VARROA TREATMENTS: MODE OF ACTION AND RESISTANCE
Dr Pablo German, Chief Technical Officer, Pheromite
How do the different varroa treatments kill the mites? Why do they kill the mites and do not kill the bees? Can mites become resistant to a particular treatment? Do we care about answering all these questions?
Most beekeepers do care for several reasons. First, we have the natural curiosity of wanting to understand how things work. Second, the more we know about our varroa mite enemy and the weapons we use, the better we will be able to fight against it. Third, we want to understand what secondary effects the treatments may have on the bees. Finally, the mode of action can give clues about the ability of the mites to develop resistance against the treatments.
In spite of the importance of this topic, there are no good summaries on how different treatments affect the mites. There are also unsupported opinions circulating on the Internet.
In this article, I review the scientific literature and summarise the mode of action of different varroa treatments as the knowledge currently stands. Some of the treatments act as the chemicals are absorbed within the body of the mite, others have direct physical effects upon contact, and others stimulate defensive behaviours from the bees.
The synthetic chemicals tau-fluvalinate and flumethrin (Apistan® and Bayvarol®) belong to the family of pyrethroids that includes a large number of insecticides used domestically and in agriculture.
Figure 1. Tau-fluvalinate.
Figure 2. Flumethrin.
They work by producing an over-excitation of the nervous system of the mite. In particular, they bind to the voltage-gated sodium channel, present on the membrane of neurons. The inability of the channel to close and reset the neuron to the resting state leads to paralysis and death . Imagine if all your muscles contracted at the same time: you wouldn’t be able to move and breathe.
The reason why tau-fluvalinate and flumethrin are such powerful weapons against varroa is that these compounds have a high affinity for the varroa mite voltage-gated sodium channel. Interestingly, a recent study reported that tau-fluvalinate has even higher affinity for the honey bee voltage-gated sodium channel . The established safety profile of flumethrin in bees suggests that the bees have detoxification mechanisms that prevent the harmful effects . The high affinity for one single target makes tau-fluvalinate and flumethrin very effective at killing the mite, while at the same time being relatively safe for humans .
Unfortunately, this high affinity for one single target also enables mites to become resistant to tau-fluvalinate and flumethrin with a single DNA mutation in the voltage-gated sodium channel. Random mutations occur all the time, so one single DNA mutation in one gene is an event likely to occur when thousands of mites are breeding in one single beehive.
In the presence of tau-fluvalinate and flumethrin, only mites with specific mutations in the voltage-gated sodium channel are able to survive and continue reproducing. The relatively high likelihood of a single mutation in a single gene to occur, explains why resistance to tau-fluvalinate and flumethrin has been broadly reported around the world –. In fact, several single mutations in the voltage-gated sodium channel have been identified that produce tau-fluvalinate- and flumethrin-resistant varroa mites , , .
The synthetic chemical widely used for treating varroa mites is the contact pesticide amitraz (Apivar® and Apitraz®).
Figure 3. Amitraz.
The evidence of the mode of action of amitraz on varroa mites comes from insects and other mites and points to effects on octopamine receptors –. The role of octopamine in insects and mites is similar to the role of noradrenaline in humans, which is to trigger the fight-or-flight response. When you are startled by something, your body releases noradrenaline, which binds to the noradrenaline receptor present in tissues and organs throughout your body. Your heart pumps faster, your muscles release quick sources of energy, and you get ready to fight or flee.
A similar stress response occurs in insects and mites when octopamine is released, which binds to the octopamine receptors . Amitraz seems to act by binding to the octopamine receptor(s), which leads to an acute stress response with different effects in insects and mites.
Most beekeepers have noticed that amitraz is slower at killing mites than flumethrin, for example. The reason for this seems to be that by causing this stress response, the mite does not die immediately but its behaviour is completely altered, which leads to death later on. Amitraz is said to act by sub-lethal effects rather than by lethal effects . Humans, and in fact all vertebrates, do not have octopamine receptors, which is the reason why amitraz is relatively safe for humans.
The relatively slow and low onset of varroa mite resistance to amitraz—when compared to resistance to flumethrin for example— seems to indicate that amitraz acts on more targets than just one type of octopamine receptor. Indeed, resistance to amitraz has been reported in fewer cases than the previous two miticides, and studies have shown that the level of resistance is lower as well (the dose of amitraz needed to kill amitraz-resistant mites is not that much higher) –. In fact, amitraz is still the most effective miticide used in the USA, despite resistance having been reported two decades ago. This seems to point to the fact that one single mutation in one gene is not enough to provide resistance. Although point mutations in amitraz-resistant organisms have been identified, evidence from a cattle tick indicates that resistance to amitraz occurs both by mutations in the octopamine receptor and enhanced metabolism in getting rid of amitraz –. In spite of the lower resistance to amitraz by the varroa mite, alternating amitraz with other treatments is still necessary.
So far we have only talked about synthetic chemicals. Other chemicals present in nature are known as ‘organic’. Plants, in particular, constantly have to evolve ways to survive against pests. Hence, it is not surprising that several chemicals from plants have insecticide and miticide effects. In contrast with synthetic chemicals that are generally designed against one particular target, plants have to fight against many different pests at the same time. This makes their chemicals more broad spectrum, usually affecting several targets. Essential oils have been shown to have insecticidal effects and thymol, derived from thyme, is most commonly used against the varroa mite. As with previous treatments, most of what we know about how thymol works comes from evidence on insects.
Figure 4. Thymol.
Similar to amitraz, some essential oils also appear to have neurotoxic effects by binding and affecting the function of octopamine receptors , . In addition, thymol binds to tyramine receptors, which are related to the octopamine receptors but whose function is not entirely understood . There is further evidence that thymol affects the function of gamma-aminobutyric acid (GABA) receptors in insects, which are also important for nerve signal transmission .
The presence of multiple targets for thymol makes it more difficult for resistance to occur. In fact, there are no published reports of mite resistance to thymol. This does not mean that resistance to thymol is impossible. One way in which resistance could arise would be by improvement in the detoxification system of the mite. Therefore, it is still best practice to alternate thymol with other treatments.
Other popular miticides used against varroa are organic acids. Formic acid is a volatile acid that works in the hive as a fumigant.
Figure 5. Formic acid.
Initially it was observed that formic acid affects respiration in the mite  and this was linked to previous studies suggesting that formic acid inhibits cytochrome c and the electron transport chain in the mitochondria. In addition, formic acid was also suggested to have neurotoxic effects in flies . Later studies seem to suggest that formic acid kills insects, and probably varroa mites, by disrupting the mitochondria in the cells .
What happens when mitochondria in the mite are disrupted? Mitochondria are present within cells and carry out cellular respiration and energy production. When the mitochondria are disrupted, the cells cannot function. This probably leads to neurotoxic effects by disrupting the mitochondria in the neurons and inhibition of respiration . Formic acid seems to cause mitochondria disruption by the physico-chemical effects of low pH. It has been suggested that the bees have higher metabolic and buffering capacity against the acid, which explains why formic acid affects mites more than bees . This mode of action suggests that resistance is not likely to occur as several changes would be needed in the mite. No mite resistance to formic acid has been reported.
Oxalic acid is the other common organic acid. As opposed to formic acid that kills mites with the acid vapours, the main way in which oxalic acid kills mites seems to be by direct contact.
Figure 6. Oxalic acid.
There were some reports that oxalic acid may damage the mouthparts of the mite . However, there is no scientific evidence for this and the origin of this concept seems to be a manipulated picture published on the Internet. What we do know is that oxalic acid needs to be in direct contact with the mite  and is distributed around the hive via bee-to-bee contact .
Given that oxalic acid has been shown to affect mitochondria in mammals  and that mitochondria are sensitive to acids , it is likely that oxalic acid, as in the case of formic acid, also affects the varroa mite by disrupting or affecting mitochondrial function due to the effects of low pH. This physico-chemical mode of action would explain why there have been no reports of mites resistant to oxalic acid.
There is evidence that sugar dusting with powdered sugar helps increase mite fall and reduce mite numbers –. Sugar dusting seems to act in two ways. First, it affects the mite’s ability to cling to bees and they fall off. Second, it stimulates bees grooming themselves and grooming each other, which also produces mites to fall off , –. Given the physical mode of action, resistance to sugar dusting is not possible. However, sugar dusting has been said to have a small effect in reducing mite levels and may only be useful as a complementary method together with other methods.
Food-grade mineral oil
There is very little literature on the use of food-grade mineral oil (FGMO) for varroa control. However, some beekeepers like to use it either by fogging with thermal insect foggers or with cords. FGMO only affects phoretic mites (mites on bees) and it needs to be applied often to have any effect. Regarding the mode of action, some comments on the Internet point to the oil blocking the pores in the mite’s cuticle and preventing gas exchange, which affects breathing. The cuticle of the mite seems to make it more susceptible than bees. If this physical mode of action is correct, resistance is very unlikely. It is possible that the oil also stimulates bee grooming behaviour.
The mode of action of different varroa treatments has not been studied in detail for most treatments. However, we can still get an idea from studies in insects and other mite species. Different treatments have different modes of action: either chemically after being absorbed, physically by direct contact, or by stimulating defensive behaviours from the bees.
The synthetic chemicals are absorbed by the mite and tend to affect one single protein target, such as the voltage-gated sodium channel (flumethrin and fluvalinate) and octopamine receptors (amitraz). This specificity on single targets makes it highly likely that the mites will develop resistance by mutations in those targets, as has indeed been reported for all of them. In addition, mites can also develop resistance with detoxification enzymes that degrade or get rid of these chemicals from the body.
The organic chemicals act by absorption or direct contact and seem to act by lowering the pH in the case of organic acids or on more than one target in the case of thymol, making them less specific against varroa mites. This is a logical consequence of the fact that these chemicals are synthesized by plants to fight against different types of insects and pests and not against mites in particular. Indeed, thymol seems to act by affecting octopamine, tyramine, and GABA receptors, while formic and oxalic acid by lowering the pH. The action on more than one target or by physico-chemical effects makes resistance to these treatments less likely. In fact, there are no reports of resistance to these treatments. However, alternation with other treatments is still recommended.
Finally, the less-popular icing sugar and food- grade mineral oil treatments seem to affect the mite by physical effect due to the direct contact and by stimulating bee grooming behaviours. This means that resistance to these treatments is very unlikely to arise.
|Treatment||Mode of Action||Resistance?|
|Flumethrin/fluvalinate||Voltage-gated sodium channel||Yes|
|Thymol||Octopaminergic system, tyramine and GABA receptors||Less likely|
|Formic acid||Mitochondrial disruption, neurotoxic||Less likely|
|Oxalic acid||Mitochondrial function||Less likely|
|Sugar dusting||Grooming, affects ability of mites to hold on to bees||Not likely|
|Food-grade mineral oil||Suffocation, grooming||Not likely|
Table 1. Various treatments, their mode of action and likelihood of varroa resistance.