Boosting efficacy of natural acaricides using mesoporous nanoformulations: sustainable therapy against bee varroatosis

Mesoporous silica (MS)-based powders are a class of materials characterized by large surface area and pores sized 2-50 nm, with either ordered (OMS) or non-ordered (NOMS) mesostructure. MS can adsorb at their surface large amounts of compounds, and some can act as release modulating agents, offering convenient and relatively low-cost solutions for the controlled release of active agents. Despite these advantages, as of today, only some NOMS — but no OMS — are approved as excipients in pharmaceutical products, and only some of them possess a retentive mesostructure. Recently, Sil-Sol® (SS) — a pharma-grade NOMS — was identified as both having high loading capability and controlled release potentials. In this work, we explored the potentials of SS as a carrier for the sustained release of Volatile Essential Oil Components (VEOCs). VEOCs are an eco-friendly alternative to synthetic acaricides adopted by honeybee keepers to control Varroa destructor , a parasitic mite currently representing a threat to bee populations worldwide. However, VEOC rapid volatilization imposes repeated hive administrations, increasing labor burden. Here we optimized and characterized SS blends with 4 VEOCs (thymol, menthol, camphor and eucalyptol); we showed that SS mesosorption reduces all VEOC volatilization rates at all temperature tested (33 °C, 45 °C and 60 °C). By Arrhenius plot elaboration of the release data, we showed that TH, CA and EU (but not ME) also interact chemically with the SS surface, this affecting the retaining properties of their blends beyond the material's mesostructure. Optimized formulations were applied in vivo within a pilot field study, demonstrating that the use of SS/VEOC blends allows reducing the frequency of administration to the hives with respect to current VEOC based products. SS-based delivery systems may represent a viable, scalable, and regulatory-compliant strategy for improving the practicality and efficiency of VEOC-based Varroa control in beekeeping.