The downside of amitraz is that it is somewhat more toxic to bees than fluvalinate, and shuts down the queen’s egglaying during treatment (Henderson 1998; pers comms). This is not surprising, since Bloomquist (1996) states, “Amidines cause an overstimulation of octopaminergic synapses in insects, resulting in tremors, convulsions, and continuous flight behavior in adult insects. Moreover, these compounds have the ability to cause a true anorexia in insects and also suppress reproduction.”
The upside to amitraz is that it degrades quickly in honey and beeswax (Fries, et al 1998), and therefore leaves virtually no residues (although one of its metabolites, DMF, is quite stable, and is a common contaminant of European honey (Shroeder, et al 2004)). Vesely, et al (2004) analyzed beeswax in the Czech Republic for amitraz and its degradation products. They concluded “these quantities do not present amitraz as hygienic risk for bees and for humans even after 20 years of its systematic application.”
I’ve spent some time trying to find any reason not to reregister amitraz as a varroacide in the U.S. I’m not a big synthetic chemical fan, but I think that amitraz deserves a second look. It works well against the mite, appears slow to promote resistance, doesn’t appear to contaminate honey or wax to any extent, and is relatively nontoxic to humans and bees.