4.2.1. Acaricide treatment
Use an effective acaricide > 95 % product as per manufacturer recommendation. Beware of resistance of mites to this product, see section '3.6.3. Bioassays to quantify the susceptibility of the varroa mite to acaricides ' for methods on how to test mite susceptibility to acaricides.
1. A protected bottom board should be used to prevent bees removing the fallen mites.
The protection is typically a wire screen with 3-4mm holes covering the whole surface of the board, leaving no access for bees to the fallen mites.
2. Ant protection should be put in place to prevent their access to the hives and predation on fallen mites and therefore biasing the number of mites counted.
3. Given the rapid action of efficient acaricides and to ease counting, mite fall should be assessed daily.
See sections 4.2.4. ‘Natural mite fall’ and 4.2.5. ‘Sub-sampling mites’, to count mites on a bottom board.
If the active ingredient used is persistent enough (i.e. the treatment still in place or if residues persist in the hive) and do not penetrate in the cell through the capping (e.g. most synthetic acaricides), the mites that entered cells just before the treatment become exposed upon their emergence with their bee host and die within a few days. Mite fall should thus be counted for at least 3 weeks, this period covering the development times of pupae and the time necessary for mite fall to decrease to pre-treatment levels. The same counting period should be covered if a non-persistent acaricide is used that also kills mites in the cells (e.g. formic acid). Indeed, mites dead in the cells will only be released and fall on the bottom board to be counted upon emergence of their host bee. In case the product is not persistent and does not affect mites in cell (e.g. oxalic acid), colonies without capped brood must be treated. Absence of capped brood can be obtained by caging the queen 22 days before the planned treatment. All mites being in the phoretic phase, mites should fall for a shorter period (since none are trapped in cells). Mite fall count can therefore stop when it decreased to pre-treatment levels.
Pros: efficient, relatively low workload.
Cons: slow, dead mites, and in case of use of persistent acaricides, contaminated colonies cannot be used further; in case of queen caging, the development of varroa population before treatment can be slightly affected by the interruption of brood rearing.