Honeybees are the most prevalent insect pollinator species; they pollinate a wide range of crops. Colony collapse disorder (CCD), which is caused by a variety of biotic and abiotic factors, incurs high economic/ecological loss. Various ecological stressors are microbial infections, exposure to pesticides, loss of habitat, and improper beekeeping practices that are claimed to cause these declines. Honeybees have an innate immune system, which includes physical barriers and cellular and humeral responses to defend against pathogens and parasites. Exposure to various stressors may affect this system and the health of individual bees and colonies.
1. Introduction
The European honeybees (
Apis mellifera L.) is considered one of the most important agricultural pollinators worldwide. They play a key role in food productivity by pollinating various plants
[1][2]. One-third of a person’s diet comes from insect-pollinated plants, and honeybees are responsible for the pollination of over 80% of flowering plants. Without honeybees pollination, crop yields would decrease by >90%
[3]. Therefore, there is major international concern related to bee colony loss
[4][5].
Colony collapse disorder (CCD), which first emerged in the US in 2006, caused huge colony losses and posed challenges for crop pollination, which is the major service of the apicultural industry in North America
[6]. The observed losses between 1961 and 2007 recorded in Europe and North America were 26.5% and 49.5%, respectively. Honeybee colonies have increased, primarily in Asia (426%), Africa (130%), South America (86%), and Oceania (39%)
[7]. The following factors have been implicated in honeybee losses in different parts of the world: honeybee diseases, parasites, in-hive chemical substances, agrochemicals, genetically modified (GM) plants, modified land-use, changed and alteration in the cultural practices, beekeeping practices, as well as the climate change
[8][9][10][11][12][13].
2. Honeybee Immunity
The widespread agreement on the multifactorial origins of colony collapse and its frequent correlation with high pathogen and parasite loads indicate that the immune system is the most targeted, and its activity can be altered by a variety of stressors
[14]. Bees have an innate immune system, which includes physical barriers, cellular, and humoral responses to defend against pathogens and parasites. The physical barrier includes an exoskeleton cuticle and the peritrophic membranes lining the digestive tract to prevent the entry of pathogenic organisms into the body cavity
[15]. The recognition of pathogen-associated molecular patterns by recognition receptors triggers the innate immune system
[16]. As a result, hemocytes that represent the primary mediators of cellular immunity will be activated, including phagocytosis, nodule formation, encapsulation, as well as the initiation of phenol-oxidase that regulates coagulation or melanization, or the synthesis of antimicrobial peptides (AMP), such as abaecin, apidaecin, hymenoptaecin, and defensin
[17]. The immune system of honeybees possesses orthologues for the major members of immune pathways comprising the following: Toll (transmembrane signal transduction pathway), immune deficiency (Imd), Jun-N-terminal kinase, and JAK/STAT (Janus kinase/signal transducers and activators of transcription). RNA interference (RNAi), known as RNA silencing, is an important antiviral defense mechanism in insects, including honeybees. The efficacy of RNAi-mediated treatment against honeybee viruses and the fact that honeybee viruses encode for putative virus-encoded suppressors of RNAi shows that RNAi is an important honeybee antiviral defense mechanism
[9][18]. In addition, several studies implicate the involvement of innate immune pathways (Jak-STAT, Toll, and Imd) and non-sequence-specific dsRNA-mediated antiviral defense as part of the immune responses in honeybee
[19].
Bees are creatures that exhibit social immunity to prevent parasite infection from spreading among colony members. Honeybee workers use hygienic practices to remove diseased brood
[20]. Furthermore, social fever develops when bees cooperate to raise the temperature within the colony to resist the heat-sensitive fungal disease
Ascosphaera apis, known as chalkbrood disease
[21]. Grooming, which is the physical removal of parasitic mites from the bodies of adult bees by individual workers or their nest-mates, is one of the most important defense modes against the ectoparasitic mite
Varroa destructor [22]. Other mechanisms of social immunity such as propolis collection is used for nest constriction. It helps in declining investment in the immune response of 7-day-old bees and enriches the health and productivity of the colony
[23]. Previous study showed that a propolis envelope reduced the clinical symptoms of American foulbrood (AFB) two months after the challenge, compared with those of colonies without a propolis envelope. Additionally, it protected the brood from pathogenic infection
[24]. Glucose oxidase (GOX) is an antiseptic enzyme found in nectar and larval diet, and an additive to prolong the products’ shelf life. In the hypopharyngeal glands, GOX is a catalytic enzyme that catalyzes the conversion of β- d- glucose to gluconic acid and hydrogen peroxide (H
2O
2)
[25] and provides the social immunity. H
2O
2 functions as an antiseptic, preventing pathogen growth in honeybee larval diet
[26].
3. Main Causes of Honeybee Colony Losses
3.1. Varroa Mite
The ectoparasitic mite
V. destructor is considered one of the most important factors behind the recent high annual loss of honeybee colonies. The mite directly damages bees by feeding primarily on honeybee fat body tissue and not hemolymph
[27][28].
The physical damage caused by the mite has been reported to suppress the bees’ immune response
[29]. Varroa parasitism has inhibited the expression of genes encoding immunity (hymenoptaecin and defensin), longevity, and stem cell proliferation in honeybees
[30].
The mite is unlikely to cause the collapse of hives; however, it acts as a vector for a cocktail of viral disease agents, which are among the probable causes for CCD, including the deformed wing virus (DWV), kashmir bee virus, sacbrood virus (SBV), acute bee paralysis virus, and Israeli acute paralysis virus (IAPV)
[31][32][33][34].
3.2. Nosema spp.
Nosemosis is a disease that affects honeybees and is caused by intracellular parasites (
Nosema apis and
Nosema ceranae) that infect the adults’ midgut epithelial cells
[35].
N. ceranae infection is highly pathogenic for honeybee colonies, significantly reducing the colony size, brood rearing, and honey production, and increasing winter mortality. In persistent infections, the pathogen can impact colony performance by reducing a colony’s ability to regulate hive temperature or by killing the entire colony
[36]. For the European honeybee,
N. ceranae reduced homing and orientation skills, and altered the metabolism of forager bees
[37].
3.3. Viral Pathogens
For honeybees worldwide, over 24 viruses have been discovered, some of which can have major health repercussions
[38]. A highly prevalent and relatively virulent virus transmitted by
V. destructor which impacts the health of honeybee colonies worldwide is DWV
[10]. DWV-induced honeybee loss, coupled with a long-term decline in beekeeping, has become a serious threat to the adequate provision of pollination services, which threatens food security and ecosystem stability
[39].
3.4. Pesticides
The exposure of honeybees to pesticides compromises their immune responses, navigation ability, learning, and memory
[40]. Pesticides in sub-lethal quantities can be harmful to honeybees, as it may not kill bees but reduce their performance and survival during foraging. Bees exposed to pesticides had high vulnerability to infections and hence became means to spread diseases to other parts of the colony or other colonies via the shared use of flowers
[8].
3.5. Malnutrition
Food resources are collected in large quantities by bee colonies to prepare for scarcity and are stored as honey and bee bread. This is because nutrition deficiency and deterioration can affect development and bee lifespan and increase the likelihood of infestation by a parasite, virus, or disease, resulting in honeybee mortality
[41][42]. Bees fed only on water and sugar (low protein diet) exhibited higher mortality and viral load rates, compared with bees fed on higher pollen diets. These results showed that poor nutrition can suppress immunity and that a different host’s nutrition can alter specific components of the immune system
[43]. Supplementary nutrition such as homemade sugar syrups can cause undesirable effects on bee health due to the presence of toxic compounds, such as hydroxylmethylfurfural (HMF). In addition, the preparation conditions of these sugar syrups such as temperature or addition of an acidifying substance resulted in a higher amount of HMF and bee mortality
[44].
3.6. Other Causes
Heavy metal pollution originates from various sources. Heavy metals may be ingested by honeybees through water sources and by foraging for nectar and pollen from plants that have already stored heavy metals
[45]. Cd, Pb, As, Hg, Ni, and Cr are particularly harmful to living beings due to their high toxicity. Furthermore, elements such as Cu, Fe, Se, Zn, Mn, and Co, which are required for a range of biochemical and physiological processes, can be hazardous to bee colonies
[46].
Urbanization is one of the biggest challenges to wild plants and pollinators, including bees
[47]. Heavy metals such as Pb and Cd are found in high concentrations in urban areas. In urban areas, bees were more susceptible to disease transmission, particularly the black queen cell virus (BQCV) and fungal pathogen
N. ceranae. This effect did not appear to be mediated by immunity, as determined by immune-gene expression, which was not affected by urbanization
[48]. Moreover, there was no evidence of immunocompetence differences between managed and feral bees
[49].
Nanoparticles (NPs), which are used in a variety of industrial applications, negatively impact human and animal health, including honeybees. CdO or PbO NPs produced histological and cellular abnormalities in honeybee workers’ midgut epithelial cells
[50]. When exposed to CdO or PbO NPs, separately or in combination, acetylcholinesterase (AChE) activity and the expression of a variety of stress-related detoxifying enzymes were inhibited. Furthermore, the rate of feeding and survival reduced
[51].
The rapid expansion of the telecommunications industry has resulted in a massive increase in the number of mobile phones and the rapid deployment of cell towers across the globe. According to certain studies, honeybees do not rely on the electromagnetic field (EMF) to navigate, and many apiaries that are experiencing losses are in rural areas where cell phone service is absent. The World Health Organization confirmed the same data; however, some researchers revealed that there was standard evidence that the EMF could cause damage in honeybees
[52][53]. It was associated with increased bees activity, increased inside temperature, increased queen loss, abnormal real cell production, weight loss, and reduced operculated brood
[54]. Chronic radiofrequency EMF exposure significantly reduced the hatching of honeybee queens
[55].
4. Interaction between Different Stressors Affects the Bees Immunocompetence
4.1. Interaction between Pesticides and Pathogens
Many factors including pesticides, diseases, and malnutrition lead to bees’ decline in different regions worldwide. These threats are frequently interconnected, and it is unlikely that colony losses are caused by single stressors (
Figure 1)
[56].
Figure 1. The impact on honeybees’ health when exposed to interaction between environmental and ecological stressors.
Interactions between pesticides and pathogens may play a role in increased honeybee colony losses, including CCD and other pollinator reductions worldwide
[57]. CLO inhibits NF-κB immune signaling in insects at sub-lethal dosages, and CLO and imidacloprid compromise honeybees antiviral defenses regulated by transcription factors
[58]. Although other studies estimated that increased pesticides may not always result in increased viral loads
[10], exposure to neonicotinoid pesticide imidacloprid, in the presence of the gut parasite
N. ceranae, increased the levels of enzymes such as catalase (CAT) and glutathione-S-transferase in the heads of bees. These enzymes are involved in pesticide and parasite resistance to xenobiotics and oxidative stress. Furthermore, stressors affected midgut enzymes, i.e., carboxylesterase alpha (CaE) and carboxylesterase para (CaE p), which are engaged in metabolic and detoxifying processes
[57][59][60][61]. In honeybees, the interactions between thiacloprid and
N. ceranae caused Nosema to increase, regardless of the thiacloprid dosage
[61].
4.2. Interaction between Pesticides and Poor Nutrition
The combination of low diet and chemical exposure affects bee survival synergistically (−50%). The interaction reduced food consumption (−48%), hemolymph levels of glucose (−60%), and trehalose (−27%)
[62]. Researchers have indicated that various insecticides indirectly influence honeybees’ health via diet by suppressing immunity-related genes and negatively altering NFB immunological signaling. It can significantly impair the honeybees’ immune system, reducing the bees antiviral defense regulated by this transcription factor. This may cause direct death or become easy prey to predators
[58][63][64]. Bees survival was reduced as a result of the interaction of restricted nectar and nectar availability with neonicotinoid exposures, such as CLO and TMX
[62].
4.3. Interaction between Pathogens and Poor Nutrition
Nutritional deficiencies increased pathogen load and reduced adult longevity and survival
[65]. Diseases altered foraging behavior by reducing foraging abilities or altering floral preferences. In addition, pollination services were impaired when bee populations were reduced or feeding habits were altered
[66]. The interaction between poor nutrition and pathogenic infection increased the rate of colony mortality and reduced the ability of bees to fight other stressors
[67]. Disease infection (viral, fungal, and bacterial pathogens), reduced bee nutrition (caloric needs, dietary requirements, nutrient storage, gut physiology, and microbiota), and made bees more susceptible to diseases and vice versa
[68].
4.4. Interaction between Parasites and Pathogens
The interactions between the DWV and ectoparasitic mite,
V. destructor, resulted in DWV replication and increased Varroa reproductive output
[69].
V. destructor can vector IAPV in honeybees and are capable of IAPV replication. The density of Varroa mites and the duration of exposure to the mites were positively related to the copy number of IAPV in bees. Furthermore, the mite–virus association may reduce the host immunity, promoting high levels of virus replication. Varroa mites provide a plausible route for IAPV transmission in the field and may significantly contribute to the honeybee diseases associated with CCD
[27][70].
5. Strategies to Enhance Honeybee Immunity
5.1. Fortified Nutrients
Currently, honeybees face numerous threats that hinder their survival. Special attention should be paid to beekeepers, bee supplements, and nutrition to limit the risk of viruses, other diseases, and agrochemicals. Beekeepers should support colonies with suitable supplementary feeding during dearth periods
[71]. In addition, the diversity of nutrition that comes from different natural plant sources can improve honeybee immunity, antiviral, and antimicrobial properties
[68]. A diet with pollen from different plants or high-quality single pollen organs can enhance honeybees’ immunity and survival as shown in
Figure 2.
Figure 2. Factors that can improve honeybees immunity against different stressors.
5.2. Natural Products as Alternative Sources
Essential oils such as thymol, linalool, and camphor, as well as cocktails of thymol, eucalyptol, menthol, and others, have been confirmed to be particularly efficient in suppressing Varroa mites. These types of essential oils were discovered to lower mortality rates among bees in diseased colonies [72]. Although natural products therapy has fewer side effects than chemical therapy, the efficacy of these substances varies depending on the climate and colony condition [73].
Recently, Chinese herbal medicine has demonstrated a unique antiviral effect for both human and animal life. Honeybees are at risk from SBV and Chinese sacbrood virus (CSBV). Infected larvae will not develop into pupae and will eventually die, and there is currently no effective cure for the virus [19]. Radix isatidis, a Chinese herbal remedy, was primarily utilized to treat human influenza viruses. It has recently been proved to effectively regulate CSBV by suppressing its replication, increasing immunological response, and extending the lifespan of CSBV infection larvae, thereby lowering death rates and preventing CCD [74]. DWV and Lake Sinai virus are two RNA viruses with positive strands that kill honeybees. Bees fed with polypore mushroom extracts exhibited a strong ability to diminish both virus larvae. Modified porphyrins, which are mostly produced by living organisms, can reduce spore burdens in bees and increase the survival likelihood of bees infected with RNA viruses [75].
5.3. Nanomaterials as Novel Alternative Approaches
Nanotechnology is one of the most active areas in research that has proven to be extremely versatile and has sparked a revolution in medical treatments, fast diagnoses, cellular regeneration, and medication delivery
[76][77]. It has been employed to discover novel therapies for honeybee diseases, as current antibiotics do not entirely eradicate the infection. Antibiotic use over time results in an accumulation of antibiotics in honey, which can be hazardous to human health
[78].
5.4. Organizations and Initiatives Directed to Saving the Bees
Organizations and initiatives are directed to save the bees in response to honeybee colony losses. Working groups such as COLOSS, a COST initiative funded by the EU Science Foundation, have been formed to address the global loss of managed honeybee colonies. By offering strategies to reduce the risk of this problem, COLOSS is playing an important role in identifying and mitigating colony losses
[79]. One of these strategies was adequate nutrition that was a key factor for honeybees’ growth and colony development.
6. Conclusions
Honeybees are important pollinators for humans and ecosystems. Unfortunately, CCD, a serious threat to the beekeeping industry, has recently been reported worldwide. It is caused by a variety of stressors that affect the immune system of bees, such as pathogens, insecticides, and inadequate diets. Scholars and governments have universally agreed that there is no single cause is to be blamed and that the causes are interconnected. However, further research is required to understand the mechanisms behind the interactions among different stressors and to discover more important genes and signaling pathways involved in honeybee stress responses. First, beekeepers must consider these aspects by planting floral-rich vegetation around the apiary and using proper dietary supplements. Second, experts could agree on a scientific plan for the treatment and management of the related diseases and pests, including the development of new nanotechnology-based remedies. Finally, organizations and stakeholders should pay attention to training to improve the efficiency of breeders and recent graduates in apiary management.
This entry is adapted from the peer-reviewed paper 10.3390/vetsci9050199