4.2.4. Natural mite fall
This method is based on the quantification of naturally dead mites. Counting can be exhaustive (more accurate when done with a guide, Fig. 13) when mite fall is low to medium or can be sub-sampled (using a checked pattern board; see section 4.2.5. ‘Subsampling mites to count on a bottom board’) when mite fall is high. In both cases, a formula needs to be applied to calculate the total infestation rate of the colony. Various studies gave contradictory conclusions regarding the accuracy of the natural fall method to determine total infestation rate since natural mite fall is largely determined by the amount of emerging infested brood (Lobb and Martin, 1997), but it is in general considered as a good indicator of colony infestation (see Branco et al., 2006).
Material needed: screened floor board, guide or sheet with checked pattern see 4.2.5. ‘Sub-sampling mites to count on a bottom board’.
Note: make sure the hives are inaccessible to ants or use a sticky board.
1. Pull the bottom board from underneath the colony.
2. When using non-sticky boards, shelter from the wind to count.
3. Place a guide above the board to avoid counting the same mites (Fig. 13) or count mites from the selected squares on a checkered board (see section 4.2.5. ‘Sub-sampling mites to count on a bottom board’).
4. If dead bees are present on the board, check them as they act as magnets to fallen live mites.
5. Adapt counting frequency to mite fall rate since many mites on the board are difficult to count and since increasing amount of debris accumulating over time makes counting difficult.
If frequency of visit cannot be increased and high mite numbers must be counted, use the checkered board method (see section 4.2.5. ‘Subsampling mites to count on a bottom board’).
6. Collect data for 2 weeks, average the figure to obtain mean weekly mite fall.
This period covers natural variation in mite fall due to population dynamics cycles within the host.
7. Calculate the total colony infestation rate from the weekly mite fall by multiplying the daily mite drop by 250–500 or 20–40 when brood is absent or present, respectively (Martin, 1998).
These correction factors are valid for central European conditions.
Pros: non-destructive/non-invasive, fast, no need to open the hive depending on design; reliable for non-collapsing colonies with brood; this is a method providing relative quantification that can be used for comparison between colonies within an experiment, not across studies.
Cons: sometimes unreliable, death rate may vary according to colony status, season, bee race, climate (regional variations possible), amount of brood; little is known on the influence of these factors on mite death; specific equipment necessary.