Viruses in bee colonies appear to be present at very low levels at any time, no symptoms, barely detectable with molecular techniques. DWV in particular is latent and seldom a problem without varroa mites in the picture. Feeding by varroa activates and promotes more virulent variants. The intensity, prevalence and symptoms of DWV increase with higher levels of mites. This study demonstrates very elegantly the process of activation:
https://www.academia.edu/13870608/A_Virulent_Strain_of_Deformed_Wing_Virus_DWV_of_Honeybees_Apis_mellifera_Prevails_after_Varroa_destructor_Mediated_or_In_Vitro_Transmission
A related topic that has been mentioned is the pressure from SHB in untreated colonies. Personally, I have only seen SHB invasions and takeover of colonies (with obvious reproduction and larvae feeding on stores and brood) once colonies have been weakened by varroa. Strong colonies, treated or untreated for varroa, tolerate certain levels of adult beetles and patrol every corner of the colony to prevent larvae from developing. Beekeeping mistakes like placing two combs together so that workers cannot fit in between the comb and patrol, can allow SHB larvae to take off even in strong colonies. This is an easy demonstration that bees are actively attacking SHB immatures all the time. When that breaks down, it is not a pretty sight.
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