The production of food of plant origin is critically dependent on the pollination ability of honey bees, whose health has been deteriorating for a long time, and whose population is declining. In our experiment on a honey bee brood at the 4-day larval stage, we tested the following concentrations of oxalic acid: 0% (control - applied distilled water), 0.87%, 1.75%, 3.5% and 7%, corresponding to doses of 0 mg, 2.61 mg, 5.25 mg, 10.5 mg, and 21 mg of oxalic acid per dm of honeycomb with the brood. The LC values (72 h) ranged between 3.17% and 3.33%. The different LC values obtained resulted from three different methods used to calculate this indicator. The therapeutic index (TI) of oxalic acid was set to be 1.1, indicating a high risk to the honey bee brood. We observed an increased gene expression for the detoxifying enzyme glutathione--transferase (GST), but did not detect an increased gene expression for superoxide dismutase (SOD1 and SOD2), which protects the organism from oxidative stress. A decrease in gene expression was observed for prophenoloxidase and hymenoptaecin, while defensin and lysozyme did not show significant changes. These results emphasise the need for the accurate dosage and application of oxalic acid in the treatment of varroosis.