Peter asks: .. is there any possibility that freezing the cappings with
liquid nitrogen alters them in some way that is then detected by the bees,
causing them to remove them - and they then find the dead larvae?

Response:  yes, that may be occurring.  However, liquid nitrogen does not
change the appearance (to our eyes), whereas some other freezing methods
do.  Part of this approach is consistency -- do the same damage to the same
area, each time.

However, at time/concentrations less than that required to kill the brood,
we didn't see any evidence of bees uncapping cells exposed to the liquid
nitrogen.

Good question -- don't have a really good answer. We've played around a bit
with ultrasonics and other methods of killing pupae without damaging comb
-- but haven't found a better, affordable alternative.

Jerry

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