Bombus (Hymenoptera: Apidae) Microcolonies as a Tool for Biological Understanding and Pesticide Risk Assessment

New genomic resources inform transcriptomic responses to heavy metal toxins in the common Eastern bumble bee Bombus impatiens

BACKGROUND

The common Eastern bumble bee Bombus impatiens is native to North America and is the main commercially reared pollinator in the Americas. There has been extensive research on this species related to its social biology, applied pollination, and genetics. The genome of this species was previously sequenced using short-read technology, but recent technological advances provide an opportunity for substantial improvements. This species is common in agricultural and urban environments, and heavy metal contaminants produced by industrial processes can negatively impact it. To begin to identify possible mechanisms underlying responses to these toxins, we used RNA-sequencing to examine how exposure to a cocktail of four heavy metals at field-realistic levels from industrial areas affected B. impatiens worker gene expression.

RESULTS

PacBio long-read sequencing resulted in 544x coverage of the genome, and HiC technology was used to map chromatin contacts. Using Juicer and manual curation, the genome was scaffolded into 18 main pseudomolecules, representing a high quality, chromosome-level assembly. The sequenced genome size is 266.6 Mb and BRAKER3 annotation produced 13,938 annotated genes. The genome and annotation show high completeness, with ≥ 96% of conserved Eukaryota and Hymenoptera genes present in both the assembly and annotated genes. RNA sequencing of heavy metal exposed workers revealed 603 brain and 34 fat body differentially expressed genes. In the brain, differentially expressed genes had biological functions related to chaperone activity and protein folding.

CONCLUSIONS

Our data represent a large improvement in genomic resources for this important model species-with 10% more genome coverage than previously available, and a high-quality assembly into 18 chromosomes, the expected karyotype for this species. The new gene annotation added 777 new genes. Altered gene expression in response to heavy metal exposure suggests a possible mechanism for how these urban toxins are negatively impacting bee health, specifically by altering protein folding in the brain. Overall, these data are useful as a general high quality genomic resource for this species, and provide insight into mechanisms underlying tissue-specific toxicological responses of bumble bees to heavy metals.

Consumption of pollen contaminated with field-realistic concentrations of fungicide causes sublethal effects in Bombus impatiens (Hymenoptera: Apidae) microcolonies

Bumble bees are declining across the globe. The causes of this decline have been attributed to a variety of stressors, including pesticides. Fungicides are a type of pesticide that has been understudied in the context of bumble bee health. As a result, fungicides are often applied to flowering plants without consideration of pollinator exposure. Recent work demonstrates that fungicides have sublethal effects in bumble bees, but little is known about how much fungicide it takes to cause these sublethal effects. To address this gap in the literature, we fed microcolonies of the common eastern bumble bee (Bombus impatiens CressonHymenoptera: ApidaeHymenoptera: ApidaeHymenoptera: ApidaeHymenoptera: Apidae) pollen contaminated with a range of fungicide concentrations. We chose these concentrations based on the range of fungicide concentrations in pollen and nectar that were reported in the literature. Results revealed that later-stage pupae and newly emerged males are potentially sensitive to fungicide exposure, showing smaller size and reduced fat reserves at intermediate levels of contamination. Compared to the control, intermediated levels of fungicide-contaminated pollen led to increased pupal mortality and delayed male emergence. Contrary to expectations, higher fungicide levels did not exhibit a linear relationship with negative impacts, suggesting nuanced effects. Because body size and emergence timing are important aspects of bumble bee reproductive behavior, results have implications for mating success, potentially disrupting colony development.

Lactobacillus melliventris promotes hive productivity and immune functionality in Bombus terrestris performance in the greenhouse

Bumblebees are important pollinators in agricultural ecosystems, but their abundance is declining globally. There is an urgent need to protect bumblebee health and their pollination services. Bumblebees possess specialized gut microbiota with potential to be used as probiotics to help defend at-risk bumblebee populations. However, evidence for probiotic benefits on bumblebees is lacking. Here, we evaluated how supplementation with Lactobacillus melliventris isolated from bumblebee gut affected the colony development of Bombus terrestris. This native strain colonized robustly and persisted long-term in bumblebees, leading to a significantly higher quality of offspring. Subsequently, the tyrosine pathway was upregulated in the brain and fat body, while the Wnt and mTOR pathways of the gut were downregulated. Notably, the field experiment in the greenhouse revealed the supplementation of L. melliventris led to a 2.5-fold increase in the bumblebee survival rate and a more than 10% increase in the number of flowers visited, indicating a better health condition and pollination ability in field conditions. Our study represents a first screening for the potential use of the native gut member, L. melliventris, as probiotic strains in hive supplement for bumblebee breeding, which may be a practical approach to improve immunity and hive health.

Temperature influences desiccation resistance of bumble bees

Ongoing climate change has increased temperatures and the frequency of droughts in many parts of the world, potentially intensifying the desiccation risk for insects. Because resisting desiccation becomes more difficult at higher temperatures and lower humidity, avoiding water loss is a key challenge facing terrestrial insects. However, few studies have examined the interactive effects of temperature and environmental humidity on desiccation resistance in insects. Such studies on bees (Hymenoptera: Apoidea: Anthophila) are especially rare, despite their ecological and economic importance. Here, we crossed temperature (20, 25, and 30 °C) with humidity (<5, 50, >95 % RH) manipulations and measured time to mortality, water loss rates, and the water content at mortality of bumble bees (Bombus impatiens). We found that both higher temperature and lower humidity increased water loss rates, while warmer temperatures reduced survival time and lower humidity decreased water content at mortality. Additionally, we observed large intraspecific variation in water balance traits between colonies, and larger individuals survived longer and could tolerate more water loss before mortality. This study raises important questions about the mechanisms underpinning water loss in bumble bees and suggests that frequent access to nectar may be especially important for bumble bees' water balance and survival in a warming and drying climate.

Bumblebees are resilient to neonicotinoid-fungicide combinations

Bumblebees are among the most important wild bees for pollination of crops and securing wildflower diversity. However, their abundance and diversity have been on a steady decrease in the last decades. One of the most important factors leading to their decline is the frequent use of plant protection products (PPPs) in agriculture, which spread into forests and natural reserves. Mixtures of different PPPs pose a particular threat because of possible synergistic effects. While there is a comparatively large body of studies on the effects of PPPs on honeybees, we still lack data on wild bees. We here investigated the influence of the frequent fungicide Cantus® Gold (boscalid/dimoxystrobin), the neonicotinoid insecticide Mospilan® (acetamiprid) and their combination on bumblebees. Cognitive performance and foraging flights of bumblebees were studied. They are essential for the provisioning and survival of the colony. We introduce a novel method for testing four treatments simultaneously on the same colony, minimizing inter-colony differences. For this, we successfully quartered the colony and moved the queen daily between compartments. Bumblebees appeared astonishingly resilient to the PPPs tested or they have developed mechanisms for detoxification. Neither learning capacity nor flight activity were inhibited by treatment with the single PPPs or their combination.

Exposure to a fungicide for a field-realistic duration does not alter bumble bee fecal microbiota structure

Social bees are frequently exposed to pesticides when foraging on nectar and pollen. Recent research has shown that pesticide exposure not only impacts social bee host health but can also alter the community structure of social bee gut microbiotas. However, most research on pesticide-bee gut microbiota interactions has been conducted in honey bees; bumble bees, native North American pollinators, have received less attention and, due to differences in their ecology, may be exposed to certain pesticides for shorter durations than honey bees. Here, we examine how exposure to the fungicide chlorothalonil for a short, field-realistic duration alters bumble bee fecal microbiotas (used as a proxy for gut microbiotas) and host performance. We expose small groups of Bombus impatiens workers (microcolonies) to field-realistic chlorothalonil concentrations for 5 days, track changes in fecal microbiotas during the exposure period and a recovery period, and compare microcolony offspring production between treatments at the end of the experiment. We also assess the use of fecal microbiotas as a gut microbiota proxy by comparing community structures of fecal and gut microbiotas. We find that chlorothalonil exposure for a short duration does not alter bumble bee fecal microbiota structure or affect microcolony production at any concentration but that fecal and gut microbiotas differ significantly in community structure. Our results show that, at least when exposure durations are brief and unaccompanied by other stressors, bumble bee microbiotas are resilient to fungicide exposure. Additionally, our work highlights the importance of sampling gut microbiotas directly, when possible.IMPORTANCEWith global pesticide use expected to increase in the coming decades, studies on how pesticides affect the health and performance of animals, including and perhaps especially pollinators, will be crucial to minimize negative environmental impacts of pesticides in agriculture. Here, we find no effect of exposure to chlorothalonil for a short, field-realistic period on bumble bee fecal microbiota community structure or microcolony production regardless of pesticide concentration. Our results can help inform pesticide use practices to minimize negative environmental impacts on the health and fitness of bumble bees, which are key native, commercial pollinators in North America. We also find that concurrently sampled bumble bee fecal and gut microbiotas contain similar microbes but differ from one another in community structure and consequently suggest that using fecal microbiotas as a proxy for gut microbiotas be done cautiously; this result contributes to our understanding of proxy use in gut microbiota research.

Sub-lethal but potentially devastating - The novel insecticide flupyradifurone impairs collective brood care in bumblebees

The worldwide decline in pollinating insects is alarming. One of the main anthropogenic drivers is the massive use of pesticides in agriculture. Risk assessment procedures test pesticides for mortality rates of well-fed, parasite free individuals of a few non-target species. Sublethal and synergistic effects of co-occurring stressors are usually not addressed. Here, we present a simple, wildly applicable bio-essay to assess such effects. Using brood thermoregulation in bumblebee microcolonies as readout, we investigate how this collective ability is affected by long-term feeding exposure to the herbicide glyphosate (5 mg/l), the insecticide flupyradifurone (0.4 mg/l) and the combination of both, when co-occurring with the natural stressor of resource limitation. Documenting brood temperature and development in 53 microcolonies we find no significant effect of glyphosate, while flupyradifurone significantly impaired the collective ability to maintain the necessary brood temperatures, resulting in prolonged developmental times and a decrease in colony growth by over 50 %. This reduction in colony growth has the potential to significantly curtail the reproductive chances of colonies in the field. Our findings highlight the potentially devastating consequences of flupyradifurone use in agriculture even at sub-lethal doses and underline the urgent need for improved risk assessment procedures.

A case study of the diet-microbiota-parasite interplay in bumble bees

AIMS

Diets and parasites influence the gut bacterial symbionts of bumble bees, but potential interactive effects remain overlooked. The main objective of this study was to assess the isolated and interactive effects of sunflower pollen, its phenolamides, and the widespread trypanosomatid Crithidia sp. on the gut bacterial symbionts of Bombus terrestris males.

METHODS AND RESULTS

Bumble bee males emerged in microcolonies fed on either (i) willow pollen (control), (ii) sunflower pollen, or (iii) willow pollen spiked with phenolamide extracts from sunflower pollen. These microcolonies were infected by Crithidia sp. or were pathogen-free. Using 16S rRNA amplicon sequencing (V3-V4 region), we observed a significant alteration of the beta diversity but not of the alpha diversity in the gut microbial communities of males fed on sunflower pollen compared to males fed on control pollen. Similarly, infection by the gut parasite Crithidia sp. altered the beta diversity but not the alpha diversity in the gut microbial communities of males, irrespective of the diet. By contrast, we did not observe any significant alteration of the beta or alpha diversity in the gut microbial communities of males fed on phenolamide-enriched pollen compared to males fed on control pollen. Changes in the beta diversity indicate significant dissimilarities of the bacterial taxa between the treatment groups, while the lack of difference in alpha diversity demonstrates no significant changes within each treatment group.

CONCLUSIONS

Bumble bees harbour consistent gut microbiota worldwide, but our results suggest that the gut bacterial communities of bumble bees are somewhat shaped by their diets and gut parasites as well as by the interaction of these two factors. This study confirms that bumble bees are suitable biological surrogates to assess the effect of diet and parasite infections on gut microbial communities.

Land-use-associated stressors interact to reduce bumblebee health at the individual and colony level

In agricultural landscapes, bees face a variety of stressors, including insecticides and poor-quality food. Although both stressors individually have been shown to affect bumblebee health negatively, few studies have focused on stressor interactions, a scenario expected in intensively used agricultural landscapes. Using the bumblebee Bombus terrestris, a key pollinator in agricultural landscapes, we conducted a fully factorial laboratory experiment starting at nest initiation. We assessed the effects of food quality and insecticides, alone and in interaction, on health traits at various levels, some of which have been rarely studied. Pollen with a diluted nutrient content (low quality) reduced ovary size and delayed colony development. Wing asymmetry, indicating developmental stress, was increased during insecticide exposure and interactions with poor food, whereas both stressors reduced body size. Both stressors and their interaction changed the workers' chemical profile and reduced worker interactions and the immune response. Our findings suggest that insecticides combined with nutritional stress reduce bumblebee health at the individual and colony levels, thus possibly affecting colony performance, such as development and reproduction, and the stability of plant-pollinator networks. The synergistic effects highlight the need of combining stressors in risk assessments and when studying the complex effects of anthropogenic stressors on health outcomes.

Sublethal effects of thiamethoxam on immune system cells in the workers of Bombus terrestris (Hymenoptera: Apidae)

Neonicotinoids harm especially bumblebees and other species in agricultural areas all over the world. The toxic effect of thiamethoxam from the neonicotinoid group has been little studied especially on bees. This research aimed to evaluate the effects of thiamethoxam on the immune system cells of Bombus terrestris workers. Different ratios of 1/1000, 1/100 and 1/10 of the recommended maximum application dose of thiamethoxam formed the experimental groups. Ten foraging workers were used for each dose and control groups. Contamination was ensured by spraying the prepared suspensions at different ratios to the bees for 20 s at a pressure of 1 atm. The effects of thiamethoxam on the structures of immune system cells of bumblebees and the amount of these cells were investigated after 48 h of exposure. In general, anomalies such as vacuolization, cell membrane irregularities and cell shape changes were detected in prohemocyte, plasmatocyte, granulocyte, spherulocyte and oenocytoid in each dose group. Hemocyte area measurements in all groups were examined comparatively between groups. In general, granulocyte and plasmatocyte sizes were decreased, while spherulocyte and oenocytoid were increased. It was also determined that there was a significant decrease in the amount of hemocytes in the 1 mm3 hemolymph as dose increased. The results of the study revealed that sublethal doses of thiamethoxam negatively affected hemocytes and their amounts of B. terrestris workers.

Consequences of microsporidian prior exposure for virus infection outcomes and bumble bee host health

Host-parasite interactions do not occur in a vacuum, but in connected multi-parasite networks that can result in co-exposures and coinfections of individual hosts. These can affect host health and disease ecology, including disease outbreaks. However, many host-parasite studies examine pairwise interactions, meaning we still lack a general understanding of the influence of co-exposures and coinfections. Using the bumble bee Bombus impatiens, we study the effects of larval exposure to a microsporidian Nosema bombi, implicated in bumble bee declines, and adult exposure to Israeli Acute Paralysis Virus (IAPV), an emerging infectious disease from honey bee parasite spillover. We hypothesize that infection outcomes will be modified by co-exposure or coinfection. Nosema bombi is a potentially severe, larval-infecting parasite, and we predict that prior exposure will result in decreased host resistance to adult IAPV infection. We predict double parasite exposure will also reduce host tolerance of infection, as measured by host survival. Although our larval Nosema exposure mostly did not result in viable infections, it partially reduced resistance to adult IAPV infection. Nosema exposure also negatively affected survival, potentially due to a cost of immunity in resisting the exposure. There was a significant negative effect of IAPV exposure on survivorship, but prior Nosema exposure did not alter this survival outcome, suggesting increased tolerance given the higher IAPV infections in the bees previously exposed to Nosema. These results again demonstrate that infection outcomes can be non-independent when multiple parasites are present, even when exposure to one parasite does not result in a substantial infection.

Sperm can't take the heat: Short-term temperature exposures compromise fertility of male bumble bees (Bombus impatiens)

Bumble bee (genus Bombus) populations are increasingly under threat from habitat fragmentation, pesticides, pathogens, and climate change. Climate change is likely a prime driver of bumble bee declines but the mechanisms by which changing climates alter local abundance, leading to shifts in geographic range are unclear. Heat tolerance is quite high in worker bumble bees (CT_max ∼ 48-55 °C), making it unlikely for them to experience these high temperatures, even with climate warming. However, the thermal tolerance of whole organisms often exceeds that of their gametes; many insects can be sterilized by exposure to temperatures well below their upper thermal tolerance. Male bumble bees are independent from the colony and may encounter more frequent temperature extremes, but whether these exposures compromise spermatozoa is still unclear. Using commercially-reared Bombus impatiens colonies, males were reared in the lab and spermatozoa were exposed (in vivo and isolated in vitro) to sublethal temperatures near lower and upper thermal tolerance (CT_min and CT_max, respectively). Heat exposure (45 °C for up to 85 min) reduced spermatozoa viability both for whole males (in vivo; control = 79.5 %, heat exposed = 58 %, heat stupor = 57.7 %) and isolated seminal vesicles (in vitro; control = 85.5 %, heat exposed = 62.9 %). Whole males exposed to 4 °C for 85 min (in vivo; control = 79.2 %, cold = 72.4 %), isolated seminal vesicles exposed to 4 °C for 85 min (in vitro; control = 85.5 %, cold = 85.1 %), and whole males exposed to for 4 °C for 48 h (in vivo; control = 88.7 %, cold = 84.3 %) did not differ significantly in spermatozoa viability. After<85 min at 45 °C, males had significantly reduced spermatozoa viability, suggesting that short-term heat waves below CT_max could strongly reduce the fertility of male bumble bees with potential population-level impacts.

Higher Essential Amino Acid and Crude Protein Contents in Pollen Accelerate the Oviposition and Colony Foundation of Bombus breviceps (Hymenoptera: Apidae)

Pollen is an important source of nutrition for bumblebees to survive, reproduce, and raise their offspring. To explore the nutritional requirements for the egg laying and hatching of queenright Bombus breviceps colonies, camellia pollen, oilseed rape pollen, apricot pollen, and mixtures of two or three types of pollen in equal proportions were used to feed the queens in this study. The results showed that the camellia pollen with a higher essential amino acid content was superior to the pollen with a lower essential amino acid content in the initial egg-laying time (p < 0.05), egg number (p < 0.05), larval ejection (p < 0.01), time of first worker emergence (p < 0.05), and the average weight of workers in the first batch (p < 0.01). It took less time for colonies under the camellia pollen and camellia-oilseed rape-apricot pollen mix treatments, both with a higher crude protein content, to reach ten workers in the colony (p < 0.01). On the contrary, the queens fed apricot pollen never laid an egg, and larvae fed oilseed rape pollen were all ejected-both pollens with a lower essential amino acid content. The results emphasize that the diet should be rationally allocated to meet the nutritional needs of local bumblebees at various stages when guiding them to lay eggs, hatch, and develop a colony.

Laboratory Risk Assessment of Three Entomopathogenic Fungi Used for Pest Control toward Social Bee Pollinators

The use of fungal-based biopesticides to reduce pest damage and protect crop quality is often considered a low-risk control strategy. Nevertheless, risk assessment of mycopesticides is still needed since pests and beneficial insects, such as pollinators, co-exist in the same agroecosystem where mass use of this strategy occurs. In this context, we evaluated the effect of five concentrations of three commercial entomopathogenic fungi, Beauveria bassiana, Metarhizium anisopliae, and Cordyceps fumosorosea, by direct contact and ingestion, on the tropical stingless bees Scaptotrigona depilis and Tetragonisca angustula, temperate bee species, the honey bee Apis mellifera, and the bumble bee Bombus terrestris, at the individual level. Furthermore, we studied the potential of two infection routes, either by direct contact or ingestion. In general, all three fungi caused considerable mortalities in the four bee species, which differed in their response to the different fungal species. Scaptotrigona depilis and B. terrestris were more susceptible to B. bassiana than the other fungi when exposed topically, and B. terrestris and A. mellifera were more susceptible to M. anisopliae when exposed orally. Interestingly, increased positive concentration responses were not observed for all fungal species and application methods. For example, B. terrestris mortalities were similar at the lowest and highest fungal concentrations for both exposure methods. This study demonstrates that under laboratory conditions, the three fungal species can potentially reduce the survival of social bees at the individual level. However, further colony and field studies are needed to elucidate the susceptibility of these fungi towards social bees to fully assess the ecological risks.

Biopesticides and insect pollinators: Detrimental effects, outdated guidelines, and future directions

As synthetic pesticides play a major role in pollinator decline worldwide, biopesticides have been gaining increased attention to develop more sustainable methods for pest management in agriculture. These biocontrol agents are usually considered as safe for non-target species, such as pollinators. Unfortunately, when it comes to non-target insects, only the acute or chronic effects on survival following exposure to biopesticides are tested. Although international boards have highlighted the need to include also behavioral and morphophysiological traits when assessing risks of plant protection products on pollinators, no substantial concerns have been raised about the risks associated with sublethal exposure to these substances. Here, we provide a comprehensive review of the studies investigating the potential adverse effects of biopesticides on different taxa of pollinators (bees, butterflies, moths, beetles, flies, and wasps). We highlight the fragmentary knowledge on this topic and the lack of a systematic investigation of these negative effects of biopesticides on insect pollinators. We show that all the major classes of biopesticides, besides their direct toxicity, can also cause a plethora of more subtle detrimental effects in both solitary and social species of pollinators. Although research in this field is growing, the current risk assesment approach does not suffice to properly assess all the potential side-effects that these agents of control may have on pollinating insects. Given the urgent need for a sustainable agriculture and wildlife protection, it appears compelling that these so far neglected detrimental effects should be thoroughly assessed before allegedly safe biopesticides can be used in the field and, in this view, we provide a perspective for future directions.

Protocol for Initiating and Monitoring Bumble Bee Microcolonies with Bombus impatiens (Hymenoptera: Apidae)

Populations of some bumble bee species are in decline, prompting the need to better understand bumble bee biology and for assessing the effects of environmental stressors on these important pollinators. Microcolonies have been successfully used for investigating a range of endpoints, including behavior, gut microbiome, nutrition, development, pathogens, and the effects of pesticide exposure on bumble bee health. Here, we present a step-by-step protocol for initiating, maintaining, and monitoring microcolonies with Bombus impatiens . This protocol has been successfully used in two pesticide exposure-effects studies and can be easily expanded to investigate other aspects of bumble bee biology.

A Comparison of Pollen and Syrup Exposure Routes in Bombus impatiens (Hymenoptera: Apidae) Microcolonies: Implications for Pesticide Risk Assessment

Bumble bees are important pollinators for both native plants and managed agricultural systems. Accumulating evidence has shown that pesticides, including neonicotinoids, can have a range of adverse effects on bumble bee health. Most laboratory studies that assess the effects of chronic neonicotinoid exposure on bumble bees use syrup as the delivery vehicle, rather than pollen. However, in the field, it is likely that bumble bees are exposed to neonicotinoids in both nectar (syrup) and pollen. To examine the potential for different effects based on the vehicle, we compared two studies of chronic exposure to the neonicotinoid acetamiprid in Bombus impatiens microcolonies. We examined correlations between microcolony endpoints and identified associations between the timing of colony pollen and syrup consumption and drone production. Furthermore, in line with previous results, we found that average drone weight was affected at a range of doses only when microcolonies were exposed to acetamiprid via pollen. In general, our analyses point to the importance of the treatment vehicle and suggest that critical effects on developing brood could be missed when neonicotinoid exposure occurs only through syrup.

Variations in Nutritional Requirements Across Bee Species

With 2,000 species currently recorded in Europe, bees are a highly diversified and efficient group of pollinating insects. They obtain their nutrients from nectar and pollen of flowers. However, the chemical composition of these resources, especially of pollen (e.g., protein, lipid, amino acids, fatty acids, or sterol content), is highly variable among plant species. While it is well-known that bees show interspecific variation in their floral choices, there is a lack of information on the nutritional requirements of different bee species. We therefore developed original experiments in laboratory conditions to evaluate the interspecific variations in bee nutritional requirements. We analyzed the chemical content of eight pollen blends, different in terms of protein, lipid, amino acids, and sterols total concentration and profiles. Each pollen blend was provided to four different bee model species: honey bees (Apis mellifera), bumblebees (Bombus terrestris), mason bees (Osmia bicornis and Osmia cornuta). For each species, specific protocols were used to monitor their development (e.g., weight, timing, survival) and resource collection. Overall, we found that the nutritional requirements across those species are different, and that a low-quality diet for one species is not necessarily low-quality for another one. While honey bees are negatively impacted by diets with a high protein content (~40%), bumblebees and mason bees develop normally on these diets but struggle on diets with a low total amino acid and sterol content, specifically with low concentrations of 24-methylenecholesterol and β-sitosterol. Overall, our study supports the need of conserving and/or introducing plant diversity into managed ecosystems to meet the natural nutritional preferences of bees at species and community level.

Supercooling points of freeze-avoiding bumble bees vary with caste and queen life stage

Bumble bees thrive in cold climates including high latitude and high altitude regions around the world, yet cold tolerance strategies are largely unknown for most species. To determine bumble bee cold tolerance strategy, we exposed bumble bees to a range of low temperatures and measured survival 72 h post-exposure. All bees that froze died within 72 h while only one bee died without freezing, suggesting that bumble bees are generally freeze-avoiding insects and may be slightly chill susceptible. We then assessed whether temperatures that cause internal ice formation (supercooling points, SCP) varied among bumble bee castes (drones, workers, and queens), or across queen life stages, collection elevation, species, or season. Males froze at significantly lower temperatures than workers or queens. Queens in pre-overwintering or overwintering states froze at significantly lower temperatures than queens stimulated to initiate ovary development by CO2 narcosis (i.e., "spring" queens). We also tested whether the presence of water (i.e., wet or dry) or ramping rate affected SCP. As expected, queens inoculated with water froze at significantly higher temperatures than dry queens. SCP tended to be lower, but not significantly so, at faster ramping rates (0.5 °C/min vs 0.25 °C/min). We also found no differences in SCP between queen bumble bees collected in spring and fall, between queens collected at two sites differing in elevation by 1100 m, or between three field-caught bumble bee species. Bumble bees appear to have relatively high, invariable SCPs, likely making them highly susceptible to freezing across all seasons. As bumble bees are not freeze-tolerant and appear to lack the ability to prevent freezing at temperatures much below 0 °C, they may rely on season- and caste-specific micro-habitat selection to thrive in cold climates.

Sulfoxaflor and nutritional deficiency synergistically reduce survival and fecundity in bumblebees

A range of anthropogenic factors are causing unprecedented bee declines. Among these drivers the usage of pesticides is believed to be crucial. While the use of key bee-harming insecticides, such as the neonicotinoids, has been reduced by regulatory authorities, novel, less studied substances have occupied their market niche. Understanding the threat of these chemicals to bees is, therefore, crucial to their conservation. Here we focus on sulfoxaflor, a novel insecticide, targeting the same neural receptor as the neonicotinoids. In stark contrast to the growing concerns around its negative impacts on bee health, a recent assessment has resulted in the extension of its authorisations across the USA. However, such assessments may underestimate risks by overlooking interactive impacts of multiple stressors. Here we investigated co-occurring, lethal and sublethal risks of sulfoxaflor and a dietary stress for bumblebees (Bombus terrestris), a key pollinator. Specifically, we employed a novel microcolony design, where, for the first time in bees, pesticide exposure mimicked natural degradation. We orally exposed workers to sulfoxaflor and a sugar-deficient diet in a fully factorial design. Field realistic, worst-case sulfoxaflor exposure caused a sharp increase in bee mortality. At sublethal concentrations, sulfoxaflor negatively affected bee fecundity, but not survival. Nutritional stress reduced bee fecundity and synergistically or additively aggravated impacts of sulfoxaflor on bee survival, egg laying and larval production. Our data show that non-mitigated label uses of sulfoxaflor may have major, yet severely neglected effects on bumblebee health, which may be exacerbated by nutritional stress. By unravelling mechanistic interactions of synergistic risks, our study highlights the need to overcome inherent limitations of Environmental Risk Assessment schemes, which, being based on a "single stressor paradigm", may fail to inform policymakers of the real risks of pesticide use.

Worker-Born Males Are Smaller but Have Similar Reproduction Ability to Queen-Born Males in Bumblebees

Queen-worker conflict over the reproduction of males exists in the majority of haplodiplioidy hymenpteran species such as bees, wasps, and ants, whose workers lose mating ability but can produce haploid males in colony. Bumblebee is one of the representatives of primitively eusocial insects with plastic division labor and belongs to monandrous and facultative low polyandry species that have reproductive totipotent workers, which are capable of competing with mother queen to produce haploid males in the queenright colony compared to higher eusocial species, e.g., honeybees. So, bumblebees should be a better material to study worker reproduction, but the reproductive characteristics of worker-born males (WMs) remain unclear. Here, we choose the best-studied bumblebee Bombus terrestris to evaluate the morphological characteristics and reproductive ability of WMs from the queenless micro-colonies. The sexually matured WMs showed smaller in forewing length and weight, relatively less sperm counts but equally high sperm viability in comparison with the queen-born males (QMs) of the queenright colony. Despite with smaller size, the WMs are able to successfully mate with the virgin queens in competition with the QMs under laboratory conditions, which is quite different from the honeybees reported. In addition, there was no difference in the colony development, including the traits such as egg-laying rate, colony establishment rate, and populations of offspring, between the WM- and the QM-mated queens. Our study highlights the equivalent reproductive ability of worker-born males compared to that of queens, which might exhibit a positive application or special use of bumblebee rearing, especially for species whose males are not enough for copulation. Further, our finding contributes new evidence to the kin selection theory and suggests worker reproduction might relate to the evolution of sociality in bees.

The Utility of a Bumble Bee (Bombus spp. [Hymenoptera: Apidae]) Brood Test for Evaluating the Effects of Pesticides

Risk assessment for chemicals in the United States relies upon the honey bee (Apis meliffera L. [Hymenoptera: Apidae]) as a surrogate for other bee species. There is uncertainty in extrapolating honey bee toxicity data to bumble bees due to differences in life history strategies, food consumption, and nest structure. Here we evaluated the design of a queenless bumble bee microcolony test that could be considered for generating larval toxicity data. Three microcolony studies were conducted with Bombus impatiens to evaluate the effects of exposure to 1) diflubenzuron in pollen, 2) dimethoate in pollen, and 3) dimethoate in sucrose. Immature drone bee emergence, worker survival, pollen, and sucrose utilization were measured throughout the study duration. For dimethoate, a 10-d chronic adult bumble bee study was also conducted to compare microcolony endpoints to toxicity endpoints on individual adults. Microcolonies exposed to 10 mg diflubenzuron/kg pollen produced fewer adult drones despite no effects on worker survival. Microcolonies treated with dimethoate at ≥3 mg a.i./kg pollen and ≥0.1 mg a.i./kg sucrose solution produced fewer drones. Exposure to dimethoate in the 10-d chronic adult study resulted in direct mortality to the adult workers at ≥0.1 mg a.i./kg diet. Results from the 10-d study suggest direct effects of dimethoate on workers in the microcolony will alter provisioning of diet to the brood, resulting in lower drone production in the microcolony. Our data suggest that the microcolony study is only appropriate to assess brood effects to bumble bees for substances with low toxicity to adults, as demonstrated with diflubenzuron.

Effects of air pollution exposure on social behavior: a synthesis and call for research

BACKGROUND

There is a growing literature from both epidemiologic and experimental animal studies suggesting that exposure to air pollution can lead to neurodevelopmental and neuropsychiatric disorders. Here, we suggest that effects of air pollutant exposure on the brain may be even broader, with the potential to affect social decision-making in general.

METHODS

We discuss how the neurobiological substrates of social behavior are vulnerable to air pollution, then briefly present studies that examine the effects of air pollutant exposure on social behavior-related outcomes.

RESULTS

Few experimental studies have investigated the effects of air pollution on social behavior and those that have focus on standard laboratory tests in rodent model systems. Nonetheless, there is sufficient evidence to support a critical need for more research.

CONCLUSION

For future research, we suggest a comparative approach that utilizes diverse model systems to probe the effects of air pollution on a wider range of social behaviors, brain regions, and neurochemical pathways.

Short-term lab assessments and microcolonies are insufficient for the risk assessment of insecticides for bees

Risk assessment studies addressing effects of agrochemicals on bumblebees frequently use microcolonies. These are queenless colonies consisting of workers only in which typically one worker will lay unfertilized male-destined eggs. In the first tier of risk assessment for bees, short-term laboratory experiments (e.g. microcolonies) are used, the results of which will determine whether higher tier (semi-)field experiments are needed. To evaluate the suitability of microcolonies for risk assessment, a direct comparison between different assessment methods for the neonicotinoid pesticides acetamiprid and thiacloprid was made: microcolonies and queenright colonies under short-term laboratory conditions, queenright colonies under long-term laboratory conditions, and queenright colonies under field conditions. Here, we demonstrate that results from microcolonies contradict results from queenright colonies. While thiacloprid negatively impacted gyne production in queenright colonies, it had a positive effect on microcolony size. By contrast, thiacloprid had no significant effect on fitness parameters of queenright colonies under short-term laboratory conditions when mostly workers are produced. These results thus highlight both the need for long term assessments, allowing evaluation of gyne production, and the risk of reaching erroneous conclusions when using microcolonies. The negative effect of thiacloprid on colony fitness was confirmed under field conditions, where thiacloprid affected the production of reproductives, colony weight gain, worker weight, and foraging behaviour. For acetamiprid, a negative trend on colony fitness could only be shown in a field setup. Therefore, field-realistic setups, which allow colonies to forage freely, are most appropriate to assess sublethal effects of pesticides affecting behaviour and learning.

Thermal niches of specialized gut symbionts: the case of social bees

Responses to climate change are particularly complicated in species that engage in symbioses, as the niche of one partner may be modified by that of the other. We explored thermal traits in gut symbionts of honeybees and bumblebees, which are vulnerable to rising temperatures. In vitro assays of symbiont strains isolated from 16 host species revealed variation in thermal niches. Strains from bumblebees tended to be less heat-tolerant than those from honeybees, possibly due to bumblebees maintaining cooler nests or inhabiting cooler climates. Overall, however, bee symbionts grew at temperatures up to 44°C and withstood temperatures up to 52°C, at or above the upper thermal limits of their hosts. While heat-tolerant, most strains of the symbiont Snodgrassella grew relatively slowly below 35°C, perhaps because of adaptation to the elevated body temperatures that bees maintain through thermoregulation. In a gnotobiotic bumblebee experiment, Snodgrassella was unable to consistently colonize bees reared at 29°C under conditions that limit thermoregulation. Thus, host thermoregulatory behaviour appears important in creating a warm microenvironment for symbiont establishment. Bee-microbiome-temperature interactions could affect host health and pollination services, and inform research on the thermal biology of other specialized gut symbionts.

Impacts of Neonicotinoids on the Bumble Bees Bombus terrestris and Bombus impatiens Examined through the Lens of an Adverse Outcome Pathway Framework

Bumble bees (Bombus sp.) are important pollinators for agricultural systems and natural landscapes and have faced population declines globally in recent decades. Neonicotinoid pesticides have been implicated as one of the reasons for the population reductions in bumble bees and other pollinators due to their widespread use, specificity to the invertebrate nervous system, and toxicity to bees. Adverse outcome pathways (AOPs) are used to describe the mechanism of action of a toxicant through sequential levels of biological organization to understand the key events that occur for a given adverse outcome. We used the AOP framework to organize and present the current literature available on the impacts of neonicotinoids on bumble bees. The present review focuses on Bombus terrestris and B. impatiens, the 2 most commonly studied bumble bees due to their commercial availability. Our review does not seek to describe an AOP for the molecular initiating event shared by neonicotinoids, but rather aims to summarize the current literature and determine data gaps for the Bombus research community to address. Overall, we highlight a great need for additional studies, especially those examining cellular and organ responses in bumble bees exposed to neonicotinoids. Environ Toxicol Chem 2021;40:309-322. © 2020 SETAC. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Overview of the testing and assessment of effects of microbial pesticides on bees: strengths, challenges and perspectives

Currently, there is a growing interest in developing biopesticides and increasing their share in the plant protection market as sustainable tools in integrated pest management (IPM). Therefore, it is important that regulatory requirements are consistent and thorough in consideration of biopesticides' unique properties. While microbial pesticides generally have a lower risk profile, they present special challenges in non-target organism testing and risk assessment since, in contrast to chemical pesticides, their modes of action include infectivity and pathogenicity rather than toxicity alone. For this reason, non-target organism testing guidelines designed for conventional chemical pesticides are not necessarily directly applicable to microbial pesticides. Many stakeholders have recognised the need for improvements in the guidance available for testing microbial pesticides with honey bees, particularly given the increasing interest in development and registration of microbial pesticides and concerns over risks to pollinators. This paper provides an overview of the challenges with testing and assessment of the effects of microbial pesticides on honey bees (Apis mellifera), which have served as a surrogate for both Apis and non-Apis bees, and provides a foundation toward developing improved testing methods.

Effects of the Neonicotinoid Acetamiprid in Pollen on Bombus impatiens Microcolony Development

Honey bees and other wild bee species including bumble bees have experienced population declines in recent decades. Although many stressors are implicated in bee population declines, much attention has focused on neonicotinoid pesticides, which are widely used and known to be toxic to pollinators. One neonicotinoid, acetamiprid, has been studied very little in bumble bees, despite its use on bumble bee-pollinated crops. We assessed the impacts of acetamiprid to the North American bumble bee Bombus impatiens using the microcolony model. We examined nest growth, development, and subsequent nest productivity as measured by drone production. We found that high concentrations of acetamiprid in pollen (4520 µg/kg) significantly impacted nest growth, development, and, ultimately, reproduction (drone production). We found the no-observable-adverse effect level to be 45.2 µg/kg. Overall, acetamiprid has the potential to negatively impact reproductive endpoints for B. impatiens. However, effects occurred at concentrations substantially higher than expected environmental concentrations that would be achieved when following label rates. Further work is required to assess the effects of this pesticide on B. impatiens via alternate routes of exposure and on queenright colonies. Environ Toxicol Chem 2020;39:2560-2569. © 2020 SETAC. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Effects of the neonicotinoid acetamiprid in syrup on Bombus impatiens (Hymenoptera: Apidae) microcolony development

Worldwide, many pollinator populations are in decline. Population reductions have been documented for the agriculturally important honey bee (Apis mellifera), and other bee species such as bumble bees that are also critical for pollinating crops and natural landscapes. A variety of factors contribute to the observed population reductions, including exposure to agrochemicals. In recent decades, neonicotinoid pesticide use has dramatically increased, as have concerns regarding the safety of these chemicals for pollinator health. Here we assessed the toxicity of the neonicotinoid acetamiprid to the bumble bee Bombus impatiens, a species commercially available for use in agricultural settings in North America. Using the microcolony model, we examined nest growth, development and subsequent nest productivity as measured by drone production. We found that high concentrations of acetamiprid in syrup (11,300 μg/L) significantly impacted nest growth and development, and ultimately drone production, and exposure to 1,130 μg/L acetamiprid also significantly decreased drone production. The no observable adverse effect level was 113 μg/L. Overall, acetamiprid delivered in syrup can negatively impact B. impatiens nest development and productivity, however only at concentrations above which would be expected in the environment when used according to label rates.

The Importance of Males to Bumble Bee (Bombus Species) Nest Development and Colony Viability

Bumble bee population declines over the last decade have stimulated strong interest in determining causative factors and necessary conservation measures. Research attention has largely been directed toward bumble bee worker and queen health and their contributions to population stability, while male bees (i.e., drones) have typically been overlooked regarding their role in influencing colony fitness and longevity. In this review we assess existing literature on the diverse role of males within bumble bee nests and their importance to queen health and fitness, as well as to overall nest success. The implications of reproductive measures, including sperm transfer, mating behavior, mating plugs, and male immunity, among other topics, are examined. Overall, bumble bee males are found to drive colony function in a unique manner. Current knowledge gaps pertaining to the role of males are discussed. We highlight the importance of drones to queen success and fitness in many ways, and suggest future research exploring impacts of this often-neglected caste.

Bumble bees damage plant leaves and accelerate flower production when pollen is scarce

Maintaining phenological synchrony with flowers is a key ecological challenge for pollinators that may be exacerbated by ongoing environmental change. Here, we show that bumble bee workers facing pollen scarcity damage leaves of flowerless plants and thereby accelerate flower production. Laboratory studies revealed that leaf-damaging behavior is strongly influenced by pollen availability and that bee-damaged plants flower significantly earlier than undamaged or mechanically damaged controls. Subsequent outdoor experiments showed that the intensity of damage inflicted varies with local flower availability; furthermore, workers from wild colonies of two additional bumble bee species were also observed to damage plant leaves. These findings elucidate a feature of bumble bee worker behavior that can influence the local availability of floral resources.

Impact of Diflubenzuron on Bombus impatiens (Hymenoptera: Apidae) Microcolony Development

Reliance on the honey bee as a surrogate organism for risk assessment performed on other bees is widely challenged due to differences in phenology, life history, and sensitivity to pesticides between bee species. Consequently, there is a need to develop validated methods for assessing toxicity in non-Apis bees including bumble bees. The usefulness of small-scale, queenless colonies, termed microcolonies, has not been fully investigated for hazard assessment. Using the insect growth regulator diflubenzuron as a reference toxicant, we monitored microcolony development from egg laying to drone emergence using the Eastern bumble bee Bombus impatiens (C.), a non-Apis species native to North America. Microcolonies were monitored following dietary exposure to diflubenzuron (nominal concentrations: 0.1, 1, 10, 100, and 1,000 µg/liter). Microcolony syrup and pollen consumption was significantly reduced by diflubenzuron exposure. Pupal cell production was also significantly decreased at the highest diflubenzuron concentration assessed. Ultimately, diflubenzuron inhibited drone production in a concentration-dependent manner and a 42-d 50% inhibitory concentration (IC50) was determined. None of the dietary concentrations of diflubenzuron tested affected adult worker survival, or average drone weight. These data strengthen the foundation for use of this methodology, and provide valuable information for B. impatiens; however, more work is required to better understand the utility of the bumble bee microcolony model for pesticide hazard assessment.