Hi Bill - You said 

"washed 10 mites in a 300 bee sample"

Out of interest where are you taking the 300 bee sample from in the hives - a brood frame?  I have taken counts from there lately getting ready for August treatments as well and found 3-8. Also treated with oxcilic acid in Dec.

> "if I find even 1 or 2 relatively high mite counts in a yard of 24 hive is it best to treat
> the whole yard?   I am inclined to treat the whole yard"

I think its a game of numbers (colonies vs. labor vs. $) Bill.  If you can stand to possible loose some or all of the colonies that you didn't treat and make splits next spring to replace the then treat only the ones that you had hi hits on and gamble the rest (cost of bees+labor+losse of honey yeald+loss of pollination services, etc.)  Or take the approach to test them all (higher labor cost) and treat only the ones with hi hits.  As I only have a few hives I take the second approach.  I also have an adversion to taking on what I persieve as "too much risk" and would be willing to spend the $4-10/colony that it takes to treat them all.  But you bigger guys have a lot more on your plate so I guess you have to take on more risk!

Scott
Bay Area, CA
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