Effects of Oral and Volatile Peppermint Oil Exposure on Feeding and Hypopharyngeal Gland Development in Young Worker Honey Bees (Apis mellifera)

Protein nutrition during early adulthood is closely associated with hypopharyngeal gland (HPG) development in worker honey bees, but whether peppermint oil (PO) influences this physiology and whether its effects depend on exposure route remain unclear. Newly emerged Apis mellifera workers were exposed for up to six days to dietary PO (0.05%, w/w), volatile PO (0.05% in paraffin oil, v/v), both routes, or control conditions. Protein-patty mass disappearance and recorded deaths were monitored daily, while body and head weight, HPG acinus area, feeding- and nutrient-associated gene expression in the brain and fat body, HPG mrjp1 expression, and MRJP1 protein abundance were assessed at designated sampling points. PO-exposed cages showed greater cumulative protein-patty mass disappearance and significantly altered feeding- and nutrient-associated gene expression, although individual molecular responses varied among exposure routes. All PO-treated groups had larger HPG acinus areas than controls on days 4 and 7; mrjp1 expression was significantly higher in PO-F on day 4 and in all PO-treated groups on day 7. MRJP1 protein abundance was also increased in all PO treatments. Volatile exposure showed numerically higher mortality on observation day 3, but cumulative mortality did not differ significantly from the control. These results indicate that dietary and volatile PO exposure were associated with changes in feeding-related measures and HPG development in young worker honey bees.