Percent lipid is associated with body size but not task in the bumble bee Bombus impatiens

Nutritional inequalities structure worker division of labor in social insects

Eusocial insect societies are fundamentally non-egalitarian. The reproductive caste 'wins' in terms of resource accumulation, whereas non-reproductive workers 'lose'. Here, we argue that the division of labor among workers is also organized by nutritional inequalities. Across vastly different social systems and a variety of hymenopteran species, there is a recurrent pattern of lean foragers and corpulent nest workers. Experimental manipulations confirm causal associations between nutritional differences, associated molecular pathways, and behavioral roles in insect societies. The comparative and functional genomic data suggest that a conserved toolkit of core metabolic, nutrient storage, and signaling genes has evolved to regulate the social insect division of labor. Thus, the unequal distribution of food resources can be considered a fundamental organizing factor in the social insect division of labor.

Ecological Drivers and Consequences of Bumble Bee Body Size Variation

Body size is arguably one of the most important traits influencing the physiology and ecology of animals. Shifts in animal body size have been observed in response to climate change, including in bumble bees (Bombus spp. [Hymenoptera: Apidae]). Bumble bee size shifts have occurred concurrently with the precipitous population declines of several species, which appear to be related, in part, to their size. Body size variation is central to the ecology of bumble bees, from their social organization to the pollination services they provide to plants. If bumble bee size is shifted or constrained, there may be consequences for the pollination services they provide and for our ability to predict their responses to global change. Yet, there are still many aspects of the breadth and role of bumble bee body size variation that require more study. To this end, we review the current evidence of the ecological drivers of size variation in bumble bees and the consequences of that variation on bumble bee fitness, foraging, and species interactions. In total we review: (1) the proximate determinants and physiological consequences of size variation in bumble bees; (2) the environmental drivers and ecological consequences of size variation; and (3) synthesize our understanding of size variation in predicting how bumble bees will respond to future changes in climate and land use. As global change intensifies, a better understanding of the factors influencing the size distributions of bumble bees, and the consequences of those distributions, will allow us to better predict future responses of these pollinators.

Field-realistic concentrations of a neonicotinoid insecticide influence socially regulated brood development in a bumblebee

The systemic neonicotinoid insecticides are considered as one of the key culprits contributing to ongoing declines in pollinator health and abundance. Bumblebees are among the most important pollinators of temperate zone plants, making their susceptibility to neonicotinoid exposure of great concern. We report that bumblebee (Bombus terrestris) colonies exposed to field-realistic concentrations of the commonly used neonicotinoid Imidacloprid grew slower, consumed less food, and produced fewer workers, males and gynes, but unexpectedly produced larger workers compared to control colonies. Behavioural observations show that queens in pesticide-treated colonies spend more time inactive and less time caring for the brood. We suggest that the observed effects on brood body size are driven by a decreased queen ability to manipulate the larva developmental programme. These findings reveal an intricate and previously unknown effect of insecticides on the social interactions controlling brood development in social insect colonies. Insecticide influences on the social mechanisms regulating larval development are potentially detrimental for bumblebees, in which body size strongly influences both caste differentiation and the division of labour among workers, two organization principles of insect societies.

Decreased efficiency of pollen collection due to Sulfoxaflor exposure leads to a reduction in the size of bumble bee workers in late European summer

Bumble bees (Bombus terrestris) are important pollinators of wild and crop plants. Despite their importance in the process of fruit and seed production on crop sites, their activity may be impaired due to exposure to pesticides. This species has a yearly life cycle and colony success may rely on effective foraging of workers on ruderal plants late in summer when most crops are no longer flowering. In the current study, we investigated the effect of chronic exposure to Sulfoxaflor on aspects of the foraging behavior of bumble bees and whether Sulfoxaflor influences the body size of workers of B. terrestris in a crop landscape. We found that 2 weeks of continuous exposure to Sulfoxaflor influenced workers’ foraging dynamics and collection of resources. However, there was no evidence that the 5 ppb dose of the pesticide impacted the ability of bees to handle flowers with different traits. Workers from colonies exposed to Sulfoxaflor were smaller. The effect on worker size may be explained as a consequence of the reduced pollen income per unit of worker foraging. Thus, if the effects of Sulfoxaflor applied directly to crops had the same effect as that observed on commercial bumble bees after our chronic exposure, it might negatively impact colony success due to the impact on pollen collection and the reduction in the size of workers.

Gene expression and epigenetics reveal species-specific mechanisms acting upon common molecular pathways in the evolution of task division in bees

A striking feature of advanced insect societies is the existence of workers that forgo reproduction. Two broad types of workers exist in eusocial bees: nurses who care for their young siblings and the queen, and foragers who guard the nest and forage for food. Comparisons between these two worker subcastes have been performed in honeybees, but data from other bees are scarce. To understand whether similar molecular mechanisms are involved in nurse-forager differences across distinct species, we compared gene expression and DNA methylation profiles between nurses and foragers of the buff-tailed bumblebee Bombus terrestris and the stingless bee Tetragonisca angustula. These datasets were then compared to previous findings from honeybees. Our analyses revealed that although the expression pattern of genes is often species-specific, many of the biological processes and molecular pathways involved are common. Moreover, the correlation between gene expression and DNA methylation was dependent on the nucleotide context, and non-CG methylation appeared to be a relevant factor in the behavioral changes of the workers. In summary, task specialization in worker bees is characterized by a plastic and mosaic molecular pattern, with species-specific mechanisms acting upon broad common pathways across species.

Care-giver identity impacts offspring development and performance in an annually social bumble bee

BACKGROUND

The developmental fates of offspring have the potential to be influenced by the identity of their care-givers and by the nature of the care that they receive. In animals that exhibit both parental and alloparental care, such as the annually eusocial insects, the influence of care-giver identity can be directly assessed to yield mechanistic and evolutionary insights into the origins and elaboration of brood care. Here, we performed a comparative investigation of maternal and worker brood care in bumble bees, a pollinator group where mothers (queens) rear the first offspring in the nest, and then daughters (workers) assume this role upon their emergence. Specifically, we compared the effects of queen and worker brood care on offspring development and also offspring performance, for a set of traits related to sensory biology, learning, and stress resistance.

RESULTS

We found that queen-reared workers were smaller-bodied than worker-reared offspring, suggesting that bumble bee queens influence body size determination in their offspring. We also found that queen-reared workers were more resistant to starvation, which might be beneficial for early nesting success. These maternal influences could not be explained by feeding rate, given that we detected a similar offspring feeding frequency in both queens and workers.

CONCLUSION

Bumble bee queens have a unique influence on the development of the first offspring in the nest, which they rear, relative to worker-reared workers. We propose that bumble bee brood care has been shaped by a suite of evolutionary and ecological factors, which might include a maternal influence on traits that promote survival of incipient colonies.

Pollen defenses negatively impact foraging and fitness in a generalist bee (Bombus impatiens: Apidae)

Plants may benefit from limiting the community of generalist floral visitors if the species that remain are more effective pollinators and less effective pollenivores. Plants can reduce access to pollen through altered floral cues or morphological structures, but can also reduce consumption through direct pollen defenses. We observed that Eucera (Peponapis) pruinosa, a specialist bee on Cucurbita plants, collected pure loads of pollen while generalist honey bees and bumble bees collected negligible amounts of cucurbit pollen, even though all groups of bees visited these flowers. Cucurbit flowers have no morphological adaptations to limit pollen collection by bees, thus we assessed their potential for physical, nutritional, and chemical pollen traits that might act as defenses to limit pollen loss to generalist pollinators. Bumble bee (Bombus impatiens) microcolonies experienced reduced pollen consumption, mortality, and reproduction as well as increased stress responses when exposed to nutritional and mechanical pollen defenses. These bees also experienced physiological effects of these defenses in the form of hindgut expansion and gut melanization. Chemical defenses alone increased the area of gut melanization in larger bees and induced possible compensatory feeding. Together, these results suggest that generalist bumble bees avoid collecting cucurbit pollen due to the physiological costs of physical and chemical pollen defenses.

A Review of Native Wild Bee Nutritional Health

Declines in wild and managed bee species richness and abundances have been observed throughout Europe and North America in recent decades. These declines have led to questions regarding pollination of wild and cultivated plants. In response to these concerns, efforts towards the conservation of pollinators have been initiated. Part of this conservation effort should be to provide the basic nutritional needs for bees. Nutrition plays one of the most important roles in bee growth, development, and reproduction. There is a large body of information regarding honey bee nutrition, whereas we lack nutritional information on native wild bees. Our knowledge of bumble bee nutritional needs has increased since the introduction of commercial rearing and sale of certain bumble bee species; however, there is still a lack of basic nutritional guidelines such as minimum dietary needs of proteins, amino acids, lipids, and sterols. The large difference in physiology and life history between honey bees and North American wild bees suggests that their nutritional requirements could be quite different.

The geometric framework for nutrition reveals interactions between protein and carbohydrate during larval growth in honey bees

In holometabolous insects, larval nutrition affects adult body size, a life history trait with a profound influence on performance and fitness. Individual nutritional components of larval diets are often complex and may interact with one another, necessitating the use of a geometric framework for elucidating nutritional effects. In the honey bee, Apis mellifera, nurse bees provision food to developing larvae, directly moderating growth rates and caste development. However, the eusocial nature of honey bees makes nutritional studies challenging, because diet components cannot be systematically manipulated in the hive. Using in vitro rearing, we investigated the roles and interactions between carbohydrate and protein content on larval survival, growth, and development in A. mellifera We applied a geometric framework to determine how these two nutritional components interact across nine artificial diets. Honey bees successfully completed larval development under a wide range of protein and carbohydrate contents, with the medium protein (∼5%) diet having the highest survival. Protein and carbohydrate both had significant and non-linear effects on growth rate, with the highest growth rates observed on a medium-protein, low-carbohydrate diet. Diet composition did not have a statistically significant effect on development time. These results confirm previous findings that protein and carbohydrate content affect the growth of A. mellifera larvae. However, this study identified an interaction between carbohydrate and protein content that indicates a low-protein, high-carb diet has a negative effect on larval growth and survival. These results imply that worker recruitment in the hive would decline under low protein conditions, even when nectar abundance or honey stores are sufficient.

Specialization on pollen or nectar in bumblebee foragers is not associated with ovary size, lipid reserves or sensory tuning

Foraging specialization allows social insects to more efficiently exploit resources in their environment. Recent research on honeybees suggests that specialization on pollen or nectar among foragers is linked to reproductive physiology and sensory tuning (the Reproductive Ground-Plan Hypothesis; RGPH). However, our understanding of the underlying physiological relationships in non-Apis bees is still limited. Here we show that the bumblebee Bombus terrestris has specialist pollen and nectar foragers, and test whether foraging specialization in B. terrestris is linked to reproductive physiology, measured as ovarian activation. We show that neither ovary size, sensory sensitivity, measured through proboscis extension response (PER), or whole-body lipid stores differed between pollen foragers, nectar foragers, or generalist foragers. Body size also did not differ between any of these three forager groups. Non-foragers had significantly larger ovaries than foragers. This suggests that potentially reproductive individuals avoid foraging.

Summer and fall ants have different physiological responses to food macronutrient content

Seasonally, long-lived animals exhibit changes in behavior and physiology in response to shifts in environmental conditions, including food abundance and nutritional quality. Ants are long-lived arthropods that, at the colony level, experience such seasonal shifts in their food resources. Previously we reported summer- and fall-collected ants practiced distinct food collection behavior and nutrient intake regulation strategies in response to variable food protein and carbohydrate content, despite being reared in the lab under identical environmental conditions and dietary regimes. Seasonally distinct responses were observed for both no-choice and choice dietary experiments. Using data from these same experiments, our objective here is to examine colony and individual-level physiological traits, colony mortality and growth, food processing, and worker lipid mass, and how these traits change in response to variable food protein-carbohydrate content. For both experiments we found that seasonality per se exerted strong effects on colony and individual level traits. Colonies collected in the summer maintained total worker mass despite high mortality. In contrast, colonies collected in the fall lived longer, and accumulated lipids, including when reared on protein-biased diets. Food macronutrient content had mainly transient effects on physiological responses. Extremes in food carbohydrate content however, elicited a compensatory response in summer worker ants, which processed more protein-biased foods and contained elevated lipid levels. Our study, combined with our previously published work, strongly suggests that underlying physiological phenotypes driving behaviors of summer and fall ants are likely fixed seasonally, and change circannually.

Behavioural syndromes and social insects: personality at multiple levels

Animal personalities or behavioural syndromes are consistent and/or correlated behaviours across two or more situations within a population. Social insect biologists have measured consistent individual variation in behaviour within and across colonies for decades. The goal of this review is to illustrate the ways in which both the study of social insects and of behavioural syndromes has overlapped, and to highlight ways in which both fields can move forward through the synergy of knowledge from each. Here we, (i) review work to date on behavioural syndromes (though not always referred to as such) in social insects, and discuss mechanisms and fitness effects of maintaining individual behavioural variation within and between colonies; (ii) summarise approaches and principles from studies of behavioural syndromes, such as trade-offs, feedback, and statistical methods developed specifically to study behavioural consistencies and correlations, and discuss how they might be applied specifically to the study of social insects; (iii) discuss how the study of social insects can enhance our understanding of behavioural syndromes-research in behavioural syndromes is beginning to explore the role of sociality in maintaining or developing behavioural types, and work on social insects can provide new insights in this area; and (iv) suggest future directions for study, with an emphasis on examining behavioural types at multiple levels of organisation (genes, individuals, colonies, or groups of individuals).