Honeybee Medicine Cabinet

Introduction

The coevolutionary dependencies between pollinators and plants has enabled pollinators including honeybees to derive benefits from pollen and nectar but today they face challenges from declining natural flowering habitats and diverse floral (polyfloral) resources. They are also affected by increasing metabolic stressors, such as the exposure to plant protection products (agrochemicals), dietary limitation, monofloral diets, and pressure of disease and pathogen interactions.

Pollen and nectar gathered by foraging honeybees provide essential macronutrients (proteins, carbohydrates, and lipids) that ensure proper brood development. In addition to these macronutrients, honeybees also obtain various plant secondary metabolites (phytochemicals) from floral nectar and pollen. Specific phytochemicals in pollen and nectar provide a variety of health benefits to honeybees, including extended longevity, improved pathogen and environmental stress tolerance, pesticide detoxification, building gut microbiome abundance and enhanced cognition and memory.  It should be noted that phytochemical composition varies between plant species suggesting that the negative impact levels of agrochemicals on honeybees could vary based on the phytochemicals in the pollen and nectar of that crop, and hence the effects may vary across crops and time of the year. The ability of the honeybee to tackle these historically recent environmental challenges is strongly dependent on the diversity and availability of their sources of healthy nutrition.

Social Immunity

Nest homeostasis which honeybees continually work to achieve for themselves, that is, constant temperature, humidity, and respiratory gases, as well as rich food stores and vast amounts of brood also create excellent growth conditions for microorganisms. Furthermore, the large number of closely related individuals in tight proximity with high interaction frequencies promotes pathogen transmission. Honeybees also have a reduced set of immune genes compared to other insect species. This deficiency can partially be compensated by their “social immunity” resulting from highly adaptive hygienic behavior and workers behavior towards infected colony members. For instance, it’s been shown that collecting antimicrobial propolis (plant resins) and applying it to the inside of the hive also contributes to the “social immunity” at colony level and the decrease of infectious microorganisms.

Besides their natural immune defense, honeybees evolved behavioral defenses to combat infections. Foraging of antimicrobial plant compounds play a key role for this “social immunity” behavior. Secondary plant metabolites in floral nectar are known for their antimicrobial effects. Yet, these compounds are highly plant specific, and the effects on honeybee health will depend on the floral origin of the honey produced. As worker bees not only feed themselves, but also the larvae and other colony members, honey is a prime candidate acting as self-medication agent in honeybee colonies to prevent or decrease infections. The specificity of the monofloral honeys and the strong antimicrobial potential of the polyfloral honey suggest that the diversity of honeys in the honey stores of a colony may be highly adaptive for its “social immunity” against the diverse suite of pathogens they encounter in nature. The polyfloral diversity may therefore assist their genetic variance in the fight against infection and parasites.

Medicinal Selectivity

As mentioned above, nectar contains many secondary plant metabolites including various aromatic acids and diverse phenols with high antimicrobial activity. These plant derived honey compounds are not only highly plant specific, but also depend on seasonal and environmental factors as well as processing and storage by honeybees. Under natural conditions, when the beekeeper does not extract the honey from the hive, the honey stores of a colony will therefore contain a variety of honeys from many different plant sources with variable composition of secondary plant metabolites and variance in antimicrobial competence. Once stored inside the hive, the different honeys are available independent of the foraging season. Hence, different honeys can potentially be chosen by the worker bees not only to satisfy their carbohydrate needs for food, but also for their antimicrobial activity.

Given this variance in antimicrobial compounds among different honeys, they may well have specific efficacies against various bacterial pathogens. It would be highly adaptive if honeybees took advantage of this variability in the honey store using the different honeys to fight various pathogens. The use of this potential for an efficient selective self-medication would considerably increase the “social immunity” of the colony. Indeed, honeybees have been shown to selectively choose among several honey types depending on their health status and the higher potential to reduce their infection.

As honey is the central nutrient for developing larvae, the diversity in the honey stores may serve as a richly stocked natural “in-hive pharmacy” against a broad variety of brood diseases. During the first 2 days after hatching from the egg, the larval diet mainly consists of components secreted from the hypopharyngeal food glands of nurse bees, presumably mixed with honey. However, beginning with the third day honey and pollen is added to the diet and directly fed to the worker larvae. Thus, the nurse bees are in the central position of the intracolonial food web and provide a mechanism to promote the colony’s health status by selectively feeding specific honeys in response to specific infections.

In addition, honey provides phytochemicals that can promote overall colony health in several ways. It was shown that nurse bees infected with Nosema preferentially consume sunflower honey, which has a high antimicrobial activity. Caffeine, an alkaloid found in the nectar of the Rubiaceae plant family, among others, can enhance memory in honeybees. Moreover, phytochemicals in nectar, honey, pollen, or propolis can offer other health benefits. The phenolic acid p-coumaric acid, a constituent of many honeys, upregulates both detoxification genes and immunity genes in larval and adult honey bees.  Quercetin, a flavonol found in many honeys, essentially all pollen, and in propolis in many parts of the world, also upregulates at least 12 genes detoxifying and enhances longevity of workers exposed to insecticides.

Diverse Foraging

The honeybee is a highly complex (eusocial) species whose foragers collect food to meet hive requirements and adjust their food-gathering behavior according to these collective needs. Foragers are the first members of the colony to encounter and evaluate potential food resources and to make decisions about whether to bring them back to the hive. Phytochemicals in nectar and pollen can both attract pollinators and repel inappropriate floral visitors, including honeybees.

Many secondary plant metabolites with known antimicrobial potential have been found in honey. These compounds have been shown to be highly plant specific regarding which polyphenols and flavonoids that interfere with pathogen growth. Furthermore, the strength of antimicrobial effect depends on the interaction among these different polyphenols and flavonoids. It’s been shown that polyfloral honey had about double the polyphenol concentration and enhanced antimicrobial activity compared to the monofloral honeys from a single crop. It’s also noted that the honeybee’s specific lactic acid bacteria within their honey stomach have been shown to play an important role in producing antimicrobial substances in honey. The composition of the lactic acid bacteria microflora varies depending on floral sources, thus the substances the specific flora produces.

Pollination

Recently, growing evidence for strong declines of insect abundance and richness, including in pollinators, underpin concerns for the disruption of ecosystem services. Among pollinators, honeybees provide crucial ecosystem services to both wild and cultivated plants. Several reports indicate the possible reasons for poor honeybee health such as exposure to a wide range of plant protection products (PPPs), dietary limitation, monofloral diets and pressure of diseases and pathogens or maybe of a combination of factors. Phytochemicals available to foraging bees depends on the plant species in the foraging range. Each plant species offers a different mix of phytochemicals in its pollen and nectar and the mix depends on the physiological status of the plant (e.g., drought-induced responses). Therefore, the effect of PPP exposure on pollinating bees may not be dependent solely on the dose but also on the plant species that are receiving the PPP application and are in the foraging range of bees.

For their part, herbicides and fungicides have been comparatively understudied relative to the frequency with which they are documented as hive contaminants. Chlorothalonil is among the most frequently encountered contaminant in beehives, especially in wax and in pollen. The longstanding assumption has been that fungicides and herbicides, with relatively low acute toxicity relative to pesticides formulated to kill arthropods, are considered to be safe for bees. Nonetheless, fungicides and herbicides can have unexpected undesirable impacts on honeybees. The herbicide atrazine alters acetylcholinesterase activity (neurotransmission) in honeybees and exposure to glyphosate reduces sensitivity to sucrose and interferes with learning performance and navigation ability. Moreover, bees consuming food contaminated with the fungicide chlorothalonil experience higher rates of infection by the parasite Nosema, reduced queen body size, fewer workers, and smaller colony size.  Chlorothalonil also synergizes tau-fluvalinate, a pyrethroid acaricide used in beehives for varroa control, and increases its toxicity to honeybees.

Evidence shows that the impacts of sublethal doses of PPPs differ depending on the phytochemical supplement in the worker honeybee diet.  Although phytochemicals in pollen and nectar may help improve the ability of honeybees to tolerate the PPP, the benefits depend on the specific phytochemical and the dose of the PPP. Similar results have been reported in previous studies, where access to phytochemicals in the diet allowed bees to live longer even when workers are exposed to sublethal doses.

Substitutes and Malnutrition

Nectar and pollen, both raw and in their processed forms as honey and beebread, have long been considered the principal natural sources of carbohydrate and protein for honeybees. Current beekeeping practices have led to the creation of substitutes or supplements for honey and pollen, notably sucrose or fructose for honey and soy flour diet for pollen. However, phytochemicals clearly serve important functions beyond carbohydrate and protein nutrition for honeybees and their absence from dietary supplements or substitutes may have effects on honeybee health that are yet undetermined.  Among the phytochemicals present in most pollens and in honey from a diversity of nectar sources, the phenolic acid p-coumaric acid and the flavonol quercetin upon ingestion upregulate expression of a diversity of xenobiotic-metabolizing genes (detoxifying), including enzymes, in both adults and larvae. In addition to influencing detoxification capacity, phytochemicals may affect the lifespan of bees, as they are known to do in other organisms since the longevity of adults exposed to pesticides during larval stage depends on pollen nutrition. For instance, in Brazil they found that honeybee longevity is 6.6% greater in hives with more propolis present; propolis typical of this region has been shown to be rich in quercetin (natural antioxidant), along with phenolic acids. It’s also been shown that quercetin can reduce toxicity of tau-fluvalinate, a broad-spectrum pyrethroid acaricide (insecticide). Previous studies have also shown that malnutrition can increase sensitivity to pesticides, reduce immunocompetence and alter gene expression in protein metabolism and oxidation-reduction in fat body.

Together, these findings suggest that substituting sugar syrups for honey or yeast/soy flour patties for pollen may not only cause malnutrition but may also have unanticipated effects on lifespan in the presence of environmental stressors. There is also enough evidence of antagonistic interactions or negative effects that simply augmenting honeybee sugar substitutes or soy flour substitutes with phytochemicals added is inadvisable without additional information on the mechanisms by which phytochemicals can enhance longevity or increase pesticide toxicity. They also suggest that honey is more than a fuel source and that pollen is more than merely a protein source for the bees; the phytochemicals of honey and pollen appear to play an essential role in honeybee metabolic detoxification, particularly in the presence of life shortening pesticides.

Honey vs. Sucrose

Beekeepers often feed high fructose corn syrup (HFCS) or sucrose after harvesting honey or during periods of nectar dearth. We report that, relative to honey, chronic feeding of either of these two alternative carbohydrate sources elicited hundreds of differences in gene expression in the fat body, a peripheral nutrient-sensing tissue analogous to vertebrate liver and adipose (fatty) tissues. These expression differences included genes involved in protein metabolism and oxidation-reduction, including some involved in amino acid metabolism. Differences between HFCS and sucrose diets were much more subtle and included a few genes involved in carbohydrate and lipid metabolism. Our results suggest that bees receive nutritional components from honey that are not provided by alternative food sources widely used in apiculture and that the constituents in honey differentially regulate physiological processes and that sucrose and HFCS may not be equivalent nutritional substitutes to honey.  Studies further show honey induces gene expression changes on a more global scale. These changes may have toxicological relevance under natural conditions in contemporary agroecosystems, where bees are routinely exposed to toxins and pesticides.

Central to honeybee health is nutrition. Malnutrition in honeybee colonies can result from maintaining densities of colonies that are too high for available flora or placement of colonies for pollination of crops that are deficient in pollen or nectar or have low nutritive value. Poor nutrition can make bees more susceptible to pesticides and lead to a compromised immune system making bees more vulnerable to diseases. Contrary to our expectations, we found the expression profile of honey-fed-bees resembled the less well-nourished foragers rather than the more well-nourished nurse bees. These results suggest that there may be compounds in honey that modulate honeybee physiology towards a forager-like state.  A state that may be healthier for bees facing a growing list of stressors and the need to be more streamline during flight.   This is similar to how they prepare the overwintered queen for swarming.

Why Leave Honey?

The colony’s food source selectivity in terms of nectar, as selective decision behavior by foraging bees, is a process of natural selection among alternative nectar sources including effectiveness and communication. Hence, the workers may also choose from a complex mix of different honeys because honey stores will overlap with seasonally changing flower availability. Studies show that low concentrations of secondary phytochemicals elicit a significant feeding preference, confirming the mechanism of selectively choosing between specific nectar resources regardless of availability. This qualitative variance in the honey stores of the colony may be of considerable importance for colony health whenever it is exposed to various pathogenic bacteria.

If variable honey stores facilitate colony health, this would not only be an important evolutionary achievement of honeybee colonies, it would also have profound consequences for beekeeping practices. Apiculturists might take advantage of specific honey flows to protect their colonies against specific diseases. In addition, beekeepers should be aware that the exclusive production of monofloral honeys may have negative consequences for colony health. Also, the feeding of sugar as a food source over winter may enhance the propensity of the colony to be infected by pathogens. In conclusion, floral biodiversity providing the nectar source for the colony will have direct implication for colony health with similar importance as the genetic diversity of the honeybee.

Glossary

• Abiotic Stress – is the negative impact of non-living factors on the organisms such as temperature, sunlight, wind, salinity, flooding, drought, and agrochemicals such as pesticides.
• Biotic Stress – The stressthat occurs because of damage done to an organism by other living organisms, such as bacteria, viruses, fungi, parasites, beneficial and harmful insects, weeds, and cultivated or native plants.
• Eusociality – The highest level of organization of sociality. It is defined by the following characteristics: cooperative brood care, overlapping generations within a colony of adults, and a division of labor into reproductive and non-reproductive groups. The division of labor creates specialized behavioral groups within an animal society which are sometimes referred to as ‘castes’.
• Gut Microbiome – the microorganisms, including bacteria, archaea, fungi, and viruses, that live in the digestive tract.
• Metabolites – a metabolite is an intermediate or end product of metabolism. The term is usually used for small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, catalytic activity of their own, defense, and interactions with other organisms. A primary metabolite is directly involved in normal “growth”, development, and reproduction.
• Monofloral – Made wholly or predominantly from the nectarof a single plant species.
• Pathogen – An agent that causes disease, especially a virus, bacterium, or fungus.
• Phytochemicals – chemical compounds produced by plants, generally to help them resist fungi, bacteria, and plant virus infections, and also consumption by insects and other animals.
• Plant Protection Product – It protect plants from weeds, fungi, or insects. In general, a pesticide is a chemicalor biological agent (such as a virus, bacterium, or fungus) that deters, incapacitates, kills, or otherwise discourages pests.
• Polyfloral – Made from the nectar of an assortment of plants.
• Quercetin – A natural antioxidant present in many fruits, vegetables, and grains.
• Xenobiotic – A chemical substance found within an organism that is not naturally produced or expected to be present within the organism.

Referenced Materials

• Diversity of honey stores and their impact on pathogenic bacteria of the honeybee, Apis mellifera
• Pathogen-associated self-medication behavior in the honeybee Apis mellifera
• Impacts of Dietary Phytochemicals in the Presence and Absence of Pesticides on Longevity of Honey Bees (Apis mellifera)
• Context-Dependent Effect of Dietary Phytochemicals on Honey Bees Exposed to a Pesticide, Thiamethoxam
• High nutritional status promotes vitality of honey bees and mitigates negative effects of pesticides
• Metabolomics unveils the influence of dietary phytochemicals on residual pesticide concentrations in honey bees
• Dietary Phytochemicals, Honey Bee Longevity and Pathogen Tolerance
• Behavioral responses of honey bees (Apis mellifera) to natural and synthetic xenobiotics in food
• Increase in longevity and amelioration of pesticide toxicity by natural levels of dietary phytochemicals in the honey bee, Apis mellifera
• Honey Has Numerous Health Benefits for Bees
• Diet-dependent gene expression in honey bees: honey vs. sucrose or high fructose corn syrup
• Scientists track gene activity when honey bees do and don’t eat honey
• Missing Microbes in Bees: How Systematic Depletion of Key Symbionts Erodes Immunity