Last night we had Howland Blackiston is Chat - the Author of Beekeeping for Dummies BIGGEST point of the 2 hour chat session was to NOT TREAT for any conditions UNLESS you are SURE the condition exists.
Treating for Varroa when none is present 1) is costly and 2) creates a less effective treatment if/when Varroa infests the hive.
This can be said with any treatment of chemicals - more natural eccential oils, etc., are less of an issue.
The other point was that a SCREENED BOTTOM BOARD is NOT for reducing Varroa - but for MONITORING them. The effectiveness of SBB on Varroa reduction is practically NIL - relative to the infestation numbers.
SBB though is a great method of venting the hive and can be used to prevent swarm modes grom occuring.
Howland even spoke frankly about his own patented inner hive feeder - saying some people have reported bees drowning in the screening, but far less than other similar techniques using a walk-down system of feeding.
Howland also stated he currently keeps 4 hives, which is the right amount for him to tend. He states that the property size doesn't matter nearly as much as does the beekeepers ability to tend the number of hives he/she has.