4.2.3.3. Evaluation of total mite population size in the colony

From the infestation rate of adult bees or from that of brood, it is possible to calculate the infestation rate of the whole colony (Martin, 1998). However, it is more accurate to assess both parameters based on samples and revert to the total for the colony after estimating the amount of adults and brood.

1. Measure colony strength (see the BEEBOOK paper on estimating colony strength by Delaplane et al. (2013)) on the same dates as sampling for determining mite infestation rates, so as to accurately calculate the total colony infestation rate (Fries et al., 1991b).

2. Multiply the total number of bees in the colony (e.g. 9,356) by the proportion of infested workers in the sample investigated (e.g. 0.107) to obtain the size of the varroa population in the phoretic phase in the colony (1,001).

3. Multiply the total number of sealed brood cells in the colony (e.g. 12,035) by the proportion of infested cells in the sample investigated (0.071) to obtain the size of the varroa population in the reproductive phase in the colony (855).

4. Add adult and brood infestation numbers to obtain mite population size in the colony (1,001 + 855 = 1,856).

With this information, the mite distribution between the phoretic and reproductive phases  can be determined as the proportion of either mites on adults, or mites in brood, in relation to the total mite population within the colony (in our example 54 and 46 % respectively). Mite distribution patterns can be used to determine the effect of brood attractiveness on varroa for example.