1. Honeybee immunity: components and costs

The honeybee immune system is primarily innate and includes physical barriers, cellular responses and humoral mechanisms. The exoskeleton and peritrophic membranes act as first-line barriers preventing pathogen entry. Once pathogens are recognized via pathogen-associated molecular patterns, immune pathways such as Toll (Toll-like receptor signaling pathway), Imd (immune deficiency pathway), JNK (Jun N-terminal kinase pathway) and JAK/STAT (Janus kinase / signal transducer and activator of transcription pathway) are activated, leading to phagocytosis, encapsulation, melanization and the production of antimicrobial peptides (e.g., abaecin, defensin, hymenoptaecin).

RNA interference represents a key antiviral defense mechanism in honeybees. The fact that honeybee viruses encode suppressors of RNAi highlights the biological relevance of this pathway. In addition to individual immunity, social immunity mechanisms—such as hygienic behaviour, grooming against Varroa destructor, social fever against fungal pathogens, propolis envelope formation and glucose oxidase activity in larval food—contribute to colony-level protection.

Immune activation is physiologically costly. Nutrition strongly modulates immune competence, and the type, quality and diversity of pollen intake influence the balance between immune investment and other life-history traits. The review emphasizes that maintaining immune function requires sufficient and diverse nutritional resources.

2. Major stressors affecting immunocompetence

Varroa destructor is identified as a central driver of colony losses. The mite feeds primarily on fat body tissue, causes physical damage, suppresses immune-related gene expression and acts as an efficient vector for multiple viruses, including deformed wing virus (DWV) and Israeli acute paralysis virus (IAPV). Varroa infestation is closely linked to increased viral replication and reduced longevity. Nosema spp., particularly Nosema ceranae, infect midgut epithelial cells and reduce colony size, brood rearing and honey production. Infection alters pheromonal regulation and metabolic processes, and immune responses may be initiated early but become compromised during persistent infection.

Viral pathogens such as DWV, sacbrood virus and IAPV modulate immune signaling pathways and may interfere with vitellogenin expression, melanization responses and NF-κB signaling. Their impact is often amplified in the presence of Varroa.

Pesticides, especially neonicotinoids, can impair navigation, learning, immune gene expression and antiviral defenses at sublethal doses. However, field studies report variable outcomes, and effects may depend on dose, timing and environmental context.

Malnutrition reduces longevity and increases susceptibility to pathogens. Diets limited to sugar without adequate protein suppress immunity. Preparation conditions of sugar syrups may generate toxic compounds such as hydroxymethylfurfural, which can increase mortality. Monofloral diets and monoculture landscapes are associated with altered gut microbiota and increased pathogen susceptibility under laboratory conditions.

Additional stressors discussed include heavy metals, nanoparticles, electromagnetic fields and improper beekeeping practices. These factors may influence cellular immunity, enzyme activity or colony performance, though evidence varies in strength and field relevance.

3. Interactions between stressors: synergistic and context-dependent effects

The review highlights that colony losses are rarely attributable to single stressors. Interactions between pesticides and pathogens can suppress immune signaling and increase viral replication. For example, neonicotinoids may inhibit NF-κB pathways and alter enzyme activity involved in detoxification and oxidative stress, thereby modifying host–pathogen dynamics. Combined exposure to poor nutrition and pesticides can synergistically reduce survival, food consumption and carbohydrate levels in hemolymph. Nutritional restriction may amplify the impact of chemical exposure by weakening antiviral defenses.

Interactions between parasites and viruses are particularly critical. Varroa infestation enhances DWV replication and may destabilize immune–virus dynamics, increasing mortality risk. Similarly, density and exposure duration of mites correlate with viral copy numbers.

Laboratory and field studies summarized in the review demonstrate that effects are often dose-dependent, life-stage specific and context-sensitive. In some cases, synergistic effects are observed; in others, antagonistic or no additive effects are reported, underscoring the complexity of multifactorial stress environments.

4. Nutritional and biological strategies to support immunity

Diverse and high-quality pollen diets enhance immunocompetence and pathogen resistance. Mixed pollen sources can improve larval resistance to fungal pathogens and reduce mortality following viral challenge. Protein availability is repeatedly identified as a key determinant of immune performance and colony survival. Artificial supplements based on algae, marine products, amino acids or commercial protein patties have been investigated. Some formulations reduced Nosema spore loads, improved antioxidative protection or enhanced colony strength, while others showed limited or inconsistent benefits compared with natural pollen. Dietary phytochemicals such as p-coumaric acid, indole-3-acetic acid and abscisic acid have been associated with improved survival or overwintering performance under specific experimental conditions. However, responses vary and depend on colony context and stress exposure.

Natural products, including essential oils (e.g., thymol-based formulations), plant extracts and certain mushroom extracts, are explored as alternatives for controlling Varroa, viral infections and bacterial brood diseases. Efficacy may vary with climate and colony status. Similarly, nanomaterials such as silver nanoparticles have shown antimicrobial activity in experimental settings, but further research is required to assess safety and long-term impacts on bees and hive products.

International initiatives such as COLOSS emphasize coordinated monitoring, beekeeper education and the exchange of feeding and management strategies to mitigate stress during critical periods such as overwintering.

5. Conclusions

The review concludes that colony collapse disorder and broader colony losses result from multifactorial interactions affecting immune competence. Pathogens, parasites, pesticides and nutritional stress are interconnected, and their combined effects may amplify negative outcomes. While several nutritional, natural-product and technological approaches show promise, their effectiveness depends on ecological context and integrated management. Continued research is needed to clarify molecular pathways, dose–response relationships and long-term colony-level consequences. Evidence-based, coordinated strategies involving beekeepers, researchers and organizations are essential to improve colony resilience.

6. Practical recommendations

Varroa acts as a major viral vector → increased viral load and impaired immunity → rigorous and regular monitoring and control of infestation is essential. A protein-poor diet reduces immunocompetence → colonies become more susceptible to viruses and Nosema → promote diverse floral resources and supplement during forage shortages. Combined exposures (pesticides + pathogens) may produce synergistic effects → increased risk even at sublethal doses → minimize chemical exposures as far as possible.

• Dietary supplements show context-dependent effects → benefits are sometimes observed but not universal → prioritize natural pollen when available.
• Some natural products (essential oils, plant extracts) show efficacy against Varroa or Nosema under experimental conditions → efficacy depends on climate and colony status → apply cautiously and according to validated recommendations.
• Training and beekeeping experience reduce winter losses → improved early detection of diseases and better-adapted management → invest in continuing education.

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See also:

• Infernal cascade: Chronicle of an announced death
• Brood break: Varroa control
• Factsheet: 1.3.3 Oxalic acid sublimation
• Factsheet: 1.2.1 Liebig dispenser
• Factsheet: 2.1 American foulbrood
• Biotic and abiotic stresses on honeybee health

 

Scientific basis (selection)

El-Seedi et al., 2022, Bee Stressors from an Immunological Perspective and Strategies to Improve Bee Health, Veterinary Sciences.

Nazzi et al., 2012, Synergistic parasite–pathogen interactions mediated by host immunity can drive the collapse of honeybee colonies, PLoS Pathogens.

Dolezal & Toth, 2018, Feedbacks between nutrition and disease in honey bee health, Current Opinion in Insect Science.

Goulson et al., 2015, Bee declines driven by combined stress from parasites, pesticides, and lack of flowers, Science.