21
Aug
Study Shows Synergistic Effects of Pesticides and Mites in Bees, Adding to Science on Colony Decline
(Beyond Pesticides, August 21, 2025) The presence of Varroa mites in combination with the neonicotinoid insecticide imidacloprid increases the risk of bee mortality and disrupts the larval gut microbiome, according to a study of the synergy (a greater combined effect) between Varroa destructor, a parasitic mite that attacks and feeds on honey bees, and imidacloprid. The study in Pesticide Biochemistry and Physiology adds to the growing body of science on the severely declining bee population by investigating the toxic effects of both the parasites and pesticide stressors in honey bees (Apis mellifera). “Given that V. destructor may increase bees’ sensitivity to imidacloprid by compromising their physiological health and immunity, this study systematically assesses the effects of V. destructor infestation and imidacloprid exposure on honey bee survival, detoxification enzyme activity, and gut microbiota,†the authors explain.
The intestinal tract and gut microbiome are crucial for digestion, metabolism, nutrient absorption, immune regulation, and pathogen defense. Within honey bees, the gut microbiome is “highly susceptible to external environmental stressors, such as pesticide exposure and parasitic infections [and] these disturbances can lead to microbial imbalances, ultimately affecting bee health.†(See studies here and here.)
Previous research earlier this year, captured in Daily News Variability in Effect of Pesticides on Bumblebee Survival Tied to Gut Microbiome Health, shows intraspecific differences (between individuals of a species) in wild bumblebees (Bombus vosnesenskii) exposed to an herbicide (glyphosate), a fungicide (tebuconazole), and an insecticide (imidacloprid), with gut microbiome health as a factor. Additional coverage (see here, here, here, and here) highlights studies that show how pesticide exposure disturbs and shifts the abundance of certain microbes in the bee gut microbiome. These disturbances primarily occur in one of two ways—either by directly harming microbes or indirectly harming the host (bee) and subsequently shifting the microbiome.
Studying environmental stressors in combination is an important area of research, as organisms are not subjected to contaminants or parasites singularly. As the current study points out, “Honey bee health is affected by a variety of environmental factors, with Varroa destructor parasitism and pesticide exposure being important factors contributing to colony decline.†The loss of honey bees, and other pollinators, threatens crop productivity, natural plant diversity, and insect diversity.
The decline of pollinators is a major ecological crisis and contributes to the “insect apocalypse†currently occurring. There is a complex interplay of multiple factors in this decline, including pesticide exposure from chemical-intensive agriculture and land management, parasite infestations, and habitat degradation, among others. “Among the many stressors, pesticides and parasites are considered the primary drivers of the severe decline in honey bee populations,†the study authors note. (See more on What the Science Shows on Biodiversity.)
Neonicotinoid insecticides, in particular, are linked to adverse impacts on pollinators. Imidacloprid is associated with increased risks to not only pollinators but to human health and other wildlife as well. (See Daily News coverage on pollinators and imidacloprid here and here.)
Studies show that exposure to low doses of imidacloprid can impair honey bee cognition, foraging behavior, navigation, and overall survival. (See here, here, here, here, and here.) “While prolonged low-dose exposure may not cause immediate mortality, it can weaken their immune system, making them more susceptible to pathogens and other environmental stressors,†the researchers state. They continue, “[O]ur previous research further demonstrated that imidacloprid reduces the survival and learning ability of young bees, disrupts gut microbiota diversity, and leads to metabolic imbalances.â€
Varroa mites parasitize honey bee larvae, pupae, and adults by extracting nutrients from them. Infestations of these mites “weakens individual bees, disrupts their growth and development, and ultimately compromises the health of the entire colony.†When combining this weakened state with exposure to pesticides, the honey bees experience cascading adverse health effects.
The current study collected honey bees from a research apiary, assessed the degree of Varroa destructor infestation across the colony, and selected colonies that were categorized as highly infested (if rates exceeded the threshold of 20%). Both forager bees and larvae to rear and become newly emerged bees were included for the acute (7-day) and chronic (14-day) exposure tests. After assessing mortality throughout the experiments, DNA extraction and sequencing was also conducted to analyze the intestinal flora of the honey bees.
“Our results showed that imidacloprid significantly reduced bee survival and that the synergistic effect of V. destructor and imidacloprid further exacerbated the risk of bee mortality,†the authors say. The data shows that as imidacloprid concentrations increase, the mortality rate of bees also increases. “This indicates that imidacloprid is highly toxic to bees, especially when exposed for a long period of time or at high concentrations, and can significantly reduce the survival rate of bees,†the researchers state. (See studies here and here.)
They continue, “At the same time, the presence of V. destructor reduced the resistance of bees to pesticides and exacerbated bee mortality, suggesting that V. destructor infestation may have a synergistic effect with pesticide exposure.†(See additional studies here and here.) The analysis also shows impaired immune defenses through altered enzyme activity levels, as well as reduced diversity of gut flora in the newly emerged bees. “These results indicated that V. destructor infection significantly affected the gut flora composition of newly emerged bees, especially the abundance of Proteobacteria and Firmicutes changed significantly, accompanied by significant fluctuations of some key genera,†the researchers explain.
The combination of exposure to Varroa mites and imidacloprid “significantly altered the diversity and composition of the flora in the intestinal tract of honeybees compared to healthy bees,†which is consistent with other studies that suggest these implications occur as a result of physical damage, immune system suppression, or pathogen transmission from the mites that may indirectly alter the microbial community of honey bees, thereby affecting immune function and overall health. (See here and here.)
The authors hypothesize that the suppression of immune and detoxification pathways leads to the bees’ inability to metabolize or resist the toxic effects of imidacloprid and results in impacts on survival. “This study shows that V. destructor infection and imidacloprid exposure significantly affect honey bee survival, gut enzyme activity, and the diversity and function of the gut microbiota through synergistic effects,†they conclude. “V. destructor may exacerbate immunosuppression and physiological damage in honey bees by altering the structure of their gut flora, while imidacloprid exposure may respond to toxicants by enhancing detoxifying enzyme activity, but may also lead to damage to the immune system.â€
Beyond Pesticides, in a similar Daily News titled Neonicotinoid Insecticides Contribute to Honey Bee Vulnerability to Parasitic Varroa Mites, previously covered a 2024 study published in the Journal of Insect Science that addresses the debate on whether neonicotinoid insecticides or Varroa mites are more detrimental to the survival of bees. The evidence suggests that neonicotinoids are not only harmful individually but can increase vulnerability to parasitism from mites in western honey bees, showing heightened risks with combined exposure.
Additional research cited in the current study includes:
- One study finds V. destructor infestations “reduce bees’ tolerance to pesticides, making them more susceptible to pesticide-induced mortality.â€
- Studies (see here and here) find synergistic and additive effects of acetamiprid, another neonicotinoid insecticide, and parasites on honey bees, with bee mortality risk increasing as the number of stressors rises.
- “Similarly, exposure to both chlorothalonil [an organochlorine fungicide] and V. destructor significantly reduced worker survival, with observed synergistic effects.†(See study here.)
- Research also indicates that exposure to neonicotinoid insecticides such as thiamethoxam increases parasitism and heightened risks to honey bee populations.
To mitigate these adverse health effects, Beyond Pesticides urges the widespread adoption of organic agriculture and land management. Not only does this holistic solution remove the use of petrochemical pesticides and synthetic fertilizers that threaten honey bees and other pollinators, but it also protects and enhances biodiversity and mitigates both the climate change and public health crises.
For more information on the direct impacts of organic practices on pollinators, see Study Adds to Wide Body of Science Highlighting Benefits of Organic for Insect Biodiversity. Additional health and environmental benefits are available here and here.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source:
Wang, Y. et al. (2025) Varroa destructor infestation amplifies imidacloprid vulnerability in Apis mellifera, Pesticide Biochemistry and Physiology. Available at: https://www.sciencedirect.com/science/article/abs/pii/S0048357525003293.
