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NEW ZEALAND BEEKEEPER, MAY 2017
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 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 physico- chemical effects on more than one target, 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, formic acid disrupts the mitochondria in cells, perhaps as a consequence of low pH, and oxalic acid may also act by affecting mitochondrial function. The action on more than one target or by physico-chemical effects that disrupt cell structures 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.
A gorgeous frame of honey. Photo: Jo Telfar.
Conclusion The mode of action of different varroa
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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.
Treatment
Mode of Action
Resistance?
Flumethrin/fluvalinate Voltage-gated sodium channel
Yes
Amitraz
Octopamine receptor
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.
References Complete article with references is available on request from the author: pablo.german@pheromite.com
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