Natural Mite Drop Count
Lexikon
The natural mite drop count records how many varroa mites die on their own and fall onto the board beneath the bee colony, without any bees needing to be removed or the colony opened.
The natural mite drop count records how many varroa mites die on their own and fall onto the board beneath the bee colony, without any bees needing to be removed or the colony opened. It is thus one of the least invasive methods of infestation monitoring and is particularly suited to regular observation repeated over the season.
For evaluation, the number of dead mites found on the board is divided by the number of days the board lay in the colony, giving the average daily mite drop. However, because the natural mite drop reflects only a fraction of the total mite population in the colony, this daily figure is often multiplied by an extrapolation factor to estimate the approximate total infestation. The Bavarian Institute for Bee Research in Veitshöchheim names different factors depending on the season for this purpose, 100 to 300 from May to September and 500 in October and November, and the Lower Saxony institute in Celle gives 120 as a rule of thumb for summer. The higher factor in autumn means that, in relation to the population, only few mites die and fall off on their own at that time.
The threshold values considered critical differ greatly by season and source. For the survey at the end of June or beginning of July, the Swiss specialist site bienen.ch names a natural mite drop of more than ten mites per day as a signal for prompt action, while at up to seven mites per day, gentler methods such as complete brood removal may still be considered. For the period from November, the Bavarian institute in Veitshöchheim gives a markedly lower value of 0.5 mites per day, above which a winter treatment is recommended in the broodless colony; the Lower Saxony institute in Celle sets this limit at one mite per day.
A common mistake with the natural mite drop count is carrying out only a single, brief check and drawing a final decision from it. Since weather, brood activity and random fluctuations affect the daily mite drop, the specialist sources agree in recommending that the board be evaluated over several days and the measurement repeated several times over the course of the season, rather than relying on a single reading.