Combination Effects of Varroa Treatments and Pesticides on Honey Bees

DISCLAIMER: The blog below is generally created from the research listed in the “Referenced Material” section.  This is not my research. All credit is given to these studies and researchers who are helping beekeepers better understand how we can improve our management practices.

In our often over medicated world, we often find ourselves having discussions with our doctors or pharmacists about the possible negative combination impacts of the drugs we are taking or what might happen when we introduce a new one even if the addition itself is seen as completely safe.  Even taking an aspirin/Tylenol after getting home from work might be overdosing us as we learn that these medicines might already be in something we have taken previously in the day.  Think about the lists of serious risks you often hear in any new medicine advertisement.  Think about how many people are fighting with food and medicine allergies to their pets these days. Everything, I mean everything, has both positive and negative effects on us or our pets…and our honeybees.

Below is a quick overview of a topic that often is not discussed when we listen to a beekeeping presentation of a new treatment or management method.  Why this is, I’m not sure but studies are out there, and the topic of combination effects should be understood by any beekeeper who is or will be considering treatments for their bees.  There are even new studies pointing out the negative effect of improperly applied treatments per product descriptions.  Straying from very strict adherence to product application instructions and environmental requirements could also make treatments more harmful than intended even expanding the combination effects already present.  Chemicals that we need to protect ourselves from to apply should be considered with great care as we apply them to another unprotected biological species. Nothing will be totally safe even those called out as safe because they are listed as organic.  The question we should ask is will the combination effect do more harm than good.

Will the treatment help our honeybees thrive or just survive.

Combination Effects of Varroa Treatments and Pesticides on Honey Bees

Combining acaricides (mite killers) with other pesticides (insecticides, fungicides) significantly harms honey bees, often creating synergistic effects where the mixture is far more toxic than individual chemicals, impacting survival, detoxification, immunity, and behavior, even at low, realistic doses, leading to systemic damage and increased colony collapse risk. These chemicals interact, suppressing immune responses, altering metabolism, and making bees vulnerable to diseases and stressors, with effects varying from lethal to sublethal (hyperactivity, reduced feeding).

Acaricides

• Purpose: Primarily used to control Varroa destructor, a significant pest of honey bees.
• Types: Include both synthetic and organic options.
• Impact: Can harm bee health, especially if used improperly or in high concentrations.

Pesticides

• Purpose: Designed to eliminate herbivorous pests but can also affect non-target insects like honey bees.
• Application Methods: Spraying and seed coating are common methods that lead to bee exposure.
• Exposure Routes: Bees can be exposed through ingestion, contact, or inhalation.

Key Combination Effects:

• Synergistic Toxicity: Mixtures often show “more than additive” effects, meaning the combined harm is greater than the sum of each chemical’s individual harm, as seen with acaricide-fungicide-insecticide cocktails.
• Metabolic & Detoxification Disruption: Combinations interfere with detoxification enzymes and stress response pathways, overwhelming the bees’ systems.
• Immune Suppression: Acaricides and other pesticides can weaken immune function, making bees more susceptible to viruses and pathogens.
• Behavioral Changes: Exposure can cause hyperactivity (more active/crossing lines) or reduced feeding, impairing foraging and colony function.
• Bioaccumulation: Residues build up in wax, pollen, and honey, creating chronic exposure for bees and mites, increasing resistance and overall colony burden.

Examples of Combinations Studied:

• Acaricides + Fungicides: Can heighten toxicity; some fungicides might even offer weak protection at very low doses but become harmful chronically.
• Acaricides + Insecticides (e.g., Coumaphos + Imidacloprid): Reduced food consumption and increased mortality.
• Insecticide + Fungicide + Herbicide: Showed high mortality and global systemic impacts on bee physiology in winter bees.

In essence, the “chemical cocktail” effect, where multiple pesticides and acaricides interact, poses a significant threat to honey bee health, often exceeding the danger of single-chemical exposure.

Reference Materials

• Mixtures of an insecticide, a fungicide and a herbicide induce high toxicities and systemic physiological disturbances in winter Apis mellifera honey bees
• Effects of coumaphos and imidacloprid on honey bee (Hymenoptera: Apidae) lifespan and antioxidant gene regulations in laboratory experiments
• Effects of synthetic and organic acaricides on honey bee health: A review
• Acaricide, Fungicide and Drug Interactions in Honey Bees (Apis mellifera)
• Combined toxic mechanism and risk assessment triggered by acetamiprid and tetraconazole in honey bees
• Single and combined exposure to ‘bee safe’ pesticides alter behaviour and offspring production in a ground-nesting solitary bee (Xenoglossa pruinosa)
• Direct effect of acaricides on pathogen loads and gene expression levels in honey bees Apis mellifera
• Pesticide–Virus Interactions in Honey Bees: Challenges and Opportunities for Understanding Drivers of Bee Declines
• Four Common Pesticides, Their Mixtures and a Formulation Solvent in the Hive Environment Have High Oral Toxicity to Honey Bee Larvae
• Impact of common acaricide exposure of honey bee colonies for Varroa control on royal jelly metabolome and proteome
• Assessing the effects of a commercial fungicide and an herbicide, alone and in combination, on Apis mellifera: Insights from biomarkers and cognitive analysis
• Pesticides and habitat loss additively reduce wild bees in crop fields
• Understanding the Effects of Sublethal Pesticide Exposure on Honey Bees: A Role for Probiotics as Mediators of Environmental Stress
• The Exposure of Pesticides to Honeybees: A Global Threat to Food Security
• Effects of synthetic and organic acaricides on honey bee health: A review