Varroa destructor is the main parasitic threat to Apis mellifera colonies worldwide and represents a major concern for honey bee health and apicultural sustainability. Oxalic acid (OA) is widely used for the control of varroosis; however, its efficacy is strongly influenced by the presence of capped brood. This pilot field study aimed to evaluate the efficacy and safety of a combined treatment protocol based on Api-Bioxal® administered by trickling and sublimation. The study was carried out in a permanent apiary located in the province of Bergamo (Northern Italy) from October 2024 to February 2025 and involved ten A. mellifera colonies housed in Dadant-Blatt hives. The experimental protocol consisted of one OA trickling treatment followed by two OA sublimations and a control treatment with amitraz. Treatment-induced mite fall was recorded daily for 117 days; brood presence, ambient temperature and pollen influx were monitored throughout the study period, and colony strength was assessed at the beginning and end of the trial using the Liebefeld method. Differences between pre- and post-treatment measurements were analysed using the Wilcoxon signed-rank test. An overall mean of 563.6 Varroa mites per colony was recorded as treatment-induced mite fall. The highest efficacy of OA was observed when treatments were applied in broodless conditions, whereas the presence of capped brood markedly reduced acaricidal efficacy. No abnormal honey bee mortality or adverse effects on colony health were observed. Colony strength increased in 8 out of 10 colonies, with statistically significant differences between pre- and post-treatment assessments (p < 0.012). These results indicate that Varroa destructor populations remain susceptible to oxalic acid when treatments are properly timed in relation to brood presence. A combined protocol of OA trickling and sublimation may enhance varroosis control without negative effects on colony health. Further large-scale studies are required to confirm these preliminary findings and to support evidence-based adjustments to national Varroa control guidelines.
Loglio et al. (Fri,) studied this question.