Honeybee economics: optimisation of foraging in a variable world

A New Isolated Fungus and Its Pathogenicity for Apis mellifera Brood in China

In this article, we report the pathogenicity of a new strain of fungus, Rhizopus oryzae to honeybee larvae, isolated from the chalkbrood-diseased mummies of honeybee larvae and pupae collected from apiaries in China. Based on morphological observation and internal transcribed spacer (ITS) region analyses, the isolated pathogenic fungus was identified as R. oryzae. Koch's postulates were performed to determine the cause-and-effect pathogenicity of this isolate fungus. The in vitro pathogenicity of this virulent fungus in honeybees was tested by artificially inoculating worker larvae in the lab. The pathogenicity of this new fungus for honeybee larvae was both conidial-concentration and exposure-time dependent; its highly infectious and virulent effect against the larvae was observed at 1 × 105 conidia/larva in vitro after 96 h of challenge. Using probit regression analysis, the LT50 value against the larvae was 26.8 h at a conidial concentration of 1 × 105 conidia/larva, and the LC50 was 6.2 × 103 conidia/larva. These results indicate that the new isolate of R. oryzae has considerable pathogenicity in honeybee larvae. Additionally, this report suggests that pathogenic phytofungi may harm their associated pollinators. We recommend further research to quantify the levels, mechanisms, and pathways of the pathogenicity of this novel isolated pathogen for honeybee larvae at the colony level.

Identification of ecdysone receptor target genes in the worker honey bee brains during foraging behavior

Ecdysone signaling plays central roles in morphogenesis and female ovarian development in holometabolous insects. In the European honey bee (Apis mellifera L.), however, ecdysone receptor (EcR) is expressed in the brains of adult workers, which have already undergone metamorphosis and are sterile with shrunken ovaries, during foraging behavior. Aiming at unveiling the significance of EcR signaling in the worker brain, we performed chromatin-immunoprecipitation sequencing of EcR to search for its target genes using the brains of nurse bees and foragers. The majority of the EcR targets were common between the nurse bee and forager brains and some of them were known ecdysone signaling-related genes. RNA-sequencing analysis revealed that some EcR target genes were upregulated in forager brains during foraging behavior and some were implicated in the repression of metabolic processes. Single-cell RNA-sequencing analysis revealed that EcR and its target genes were expressed mostly in neurons and partly in glial cells in the optic lobes of the forager brain. These findings suggest that in addition to its role during development, EcR transcriptionally represses metabolic processes during foraging behavior in the adult worker honey bee brain.

A mixed model of heat exchange in stationary honeybee foragers

During foraging honeybees are always endothermic to stay ready for immediate flight and to promote fast exploitation of resources. This means high energetic costs. Since energy turnover of foragers may vary in a broad range, energetic estimations under field conditions have remained uncertain. We developed an advanced model, combining the benefits of mechanistic and correlative models, which enables estimation of the energy turnover of stationary foragers from measurements of body surface temperature, ambient air temperature and global radiation. A comprehensive dataset of simultaneously measured energy turnover (ranging from 4 to 85 mW) and body surface temperature (thorax surface temperature ranging from 33.3 to 45 °C) allowed the direct verification of model accuracy. The model variants enable estimation of the energy turnover of stationary honeybee foragers with high accuracy both in shade and in sunshine, with SD of residuals = 5.7 mW and R² = 0.89. Its prediction accuracy is similar throughout the main range of environmental conditions foragers usually experience, covering any combination of ambient air temperature of 14-38 °C and global radiation of 3-1000 W m^-2.

Comparative transcriptome analysis of adult worker bees under short-term heat stress

High temperature affects behavior, physiology, survival, and the expression of related genes in adult honeybees. Apis mellifera is the common pollinator in greenhouse and is susceptible to high temperature stress. To further explore the molecular basis related to heat stress, we compared the transcriptome profiles of adult worker bees at 25 and 45°C, and detected the expression patterns of some differentially expressed genes (DEGs) in different tissues by q RT-PCR. Differential expression analysis showed that 277 DEGs were identified, including 167 genes upregulated and 110 genes downregulated after heat stress exposure in adult worker bees. In GO enrichment analysis, DEGs were mostly enriched for protein folding, unfold protein binding, and heme binding terms. Protein processing in endoplasmic reticulum and longevity regulating pathway-multiple species were significantly enriched in KEGG. The expression levels of 16 DEGs were consistent with the transcriptome results. The expression patterns of 9 DEGs in different tissues revealed high levels in the thorax, which was supposed that the thorax may be the most important part in the response to heat stress. This study provided valuable data for exploring the function of heat resistance-related genes.

Energetics of Paper Wasps (Polistes sp.) from Differing Climates during the Breeding Season

Paper wasps are widely distributed in Europe. They live in the warm Mediterranean, and in the harsh Alpine climate. Some species are very careful in their choice of nesting sites to ensure a proper development of the brood. We investigated microclimate conditions at the nests of three species (P. dominula, P. gallicus, P. biglumis) from differing climates, in order to characterize environmental conditions and conduct energetic calculations for an entire breeding season. The mean ambient nest temperature differed significantly in the Mediterranean, temperate, and Alpine habitats, but in all habitats it was about 2 to 3 °C above the standard meteorological data. The energetic calculations of adult wasps' standard and active metabolic rate, based on respiratory measurements, differed significantly, depending on the measured ambient temperatures or the wasps' body temperatures. P. gallicus from the warm Mediterranean climate exhibited the highest energetic costs, whereas P. biglumis from the harsh Alpine climate had the lowest costs. Energetic costs of P. dominula from the temperate climate were somewhat lower than those in the Mediterranean species, but clearly higher than those in the Alpine species. Temperature increase due to climate change may have a severe impact on the wasps' survival as energetic costs increase.

Honey Bee Nutrition

Optimal nutrition is crucial for honey bee colony growth and robust immune systems. Honey bee nutrition is complex and depends on the floral composition of the landscape. Foraging behavior of honey bees depends on both colony environment and external environment. There are significant gaps in knowledge regarding honey bee nutrition, and hence no optimal diet is available for honey bees, as there is for other livestock. In this review, we discuss (1) foraging behavior of honey bees, (2) nutritional needs, (3) nutritional supplements used by beekeepers, (4) probiotics, and (5) supplemental forage and efforts integrating floral diversity into cropping systems.

Honey robbing: could human changes to the environment transform a rare foraging tactic into a maladaptive behavior?

Human environmental modifications have outpaced honey bees' ability to evolve adaptive regulation of foraging tactics, possibly including a tactic associated with extreme food shortage, honey robbing. Honey robbing is a high risk, high reward, and understudied honey bee tactic whereby workers attack and often kill neighboring colonies to steal honey. Humans have exacerbated the conditions that provoke such robbing and its consequences. We describe robbing as an individual-level and colony-level behavioral syndrome, implicating worker bees specialized for foraging, food processing, and defense. We discuss how colony signaling mechanisms could regulate this syndrome and then explore the ecological underpinnings of robbing-highlighting its unusual prevalence in the commonly managed Apis mellifera and outlining the conditions that provoke robbing. We advocate for studies that identify the cues that modulate this robbing syndrome. Additionally, studies that apply behavioral ecology modeling approaches to generate testable predictions about robbing could clarify basic bee biology and have practical implications for colony management.

Coping with the cold and fighting the heat: thermal homeostasis of a superorganism, the honeybee colony

The worldwide distribution of honeybees and their fast propagation to new areas rests on their ability to keep up optimal ‘tropical conditions’ in their brood nest both in the cold and in the heat. Honeybee colonies behave like ‘superorganisms’ where individuals work together to promote reproduction of the colony. Social cooperation has developed strongly in thermal homeostasis, which guarantees a fast and constant development of the brood. We here report on the cooperation of individuals in reaction to environmental variation to achieve thermal constancy of 34–36 °C. The measurement of body temperature together with bee density and in-hive microclimate showed that behaviours for hive heating or cooling are strongly interlaced and differ in their start values. When environmental temperature changes, heat production is adjusted both by regulation of bee density due to migration activity and by the degree of endothermy. Overheating of the brood is prevented by cooling with water droplets and increased fanning, which start already at moderate temperatures where heat production and bee density are still at an increased level. This interlaced change and onset of different thermoregulatory behaviours guarantees a graded adaptation of individual behaviour to stabilise the temperature of the brood.

Relation between activity, endothermic performance and respiratory metabolism in two paper wasps: Polistes dominula and Polistes gallicus

Climate change is expected to produce shifts in species distributions as well as behavioural and physiological adaptations to find suitable conditions or to cope with the altered environment. The paper wasps Polistes dominula and Polistes gallicus are closely related species, native in the European Mediterranean region and North Africa. P. dominula has expanded its range to the relatively cooler climates of Northern and Eastern Europe, but P. gallicus remained in its original distribution area. In order to reveal their metabolic adaptation to the current climate conditions, and the impact on energy demand at future climate conditions, we investigated the respiratory metabolic rate (CO₂ production) of P. dominula from Austria and P. gallicus from Italy. In contrast to the metabolic cold adaptation hypothesis their standard metabolic rate was nearly the same and increased in a typical exponential course with increasing ambient temperature. The metabolic rate of active wasps was higher than the standard metabolic rate and increased with the wasps' activity. There was no obvious difference in the active metabolism between the two species, with the exception that some P. gallicus individuals showed some extraordinary high values. A simultaneous measurement of metabolic rate and body temperature revealed that increased CO₂ production was accompanied by endothermic activity. The two investigated populations of paper wasps are quite similar in their metabolic response to temperature, although they live in different climate regions. The spread of P. dominula into cooler regions did not have significant influence on their active and standard metabolic rate.

The Respiratory Metabolism of Polistes biglumis, a Paper Wasp from Mountainous Regions

European Polistine wasps inhabit mainly temperate and warm climate regions. However, the paper wasp Polistes biglumis represents an exception; it resides in mountainous areas, e.g., in the Alps and in the Apennines. In these habitats, the wasps are exposed to a broad temperature range during their lifetime. We investigated whether they developed adaptations in their metabolism to their special climate conditions by measuring their CO₂ production. The standard or resting metabolic rate and the metabolism of active wasps was measured in the temperature range which they are exposed to in their habitat in summer. The standard metabolic rate increased in a typical exponential progression with ambient temperature, like in other wasps. The active metabolism also increased with temperature, but not in a simple exponential course. Some exceptionally high values were presumed to originate from endothermy. The simultaneous measurement of body temperature and metabolic rate revealed a strong correlation between these two parameters. The comparison of the standard metabolic rate of Polistes biglumis with that of Polistes dominula revealed a significantly lower metabolism of the alpine wasps. This energy saving metabolic strategy could be an adaptation to the harsh climate conditions, which restricts foraging flights and energy recruitment.

Thermoregulatory syndromes of two sympatric dung beetles with low energy costs

In heterotherm insects, endothermy implies a high energy cost due to the generation and regulation of body temperature during different activities such as flight, food location, fighting and even walking. We studied the thermoregulation process and the cost of the thermoregulation strategies in two sympatric dung beetles, Sulcophanaeus batesi and S. imperator under heat and cold stress conditions. We used a set of physiological variables to exemplify the capacity of thermolimit respirometry combined with infrared thermography to derive relevant variables capable of describing different thermoregulation syndromes. Habitat use and thermal niche differed notably between S. batesi and S. imperator, reflecting their contrasted thermal requirements. In S. imperator, thermal specialization for high temperatures was observed, being active mainly during the warmer period of the day. On the other hand, thermal adaptation in S. batesi allows its preference for cold exhibiting a morning activity periods, avoiding higher temperatures. The thermophilic strategy used by Sulcophanaeus imperator minimized the energy expenditure produced during the cooling of the body by respiration without thereby endangering higher thermal limits. In this case, S. batesi, the species with a preference for the coldest environments, presented the lowest thermal limits, although the energy cost needed to stay active during cooling was significantly lower than that in S. imperator. Sulcophanaeus imperator and S. batesi showed evident 'economizing' strategies associated with hot and cold environmental conditions, respectively. In contrast, if both species experience a deviation from their thermal optimum, a decrement in their performance could be produced.

Thermal efficiency extends distance and variety for honeybee foragers: analysis of the energetics of nectar collection and desiccation by Apis mellifera

The desiccation of nectar to produce honey by honeybees ( Apis mellifera L.) is an energy-intensive process, as it involves a quasi-isothermal change in the concentration of sugars from typically 20 to 80% by vaporization (honey ripening). This analysis creates mathematical models for: the collected nectar to honey ratio; energy recovery ratio; honey energy margin; and the break-even distance, which includes the factors of nectar concentration and the distance to the nectar from the nest; energetics of desiccation and a new factor, thermal energy efficiency (TEE) of nectar desiccation. These models show a significant proportion of delivered energy in the nectar must be used in desiccation, and that there is a strong connection between TEE and nest lumped thermal conductance with colony behaviour. They show the connection between TEE and honeybee colony success, or failure, in the rate of return, in terms of distance or quality of foraging. Consequently, TEE is a key parameter in honeybee populations and foraging modelling. For bee keeping, it quantifies the summer benefits of a key hive design parameter, hive thermal conductance and gives a sound theoretical basis for improving honey yields, as seen in expanded polystyrene hives.

The energetics and thermoregulation of water collecting honeybees

Honeybees need water for different purposes, to maintain the osmotic homeostasis in adults as well as to dilute stored honey and prepare liquid food for the brood. Water is also used for cooling of the hive. Foraging in endothermic insects is energy-intensive and the question arises how much energy bees invest in a resource without any metabolically usable energy. We investigated the energy demand of water collecting bees under natural conditions. The thermoregulation and energetic effort was measured simultaneously in a broad range of experimental ambient temperatures (Ta = 12-40 °C). The thorax temperature as well as the energetic turnover showed a great variability. The mean Tthorax was ranging from ~ 35.7 °C at 12 °C to nearly 42.5 °C at 40 °C. The energy turnover calculated from CO2-release was highest at a Ta of 20 °C with about 60 mW and lowest at 40 °C with about 22 mW per bee. The total costs during collection decreased from 10.4 J at 12 °C to 0.5 J at 40 °C. The energetic effort of the water collectors was comparable with that of 0.5 M sucrose foraging bees. Our investigation strongly supports the hypothesis that the bees' motivational status determines the energetic performance during foraging.

Foraging strategy of wasps - optimisation of intake rate or energetic efficiency?

In endothermic wasps, foraging is an expensive activity. To maximise the benefit for the colony, wasps can optimise either the intake rate or energetic efficiency of a foraging trip. We investigated the foraging behaviour of vespine wasps under variable environmental and reward conditions. We trained them to forage for 0.5 mol l^-1 sucrose solution from an artificial flower in a flow-through respiratory measurement chamber, and simultaneously measured their body temperature using infrared thermography to investigate interactions between thermoregulation and energetics. Measurement of carbon dioxide release (for energetic calculations) and load mass enabled the direct determination of foraging efficiency. An unlimited reward increased the wasps' energetic effort to increase the suction speed through high body temperatures. With reduced reward (limited flow), when an increased body temperature could not increase suction speed, the wasps decreased their body temperature to reduce the metabolic effort. Solar heat gain was used differently, either to increase body temperature without additional metabolic effort or to save energy. The foraging efficiency was mainly determined by the flow rate, ambient temperature and solar heat gain. In shade, an unlimited sucrose flow and a high ambient temperature yielded the highest energetic benefit. A limited flow reduced foraging efficiency in the shade, but only partly in sunshine. Solar radiation boosted the efficiency at all reward rates. Wasps responded flexibly to varying reward conditions by maximising intake rate at unlimited flow and switching to the optimisation of foraging efficiency when the intake rate could not be enhanced due to a limited flow rate.

Cranking up the heat: relationships between energetically costly song features and the increase in thorax temperature in male crickets and katydids

Sexual displays of acoustically signalling insects are used in the context of mate attraction and mate choice. While energetic investment in sound production can increase the reproductive success of the sender, this entails metabolic costs. Resource allocation to sexually selected, reproductive traits can trade off against allocation to naturally selected traits (e.g. growth, immunity) when individuals' energy budgets are limited. Estimating the magnitude of the costs invested in acoustic signalling is necessary to understand this trade-off and its influence on fitness and life history. To compare the costs associated with acoustic signalling for two ensiferan species, we simultaneously took respiratory measurements to record the rate of CO2 production and used infrared thermography to measure the increase in thorax temperature. Furthermore, to identify what combinations of acoustic parameters were energetically costly for the sender, we recorded the calling songs of 22 different cricket and katydid species for a comparative analysis and measured their thorax temperature while they sang. Acoustic signalling was energetically costly for Mecopoda sp. and Anurogryllus muticus, requiring a 12- and 16-fold increase over resting levels in the CO2 production rate. Moreover, calling increased thorax temperature, on average by 7.6 and 5.8°C, respectively. We found that the song intensity and effective calling rate, not simply the chirp/trill duty cycle or the pulse rate alone, were good predictors for the thorax temperature increase in males.

Modelling seasonal effects of temperature and precipitation on honey bee winter mortality in a temperate climate

Insect pollinators are essential to global food production. For this reason, it is alarming that honey bee (Apis mellifera) populations across the world have recently seen increased rates of mortality. These changes in colony mortality are often ascribed to one or more factors including parasites, diseases, pesticides, nutrition, habitat dynamics, weather and/or climate. However, the effect of climate on colony mortality has never been demonstrated. Therefore, in this study, we focus on longer-term weather conditions and/or climate's influence on honey bee winter mortality rates across Austria. Statistical correlations between monthly climate variables and winter mortality rates were investigated. Our results indicate that warmer and drier weather conditions in the preceding year were accompanied by increased winter mortality. We subsequently built a statistical model to predict colony mortality using temperature and precipitation data as predictors. Our model reduces the mean absolute error between predicted and observed colony mortalities by 9% and is statistically significant at the 99.9% confidence level. This is the first study to show clear evidence of a link between climate variability and honey bee winter mortality.