*2.3. Meteorological Conditions*

Data on weather conditions (air temperature (◦C), relative air moisture (%), number of days with rain; average values per month) during monitored beekeeping season were obtained from Croatian Hydrometeorological Department, from local climatic-meteorological stations (Osijek, Krapina, Pazin, Gospi´c, Sinj). All parameters were measured three times per day at 7:00 a.m., 2:00 p.m., and 9:00 p.m.

#### *2.4. Estimating the Strength of Honeybee Colonies*

To estimate the strength of honey bee colonies, a Liebefeld method was performed, with visual determination of number of adult honey bees and brood sealed in beeswax combs [28]. The estimation of honey bee colonies was conducted three times (prior—I, during—II, and after experimental summer treatments—III), during morning hours, between 9:00 and 10:00 a.m. before the first massive forage flights of honey bees. Strength of honey bee colonies was estimated as follows: EA1—19 May, 25 July, and 16 September; EA2—29 April, 17 July, and 8 September; EA3—14 May, 16 July, and 7 September; EA4—20 May, 10 July, and 2 September; and EA5—28 April, 15 July, and 10 September. Owing to easier assessment with adult honey bees or brood-covered comb areas, the frame for the LR hive was used and prior divided with a plastic grid into 1 dm<sup>2</sup> quadrants.

#### *2.5. Colony Examinations, Mite Counts and Treatments <sup>E</sup>*ffi*cacy*

Clinical examination of honeybee colonies included visual inspection of honeybee brood and adult bees, the behaviour of adult bees, as well as activity of bees at the entrances of hives. Fallen *V. destructor* mites were counted during the pretreatment, treatment, and a particular number of days after each treatment, and the sum of those results calculated after the final treatment was considered the total mite drop. The proportion of mites falling after each treatment to the total number of fallen mites was estimated in percentages (%). The e fficiency of each experimental treatment (A, B, C, D, E) was estimated according to the recommendations of the EMA [27].
