Phenotypic mechanisms of varroa resistant honey bees: from brood signalling to colony-level responses

The parasitic mite Varroa destructor poses a major threat to global honey bee (Apis mellifera) populations, undermining colony health through parasitism and virus transmission. While most managed colonies rely on chemical treatments, several naturally surviving populations in Europe have persisted for decades without interventions, offering insights into host–parasite interactions and resistance mechanisms. This thesis investigates three such populations from Sweden, France, and Norway, focusing on suppressed mite reproduction (SMR) and chemical communication. In Swedish and French populations, reduced mite reproductive success was primarily driven by brood traits rather than adult worker behaviour. Caging experiments confirmed that adult interaction was unnecessary to elicit SMR, underscoring intrinsic brood characteristics. In the Gotland population, resistant larvae exhibited distinct cuticular chemical profiles, notably reduced brood ester pheromones (BEP) at times critical for mite oogenesis. Additionally, the ratio of fatty acid ethyl esters (FAEE) to fatty acid methyl esters (FAME) increased when brood was infested, demonstrating phenotypic plasticity in pheromone expression. Even when treated for varroa, Gotland colonies maintained low mite reproduction, indicating SMR is partly constitutive.  These findings suggest both fixed and plastic resistance traits coexist, offering adaptive flexibility under parasite pressure. This work advances understanding of host–parasite coevolution and highlights brood pheromone modulation as a potential, heritable mechanism of resistance, informing future breeding strategies for mite-resistant bees.