Final Mite Treatment
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The last varroa treatment of the year only works in a broodless colony, since oxalic acid does not penetrate capped cells.
The last varroa treatment of the year only works in a broodless colony, since oxalic acid does not penetrate capped cells. Rules of thumb such as three weeks after the first frost are considered unreliable, and a spot check is not enough: every colony should be checked for broodlessness, at least one day before treatment, because pulling combs tears the winter cluster apart.
In Germany, trickling, spraying, and, since autumn 2023, also vaporization with an agent approved for this are permitted. When trickling, about 5 milliliters of ready-to-use solution is given per occupied comb space, roughly 30 to 50 milliliters in total depending on colony size. The institutes' recommendations differ on temperature and timing; they only agree that the bees should be sitting tightly in the cluster and must not be flying.
Trickling is done only once during winter. Higher doses and repetitions can weaken colonies or cost them. The treatment-induced mite drop lasts four to five weeks; only the natural drop afterward is therefore assessed, with less than half a mite a day given as the mark of success. Oxalic acid is highly toxic and penetrates the skin; an FFP3 mask is mandatory when vaporizing. What remains decisive is the treatment concept of the responsible bee institute.