>  Before you get the microscope slide out, you can 
>  tell from inspecting the midgut:

No, you can't.

The midgut test was completely discredited years ago.

Here's the problem - the "mid-gut test" was thought 
to be a good idea, but was later found to only reveal 
evidence of the very worst cases of Nosema

The microscope approach is the way to go, and even the
cheapest child's microscope you can find will work just 
fine.  (But do get yourself a nice bright light source 
for it, like a halogen desk lamp.

I dunno about the 400x - that would make for a very 
small field of view.  I've always used 100x.  But 
clearly, 1000x is too strong, and less than 100x might 
not reveal the detail.

Now, Eric Mussen (who seems to be the US specialist 
on the subject of Nosema) says that it is hard to 
"calibrate" one's home microscope with lab results, 
and he's right - when you focus the microscope, you 
are focusing on a very narrow range within the drop
of liquid - like maybe a 1-foot slice of a 20-foot
deep "pond".  So you have to look all over the drop,
and may see only a few paramecium-like Nosema spores.

The good news is that nothing else looks even close
to a Nosema spore.  The other good news is that it
does not matter which kind of Nosema you see, the
treatment is exactly the same.  So, you really don't
need to care which kind of Nosema is which.

Now Eric wants to convince us all to look at some
bees from a sample, and send the rest of the sample
in for analysis, and thereby calibrate how many 
spores we see with how many spores the lab tests
reveal, but again, not to worry - Nosema has been
so bad recently, I'd treat if I saw ANY spores
in ANY samples.  Nosema has been just that prevalent
in the US and Canada recently.

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