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NEW ZEALAND BEEKEEPER, MAY 2017
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 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 possible that oxalic acid also affects the varroa mite by disrupting or affecting mitochondrial function. In any case, a physico-chemical mode of action would explain why there have been no reports of mites resistant to oxalic acid. Sugar dusting 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
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. Thymol 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. 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
Figure 4. Thymol.
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. Formic acid 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.
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