An ethanol extract of Lasius fuliginosus ant nests was found to be highly effective against nosemosis in apiary honey bees. Two concentrations of the extract (300 mg and 1500 mg/100 ml sugar syrup) were tested on honey bees infected by nosemosis, and both significantly reduced the number of Vairimorpha ceranae DNA copies. Administration of the extract also had a positive effect on the honey bees’ immune system, increasing the expression of genes encoding immune peptides (abaecin, defensin-1, hymenoptaecin). The extract was subjected to detailed biochemical analysis for the presence of total carbohydrates (22.1 µg/ml), proteins (26.7 µg/ml), and phenolic compounds (63.8 μM) and also to assess its reducing activity (69.2% for DPPH radicals and 94% for the ABTS method). FTIR analysis revealed a number of groups and types of organic compounds (including fatty acids, triglycerides, formic acid, and phenols), the presence of which may have a decisive influence on the preparation’s high level of biological activity.