Immune gene expression in developing honey bees (Apis mellifera L.) simultaneously exposed to imidacloprid and Varroa destructor in laboratory conditions

The impact of widespread pesticide use in agriculture has been a major concern to the beekeeping industry. Accumulating evidence suggests that pesticides have a negative impact on honey bees. Additionally, honey bees exposed to different stressors, such as parasites and the pathogens they vector, may further affect their health. The aim of this study was to investigate the combined or sole effect of two stressors on developing honey bees under laboratory conditions where larvae were per os treated with realistic field doses of imidacloprid and later infested with Varroa destructor. Here, we present a gene expression profile of 15 immune-related genes in different honey bee development stages (white-eyed pupae, brown-eyed pupae, and emerged honey bees). Our results show that pesticide consumed in the larval stage lead to a decrease in immune response of bees in early development stages (white- and brown-eyed pupae) and later to an increase in honey bee immune response, and thus honey bee health may be challenged. The gene expression patterns of Varroa infested honey bees changed during development with the number of significantly differentially expressed genes increasing from white-eyed pupae to newly emerged honey bees. The effect of both stressors has a significant effect on antimicrobial peptides and genes involved in defense (apidaecin, hymenoptaecin, defensin-1, lysozyme-2, and PPO) in different developmental stages. All tested honey bees exposed to Varroa had significantly elevated DWV loads. Overall, our data showed changes in immune-related gene expression of three honey bee development stages, exposed to each of two stressors alone or combined, which varies depending on the developmental stage.