Parasites delay worker reproduction in bumblebees: consequences for eusociality

Workers in eusocial insects usually tend the brood of the queenand so achieve representation in the next generation throughaiding relatives to reproduce. However, workers of some eusocialspecies, such as bumblebees, are capable of reproductive activityeven in the presence of the queen (in queen-right colonies),and worker reproduction is associated with aggressive behaviorsand egg cannibalism, both of which reduce colony efficiency.Thus, factors that affect worker ovariandevelopment, a preconditionfor reproduction, can influence social harmony and colony productivity.Parasites are a ubiquitous and important part of the bioticenvironment of all organisms. Here we show that parasites playan important role in the reproductive physiology of worker bumblebeesin queen-right colonies of Bombus terrestris, affecting thepattern and timing of ovarian development and oviposition. Workersfrom colonies parasitized with the intestinal trypanosome Crithidiabombi had less developed ovaries than workers of the same agefrom unparasitized colonies. In addition, parasitized colonieswere smaller than unparasitized colonies for about the firsthalf of colony development. This generated further demographiceffects such that workers were on average younger in parasitizedthan in unparasitized colonies around the time of the onsetof worker oviposition, and worker oviposition occurred significantlylater in parasitized colonies. Workers in parasitized coloniestherefore had lower individual reproductive potential and werecooperative for a larger proportion of the colony cycle thanthose in unparasitized colonies. In this system, where transmissionof the parasite between years probably occurs only in infested,young queens, this effect may represent an adaptation on thepart of the parasite to ensure its successful passage throughthe winter. Parasites, by reducing the cost of worker cooperation,may facilitate queen control over her worker force and playan important role in moderating the social organization of eusocialinsect colonies.     

Fitness and the level of homozygosity in a social insect

To date very few studies have addressed the effects of inbreeding in social Hymenoptera, perhaps because the costs of inbreeding are generally considered marginal owing to male haploidy whereby recessive deleterious alleles are strongly exposed to selection in males. Here, we present one of the first studies on the effects of queen and worker homozygosity on colony performance. In a wild population of the ant Formica exsecta, the relative investment of single‐queen colonies in sexual production decreased with increased worker homozygosity. This may either stem from increased homozygosity decreasing the likelihood of diploid brood to develop into queens or a lower efficiency of more homozygous workers at feeding larvae and thus a lower proportion of the female brood developing into queens. There was also a significant negative association between colony age and the level of queen but not worker homozygosity. This association may stem from inbreeding affecting queen lifespan and/or their fecundity, and thus colony survival. However, there was no association between queen homozygosity and colony size, suggesting that inbreeding affects colony survival as a result of inbred queens having a shorter lifespan rather than a lower fecundity. Finally, there was no significant association between either worker or queen homozygosity and the probability of successful colony founding, colony size and colony productivity, the three other traits studied. Overall, these results indicate that inbreeding depression may have important effects on colony fitness by affecting both the parental (queen) and offspring (worker) generations cohabiting within an ant colony.     

Larger laboratory colonies consume proportionally less energy and have lower per capita brood production in Temnothorax ants

Colony size can affect individual- and colony-level behavioral and physiological traits in social insects. Changes in behavior and physiology in response to colony growth and development can affect productivity and fitness. Here, we used respirometry to study the relationship between colony size and colony energy consumption in Temnothorax rugatulus ants. In addition, we examined the relationship between colony size and worker productivity measured as per capita brood production. We found that colony metabolic rate scales with colony size to the 0.78 power and the number of brood scales with the number of workers to the 0.49 power. These regression analyses reveal that larger ant colonies use proportionally less energy and produce fewer brood per worker. Our findings provide new information on the relationships between colony size and energetic efficiency and productivity in a model ant genus. We discuss the potential mechanisms giving rise to allometric scaling of metabolic rate in ant colonies and the influence of colony size on energy consumption and productivity in general.     

Colony social organization of <Emphasis Type="Italic">Lasioglossum malachurum</Emphasis> Kirby (Hymenoptera,Halictidae) in southern Greece

Summary We studied the nesting and social biology of two aggregations of the obligately eusocial halictine bee Lasioglossum malachurum at Agios Nikolaos Monemvasias (ANM) in southern Greece. Observations and nest excavations carried out from May to June 2000, revealed social and demographic variation between aggregations and years at ANM, as well as notable differences between these and other European populations. In southern Greece, the colony cycle includes multiple broods: the first two broods comprise only workers, whereas the third brood comprises workers, males, and gynes. Although pleometrosis is unknown in other populations, in the ANM region, as many as 10% of nests have more than one foundress. Newly emerged workers and gynes exhibit non-overlapping size distributions, but a few queens are worker-sized, indicating that workers occasionally overwinter and become foundresses. Although the vast majority of workers are unmated and most exhibit no ovarian development, an increase in worker ovarian development at the time of male production suggests that many males may develop from worker-laid eggs. Worker reproduction seems to be inhibited by the presence of queens, and annual variation in queen mortality may underlie annual variation in worker ovarian development. Across Europe, the major demographic and social differences among L. malachurum populations are in the number of worker broods and the extent of worker ovarian development. This contrasts with the results of a principal components analysis of social traits among 15 social L. (Evylaeus) populations, which shows that interspecific social variation is defined by the proportion of males in the early brood, the proportion of workers mated, queen-worker size dimorphism, gyne overwintering locale, and the proportion of workers with developed ovaries.Received 17 June 2002; revised 16 January 2002; accepted 27 January 2003.     

The Effect of Queen and Worker Adoption on Weaver Ant (Oecophylla smaragdina F.) Queen Fecundity

Incipient ant colonies are often under fierce competition, making fast growth crucial for survival. To increase production, colonies can adopt multiple queens (pleometrosis), fuse with other colonies or rob brood from neighboring colonies. However, different adoption strategies might have different impacts such as future queen fecundity or future colony size. O. smaragdina queen production was measured in incipient colonies with 2, 3 or 4 founding queens, following the transplantation of 0, 30 or 60 pupae from a donor colony. Pupae developed into mature workers, resulting in increased worker/queen ratios in pupae transplanted treatments and leading to increases in the per capita queen production. Conversely, more queens did not induce increased per capita fecundity. Thus, brood robbing added individuals to the worker force and increased future production of resident queens, whereas queen adoption increased the colony’s future production, but not the production of individual queens.     

Regulation of worker activity in a primitively eusocial wasp, Ropalidia marginata

Ropalidia marginala, a tropical, primitively eusocial, polistinewasp, is unusual in that the queen (the sole egg-layer) is neitherthe most behaviorally dominant nor the most active individualin the colony. The queen by herself rarely ever initiates interactionstoward her nest mates or unloads returning foragers. There arealways a few workers in the colony who are more dominant andactive than the queen. Absence of the queen from her colonydoes not affect colony maintenance activities such as foragingor brood care, but it always results in one individual becomingvery aggressive and dominant. The dominant worker becomes thenext queen if the original queen does not return. The queendoes not appear to play any significant role in colony activityregulation. Instead, colony activities appear to be regulatedby several mechanisms including dominance behavior toward foragers,feeding of larvae, and the unloading of returning foragers,all mediated by workers themselves. Regulation of colony maintenanceappears to be based on direct evaluation of the needs of thecolony by the workers themselves. The queen however has perfectreproductive control over all workers; workers never lay eggsin the presence of the queen. It appears therefore that themechanisms involved in regulation of worker activity and workerreproduction are separate in R marginata. These findings contrastwith other primitively eusocial species where the queen actsas a "central pacemaker" and controls both worker activity andworker reproduction.     

Queen recruitment and split sex ratios in polygynous colonies of the ant Formica exsecta

Sex ratios in social insects have become a general model for tests of inclusive fitness theory, sex ratio theory and parent–offspring conflict. In populations of Formica exsecta with multiple queens per colony , sex ratios vary greatly among colonies and the dry-weight sex ratio is extremely male-biased, with 89% of the colonies producing males but no gynes (reproductive females). Here we test the queen-replenishment hypothesis, which was proposed to explain sex ratio specialization in this and other highly polygynous ants (i.e. those with many queens per nest). This hypothesis proposes that, in such ants, colonies produce gynes to recruit them back into the colony when the number of resident queens falls below a given threshold limiting colony productivity or survival. We tested predictions of the queen-replenishment hypothesis by following F. exsecta colonies across two breeding seasons and relating the change in effective queen number with changes in sex ratio, colony size and brood production. As predicted by the queen-replenishment hypothesis, we found that colonies that specialized in producing females increased their effective queen number and were significantly more likely to specialize in male production the following year. The switch to male production also coincided with a drop in productivity per queen as predicted. However, adoption of new queens did not result in a significant increase in total colony productivity the following year. We suggest that this is because queen production comes at the expense of worker production and thus queen production leads to resource limitation the following year, buffering the effect of greater queen number on total productivity.     

Development rate and brood production in haplo- and pleometrotic colonies of <Emphasis Type="Italic">Oecophylla smaragdina</Emphasis>

Pleometrosis (colony founding by multiple queens) may improve life history characteristics that are important for early colony survival. When queens unite their initial brood, the number of workers present when incipient colonies open may be higher than for single queen colonies. Further, the time until the first worker emerges may shorten. For territorial species and species that rob brood from neighbouring colonies, a faster production of more workers may improve the chance of surviving intraspecific competition. In this study, the time from the nuptial flight to the emergence of the first worker in incipient Oecophylla smaragdina Fabr. colonies founded by 1–5 queens was compared and the production of brood during the first 68 days after the nuptial flight was assessed. Compared to haplometrotic colonies, pleometrotic colonies produced 3.2 times more workers, their first worker emerged on average 4.3 days (8%) earlier and the queen’s per capita egg production almost doubled. Further, colony production was positively, correlated with the number of founding queens and time to worker emergence was negatively correlated. These results indicate that pleometrotic O. smaragdina colo-nies are competitively superior to haplometrotic colonies as they produce more workers faster and shorten the claustral phase, leading to increased queen fecundity.     

Worker policing persists in a hopelessly queenless honey bee colony (<Emphasis Type="Italic">Apis mellifera</Emphasis>)

Summary In queenright colonies of Apis mellifera, worker policing normally eliminates worker-laid eggs thereby preventing worker reproduction. However, in queenless colonies that have failed to rear a replacement queen, worker reproduction is normal. Worker policing is switched off, many workers have active ovaries and lay eggs, and the colony rears a last batch of male brood before dying out. Here we report a colony which, when hopelessly queenless, did not stop policing although a high proportion of workers had active ovaries (12.6%) and many eggs were laid. However, all these eggs and also worker-laid eggs transferred from another colony were policed. This unusual pattern was repeated eight weeks later by a second queenless colony made using worker bees from the same mother colony, which strongly suggests genetic determination.Received 19 May 2003; revised 11 September 2003; accepted 23 September 2003.     

Optimal investment allocation in primitively eusocial bees: a balance model based on resource limitation of the queen

The classical model of colony dynamics developed by Macevicz and Oster predicts that optimal colony fitness in annual eusocial insects is achieved by a bang-bang strategy of reproduction: exclusive production of workers (ergonomic phase) followed by exclusive production of sexuals (reproductive phase). We propose an alternative model that assumes colony development in discrete broods and a limited overall investment potential of the queen. Based on the costs for producing eggs, workers, and sexuals and efficiency of individuals we predict the optimal number of workers and sexuals in the colony for each brood of the colony cycle that maximizes overall colony fitness. To link our model assumptions to the real world we chose model parameters according to field data of the halictid bee Lasioglossum malachurum. However, our model is representative of a large number of species with an annual life cycle and with discrete broods. Our model shows that the optimal partitioning of resources, i.e. the optimal workers/sexuals ratio depends on rearing cost for sexuals as well as productivity of workers but not on the queens’ total investment, egg cost, or rearing cost for workers. In complete accordance to Macevicz and Oster we predict a bang-bang reproduction strategy despite the differences in the basic assumptions. Potential deviations from this strategy and transitions from social to solitary breeding are discussed in the framework of our model. Received 31 October 2006; revised 29 March 2007; accepted 17 April 2007.     

Gene expression patterns associated with caste and reproductive status in ants: worker‐specific genes are more derived than queen‐specific ones

Variation in gene expression leads to phenotypic diversity and plays a central role in caste differentiation of eusocial insect species. In social Hymenoptera, females with the same genetic background can develop into queens or workers, which are characterized by divergent morphologies, behaviours and lifespan. Moreover, many social insects exhibit behaviourally distinct worker castes, such as brood‐tenders and foragers. Researchers have just started to explore which genes are differentially expressed to achieve this remarkable phenotypic plasticity. Although the queen is normally the only reproductive individual in the nest, following her removal, young brood‐tending workers often develop ovaries and start to reproduce. Here, we make use of this ability in the ant Temnothorax longispinosus and compare gene expression patterns in the queens and three worker castes along a reproductive gradient. We found the largest expression differences between the queen and the worker castes (~2500 genes) and the smallest differences between infertile brood‐tenders and foragers (~300 genes). The expression profile of fertile workers is more worker‐like, but to a certain extent intermediate between the queen and the infertile worker castes. In contrast to the queen, a high number of differentially expressed genes in the worker castes are of unknown function, pointing to the derived status of hymenopteran workers within insects.     

<Emphasis Type="Italic">Polistes japonicus</Emphasis> (Hymenoptera,Vespidae) queens monopolize ovipositing but are not the most active aggressor in dominant-subordinate interactions

In order to elucidate the dominant–subordinate relationship between the foundress and workers, five colonies of the paper wasp Polistes japonicus were observed in a netted and covered cage located outdoors. The number of workers in each colony ranged from four to eight. Workers were divided into first and second broods. Abdominal wagging and ovipositing were performed almost exclusively by the foundress throughout colony development. However, an analysis of aggressive encounters indicated that although the foundress hardly received dominance behaviors (aggression) from workers, it lacked either partially or completely the following characteristics of the queen that are usually seen in paper-wasp colonies with independent-founding queens (except in one colony that produced no second brood): the queen being socially dominant over any worker (the queen had more wins than losses in one-on-one dominance contests with any worker), exhibiting the highest frequency of dominance behaviors, and directing dominance behaviors primarily toward the socially most-dominant worker. In particular, during the mixed-brood period (when all first- and second-brood workers were present on the nest) the foundress hardly exhibited dominance behaviors toward socially dominant workers (mainly second brood) but frequently directed dominance behaviors toward socially subordinate workers (mainly first brood). The foundress disappeared in two colonies before the reproductives emerged; in these colonies the socially most-dominant worker inherited the colony and laid many eggs. The frequency of abdominal wagging by these two foundresses decreased during colony development, while it did not in the other colonies. This suggests that abdominal wagging provides information about the vigor of the performer. The superseder was socially dominant over all other workers, but spent little time wagging its abdomen and allowed some workers to lay eggs.     

Differential proteomics in dequeened honeybee colonies reveals lower viral load in hemolymph of fertile worker bees

The eusocial societies of honeybees, where the queen is the only fertile female among tens of thousands sterile worker bees, have intrigued scientists for centuries. The proximate factors, which cause the inhibition of worker bee ovaries, remain largely unknown; as are the factors which cause the activation of worker ovaries upon the loss of queen and brood in the colony. In an attempt to reveal key players in the regulatory network, we made a proteomic comparison of hemolymph profiles of workers with completely activated ovaries vs. rudimentary ovaries. An unexpected finding of this study is the correlation between age matched worker sterility and the enrichment of Picorna-like virus proteins. Fertile workers, on the other hand, show the upregulation of potential components of the immune system. It remains to be investigated whether viral infections contribute to worker sterility directly or are the result of a weaker immune system of sterile workers.     

New method to stimulate the onset of Bombus terrestris (Hymenoptera: Apidae) rearing: Using worker helpers in the presence of frozen pupae

The failure of bumblebee queens to enthusiastically start a colony under laboratory conditions may be due to lack of oviposition during an experiment, a long delay in oviposition from the termination of hibernation, and failure to rear a large first brood. In the present study, the use of frozen male pupae to start the colony with the assistance of bumblebee workers rather than fresh, young, male pupae was investigated under controlled room temperature and humidity conditions. The period of initiation of the colony decreased with an increase in the number of worker helpers from one to six. The period was as short as 3.9 days in the presence of six worker helpers. Second and third broods also started earlier with the help of workers. The rate of first worker production per egg cup was double that of the normal method (i.e. without worker helpers). Egg eating behavior of the queen was not found in the first brood if more than two workers helped the development of the larvae. Four worker helpers were found to be sufficient, as they could produce 100% colony foundation and 91.46% first worker production colonies. This study showed that using frozen old pupae is a good stimulator for colony foundation in the presence of four bumblebee worker helpers.     

Selectable components of sex allocation in colonies of the honeybee (Apis mellifera L.)

Colonies of social insects that undergo fission as a componentof reproduction produce large excesses of males. Hypothesesto explain this phenomenon have assumed that the workers thatconstitute the entourage for the new queen (or queens) representinvestment in female reproductives. Selection for optimal colonysex allocation then leads to an increase in production of malesthat balances the investment in females based on their relativereproductive values. We show that the construction of comb dedicatedto the production of males (drone comb) versus workers (workercomb) is a component of sex investment under the control ofcolony workers. Relative comb construction was highly correlatedwith the relative investment in male and worker brood. Coloniesthat invested relatively more in their total numbers of malesinvested less in the dry weight of individual workers. Coloniesthat had more adult workers produced a greater number of malesand workers, but colony size did not affect the proportionalinvestment in drone comb or brood. Genetic variability was foundfor the number of adult workers in colonies, the amount of dronecomb produced, the amount of worker comb produced, and the dryweight of adult workers, suggesting that sex allocation is aselectable trait in honeybees.     

Elimination of sexual brood in the Argentine antLinepithema humile: queen effect and brood recognition

Mated queens of the antLinepithema humile (Iridomyrmex humilis Mayr) introduced into dequeened colony fragments rearing sexual brood elicited worker aggression resulting in queen larvae being bitten and eliminated. By contrast, male larvae were spared. Regarding queen brood, killing mainly concerned small and medium sized larvae. A large proportion of the large larvae escaped extermination, and prepupae and pupae were spared. These data suggest that workers were able to discriminate sex, caste and age of the brood. That a queen pheromone may be involved was shown by experiments using whole or cut corpses that were either rinsed or not rinsed in pentane. The pheromone eliciting worker aggressive behaviour was shown to act over a short distance, suggesting that it is somewhat volatile. Similarities and differences between this new queen pheromone and other known queen pheromones acting on queen production or worker attraction are discussed as well as the origin of the signals underlying the recognition of the larval classes.     

THE EVOLUTION OF WORKER STERILITY IN HONEY BEES: AN INVESTIGATION INTO A BEHAVIORAL MUTANT CAUSING FAILURE OF WORKER POLICING

Normally, worker honey bees (Apis mellifera) only lay eggs when their colony is queenless. When a queen is present, worker egg-laying is controlled by mutual “policing” behavior in which any rare worker-laid eggs are eaten by other workers. However, an extremely rare behavioral phenotype arises in which workers develop functional ovaries and lay large numbers of eggs despite the presence of the queen. In this study, microsatellite analysis was used to determine the maternity of drones produced in such a colony under various conditions. One subfamily was found to account for about 90% of drone progeny, with the remainder being laid by other subfamilies or the queen. No evidence of queen policing was found. After a one-month period of extreme worker oviposition in spring, the colony studied reverted to normal behavior and showed no signs of worker oviposition. However, upon removal of the queen, workers commenced oviposition very quickly. Significantly, the subfamily that laid eggs when the queen was present did not contribute to the drone production when the colony was queenless. However, another subfamily contributed a disproportionately large number of drones. The frequency of worker oviposition appears to be determined by opposing selective forces. Individual bees benefit from personal reproduction, whereas other bees and the colony are disadvantaged by it. Thus a behavioral polymorphism can be maintained in the population in which some workers can escape worker policing, with balancing selection at the colony level to detect and eliminate these mutations.     

The impact of caging the queens during the flow season on some biological activities of honeybee colonies

This study was achieved in a private apiary located in a banana farm in Sa El Hagar, Basioun, Gharbia, Egypt from August 15, 2019 to May 25, 2020, including the banana (Musa sp., Musaceae) flow season (August and September) and extend to Egyptian clover (Trifolium alexandrinum L., Fabaceae) flow season (May). The study aimed to evaluate the effect of confining the queen during the banana flow season on the brood rearing, honey yield, and activation of worker's ovaries. Also, we determined the negative impact of caging the queen during the banana flow season on the activity of the colony in brood rearing, storing pollen, and honey yield after releasing the queen on 5 October, extending to the next flow season in May. The obtained results showed that the honeybee colonies with the caged queen produced significantly more honey yield and less brood production than the free queen ones during the banana flow season. Also, the caging of the queen did not affect the colony strength after releasing the queen despite the partial development of the ovaries of some workers, but they did not lay eggs. In addition, releasing the queens suppressed the ovaries of the laying workers. It can be concluded that caging the queen during the banana flow season helps the colonies to produce more honey yield without effect on the colony strength after releasing the queen despite the ovaries development of few workers without egg-laying.     

Associations between reproduction and work in workers of the Asian hive bee Apis cerana

If a honey bee (Apis spp.) colony becomes queenless, about 1/3 of young workers activate their ovaries and produce haploid male-producing eggs. In doing so queenless workers maximize their inclusive fitness because the normal option of vicarious production of relatives via their queen’s eggs is no longer available. But if many workers are engaged in reproduction, how does a queenless colony continue to feed its brood and forage? Here we show that in the Asian hive bee Apis cerana hypopharyngeal gland (HPG) size is larger in queenless workers than in queenright workers and that bees undertaking brood-rearing tasks have larger HPG than same-aged bees that are foraging. In queenless colonies, workers with a smaller number of ovarioles are more likely to have activated ovaries. This reinforces the puzzling observation that a large number of ovarioles reduces reproductive success in queenless A. cerana. It further suggests that reproductive workers either avoid foraging or transition to foraging later in life than non-reproductive workers. Finally, our study also showed that ovary activation and larger-than-average numbers of ovarioles had no statistically detectable influence on foraging specialization for pollen or nectar.     

Maternity of emergency queens in the Cape honey bee,Apis mellifera capensis

During reproductive swarming, some workers of the Cape honey bee, Apis mellifera capensis, lay eggs in queen cells, many of which are reared to maturity. However, it is unknown if workers are able to lay in queen cells immediately after queen loss during an episode of emergency queen rearing. In this study we experimentally de‐queened colonies and determined the maternity of larvae and pupae that were reared as queens. This allowed us to determine how soon after queen loss workers contribute to the production of new queens. We were further interested to see if workers would preferentially raise new queens from queen‐laid brood if this was introduced later. We performed our manipulations in two different settings: an apiary setting where colonies were situated close together and a more natural situation in which the colonies were well separated. This allowed us to determine how the vicinity of other colonies affects the presence of parasites. We found that workers do indeed contribute to queen cell production immediately after the loss of their queen, thus demonstrating that some workers either have activated ovaries even when their colony has a queen or are able to activate their ovaries extremely rapidly. Queen‐laid brood introduced days after queen loss was ignored, showing that workers do not prefer to raise new queens from queen brood when given a choice. We also detected non‐natal parasitism of queen cells in both settings. We therefore conclude that some A. m. capensis genotypes specialize in parasitizing queen cells.