The development and the reproductive output of 26 Bombus terrestris colonies were investigated. Four important points in colony development are distinguished. These are: a) the start of egg laying by the queen, leading to the beginning of the eusocial phase, the emergence of the first workers; b) the moment which we term the switch point, at which the queen switches from laying diploid eggs (producing workers or queens) to the laying of haploid eggs (producing males); c) the onset of queen production reared from diploid eggs; d) the loss of dominance by the queen, expressed by the beginning of aggression on the part of queen and workers, worker oviposition, oophagy and the functional elimination of the queen. This we call the competition point. The onset of queen production is highly correlated with the competition point but not correlated with the switch point. The pattern in time of the egg laying by a queen differed from the rate of increase of worker numbers published by previous authors. After an initial and slow phase, in which two broods of egg cells are produced, the rate of egg laying of a queen becomes constant, whether or not the eggs are fertilized. The transformation from this rate of egg laying to the earlier descriptions of increase in number is only possible if one neglects differences between colonies in time spent in the production of the two broods as well as the highly variable time needed for development into adults. The time at which the competition point occurs is much more predictable (at day 30.8 ± 4.9 after the emergence of the first worker) than the switch point. The latter ranges from day 6 to day 32. In our colonies two groups can be discerned, one of early switching colonies (at day 9.8 ± 2.4), the other of late switching colonies (at day 23.4 ± 4.6). The occurrence of the switch cannot be predicted from preceding behavioural or demographic data. As a consequence of the early switch such colonies produce mainly males. Partly due to the unfavourable larva/worker ratio only a few queens were reared from the last laid diploid eggs. In contradiction the late switching colonies produce on average sexuals at a 1.3:1 sex ratio (1:1.7 investment ratio). Remarkable however, biomass of sexuals is equal in both types of colonies. Males are about half the weight of queens. Certain colonies invest in males only ("3 males : 0 queens"). Since the two types of colonies occurred in about equal numbers, our local population is characterized by a 4:1 1(♂:♀) sex ratio. This male biased sex ratio, also observed for other bumblebees species (B. terricola and B. melanopygus, OWEN et al., 1980; OWEN & PLOWRIGHT, 1982), contrasts with existing theoretical models based on kin selection (TRIVERS & HARE, 1976) or local mate competition (ALEXANDER & SHERMAN, 1977) arguments. A model is proposed in which the males of the early switching colonies monopolize the matings due to their early appearance in the field. In fact, the early switching colonies, by investing mainly in males, parasitize on the reproductive strategy of the late switching colonies, which are characterized by a 1:1 sex ratio. At population level the two reproductive strategies are in equilibrium at a 1:1 frequency.

Answers were sought to three questions to the regulation of oogenesis and ovipositions by the bumblebee queen and by her workers: a) Which factors have an effect on oogenesis and subsequent ovipositions at the individual level? b) What impact do social factors and the presence of the queen have on ovary activation and egg laying by workers at colony level? c) Is oogenesis in workers related to egg laying by the queen, particularly to her switch from laying diploid, female biased eggs to haploid, male producing eggs? When workers emerge, in queenright colonies, their ovaries start to develop and become increasingly up to the age of about 10 days. By that time about 40% of the bees contain ripe eggs, whereas in the other bees the oocytes remain in an intermediate stage. Neither developmental time from egg to adult nor body size has an influence in determining which workers will develop mature eggs and which will not. In adult bees the frequency of feeding larvae or the activity of the hypopharyngeal gland are not related to ovarian activity. The ovaries of bees that have no access to pollen do not develop, and the fat bodies of these bees are relatively small. Foraging in the field is correlated with low ovarian activity. However, bees that once were foragers can become laying workers later on in life, especially if they belong to the first and second brood. Social interactions between workers or between a worker and a queen stimulate activity, leading to activation of the ovaries. The queen, however, also has a slight inhibitory effect, so that in workers aged 5–8 days ovarian activity is lower than in queenless groups. In older bees from queenright groups ovarian activity levels are almost the same as in groups of queenless workers. In queenless groups some of the workers lay eggs from the age of 7 days onwards, whereas in colonies and queenright groups the queen prevents all worker ovipositions for about 31 days. The queen, therefore, has two different effects on the reproductive characteristics of the worker, a physiological one and a behavioural one. Ovarian development levels in the worker population do not affect the switch of the queen to the laying of haploid eggs, nor does this switch has any influence on the degree of ovarian activity of the worker population later on. There is also no relation between the moment of the switch and the occurrence of worker ovipositions. Résumé Etude du développement ovarien et de la ponte des ouvrières de Bombus terrestris Nous avons cherché à répondre aux trois questions suivantes concernant la régulation de l'ovo‐genèse et la ponte des reines et des ouvrières: a) quels facteurs influencent l'ovogenèse et la ponte consécutive, au niveau individuel? b) Quel est l'impact des facteurs sociaux et de la présence de la reine sur l'activité ovarienne et la ponte des ouvrières, au niveau de la colonie? c) Est‐ce que l'ovogenèse des ouvrières est liée à la ponte des reines, particulièrement lorsqu'elle passe d'une ponte en majorité à œufs diploïdes femelles, à une ponte ...

The frequency of colonies that produce diploid males after brother‐sister (50%) and nephew‐niece (37.5%) matings proves that in B. terrestris the sex is determined by a single multi‐allelic sex locus. The diploid males which develop normally into adults make up 50% of the diploid brood. In the laboratory the growth rate of colonies with diploid males is influenced only slightly. Of 41 colony founding queens caught out of a natural population, all produced a colony without any diploid males. Therefore, the number of sex alleles in this population is estimated to be at least 24. This means that in commercial rearing systems for bumble bees, involving several generations, the occurrence of diploid males can largely be prevented by a good scheme for crossings.

Patterns of sex allocation in bumblebees have been enigmatic and difficult to interpret in either a Fisherian context or in a kinselection perspective. We gathered data on several hundred laboratory-reared colonies of the bumblebee Bombus terrestris and analyzed sex allocation as a function of diapause duration and a series of variables describing colony development. Our analyses addressed both sex allocation patterns across different cohorts of laboratory colonies reared at different times and sex allocation patterns across individual colonies within these cohorts. We used path analysis to test a hypothetical model linking a sequence of colony-development variables to the crucial reproductive parameters at the end of the colony life cycle. We show that (1) population-wide patterns of sex allocation show equal investment in the sexes and are thus consistent with queen control, but not with worker control. (2) A significant part of the colony-level and cohort-specific variation in sex allocation is related to the hibernation conditions of founding queens: Queens with longer than average winter diapause produce larger cohorts of first and second brood workers, switch to haploid eggs early, and produce colonies that raise mostly males and few new queens and vice versa. (3) Colony-level sex allocation is significantly related to the time span between the switch point (date of first haploid egg laid by the queen) and the competition point (date of first haploid egg laid by one of the workers): the longer this period, the more male biased the sex ratio. (4) The breeding constraints of an annual life cycle, the short reproductive season, and the presumably high premium on early produced males imply that bumblebee workers have no realistic options to capitalize on their relatedness asymmetry toward the different kinds of reproductive brood by biasing the sex ratio.

-The first aim of this study was to investigate whether specific changes in the frequencies of behavioural characteristics are correlated with the cause or reflect the onset of the laying of haploid eggs by the queen, and whether these changes cause the onset of egg laying and aggressive behaviour by workers. The second aim was to obtain a better understanding of the occurrence of the behaviours in relation to the physiology and function of the queen and the workers.No behavioural characteristics of workers which could predict the occurrence of the queen's switch to haploid egg laying was found nor did any such behaviour indicate that the switch had already occurred. The onset of worker oviposition was not found to be the consequence of the level or the type of agonistic actions between the workers and the queen.With the onset of worker oviposition the agonistic behaviours called trembling and buzzing, which are directed at the queen, disappear. At the same time the frequency of a non-directed behaviour, called humming, increases. This behaviour is seen particularly in the small housebees with developed ovaries.Before worker oviposition begins, the frequencies with which individual workers perform agonistic behaviour are not correlated with the conditions of their ovaries nor correlated with other characteristics of the workers. However, these frequencies are related to the sequence in which workers emerge. The correlations with age are due to the fact that workers of the first brood have higher activity levels than those of the later broods. The first egg layers are to be found among the first workers.In the period of worker oviposition, humming and the aggressive behaviours of darting and attack are performed by workers with developed ovaries. These behaviours are directed particularly towards workers who also have active ovaries.If the queen has disappeared 1 worker usually becomes dominant. This worker is characterized by the relatively high frequency with which she engages in darting and attacks. In addition, this worker is characterized by a specific behaviour, called pumping, which is directed at workers that have mature eggs in their ovaries. Behavioural domination, however, is no guarantee that the bee concerned will eventually become the most productive one.Although at an apparently pre-determined moment in the ontogeny of the colony the queen loses control over worker oviposition and related behaviours, she remains dominant in various other behavioural hierarchies. It is therefore concluded that the queen never loses dominance completely.The colony cycle of Bombus terrestris consists of 3 distinct phases: a) the initiation of the colony by the solitary queen who produces a small number of workers: b) the eusocial phase in which the workers assist the queen in rearing additional workers, new queens and males; c) the competition phase during which workers lay eggs; they compete for opportunities to reproduce and destroy each other's eggs and larvae. As a result the productivity of the colony dur...