We often see that mortality levels in a given year in the various countries are rather similar. This suggests that there is a universal principle underlying the problem of increased bee mortality, including CCD. This could be the ‘environmental factor’ mentioned by Eric Mussen in his newsletter. When asked, Mussen did not want to explain or underpin this idea.
When we consider bee decline over a longer period, we observe a systematic increase in mortality levels. These levels can be explained by the individual contributions of the various pathogens and parasites. Each of these adds a nail to the coffin, until, as a consequence of the resulting high levels of infection and pressure on the immune system, the maximum tolerable unfavorable combination is reached. For example, one could argue that ‘only Varroa’ is responsible for approximately 10 - 15% of mortality, ‘Varroa + bacterium’ for 15 - 30% of mortality and ‘Varroa + bacterium + virus’ for the highest mortality percentages. Ultimately, in certain cases the combined effects can no longer be tolerated by the bee colony.
Here, the interaction between the various bee colonies and a specific situation play a key role. We suspect that among the colonies a selection takes place between healthy and weakened bees, with the result that some colonies survive the winter well, and others do not.