So you’re Treating Varroa, Now What?
For readers in Australia, Varroa mites is slowing spreading from New South Wales and into the remaining states. As of writing, surveillance teams have been deployed with sticky mats and flumethrin for Varroa screening. Yours truly recently had a check for Varroa via the sticky mat test, and the results came back positive – mites detected.
It is November, the peak brood rearing season is about to start. A shipment of flumethrin strips (Bayvarol) has arrived and will be applied to the brood boxes for 6-8 weeks. In order to treat these honey bee mites, My hives will be super-less for this period, as flumethrin has been shown to contaminate honey. Read about the results here.
Using Flumethrin in the Hive
There are several ways to apply flumethrin. The approved method for beekeeping is in strips placed within the brood rearing area of the colony, in between frames. As bees come into contact with the flumethrin, it spreads throughout the hive due to normal bee activity and killing any mites that come into contact with a flumethrin-coated surface.
Other (unapproved) methods of application are spraying it onto combs and directly coating brood (drone) cells. The reader can see why these are unapproved, the risk of off-target effects and getting the wrong comb are too great.
How Long do I Leave the Strips in for?
This question is dependant on mite infestation levels, flumethrin dose strength and the season. To use an example, my hives are to be treated with four strips per brood box (two strips for a nuc). Each strip contains 3.6 milligrams of flumethrin, and as of Spring 2023, the guidelines are to keep strips for 6-8 weeks to serve as a “control” treatment.
The New South Wales department of Primary Industries has further advice on leaving strips in place:
- 1 – 6 days for surveillance, i.e. for the purpose of detecting mites in the hive
- This is usually accompanied with a sticky mat that is taken to a lab for formal diagnosis, as in the image above.
- 3 – 6 weeks (suppression), i.e. to stop a colony from being infected.
- 6 – 8 weeks for control of an existing infestation, as mentioned above.
- I’ll admit “suppression” versus “control” is ambiguous here. “Suppression” treatments are for stopping an infection from taking hold in an otherwise uninfected hive. “Control” is applied for hives already infected and not already treated.
How Flumethrin kills Varroa
The social nature of bees means that the four strips of flumethrin placed inside the hive will rapidly spread onto all bees, affecting any adult mite that comes into contact with them. The proposed mechanism for how flumethrin kills Varroa is:
- As bees move inside the brood box, they come into contact with the flumethrin strips.
- Flumethrin adheres to their hairs and exoskeleton.
- As bees groom one another (allogrooming), the flumethrin residues are transferred from one bee to another, eventually including the drones and queen,
- Bees also groom themselves (autogrooming), which leads to the ingestion of flumethrin and subsequent risk of developing sublethal effects, covered below
- Varroa mites attach themselves to bees for feeding or transport. If they come into contact with a treated bee, they are exposed to flumethrin
- Alternatively, a mite can contract flumethrin by moving across a treated surface.
Flumethrin affects the nervous system of the mites (Davies et al., 2008), binding to their sodium channels within the nerve cells and keeping them open, leading to excessive muscular contractions. Eventually, paralysis and mite death occurs.
Flumethrin in Honeycomb
Two differing results from a study in Pakistan (Jamal et al., 2020) & China (Yu et al., 2015). Using the same analytical method (liquid chromatography), the former study found no flumethrin contaminants in the honey, but rather in the wax itself. In the latter, both the honey and the wax had detectable flumethrin residues. Both authors hypothesise that the results observed in Chinese apiaries may be due to non-compliance with flumethrin application protocols, as honey contamination is likely during periods of nectar flows (since the bees are actively making it).
Where flumethrin was detected in comb from new colonies or previously untreated hives, the authors suggest that this is due to wax recycling. Wax cappings, discarded wax and burr comb are reused and recast into new foundation frames. The EMEA’s 1998 report on veterinary medicines states that flumethrin residues are found in exposed beeswax year-round, with concentrations peaking when treatments are conducted during nectar flows.
Why Flumethrin Contaminates Honey
Many flumethrin honey contaminations are due to the reuse of old wax comb. Flumethrin is more likely to dissolve in wax than other miticides, and accumulates into higher concentrations over time . When bees store honey on comb containing flumethrin and cap it, by the time of extraction, some of the residues would have transferred into the honey itself. Once again, contamination is more likely when flumethrin is used during nectar flows.
I speculate that the typical honey extraction process can further increase flumethrin levels in the honey as the wax cappings and honey come into rapid contact with one another. Contamination is more profound when the same equipment is reused for the honey extraction process (say, your unwashed drone uncapper or the trusty electric knife of 10 years…).
Chemical Properties of Flumethrin
Flumethrin is a synthetic pyrethrin, belonging to the class pyrethroids. It can be formulated as a spray or dip (for veterinary applications), or strips in the case of miticides. The strips are stable at room temperature and do not degrade rapidly, enabling it to maintain efficacy while placed within hives.
It is fat soluble (lipophilic), allowing it to adhere onto the waxy exoskeleton of bees as well as the wax comb. It has a low volatility, and will not evaporate from the strips, ensuring a steady release of the chemical. The stability of the chemical make it suitable for use in strip form, leading to formulations like Bayvarol or ApiFlume.
Flumethrin & Bee Health
Flumethrin affects the bee nervous system by causing multiple action potentials in the nerve cells, keeping the of sodium ion channels open (Oruc et al., 2012). This miticide accumulates in beeswax over years of treatments and reuse, reaching concentration levels up to the milligram per kilo (Lozano et al., 2019). For reference, this is when honey becomes no longer acceptable for human consumption; more on that below.
Combining the high affinity of flumethrin and the recycling of beeswax means the miticide’s levels can produce sublethal effects on bees and their larvae. Liu & colleagues (2022) investigated larvae exposed to flumethrin and found a decreased expression in metabolic genes in newel-emerged honey bees. When flumethrin concentrations were 1mg /L, the immune and detoxifying genes were significantly decreased. The authors conclude that larva exposed to flumethrin developed into adults with impaired gene expressions.
As previously stated, bees brush against the flumethrin strips inside the hive, leading to residues on their bodies. As the bee grooms itself, it can ingest these residues, resulting in oral toxicity. The ingested dose and extent of oral toxicity differs in field vs laboratory studies.
Laboratory Studies on Flumethrin Toxicity
A 2012 laboratory study by Oruc and colleagues found that in oral doses of 0.25 micrograms and higher, some worker bees started exhibiting tremors and convulsions, starting at three minutes post-treatment. Affected bees were unable to move or feed themselves, producing a defunct member of the hive.
Further studies by Qi & colleagues (2020) showed reductions in the honey bee immune and metabolic response following an acute (48 hour) and chronic (14-day) exposure trial. No direct deaths were observed from flumethrin ingestion, however in the acute trial, the lifespan of bees in the treatment group was significantly decreased compared to control. Flumethrin showed high acute oral toxicity, inducing significant changes in oxidative stress, detoxification, and immune responses at sublethal doses.
Field Studies on Flumethrin Toxicity
A search for “field studies of Flumethrin toxicity” will result in plenty of results… on cats, dogs, and cattle. While I do care for our four-legged friends, the data on our six legged legionnaires is lacking.
A 2022 study by Li & colleagues attempted to mimic the field-realistic dosages of flumethrin exposure by administering 1, 0.1, 0.01 and 0 milligram per litre doses of the drug to honey bees. Two-day old larvae were treated daily with the prescribed dose and their health outcomes monitored. The authors listed a number of statistically significant and insignificant events. Viewing the results from a clinically significant standpoint, we can see that flumethrin hindered pupal growth and development, reduced the average lifespan of the adult worker bees and impaired their neurological functions, namely olfactory learning and memory.
The authors reported changes in metabolic- and immune-mediating gene expression in the adult workers as in other work. They conclude that flumethrin residues surpassing 0.01 milligram per litre, the lowest tested dose, shortened adult worker bee lifespan and imposed sublethal stressors that increased in severity with the dosage.
Flumethrin & Human Health
Pyrethroids, including flumethrin, show almost negligible acute toxicity to humans but are highly toxic to target organism. The risk of significant health effects from consuming honey containing trace levels of flumethrin is considered low. As you’ve read above, the chances of flumethrin reaching highly toxic levels in honey is low (eating comb honey is another story), as the amounts present in contaminated honey are far lower. So, the most likely outcome is allergic reactions, from mild skin irritation to full anaphylaxis (we’re all allergic to something) – again, these are rare and if you’re reading this, you would know if you are allergic to this product.
The main effects of acute pyrethroid exposure are neurotoxicity at high doses and liver hypertrophy, which are reversible if death does not occur. Symptoms of chronic pyrethroid toxicity include memory loss, immune system modifications, behavioural problems, thyroid problem, and others (Patel and Patil, 2016).
Maximum residue limits are set by regulatory bodies to define the upper limit of pyrethroid (and indeed any other toxin) that can be present in a food and still be deemed fit for sale. For flumethrin, the Australian government sets this limit at 0.005 milligram per kilogram for honey, based on recommendations form the World Health Organization. This is a regional limit and other countries will have differing regulations. For example, a 2018 proposal by the European Union aimed to set the maximum residue limit at 0.006 mg/kg.
If in doubt, or from a vulnerable population (pregnant women, rare health conditions etc…), look for organic honey, or look into becoming a beekeeper yourself… having a hive or two is not as hard as it looks!
Varroa Treatments for Non-Langstroth Hives?
Langstroth hives are focused on since they are the most popular brand worldwide. Varroa infects all honey bee colonies, and other hive types likely require chemical treatments. Even the natural beehive is not immune to infestations. Alternatives methods would be required for these hive designs, and it’s advised to consult your local authority on best practices.
The horizontal nature of log hives or the constant downward movement of the queen in Warre hives makes strip placement challenging, and the scientific literature is lacking on this topic. Studies on the effects of pesticides on honey bees are standardised using Langstroth hives, so the beekeeper will have trial multiple methods to find what works for their non-Langstroth design.
Read the whole post and found puns, typos mistakes? Voice your displeasure with a strongly-worded message to admin@tgbee.com.au, or by filling this form.