What is a common diagnostic sign of American Foulbrood?

A common diagnostic sign of American Foulbrood is the presence of dead brood, particularly in the sealed or unsealed cells. Infected larvae typically die before they can emerge as adult bees, resulting in the accumulation of dead larvae in various stages of development. A key characteristic observed is that these dead larvae are often found lying flat on the bottom of the cell or can be identified by the cell being capped but containing no healthy bees.

Additionally, when inspecting colonies, beekeepers may notice a foul odor emanating from the hive, which is another strong indicator of American Foulbrood. This disease can lead to significant loss of brood and is highly contagious among colonies.

In the context of the other options, hardened larvae might occur as a consequence of the disease but are not the primary diagnostic sign. Darkened larvae might describe some other brood diseases in terms of coloration changes, but American Foulbrood is more clearly associated with dead brood. Larvae killed in multiple stages may reflect a broader indication of illness but does not specifically pinpoint American Foulbrood as effectively as recognizing dead brood.