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Why Do Winter Bees Live Longer?

Posted December 17, 2024

Introduction

Winter can be a challenging time for beekeepers. As temperatures plummet, managing the internal conditions of beehives becomes critical and is a dance with nature. It’s clear that honey bees exhibit complex behaviors and physiological responses when managing their hive environment. The hive is in fact an extended phenotype (physical characteristic extending beyond its body) of the honey bee. Some examples are the spider web, beaver dam, termite nest, and the honeybee hive where the phenotype is the fitness of the construction for survival and reproduction of the organism. Misunderstanding this complex interaction between the colony enclosure, thermofluids (heat, radiation, water vapor, air) and honey bee behavior and physiology have resulted in management practices that don’t favor honey bee survival especially during extreme periods of stress.

For example, recent evidence indicates that bees will maintain higher levels of CO₂ when the hive configuration allows rather than reducing it. In fact, CO₂ has been shown to lower the bees’ activity to where they are in a state of deep dormancy/hibernation (diapause), conserving energy/resources through an ultra-low metabolic rate (ULMR) especially during winter. Other insights into the dynamics of hive atmospherics also offer promising methods for controlling varroa mites through the management of CO₂ levels within the hive.

Diapause

Diapause is a physiological adaptation to conditions that are unfavorable for growth or reproduction (brood). During diapause, animals become long-lived, stress-resistant, developmentally static, and non-reproductive, in the case of diapausing adults (winter bees and queen). These traits are regulated through modifications of the endocrine program (hormones) guiding development. The endocrine system is a network of glands that produce and release hormones, which are chemical messengers that regulate various functions in the body, including growth, metabolism, and mood. In insects, this typically includes changes in molting hormones, as well as metabolic signals that limit growth while skewing the organism’s energetic demands toward conservation. Specifically, honey bee diapause is an emerging scientific model to study mechanisms that determine lifespan. The induction of diapause in the bee represents a dramatic change in the normal progression of its age.

Diapause is an endogenously (internally) regulated dormant state that provides a means for insects to survive seasons of adverse environmental conditions and allows populations to synchronize periods of active development and reproduction with seasons of optimal resource availability. For many other species, diapause is a plastic response (i.e. facultative) and constitutes an alternative developmental pathway that is programmed by token signals (e.g. changes in day length, temperature, or food quality) that indicate a season of harsh conditions is coming. Unlike quiescence (at rest), a type of dormancy that is an immediate response to a change in the environment, insects that enter diapause anticipate the decline in environmental conditions and prepare by seeking an appropriate shelter and/or sequestering additional metabolic energy reserves. In addition, while quiescence ends as soon as the environment returns to “normal”, diapause persists until the diapause program is terminated by endogenously controlled processes that are not completely understood. Some aspects of the diapause phenotype are variable and species specific, but in general, diapause is characterized by developmental arrest and metabolic restructuring that includes downregulation of processes that consume energy, suppression of oxidative processes that produce energy, and upregulation of processes that enhance stress resistance. Together, the physiological and biochemical changes that occur during diapause conserve energy stores and maximize survivorship during an extended period of “suspended animation”.

Diapause is also an adaptive and plastic phenotype (set of observable characteristics or traits) that allows honey bees to persist in seasonally variable environments. Bees enter diapause in advance of unfavorable conditions and in response to predictive environmental cues. Because it allows insects to persist and adapt to new environments, diapause has been a powerful model for understanding evolution by natural selection. Moreover, natural populations often harbor ample genetic variation affecting both the capacity for and the timing of diapause. This combination of strong selection and segregating genetic variation allows diapause to rapidly evolve over contemporary timescales, including in response to changing climates, developing agricultural practices, and during biological invasions and range expansions.

In general, diapause is a critical phase of the life cycle of many insects, and likely contributed to the ecological success of this highly diverse group of animals. Many terms have been used to describe this phase of dormancy in insects, including diapause, adult diapause, reproductive diapause, hibernation, adult-wintering and overwintering. A defining feature of all these terms is an arrest in development or activity that is hormonally programmed in advance of environmental adversities such as harsh winter, dry seasons or food restriction.

Hypoxia

A recent study that held experiments over three winters has revealed a metabolism controlling function of bee-induced hypoxia (Deficiency in the amount of oxygen reaching body tissues.) in the winter cluster. Permanent low oxygen levels around 15% were found in its core. This hypoxia was actively controlled, probably via indirect mechanisms. Varying ambient oxygen levels demonstrated a causal relationship between lowered oxygen and reduced metabolic rate (MR). Under deeper ambient hypoxia the bees switched to ultra-low MR (ULMR), optional-occasional at 15% oxygen, obligatory at 7.5% oxygen. This dormancy status resembled deep diapause (hibernation) in insects. It stayed reversible after at least several days, and was terminated under normal oxygen at 59°F/15°C. Reduced MR via core-hypoxia is essential in water conserving thermoregulation of the wintering cluster. It allows bees to reconcile warm wintering in alert state—for defense of stores—with energy saving and longevity. This indicates that winter MR of bees might be related to insect diapause, and that in-body hypoxia might be functional in insect diapause.

An aspect of hive management by bees is the role of elevated CO2 levels in potentially purposely slowing the bees’ metabolic rate. By maintaining higher CO2 levels, like conditions observed in well-insulated hives with minimal top ventilation (akin to indoor storage conditions), beekeepers might influence the bees’ metabolic rates. This controlled environment can help conserve the bees’ energy during the cold months, reducing the need for them to consume their honey stores excessively. The bees could potentially enter a state of ultra-low metabolic rate (ULMR) under deep hypoxia conditions. This may show that periods of high CO2 could be reflective of bees conserving energy during periods of deep dormancy or hypoxia, not just increased activity. Less honey consumed results in much less internal moisture to manage inside the hive.

Diutinus (“Long-Lasting”) Bees

Diutinus bees are long-lived and so long-lasting bees. These are the bees that, in temperate regions, maintain the colony through the winter to the warmer days of spring. From a scientific standpoint, the key feature of these bees is that they can live for up to 8 months, in contrast to the ~30 days a worker bee lives in spring or summer. Interestingly, the fate of these bees is more about their longevity … than it is about the season. It’s worth emphasizing that diutinus bees are genetically similar to the spring/summer bees. Despite this similarity, they have unique physiological features that contribute to their ability to thermoregulate the winter cluster for months and to facilitate spring build-up as the season transitions to spring.

Four key physiological factors to consider are the levels of juvenile hormone (JH), vitellogenin (Vg) and hemolymph proteins and the size of the bees’ hypopharyngeal gland (HPG). Winter bees resemble nurse bees in having low JH levels, high levels of VG and hemolymph proteins and large HPG’s. Winter bees also differ from nurse bees in being long lived. A nurse bee will mature into a forager after ~3 weeks. A winter bee will stay in a physiologically similar state for months. One of the strongest clues about what factor(s) induces winter bee production comes from studies of free-flying summer colonies from which the brood is removed. In these, the workers rapidly change to physiologically resemble winter bees. Brood pheromone also contributes to an enhancement loop; it induces foraging which results in increased brood rearing and, consequently, the production of more brood pheromone.

Workers also produce ethyl oleate, a pheromone that slows the maturation of nurse bees, so reducing their transition to foragers. During autumn there is a reduction in forage available coupled with a reduced daylength and lower environmental temperatures. Consequently, there is less foraging by the colony. Since more foragers are present within the hive, the nurse bees are exposed to higher levels of ethyl oleate, so slowing their maturation. There’s less pollen being brought into the colony (reduced nutrition), so brood production decreases and so does the level of brood pheromone. The reduced levels of brood pheromone also reduce nurse bee maturation. All of these events are in a feedback loop. The reduction in levels of brood pheromone further reduces the level of foraging … meaning more foragers are ‘at home’, so increasing the effects of ethyl oleate having the effect of retarding worker bee maturation. The workers remain as ‘nurse-like’ long-lived winter bees.

Inside the Hive

A typical beehive during winter presents a complex microenvironment where temperature, humidity and CO₂ levels play crucial roles. Data collected from hives during the winter months showed steady CO₂ levels above 10,000 ppm (>25 times atmospheric levels). CO₂ level drops are associated with outdoor warming when bees are out of cluster, winter cluster movement during extended periods of cold, and beekeeper disturbance (e.g., cleaning entrance) events.

In a typical wood hive, clustering will start at about 10 to 15 C (50-59 F). In a well-insulated hive, clustering will not occur till outside temperatures reach -10 to -15 C (14 to 5 F). Similarly, temperatures in this location can be much warmer than those at the hive entrance or outside, emphasizing the bees’ capability to generate and conserve heat.

The temperature difference between the inside wall of the hive, especially above the bee cluster, and the external environment drives natural ventilation or convection within the hive. This process is crucial as it affects how bees manage the internal environment to protect themselves from cold and reduce energy expenditure. The entrance and internal temperature difference highlight a gradient necessary for passive airflow, which helps regulate CO2 and moisture levels inside the hive

Passive airflow uses buoyancy (warmer air rising) to facilitate air movement in the beehive. The heat emanating from the cluster causes the air around it to warm and rise, pulling in colder air in via the lower entrance. Top-vented hives will pull this air out of the colony. Hives with only a bottom entrance will vent out via the lower entrance as the rising warm air is directed toward the cooler side walls. Colder air is denser; hence it will drop. This is often called displacement ventilation in building ventilation science, vs. mixing ventilation where energy is directly inputted to force-mix the used air with fresh air.

An example of mixing ventilation (mechanical dilution) would be the bees actively fanning during the summer nectar flow. Displacement ventilation operates on natural temperature differences (in vs. out). Uninsulated hives will have negligible temperature difference with the outside temperature, thus the requirement for more upper ventilation. Mixing ventilation requires extra mechanical energy. Hence many of our energy efficient buildings have cool fresh-air diffusers at floor level and use passive heating bodies (human’s body heat) to create rising thermal plumes. The number of people (size of the cluster) in the room drives the amount of ventilation passively. This is why the size of the cluster to the volume of the hive is a critical wintering criteria. It is less about the number of bees and more about the hive volume. A small cluster will not be able to naturally ventilate a larger volume, but it will have a better chance of success in a smaller volume.

Varroa Mortality

Recent studies have shown that elevated CO2 levels, along with controlled temperature and humidity, can significantly increase the mortality of varroa mites. By maintaining a well-insulated hive that mimics the CO2-rich conditions of indoor storage, beekeepers may potentially create an environment that is not only conducive to bee health but also hostile to varroa mites. When ventilation is restricted, mean CO2 level (3.82 ± 0.31%, range 0.43-8.44%) can increase by 200% relative to standard ventilation (1.29 ± 0.31%; range 0.09-5.26%) during the winter. The overall mite mortality rates and the reduction in mean abundance of varroa mite over time is greater under restricted ventilation (37 ± 4.2%) than under standard ventilation (23 ± 4.2%) but not affected by stock of bees during this period. Restricting ventilation increased mite mortality but did not affect worker bee mortality relative to that for colonies under standard ventilation. Restricted ventilation did not affect the overall level of Nosema compared with the control. There is a significant difference in mite mortality of colonies with high CO₂ compared to colonies held at low CO₂. These results indicated that high CO₂ could increase mite mortality during the winter period, potentially improving honey bee health in properly constructed hives coming out of the winter months.

Reference Materials

How to Safely Make Frame Shoulders

Robbing Stress

Harbo Assay, Varroa Sensitive Hygiene Testing

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