Further to all this, I came cross some old data to show why I am not as
worried about tracheal or sampling more bees as some think I should be.
Here are results from Fall 1997, a year in which I sampled but never
treated:

Yard         Positive     Negative     Total Tested    Infestation level

Railway       3            47               50             High
Jahns         2            48               50             Medium
Meglis        10           40               50             Medium & Low
Falks         6            44               50             Medium
Kievers       1            49               50             Low
Dixons        4            46               50             Low

There are 6 more yards, but the above are actually some of the worst, and 4
of the next 6 were zeros.  The other two were a ten and a one.  I'm too lazy
to type them in.

The point is that I was unable to see any visible differences between yards,
or losses that I could attribute to the higher tracheal levels in some
yards.  We have varroa now, and did not then.  I understand the effects are
somewhat additive, so am lowering my threshold of concern a bit.
Nonetheless, the current results are so much better than these that I am not
too worried.

allen
http://www.internode.net/honeybee/diary/