Chalkbrood
Updated
Hard white or gray lumps the size and shape of a larva, on the landing board or rattling in the cells. It is a fungus, it is common, and it is usually telling you about the colony's condition rather than about the fungus.
What you are looking at
Larvae killed by a fungal infection and mummified. Fresh ones are chalky white, older ones go gray or black once the fungus produces spores. You will often see the bees dragging them out and dropping them at the entrance, which is how people often notice it.
The pattern is scattered rather than concentrated, and the cappings are not sunken or greasy. That distinguishes it at a glance from the foulbroods, which is the reason it is worth being able to recognize.
Why it appears
The spores are effectively everywhere and most colonies encounter them. Whether it becomes visible depends on the colony: chilled brood, damp, poor ventilation, a colony stretched too thin to keep the brood nest warm, and genetics all push it over the line.
It shows up most in spring, when a colony is expanding faster than its population can cover, and after a cold snap that forces the cluster to contract off the edges of the brood nest.
What to do
Improve the conditions rather than attacking the fungus. Reduce excess space so the bees can cover what they have, improve ventilation and drainage under the hive, and make sure the colony is not damp.
Requeening is the intervention that most reliably ends persistent chalkbrood, because susceptibility has a strong genetic component and hygienic stock removes affected larvae before the fungus sporulates. If a colony has it year after year while its neighbors do not, the queen is the variable.
A strong colony usually clears it without help once conditions improve.
What not to do
There is no approved treatment to reach for, and this is one of the cases where that absence is not a gap. Chalkbrood in a colony that is otherwise strong is a symptom worth reading, not a disease worth medicating.
Do not move visibly affected comb into a healthy colony, and do not assume a few mummies mean a sick hive. A handful at the entrance in April in a growing colony is ordinary. A brood frame with a quarter of the cells mummified is not.