4.2.3. When to sample?
There are two considerations here. The first is the best time of day to collect a sample and the second how often to sample. The best time to sample is either on sunny days, during the afternoon, when bees are actively foraging and the adult population is most clearly subdivided according to tasks/age, or at the other extreme during cold rainy days, when there is no substructuring of the population and all bees are sampled randomly, irrespective of age class. This choice depends on the design and purpose of the sampling scheme.
The frequency of sampling depends on the type survey/experiment conducted:
- For single virus geographic prevalence surveys, the best time of year for sample collection would be during the seasonal peak for the virus in question (Table 2).
- For multivirus-pathogen geographic prevalence surveys, and if only a single sample is collected, the best time would be autumn, when most viruses have a seasonal peak.
- For multivirus-pathogen surveys it is advisable to sample at least three times per season; in early spring when the colony is expanding, during peak productivity in summer and during late autumn when the colony is contracting, in order to catch the different pathogens at their peaks, observe seasonal variations in prevalence and identify possible associations between different pathogens-parasites.
- For colony-level experiments with repetitive sampling, it may be useful to take into account the natural turnover of the adult population when considering sampling frequency. During summer, both the brood stage and the adult stage last about three weeks. Sampling every three weeks therefore means that a completely new generation of adult bees is sampled each time, corresponding to the brood generation of the previous sampling point.