Patterns of gene expression during Apis mellifera worker development with varying degrees of infestation by Varroa

The parasitic mite Varroa destructor is one of the biggest health problems of the Western Honey Bee, Apis mellifera. It feeds from the bees' hemolymph and vectors several honey bee pathogens. V. destructor has also been reported to compromise honey bee immunity but available data are insufficient to support this claim. This study investigates immune gene expression of developing worker honey bees throughout the reproductive period of Varroa, comparing different infestation levels. Frames of experimental brood cells were established in three colonies during the peak months of brood production. Immediately after cell-capping, mites were manually introduced into cells with larval worker bees. Control bees were either left unmanipulated or were wounded by piercing pre-thoracic segments with a glass capillary needle to mimic the mite's initial feeding site. Developing bees were collected together with any retrievable mites daily from the experimental cells for the following ten days. Mite reproduction was assessed and bee hosts were analyzed for expression levels of ten immune genes using quantitative RT-PCR. Developmental time and experimental treatment had significant and interacting effects on gene expression that overall contradict the previously hypothesized immunosuppression of bees by V. destructor. The artificially wounded bees exhibited an increased viral load, suggesting that wounding may trigger or enable virus replication. Furthermore, the gene expression results indicate the importance of physical trauma caused by wounding and suggest complex temporal dynamics in the relationships between bee host, mite parasite, and vectored pathogens.