Attack on Varroa: Wet Silica Gel Carriers for Controlled Thymol Release in Honey Bee Varroosis Control

Background: Rapid evaporation limits the use of volatile natural products for pest control. This study aimed to develop a wet sol–gel silica formulation for sustained delivery of thymol, a volatile natural acaricide used to control Varroa destructor in honeybee colonies. Methods: Co-solvents were screened for thymol solubilization and compatibility with the silica network. The optimized formulation was evaluated for physical stability under ambient conditions and thymol release at 33 °C. Field trials in naturally infested Apis mellifera colonies assessed acaricidal efficacy under different temperature conditions in comparison with a commercial thymol formulation. Results: Triacetin, a non-volatile co-solvent, enabled the preparation of a homogeneous wet silica gel containing 25% w/w thymol, with no phase separation or thymol crystallization for at least 28 days. At 33 °C, the formulation reduced thymol volatilization compared with free thymol. Release was proportional to the square root of time, consistent with a diffusion-controlled mechanism within the triacetin-filled silica network. Estimated thymol depletion times were 62.5 ± 3.4 days for the formulation and 24.6 ± 2.6 days for free thymol. In summer trials, a single administration of 12.5 g thymol per hive yielded acaricidal efficacy comparable to that of a commercial formulation administered twice, with no macroscopically detectable adverse effects on colony health. Performance was reduced under cooler conditions. Conclusions: Wet sol–gel silica prolonged thymol release and showed potential to reduce administration frequency under summer field conditions. Temperature-dependent performance highlights the need to match release kinetics to environmental conditions to maintain acaricidal exposure.