Honey bees, vital for global pollination and agricultural crop yields, ensure food security and biodiversity.1 Nevertheless, the beekeeping industry face a significant threat from Varroa destructor—a menace to bee health and colony survival. Varroa mites infest honey bee colonies, causing considerable damage by feeding on bees’ hemolymph and fat body and transmitting harmful viruses, resulting in widespread colony losses, and jeopardizing sustainable apiculture.2
Beekeepers commonly employ Amitraz (formamidine), Coumaphos (organophosphate), and Tau-fluvalinate (pyrethroid) to combat Varroa mites.3 Clear differences with regards to in-hive residues have been observed after application of these molecules in different formulations to control varroosis in honey bee colonies.4-5
Comparing the residues of coumaphos, amitraz and pyrethroids in honey and wax involves examining their persistence, potential accumulation, and effects within beehives after treatment application. Typically, amitraz has a relatively shorter half-life compared to some pyrethroids. Its residues in honey and wax are lower compared to pyrethroids due to its quicker breakdown and elimination from the hive. While amitraz can affect bees if not used properly or if exposure levels are high, its quick degradation might lower the risk of long-term adverse effects. Besides, some pyrethroids can have acute toxicity to bees, and their residues in honey and wax might pose risks to bee health, especially if concentrations accumulate over time.6
Not only are residues in hive products relevant for consumer safety and potential toxic effects on honey bees, but they are also known to play a crucial role in the development of resistances of varroa mites against active ingredients.7-8 Pyrethroid resistance is known to develop relatively quickly, within a few years after introduction to a geographic region, in varroa mites9, and pests that are treated with pyrethroids in agriculture.10-11
On the other hand, amitraz resistance has developed much more slowly in varroa mites and insects exposed to veterinary medicines / pesticides based on amitraz compared with pyrethroid resistance. Until today, amitraz resistance has been found to occur in patches rather than in a geographically widespread pattern. Sensitivity reduction against amitraz in varroa mite populations in the field happen to a lesser degree12, and the detected reduction in treatment efficacy in the rare cases of resistance development in the field is smaller.13 In Spain, Amitraz has been authorized as a varroa treatment since 1999. But recent data collected in Spain clearly indicate that varroa mite sensitivity against amitraz is still 100%.14 A possible link between the high instability and quick degradation of amitraz residues in hive products4, 15 appears likely.
Residues from several varroa treatments and pesticides can alter bee behavior, compromise foraging efficiency, and negatively impact colony productivity. Sub-lethal exposure to pesticides residues during critical developmental stages, particularly in larvae, can lead to developmental abnormalities and weakened immune systems.16–17 The presence of residues within the hive environment poses risks not only to individual bees but also to the entire colony, potentially impacting brood development, queen fecundity, and overall colony productivity.18 Residues in hive products can also contaminate pollen and nectar, posing risks to non-target organisms and disrupting ecosystem dynamics.
Sub-lethal pesticide exposure of fluvalinate and coumaphos through wax can have adverse reproductive consequences such as reduced egg laying, early supersedure, increased queen cell rejection, and reduced ovarian weight in queen bees.19-20
The presence of coumaphos and fluvalinate in beeswax can decrease brood survival21, and the simultaneous application of coumaphos and fluvalinate can increase bee mortality22-23 and decrease three-day brood survival24.
Drones exposed to coumaphos or fluvalinate have been shown to have reduced sperm viability25, and drones exposed to fluvalinate during immature development experience increased mortality and reduced body weight and tend toward lower sperm counts26. The application of coumaphos to colonies can impact queen development and negatively impact queen health27 and residues in wax queen cells can reduce developing queen survival and weight.
Ultimately, acaricides can alter physiological functions, immune responses, and detoxification functions in the exposed bees, possibly rendering them more susceptible to pathogens and pesticides28.
Most investigations into pesticide impacts on honey bees are focused on adult bees, even though brood (eggs, larvae and pupae) is crucial to colony fitness. A robust risk assessment for any pesticide should include an evaluation of possible sublethal effects on honey bee brood29, including the considered “natural” soft compounds like Thymol, Formic and Oxalic acids. Overall, sub-lethal exposure to those “natural” acaricides can causes stress and queen mortality and affects the health, memory, behavior of honey bees, and their lifespan (adult bees and queen).30-31-32
Several studies have investigated the impact of treatment residues on bee health and surrounding biodiversity, emphasizing the importance of using legal veterinary treatments rather than illegal or unapproved medicines for bees. Research indicates that exposure to sublethal doses of unregulated or illegally used medicines can have additional detrimental effects on bee colonies, including increased susceptibility to diseases, queen failure, and elevated mortality rates among worker bees.33 Additionally, illegal treatments lacking proper authorization may have higher persistence and toxicity, leading to more severe impacts on bee health and biodiversity compared to legally approved treatments.34 Furthermore, treatment residues in or on food derived from beehives (e.g., honey, comb, wax, propolis, royal jelly, pollen) must comply with any tolerances under European regulations.
Legal veterinary treatments, when used according to prescribed guidelines and regulatory standards, undergo rigorous testing to assess their safety for bees and the environment, thereby minimizing potential adverse effects on biodiversity.
Current regulations governing the use of varroa treatments and authorized residue limits in hive products vary across regions and compounds.
For instance, amitraz, commonly used in strips or liquid solutions, is authorized for veterinary use, and shows a high level of degradation within the hive. The allowed residue limits set for amitraz in honey are within the range of 0.02-0.03 mg/kg, emphasizing its fast degradation, thus reducing its persistence in hive products.35
Flumethrin and tau-fluvalinate, pyrethroid-based treatments applied via strips, also exhibit relatively high degradation rates in hive products, with residue limits ranging from 0.01-0.02 mg/kg in honey.36 Organic acids such as oxalic acid and formic acid, often used for their natural origins, have no specified residue limits, which underlines their advantages in terms of degradation and reduced persistence in hive products.37 Thymol, derived from plant sources, also lacks specific residue limits due to its relatively rapid degradation within the hive environment.38 According to the European Commission, there is no specific MRL for oxalic acid in honey, but this in no way detracts from the guidelines for its use, especially when applied to ensure optimum effect on varroa mites and limit damage to bees. (European Commission, 2020).39
Amitraz, with its advantageous residue profile, emerges as a preferable option over Pyrethroids and Organophosphates in the context of in-hive residues as a potential risk for human consumers and the cause of toxic effects on bees. The limited level of amitraz residues in the beehive prevents the development of resistance in varroa mites.
Adopting best practices for varroa control that minimize residues while ensuring sustainable beekeeping is crucial.2 Implementing rotation and diversification of acaricides also plays a pivotal role in reducing residues and preventing the development of resistant varroa strains.40
The adoption of IPM strategies, considering mechanical interventions and judicious chemical use, plays a crucial role in minimizing residues while maintaining effective varroa control and sustainable beekeeping practices.
Brood comb renewal is an essential element to avoid the accumulation of residues over the years. It helps to limit the impact of these residues on the colonies, and also to reduce the emergence of resistances linked to prolonged contact with these residues.8