workers within each colony in their performance in both learning tasks. This extent of interindividual variation in learning ability across sensory modalities could be adaptive for colonies dealing with changeable foraging conditions. There was also significant intercolony variation in final task performance level in the olfactory learning task, and both the strength and persistence of blue preference in the colour learning task. This is the first study to demonstrate variation in olfactory learning performance across multiple bumblebee colonies using PER conditioning, suggesting this is an effective paradigm for assessing associative olfactory learning performance both within and among colonies.     

Reproductive decision-making in semelparous colonies of the bumblebee bombus terrestris

Queen and worker Bombus terrestris have different optima for the timing of gyne production. Workers, being more related to their gyne-sisters than to their sons, should ascertain that gyne production has started before attempting to reproduce. Their optimal timing for gyne production will be as early as possible, while allowing sufficient ergonomic colony growth to support gyne rearing. Queen optimum, on the other hand, should be to postpone gyne production toward the end of colony life cycle, in order to minimize the time-window available for worker reproduction. Thus, the timing of gyne production may profoundly affect the outcome of queen–worker competition over male production. In this study we investigated some of the social correlates possibly affecting this timing. It was found that neither keeping colony size constant and as low as 20 workers, nor decreasing worker average age, influenced the onset of gyne production. To test the effect of queen age we created young colonies with old queens and vice versa. When colony social composition remained unchanged, in young colonies headed by old queens gynes were produced earlier than predicted, but in the inverse situation gyne production was not delayed. When colony social composition was completely standardized queen age had a decisive effect, indicating that the timing of gyne production is both under queen influence and affected by queen age. Furthermore, queens assess colony age from the time of first worker emergence rather than from their own first oviposition. In these experiments the factors affecting gyne production also affected the onset of queen–worker conflict for male production, suggesting that both are regulated by the same causal effect. Postponing gyne production as much as possible provides another mechanism, in addition to extensive oophagy, for the queen to outcompete her workers in male production.     

Color choices by bumble bees (Bombus terrestris): innate preferences and generalization after learning

It is usually assumed that the choice behavior of bees for floral colors is influenced by innate preferences only for the first flower visits prior to any experience. After visits to rewarding flowers bees learn to associate their colors with a reward. This learning process leads to an acquired preference for the trained colors that has been believed to dominate over previous experiences and over innate preferences. This work investigates how bumble bees (Bombus terrestris) chose among artificial flowers of different colors after they had been extensively trained to other colors. The bees chose novel colors according to their similarity to the trained color if the trained color was similar to some of the test colors. This was true also if trained colors and test colors were well distinguished, so their color choice reflected generalization between colors. If the test colors were so different from the trained color that no generalization took place, choice behavior was not affected by the trained color and reflected innate preferences. The differences in choice frequencies could not be explained by physical properties of the test colors other than the dominant wavelength, a parameter taken to reflect hue perception. Preferred dominant wavelengths correspond to those observed in naive bumble bees and honeybees. Thus bumble bees show innate preferences for certain colors not only prior to color learning but also after intensive learning when choosing among very different novel colors. Color choice among similar colors, however, is controlled by generalization from the learned color. Received: 9 November 1999 / Received in revised form: 19 March 2000 / Accepted: 31 March 2000     

Food alert in bumblebees (Bombus terrestris): possible mechanisms and evolutionary implications

The return of a successful bumblebee forager stimulates nestmates to leave the nest and search for food. Here we investigate the mechanisms by which this happens. Successful Bombus terrestris foragers perform irregular runs in their nest, often lasting for several minutes. Run duration is at its maximum when food has just been discovered. Running likely serves to distribute a pheromone, since the information flow between "runners" and "recruits" can be disrupted by eliminating air exchange, while leaving other potential means of communication intact. In addition, nectar stores in the nest may be monitored continuously. A sudden influx of nectar into the nest also causes measurable increases in forager activity. The implications of bumblebee recruitment behavior for the evolution of communication in bees are discussed.     

The effect of queen-worker conflict on caste determination in the bumblebee Bombus terrestris

Endocrine analyses were used to investigate the well-known association between queen production and the onset of worker reproduction (termed the competition phase, CPh) in Bombus terrestris. Larvae that reached the age of 5 days before the CPh had a worker-like profile: low juvenile hormone (JH) biosynthesis rates and low JH hemolymph titers. In contrast, larvae that reached the age of 5 days during the CPh had a queen-like profile: high JH biosynthesis rates and high hemolymph JH levels. Larval fate could be manipulated by transplanting egg cells into host colonies with different social structures. There was a steep rise in JH production in larvae transplanted into colonies near or during the CPh. This indicates that during colony development, larvae switch from the ”worker developmental pathway” to the ”queen developmental pathway,” and that the switch is socially regulated. In small rearing groups, larvae reared with queens before the CPh developed into workers, whereas those reared with queens after the CPh developed into queens. Variation in worker type (naive or experienced) did not affect caste determination. Therefore, we hypothesize that queens produce a pheromone that directly inhibits queen differentiation by larvae. We also present two alternative scenarios that explain the timing of gyne production in B. terrestris, one based on ecological constraints and the other based on queen-worker competition. Received: 20 October 1999 / Received in revised form: 18 December 1999 / Accepted: 23 January 2000     

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.     

Seasonal activity of Bombus terrestris L. in east Mediterranean region, Turkey

Bumblebees have economical importance in most of wild and cultivated plants. They can be abundant in suitable habitats and have a broad flower choice. Bombus terrestris was collected at intervals during 2002 and 2003 from various flora and ecosystems of east Mediterranean region of Turkey. In this study, plants visited by Bombus terrestris, seasonal activities, distribution and altitudes were determined. Bombus terrestris have boon seen throughout Turkey in a wide range of habitats from sea level to 1560 m altitude within all the major native vegetation types. Prevalence of queens, workers and males of Bombus terrestris differed due to altitude. More frequently observed at 0-600 m, declining above 600 m in relation to general climate requirements.     

Worker reproductive parasitism and drift in the western honeybee Apis mellifera

When a honeybee (Apis spp.) colony loses its queen and is unable to rear a new one, some of the workers activate their ovaries and produce eggs. When a colony has a queen (i.e., it is queenright) almost all worker-laid eggs are eaten, but when hopelessly queenless, the workers become more tolerant of worker-laid eggs and rear some of them to adult drones. This increased tolerance renders a queenless colony vulnerable to worker reproductive parasitism, wherein unrelated workers enter the colony and lay eggs. Here, we show that the proportion of unrelated (non-natal) workers significantly decreases after an Apis mellifera colony becomes queenless. The remaining non-natal workers are as likely to have activated ovaries as natal workers, yet they produce more eggs than natal workers, resulting in significantly higher reproductive success for non-natal workers. In a second experiment, we provided queenless and queenright workers with a choice to remain in their own colony or to join a queenless or queenright colony nearby. The experiment was set up such that worker movement was unlikely to be due to simple orientation errors. Very few workers joined another colony, and there was no preference for workers to drift into or out of queenless or queenright colonies, in accordance with the proportion of non-natal workers declining significantly after becoming queenless in the first experiment.     

Prior experience with eggs laid by non-nestmate queens induces egg acceptance errors in ant workers

We studied the effect of prior experience to eggs laid by nestmate and non-nestmate queens on the acceptance of queen-laid eggs by worker wood ants, Formica fusca. We transferred eggs from a non-nestmate queen into colonies during early spring, when their own queen was recommencing egg laying. A few weeks later, workers from these “experienced” colonies accepted eggs of both familiar (44% acceptance) and unfamiliar (40%) non-nestmate queens much more than workers from control colonies (2%) that had only had previous contact with their own queen’s eggs. Thus, prior exposure to eggs laid by a non-nestmate queen induces much greater acceptance of all non-nestmate queen-laid eggs. Mechanistically, we hypothesize that exposure to eggs from several queens may increase acceptance by causing a highly permissive acceptance threshold of non-nestmate queen-laid eggs rather than by widening the template for acceptable queen-laid eggs. These novel results show that egg-discrimination behaviour in F. fusca is flexible and that workers respond to the diversity of eggs experienced in their colony.     

Role of juvenile hormone biosynthesis in dominance status and reproduction of the bumblebee,Bombus terrestris

Summary In Bombus terrestris females, dominant status is correlated with high levels of juvenile hormone (JH) biosynthesis and rapid oöcyte maturation. Queenright workers, which are inhibited by the dominant queen, complete the cycle of oöcyte maturation while exhibiting a continuous low rate of JH production, but their egglaying is inhibited. Measurements in foundress queens suggest that the low JH titer during oögenesis is probably not responsible for the inhibition of egg-laying. Queenless workers, kept individually, are not activated either for JH production or oöcyte maturation. In groups of three queenless workers, a dominance order becomes established and high rates of JH synthesis are observed in the dominant egg-laying workers, with low rates in subordinated workers. In groups of founder queens, also, a dominance order becomes established and results in a reduced rate of JH production in subordinated females. Correspondence to: F. Couillaud     

Mutual pheromonal inhibition among queens in polygyne colonies of the fire ant Solenopsis invicta

Summary Decrease in individual reproductive output with increasing numbers of reproductives is a general feature of social insect colonies. The previously described negative relationship between the fecundity of individual queens and number of resident queens in polygyne (multiple-queen) colonies of the fire ant Solenopsis invicta appears to result from mutual pheromonal inhibition. In an experimental test for the presence of fecundity reducing pheromones, corpses of functional (egg-laying) queens were found to effectively inhibit the fecundity of functional queens, suggesting that queen-produced pheromones suppress egg production in such queens. Evidence concerning a possible mechanism mediating this inhibition was also obtained. Treatment of queens with methoprene, a juvenile hormone (JH) analog, increased ovary development, suggesting that fecundity in functional queens may be mediated by the level of endogenous JH. These findings are consistent with the occurrence of mutual pheromonal inhibition among queens achieved by suppression of endogenous JH titers.     

Colony nutritional status modulates worker responses to foraging recruitment pheromone in the bumblebee Bombus terrestris

Foraging activity in social insects should be regulated by colony nutritional status and food availability, such that both the emission of, and response to, recruitment signals depend on current conditions. Using fully automatic radio-frequency identification (RFID) technology to follow the foraging activity of tagged bumblebees (Bombus terrestris) during 16,000 foraging bouts, we tested whether the cue provided by stored food (the number of full honeypots) could modulate the response of workers to the recruitment pheromone signal. Artificial foraging pheromones were applied to colonies with varied levels of food reserves. The response to recruitment pheromones was stronger in colonies with low food, resulting in more workers becoming active and more foraging bouts being performed. In addition to previous reports showing that in colonies with low food successful foragers perform more excited runs during which they release recruitment pheromone and inactive workers are more prone to leave the nest following nectar influx, our results indicate that evolution has shaped a third pathway that modulates bumblebee foraging activity, thus preventing needless energy expenditure and exposure to risk when food stores are already high. This new feedback loop is intriguing since it involves context-dependent response to a signal. It highlights the integration of information from both forager-released pheromones (signal) and nutritional status (cue) that occurs within individual workers before making the decision to start foraging. Our results support the emerging view that responses to pheromones may be less hardwired than commonly acknowledged. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of experience and weather on foraging rate and pollen versus nectar collection in the bumblebee, Bombus terrestris

This study examines factors that affect foraging rate of free-flying bumblebees, Bombus terrestris, when collecting nectar, and also what factors determine whether they collect pollen or nectar. We show that nectar foraging rate (mass gathered per unit time) is positively correlated with worker size, in accordance with previous studies. It has been suggested that the greater foraging rate of large bees is due to their higher thermoregulatory capacity in cool conditions, but our data suggest that this is not so. Workers differing in size were not differentially affected by the weather. Regardless of size, naïve bees were poor foragers, often using more resources than they gathered. Foraging rate was not maximised until at least 30 trips had been made from the nest. Foraging rates were positively correlated with humidity, perhaps because nectar secretion rates were higher or evaporation of nectar lower at high humidity. Temperature, wind speed and cloud cover did not significantly influence foraging rate, within the summertime range that occurred during the study. Weather greatly influenced whether bees collected pollen or nectar. Pollen was preferably collected when it was warm, windy, and particularly when humidity was low; and preferably during the middle of the day. We suggest that bees collect pollen in dry conditions, and avoid collecting pollen when there is dew or rain-water droplets on the vegetation, which would make grooming pollen into the corbiculae difficult. Availability of sufficient dry days for pollen collection may be an important factor determining the success of bumblebee colonies.Communicated by M. Giurfa     

Regulation of worker reproduction in bumblebees (Bombus terrestris): workers eavesdrop on a queen signal

In the annual bumblebee Bombus terrestris, the onset of queen-worker conflict over male production is seasonally and socially constrained. Workers will do better if they start to reproduce (the so-called competition phase) only after ascertaining that larvae are committed to gyne development but before the season ends because they gain more by rearing sister-gynes than their own sons. Here, we tested two nonmutually exclusive hypotheses as to what triggers the onset of worker reproduction: Workers can directly monitor larval development and/or workers eavesdrop on the queen signal that directs gyne development. Exposing workers to gyne larvae through a double mesh did not advance the competition phase compared to control colonies. However, when workers, but not the queen, were allowed contact with gyne larvae, both the competition phase and gyne production were advanced. Thus, while larvae do not emit a volatile pheromone that discloses their developmental route, the physical contact of workers with such larvae triggers early competition phase. However, workers exclusively exposed to worker larvae (colonies prevented from producing gyne larvae) started to reproduce at the same time as control colonies. Replacing the resident queen with an older queen (from gyne-rearing colonies) advanced the competition phase, irrespective of worker age. The results are consistent with the hypothesis that workers eavesdrop on the queen pheromones. This is adaptive because it allows workers a broader time-window for reproduction and thus to gain fitness from rearing both sister-gynes and sons before the season ends without affecting colony development.     

Transmission of a pathogen in Bombus terrestris,with a note on division of labour in social insects

Summary Parasites of social insect workers can be transmitted within the colony to other, related host individuals or, alternatively, to unrelated workers of other colonies. Division of labour affects the probability of transmission, as young individuals often work inside the nest whereas older ones often leave the nest to forage. Therefore, the relative probabilities of transmission within-vs. between-nests is also affected by the delay between host infection and the shedding of propagules, i.e. the latent period of the parasite strain. We therefore hypothesized that strains of the flagellate parasite Crithidia bombi (Trypanosomatidae, Zoomastigophorea) infecting workers of the bumble bee Bombus terrestris (Hymenoptera, Apidae) could differ in their delays and coexist in a population. This would be the case if strains that are shed after a short time delay were more efficiently transmitted to other colony members, whereas strains with long delays were more efficiently transmitted to non-related workers in the population. We tested this hypothesis by experimentally varying time delay and by allowing transmission to either sister workers from the same nest or unrelated workers from other nests. Transmission of C. bombi was measured as the number of parasitic cells shed by the exposed workers after a standard period. The results showed that relatedness as such had no effect, but that delay and nest identity were highly significant effects to explain variation in transmission success. There was a significant interaction between nest identity and delay, such that bees of some colonies acted as efficient transmitters for C. bombi under short delays and vice versa. We discuss how division of labour may affect parasitism in social insects and, vice versa, how division of labour may be under selection from the effects of parasitism, using available evidence from the literature. Correspondence to: P. Schmid-Hempel