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 -- Visit www.honeybeeworld.com/bee-l for rules, FAQ and other info ---