Formic acid combined with oxalic acid to boost the acaricide efficacy against Varroa destructor in Apis mellifera

Abstract

Beekeepers need reliable protocols to keep the Varroa infestation to sustainable levels. The goal of our study was to verify if a treatment consisting in the association of two organic acids, commonly used to control Varroa, could be a good strategy to boost their acaricide efficacy without affecting honey bee colonies' survival. We carried out a field trial in Central Italy with 56 colonies divided into seven homogeneous groups of 8 hives each. Honey bee colonies were treated with three different formic acid formulations: liquid formic acid (60%) administered with the Nassenheider Professional® dispenser, MAQS®, and Varterminator®. All of them were combined or not with two oxalic acid treatments (Api-Bioxal®). The mean acaricide efficacy of liquid formic acid (Nassenheider Professional®) alone was 73.2 ± 12.5%, while MAQS® and Varterminator® showed respectively a mean efficacy of 49.3 ± 14.9% and 81.2 ± 16.0%. The combination of the above mentioned treatments with oxalic acid increased the efficacy respectively to 92.7 ± 7.5% (Nassenheider Professional®); 89.5 ± 9.2% (MAQS®) and 89.4 ± 8.0% (Varterminator®). Unfortunately, the higher efficacy reflected an abnormal adult honey bees mortality with respect to the control group in Nassenheider Professional® and MAQS® groups. The combination of organic acids can boost the efficacy of treatments and reduce their variability but colonies strength should be carefully evaluated in order to avoid excessive losses.