Bayvarol is a relatively recent product in Canada to control Varroa. It uses the active ingredient flumethrin. Flumethrin is chemically similar to fluvalinate, the active ingredient in Apistan, which has been used for many years, and to which some Varroa developed resistance.
Paul van Westendorp recently advised that some anomalies have recently been found in the efficacy of Bayvarol (after treatment, reduction of the Varroa population was less than expected).
It is important that beekeepers record mite levels before treatment AND record mite levels after treatment, to assess whether the post-treatment mite population is low enough for good bee health.