I developed the freeze killing with liquid nitrogen - and it warrants care  
in use.  Frozen fingers are no fun - wear eye protection and rubber gloves  - 
not cloth.  Pour it on your cloth glove, and it will freeze both the  glove and 
your flesh.
 
Hygienic behavior consists of two steps - each controlled by a gene.   ID the 
dead brood, Uncap and Remove.
 
Reason I never liked the Steve T's methods of sawing out a 2" square,  
freezing  in a freezer overnight, return to the comb - the act of sawing  into the 
comb often induced hygienic behavior -not the freezing.
 
I was working for EPA at the time, and the reliability of the cut out the  
comb or pin prick tests was inconsistent.
 
The liquid nitrogen assay resolved that issue.  However, one of our  findings 
always gets forgotten - better to freeze several small areas than ONE  large 
area.  The one large area again seems to be perceived by bees as  damaged comb 
to be repaired.
 
Medhat added some other refinements that made this a bit easier for the  
average beekeeper.
 
Jerry
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