Worker drift and egg dumping by queens in wild Bombus terrestris colonies

Investigating the Foraging, Guarding and Drifting Behaviors of Commercial Bombus terrestris

Social insects have high levels of cooperation and division of labor. In bumble bees this is partly size-based, with larger bees performing tasks outside the nest and smaller bees remaining inside, although bumble bees still display considerable behavioral plasticity. The level of specialization in tasks outside the colony, including foraging, guarding and drifting (entering a foreign colony), is currently unknown for bumble bees. This study aimed to assess division of labor between outside tasks and the degree of specialization in foraging, guarding, and switching colonies in commercially reared bumble bees placed in the field. Nine factory-bought Bombus terrestris colonies were placed on three farms in Sussex, UK, between June and August 2015. Forty workers from each colony were radio-tagged and a reader on the colony entrance recorded the date, time and bee ID as they passed. The length and frequency of foraging trips and guarding behavior were calculated, and drifting recorded. The mean (±SD) length of foraging trips was 45 ± 36 min, and the mean number of foraging trips per day was 7.75 ± 7.71. Low levels of specialization in guarding or foraging behavior were found; however, some bees appeared to guard more frequently than others, and twenty bees were categorized as guards. Five bees appeared to exhibit repeated "stealing" behavior, which may have been a specialist task. The division of labor between tasks was not size-based. It is concluded that commercial bumble bees are flexible in performing outside nest tasks and may have diverse foraging strategies including intra-specific nest robbing.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10905-021-09790-0.

Multi-level social organization and nest-drifting behaviour in a eusocial insect

Stable social groups usually consist of families. However, recent studies have revealed higher level social structure, with interactions between family groups across different levels of social organization in multiple species. The explanations for why this apparently paradoxical behaviour arises appear to be varied and remain untested. Here, we use automated radio-tagging data from over 1000 wasps from 93 nests and social network analyses of over 30 000 nest visitation records to describe and explain interactions across levels of social organization in the eusocial paper wasp Polistes canadensis. We detected three levels of social organization (nest, aggregation and community) which exchange 'drifter' individuals within and between levels. The highest level (community) may be influenced by the patchiness of high-quality nesting habitats in which these insects exist. Networks of drifter movements were explained by the distance between nests, the group size of donor nests and the worker-to-brood ratios on donor and recipient nests. These findings provide some explanation for the multi-level social interactions, which may otherwise seem paradoxical. Fitness benefits across multiple levels of social organization should be considered when trying to understand animal societies.

Worker Size Diversity Has No Effect on Overwintering Success under Natural Conditions in the Ant Temnothorax nylanderi

Simple Summary

Winter is a harsh season for organisms living in temperate zones. Winter is often associated with starvation and cold temperatures, and these pressures can strongly affect organism survival. Living in groups can help these animals to cope with winter pressures. Social groups contain individuals which can vary in different ways: physiology, behavior, morphology, etc. In social insects such as ants, worker size leads to different responses to starvation and cold temperature in the laboratory. In this study, we investigated whether worker size affects colony and individual survival under natural conditions. We manipulated both worker size diversity and mean worker size within colonies of the ant Temnothorax nylanderi, reintroduced them in the field, and measured colony survival after overwintering. We found similar colony and individual (both adults and young) survival during winter between treatment colonies with reduced size diversity and/or manipulated mean worker size compared to control colonies with unmanipulated worker size. This result highlights that worker size diversity has no influence on colony performance in this species and more broadly questions the interest of worker size in social insect species with moderate worker size diversity. We discuss the potential sources of worker size diversity, including social context and selfish behavior.

Abstract

Winter is a difficult period for animals that live in temperate zones. It can inflict high mortality or induce weight loss with potential consequences on performance during the growing season. Social groups include individuals of various ages and sizes. This diversity may improve the ability of groups to buffer winter disturbances such as starvation or cold temperature. Studies focusing on the buffering role of social traits such as mean size and diversity of group members under winter conditions are mainly performed in the laboratory and investigate the effect of starvation or cold separately. Here, we experimentally decreased worker size diversity and manipulated worker mean size within colonies in order to study the effect on overwintering survival in the ant Temnothorax nylanderi. Colonies were placed under natural conditions during winter. Colony survival was high during winter and similar in all treatments with no effect of worker size diversity and mean worker size. Higher brood survival was positively correlated with colony size (i.e., the number of workers). Our results show that the higher resistance of larger individuals against cold or starvation stresses observed in the laboratory does not directly translate into higher colony survival in the field. We discuss our results in the light of mechanisms that could explain the possible non-adaptive size diversity in social species.

Substantial direct fitness gains of workers in a highly eusocial ant

Hamilton's theory of inclusive fitness suggests that helpers in animal societies gain fitness indirectly by increasing the reproductive performance of a related beneficiary. Helpers in cooperatively breeding birds, mammals and primitively eusocial wasps may additionally obtain direct fitness through inheriting the nest or mating partner of the former reproductive. Here, we show that also workers of a highly eusocial ant may achieve considerable direct fitness by producing males in both queenless and queenright colonies. We investigated the reproductive success of workers of the ant Temnothorax crassispinus in nature and the laboratory by dissecting workers and determining the origin of males by microsatellite analysis. We show that workers are capable of activating their ovaries and successfully producing their sons independently of the presence of a queen. Genotypes revealed that at least one fifth of the males in natural queenright colonies were not offspring of the queen. Most worker-produced males could be assigned to workers that were unrelated to the queen, suggesting egg-laying by drifting workers.

Body size and sperm quality in queen- And worker-produced ant males

Workers of many species of social Hymenoptera have functional ovaries and are capable of laying haploid, unfertilized eggs, at least in the absence of a queen. Except for honeybees, it remains largely unknown whether worker-produced males have the same quality as queen-produced males and whether workers benefit in direct fitness by producing their sons. Previous studies in the monogynous ant Temnothorax crassispinus revealed that a high proportion of males in natural and laboratory colonies are worker offspring. Here, we compare longevity, body size, sperm length and sperm viability between queen- and worker-produced males. We either split queenright colonies into queenright and queenless halves or removed the queen from a fraction of the queenright colonies and then examined the newly produced males. Male quality traits varied considerably among colonies but differed only slightly between queen- and worker-produced males. Worker-produced males outnumbered queen-produced males and also had a longer lifespan, but under certain rearing conditions sperm from queen-produced males had a higher viability.

The Neonicotinoid Insecticide Thiacloprid Impacts upon Bumblebee Colony Development under Field Conditions

The impacts of pesticides, and in particular of neonicotinoids, on bee health remain much debated. Many studies describing negative effects have been criticized as the experimental protocol did not perfectly simulate real-life field scenarios. Here, we placed free-flying bumblebee colonies next to raspberry crops that were either untreated or treated with the neonicotinoid thiacloprid as part of normal farming practice. Colonies were exposed to the raspberry crops for a two week period before being relocated to either a flower-rich or flower-poor site. Overall, exposed colonies were more likely to die prematurely, and those that survived reached a lower final weight and produced 46% fewer reproductives than colonies placed at control farms. The impact was more marked at the flower-rich site (all colonies performed poorly at the flower poor site). Analysis of nectar and pollen stores from bumblebee colonies placed at the same raspberry farms revealed thiacloprid residues of up to 771 ppb in pollen and up to 561 ppb in nectar. The image of thiacloprid as a relatively benign neonicotinoid should now be questioned.

Molecular tools and bumble bees: revealing hidden details of ecology and evolution in a model system

Bumble bees are a longstanding model system for studies on behaviour, ecology and evolution, due to their well-studied social lifestyle, invaluable role as wild and managed pollinators, and ubiquity and diversity across temperate ecosystems. Yet despite their importance, many aspects of bumble bee biology have remained enigmatic until the rise of the genetic and, more recently, genomic eras. Here, we review and synthesize new insights into the ecology, evolution and behaviour of bumble bees that have been gained using modern genetic and genomic techniques. Special emphasis is placed on four areas of bumble bee biology: the evolution of eusociality in this group, population-level processes, large-scale evolutionary relationships and patterns, and immunity and resistance to pesticides. We close with a prospective on the future of bumble bee genomics research, as this rapidly advancing field has the potential to further revolutionize our understanding of bumble bees, particularly in regard to adaptation and resilience. Worldwide, many bumble bee populations are in decline. As such, throughout the review, connections are drawn between new molecular insights into bumble bees and our understanding of the causal factors involved in their decline. Ongoing and potential applications to bumble bee management and conservation are also included to demonstrate how genetics- and genomics-enabled research aids in the preservation of this threatened group.

Emerging viral disease risk to pollinating insects: ecological, evolutionary and anthropogenic factors

The potential for infectious pathogens to spillover and emerge from managed populations to wildlife communities is poorly understood, but ecological, evolutionary and anthropogenic factors are all likely to influence the initial exposure and subsequent infection, spread and impact of disease. Fast-evolving RNA viruses, known to cause severe colony losses in managed honeybee populations, deserve particular attention for their propensity to jump between host species and thus threaten ecologically and economically important wild pollinator communities. We review the literature on pollinator viruses to identify biological and anthropogenic drivers of disease emergence, highlight gaps in the literature, and discuss potential management strategies. We provide evidence that many wild pollinator species are exposed to viruses from commercial species, resulting in multiple spillover events. However, it is not clear whether species become infected as a result of spillover or whether transmission is occurring within these wild populations. Ecological traits of pollinating insects, such as overlapping ranges, niches and behaviours, clearly promote cross-species transmission of RNA viruses. Moreover, we conclude that the social behaviour and phylogenetic relatedness of social pollinators further facilitate within- and between-host transmission, leaving these species particularly vulnerable to emerging diseases. We argue that the commercial use of pollinators is a key driver of disease emergence in these beneficial insects and that this must be addressed by management and policy. Synthesis and applications. There are important knowledge gaps, ranging from disease distribution and prevalence, to pathogen life history and virulence, to the impacts of disease emergence, which need to be addressed as research priorities. It is clear that avoiding anthropogenic pathogen spillover is crucial to preventing and managing disease emergence in pollinators, with far-reaching effects on our food security, ecosystem services and biodiversity. We argue that it is crucial to prevent the introduction of diseased pollinators into natural environments, which can be achieved through improved monitoring and management practices.

Chemical ecology of bumble bees

Bumble bees are of major importance, ecologically and economically as pollinators in cool and temperate biomes and as model organisms for scientific research. Chemical signals and cues have been shown to play an outstanding role in intraspecific and interspecific communication systems within and outside of a bumble bee colony. In the present review we compile and critically assess the literature on the chemical ecology of bumble bees, including cuckoo bumble bees. The development of new and more sensitive analytical tools and improvements in sociogenetic methods significantly enhanced our knowledge about chemical compounds that mediate the regulation of reproduction in the social phase of colony development, about the interactions between host bumble bees and their social parasites, about pheromones involved in mating behavior, as well as about the importance of signals, cues and context-dependent learning in foraging behavior. Our review intends to stimulate new studies on the many unresolved questions concerning the chemical ecology of these fascinating insects.

Specific recognition of reproductive parasite workers by nest-entrance guards in the bumble bee Bombus terrestris

Background

The impact of social parasites on their hosts’ fitness is a strong selective pressure that can lead to the evolution of adapted defence strategies. Guarding the nest to prevent the intrusion of parasites is a widespread response of host species. If absolute rejection of strangers provides the best protection against parasites, more fine-tuned strategies can prove more adaptive. Guarding is indeed costly and not all strangers constitute a real threat. That is particularly true for worker reproductive parasitism in social insects since only a fraction of non-nestmate visitors, the fertile ones, can readily engage in parasitic reproduction. Guards should thus be more restrictive towards fertile than sterile non-nestmate workers. We here tested this hypothesis by examining the reaction of nest-entrance guards towards nestmate and non-nestmate workers with varying fertility levels in the bumble bee Bombus terrestris. Because social recognition in social insects mainly relies on cuticular lipids (CLs), chemical analysis was also conducted to examine whether workers’ CLs could convey the relevant information upon which guards could base their decision. We thus aimed to determine whether an adapted defensive strategy to worker reproductive parasitism has evolved in B. terrestris colonies.

Results

Chemical analysis revealed that the cuticular chemical profiles of workers encode information about both their colony membership and their current fertility, therefore providing potential recognition cues for a suitable adjustment of the guards’ defensive decisions. We found that guards were similarly tolerant towards sterile non-nestmate workers than towards nestmate workers. However, as predicted, guards responded more aggressively towards fertile non-nestmates.

Conclusion

Our results show that B. terrestris guards discriminate non-nestmates that differ in their reproductive potential and respond more strongly to the individuals that are a greatest threat for the colony. Cuticular hydrocarbons are the probable cues underlying the specific recognition of reproductive parasites, with the specific profile of highly fertile bees eliciting the agonistic response when combined with non-colony membership information. Our study therefore provides a first piece of empirical evidence supporting the hypothesis that an adapted defensive strategy against worker reproductive parasitism exists in B. terrestris colonies.

Drifting behaviour as an alternative reproductive strategy for social insect workers

Restricted reproduction is traditionally posited as the defining feature of eusocial insect workers. The discovery of worker reproduction in foreign colonies challenges this view and suggests that workers' potential to pursue selfish interests may be higher than previously believed. However, whether such reproductive behaviour truly relies on a reproductive decision is still unknown. Workers' reproductive decisions thus need to be investigated to assess the extent of workers' reproductive options. Here, we show in the bumblebee Bombus terrestris that drifting is a distinct strategy by which fertile workers circumvent competition in their nest and reproduce in foreign colonies. By monitoring workers' movements between colonies, we show that drifting is a remarkably dynamic behaviour, widely expressed by both fertile and infertile workers. We demonstrate that a high fertility is, however, central in determining the propensity of workers to enter foreign colonies as well as their subsequent reproduction in host colonies. Moreover, our study shows that the drifting of fertile workers reflects complex decision-making processes associated with in-nest reproductive competition. This novel finding therefore adds to our modern conception of cooperation by showing the previously overlooked importance of alternative strategies which enable workers to assert their reproductive interests.