Tackling the varroa mite: A global threat to honey bees

Honey bees are critical pollinators supporting natural ecosystems and agricultural production. Pollination contributes to 35% of global crop output, enhancing yields for crops like almonds, coffee, and soybeans. However, honey bee populations showing colony losses at alarming rates, with colony losses hitting 40-45% annually in the United States and several European countries in recent years. Among the leading causes of these losses is the parasitic mite Varroa destructor. This invasive pathogen has spread globally since the 1960s, devastating bee colonies and threatening food security. There is a clear need for concerted analyses to highlight effective treatments for this pathogen.

To address this, in our recently published meta-analysis in (opens in a new window)Science of the Total Environment we analysed data from 138 studies across 30 countries to evaluate the efficacy of different treatments against Varroa. The analysis provides crucial insights into their effectiveness and impacts on both Varroa and honey bee health.

Map of the locations where studies used in this meta-analysis were conducted. Field based experiments are marked with a red circle, laboratory based are marked with a blue x.

Synthetic Chemicals: Effective but Harmful

Synthetic chemicals, including pyrethroids and amitraz, are widely used for Varroa management due to their high efficacy in killing mites. However, they come with significant downsides. These treatments often harm honey bees, reducing lifespan, impairing memory, and diminishing colony health. Additionally, residues from synthetic chemicals accumulate in hives, posing risks to both bees and human consumers. Over time, Varroa mites have developed resistance to these chemicals, further complicating their effectiveness.

Our analyses confirmed these findings. We found the synthetic chemicals were indeed highly effective at killing the Varroa mite, but were associated with negative health impacts for honey bees in some instances.

Agriculturally Organic Chemicals: A Balanced Approach

Agriculturally organic treatments, such as formic acid, oxalic acid, and thymol, have emerged as the main alternative treatment to synthetic chemicals. Their water-soluble and volatile nature prevents residue accumulation in hives, lowering the risk of resistance. While precise application is required to avoid harming bees, these treatments may represent a more sustainable option for Varroa control.

Our analyses found these treatments are nearly as effective as synthetic chemicals in reducing mite populations but have fewer harmful effects on bees. Therefore at the current time they are the best treatment option on balance, as they have been well studied and proven efficacy.

Biological Treatments: Understudied but Promising

Biological treatments focus on enhancing the bees’ natural resistance to Varroa or selectively breeding resistant strains. Africanised honey bees, for instance, exhibit natural resilience through behaviours like removing infested brood. To date there have been very few controlled experiments assessing their effectiveness in comparison to standard honey bee strains.

Our analyses showed that biological methods had the most positive impact on bee health, while killing less mites than chemical methods. However this was with a small sample size of studies.

While biological methods have shown significant benefits for honey bee health—especially at juvenile stages—they remain underexplored. Expanding research in this area could provide transformative solutions to Varroa management.

The Way Forward

The study highlights the urgent need for alternatives to synthetic chemicals. Agriculturally organic treatments are the best current option, balancing effectiveness and bee health. However, the international scientific community must prioritise research into biological methods and other innovative solutions to mitigate the long-term threat of Varroa.

With the recent invasion of Varroa destructor into Australia, the last large Varroa-free zone, the parasite is now a global challenge. Effective management of this invasive pest is essential not only for honey bee welfare but also for the global economy and food security. Supporting sustainable treatment options and advancing research will be critical to safeguarding these vital pollinators for future generations.

About the author

(opens in a new window)Dr Darren O’Connell is a Research Fellow in UCD School of Biology and Environmental Science. Darren’s work focuses on honey bee health, and on the ecology and evolution of solitary bees. Bluesky: (opens in a new window)‪@daroconnell.bsky.social  Twitter: (opens in a new window)@Dar_Oconnell

Further information

• Darren P. O'Connell, Kevin Healy, Jack Wilton, Cristina Botías and Julia C. Jones (2025) A systematic meta-analysis of the efficacy of treatments for a global honey bee pathogen - the Varroa mite. Science of the Total Environment. 963: 178228. (opens in a new window)https://doi.org/10.1016/j.scitotenv.2024.178228