Amitraz is the only chemical substance used by beekeepers against the Varroa mite with which beekeepers realize low bee mortality. Amitraz has been in use since 1974. It is used in the veterinary sector against ticks, mites and other parasites that infest farm animals as well as domestic pets (dogs, cats). In March 2013 in the USA, Amitraz was temporary approved for use against the Varroa mite. Even though in the Netherlands and the rest of the EU Amitraz is not permitted for use with bees, due to the lack of alternatives it is used anyway, and increasingly frequently. The temporary approval in the USA should also be considered in that light.
There is no 100% clarity about the operational mechanisms of Amitraz. Nevertheless, it is clear that the effect is quite different from that of all other insecticides used as miticide. According to some sources it works as a neurotoxin, which clings to certain receptors in the neurological system. The mouth parts, in particular, are supposed to be paralyzed, so that the mite cannot pierce the skin. It is supposed to inhibit octopamine, a neurotransmitter which mainly occurs in insects and which reduces aggression. To be effective as a neurotoxin the concentration of Amitraz has to be quite high.
According to other sources, Amitraz influences reproduction. In this case, Amitraz works in relatively low concentrations, probably via the endocrine system. We regard the inhibiting effect on reproduction as the most relevant in the application as a substance to combat the Varroa mite. The reproduction of the bees will undoubtedly also be affected. This need not lead to problems with the reproduction of the bees if it is used at certain times of the year.
Amitraz dissolves well in fats and therefore accumulates in the wax. The substance is highly unstable in watery solutions. It disintegrates in fairly well in water-soluble substances, which then decompose reasonably quickly. These substances are responsible for the neurological effects of Amitraz.
See http://www.cdpr.ca.gov/docs/risk/rcd/amitraz.pdf, which contains very detailed information about Amitraz and the possible objections to its use as a treatment against the Varroa mite.