Toxicity of oxalic acid and its toxic effect on antioxidative enzymes in honey bee larvae

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 in vivo 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 dm2 of honeycomb with the brood. The LC50 values (72 h) ranged between 3.17% and 3.33%. The different LC50 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-S-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.