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NEW ZEALAND BEEKEEPER, MAY 2017
VARROA TREATMENTS: MODE OF ACTION AND RESISTANCE PEST AND DISEASE CONTROL Dr Pablo German, Chief Technical Officer, Pheromite E-mail: pablo.german@pheromite.com
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?
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. Amitraz The synthetic chemical widely used for treating varroa mites is the contact pesticide amitraz (Apivar®). 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. Figure 3. Amitraz.
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. Tau-fluvalinate/flumethrin 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.
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.
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
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