Fig. 15. Freeze-killed brood assay: freezing brood within comb using liquid N2. Left: Dewar tank with valve to dispense liquid nitrogen, polystyrene foam cups for pouring liquid N2 into PVC pipes (black pipes in combs). Right: After 24 hours, this hygienic colony uncapped and removed >95% of the freeze-killed brood. Photos: M Spivak.