Gene expression signatures of honey bee brood reveal immune stimulation and nutritional trade-offs under oxalic acid treatment and chalkbrood disease

Varroa destructor is the most significant pest of honey bees worldwide, and oxalic acid (OA) is widely used for its control. However, its effects on brood physiology remain unclear. We examined the impact of oxalic acid-glycerine (OA-G) strips and chalkbrood (Ascosphaera apis) infection on gene expression in Apis mellifera prepupae. Twelve colonies were assigned to control, OA-G, A. apis, and combined OA-G + A. apis groups. Gene expression of antimicrobial peptides (AmPs), vitellogenin (Vg), and Vg-like B was analyzed seven days post-treatment using RT-qPCR. OA-G treatment significantly upregulated defensin1 and hymenoptaecin in both healthy and infected brood, indicating strong immune activation. In contrast, abaecin was downregulated, while apidaecin increased only under infection. Vitellogenin was consistently suppressed across all treatments, whereas Vg-like B remained unchanged. These results demonstrate differential immune and nutritional trade-offs in brood following OA exposure and infection, underscoring the complex physiological consequences of varroosis management.