Waxmoth: Galleria mellonella or Achroia Gresella
The most common occurrence of Baldbrood is caused by waxmoth larvae tunnelling underneath the cappings of sealed brood. The bees will tear down the cappings to clean away the debris left behind leaving the pupae exposed. Most of the exposed pupae will usually emerge as fully developed adults although some may be malformed.
When caused by waxmoth larvae, the pattern will be linear, otherwise if the pattern is random, this may be a result of the queen. The waxmoth larvae can often be traced and located underneath cappings that are a little paler than normal.
TREATMENT: If the condition is the result of a genetic trait of the queen, then re-queening will normally eradicate the condition.
Visual Indications of Bald Brood
In this image, the uncapped brood is the indirect result of waxmoth tunnelling through the brood nest in straight lines.
Bees will have uncapped the cells to clean them up.
When a baldbrood pattern is more random, and there is no evidence of an infestation of waxmoth, then the cause may well be in the genetics of the queen.
Re-queening with a queen of known good stock will usually cure the problem.