Re: powdred sugar treatments and a solid bottom board any point ?
If they land on a solid bottom they attach to the first bee that walks by. Powdered sugar works best when there is no sealed brood in the colony, so once brood is capped powdered sugar is not very cost/time effective. By long Langstroth box do you mean your box uses frames instead of just bars for comb to be built on?
Your next solution to mite control using no chemicals would be drone brood trapping. This helps reduce the varroa mite's population rate of increase. Once you decide to use chemicals, just try to stay away from Check Mite and Apistan as these build up in the wax comb.
37 years - 25 colonies - IPM disciple - naturally skeptic