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 specialist site hausbiene.de names different factors depending on the season for this purpose, around 200 during the brood-rearing summer period and markedly higher values of 300 to 400 during the broodless winter period, because at that time a larger proportion of the mites are moving about outside the protective brood cells and accordingly die and fall more often.
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 broodless winter period, the site Eifelvielfalt gives a rule of thumb of a markedly lower value of about 0.5 mites per day, above which a relevant burden on the colony is assumed.
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.