Nosema ceranae is an emerging pathogen of the western honey bee (Apis mellifera L.), and thus its seasonality and impact on bee colonies is not sufficiently documented for North America. This study was conducted to determine the infection intensity, prevalence, and viability of N. ceranae in >200 honey bee colonies during spring, summer, and fall, in a North American region. We also determined the relationship of N. ceranae infections with colony populations, food stores, bee survivorship, and overwinter colony mortality. The highest rates of N. ceranae infection, prevalence, and spore viability were found in the spring and summer, while the lowest were recorded in the fall. N. ceranae spore viability was significantly correlated with its prevalence and infection intensity in bees. Threshold to high levels of N. ceranae infections (>1,000,000 spores/bee) were significantly associated with reduced bee populations and food stores in colonies. Furthermore, worker bee survivorship was significantly reduced by N. ceranae infections, although no association between N. ceranae and winter colony mortality was found. It is concluded that N. ceranae infections are highest in spring and summer and may be detrimental to honey bee populations and colony productivity. Our results support the notion that treatment is justified when infections of N. ceranae exceed 1,000,000 spores/bee.

Varroa destructor is considered one of the greatest threats to the health of the honey bee, A. mellifera. In recent years, there has been a considerable increase in the number of studies on the mite and its interaction with the bee at a cellular and molecular level. However, these studies have also revealed just how complex the interaction is. A significant factor in the virulence of V. destructor is the proteins secreted in its saliva, but only a fraction of these have yet been examined. These proteins can negatively affect the bee’s immune system and promote viruses associated with mite parasitism. Initially, studies on parasitized bees concentrated on immune-related genes, but as more genes of the bees have been examined, it is clear that many other aspects of the bee are affected, such as metabolism and neural functioning. Some of those could be responsible for the detrimental changes in certain behaviors of parasitized bees, which compromises the health of the entire colony. Several viruses are associated with V. destructor parasitism, but it remains difficult to distinguish the effects of the viruses from those of the mite. Reduced immunity in parasitized bees also opens the possibility of secondary microbial infections, adding complexity to the mite-bee interaction. Further complicating studies is the impact of other factors, like agrochemicals, which can alter how V. destructor parasitism affects bee immunity, metabolism, and neural functioning. In addition, differences due to age and sex of the bee being parasitized is a factor that needs to be considered in all studies. While much has been learned in recent years about this complex interaction, the number of unanswered questions only increases.

Little is known about the combined effects of stressors on social immunity of honey bees (Apis mellifera) and related gene expression. The interaction between sublethal doses of a neurotoxin, clothianidin, and the ectoparasite, Varroa destructor, was examined by measuring differentially expressed genes (DEGs) in brains, deformed wing virus (DWV) and the proportion and intensity of self-grooming. Evidence for an interaction was observed between the stressors in a reduction in the proportion of intense groomers. Only the lowest dose of clothianidin alone reduced the proportion of self-groomers and increased DWV levels. V. destructor shared a higher proportion of DEGs with the combined stressors compared to clothianidin, indicating that the effects of V. destructor were more pervasive than those of clothianidin when they were combined. The number of up-regulated DEGs were reduced with the combined stressors compared to clothianidin alone, suggesting an interference with the impacts of clothianidin. Clothianidin and V. destructor affected DEGs from different biological pathways but shared impacts on pathways related to neurodegenerative disorders, like Alzheimer’s, which could be related to neurological dysfunction and may explain their negative impacts on grooming. This study shows that the combination of clothianidin and V. destructor resulted in a complex and non-additive interaction.