Nosema spp. parasitization decreases the effectiveness of acaricide strips (Apivar®) in treating varroosis of honey bee (Apis mellifera iberiensis) colonies

Given the key role played by honey bees in almost all terrestrial ecosystems, maintaining bee populations in adequate sanitary conditions is crucial for these essential pollinators to continue their work. From the beginning of the 21st century, beekeepers have reported a progressive increase in the overwintering mortality of honey bee colonies worldwide. Despite the failure to reach a consensus regarding the cause of this phenomenon, pathogens are thought to be strongly implicated. In the present work, we provide evidence of the negative effects of colony parasitization by Nosema spp. - primarily by N. ceranae - on the effectiveness of acaricide strips to treat Varroa destructor. The effectiveness of the Varroa mite strip treatment (Apivar (R)) was greater in colonies in which Nosema spp. parasitization had been controlled. Several studies report that infection by Nosema spp. may affect the behaviour of worker bees. As the effectiveness of Varroa strip treatment depends on bees contacting the strips and their subsequent interaction within the colony, such behavioural and social alterations could interfere with the treatment and allow more severe effects to develop in the colonies infected by Nosema. These results should be considered when assessing acaricide treatments in field conditions due to the high prevalence of both pathogens worldwide.