Although this can be a genetic trait the most usual cause of bald brood is the larvae of either the lesser or the greater wax moth tunneling below the surface of the comb, in particular under the brood cappings. The nurse bees will tear down the cell cappings to clean out the cells leaving perforated and exposed cells with brood in. Occasionally these partial cappings have a raised lip protruding from the comb surface. A typical sign of the problem will be a line of uncapped cells. Bald brood may be seen as small patches of normal developing larvae with uncapped or partially capped cells. These uncapped larvae will usually emerge as fully developed adults, although a few malformed adults may emerge due to the exposure.
Strong colonies of bees suffer less from the effects of wax moth as will good hive management. In the case of the genetic form of bald brood, re-queening the colony will usually cure the problem.
Images Courtesy The Animal and Plant Health Agency (APHA), Crown Copyright
Drone brood in worker cells
The appearance of a large amount of drone brood (shown by the domed capping), particularly in worker cells, indicate that something may be wrong. There are two reasons why a colony may exhibit drone brood in worker cells; the colony may have a failing queen which may be due to her not having been mated properly and running out of sperm in the spermatheca.
The second possibility is that the colony has been queenless for a while and female workers have begun to develop functional ovaries. The eggs that these workers lay will be unfertilised and therefore will develop into drone brood, albeit stunted ones when laid in the smaller worker cells. Looking closely you will often see multiple eggs laid in individual cells, often around the cell walls instead of or as well as at the bottom.
Image Courtesy The Animal and Plant Health Agency (APHA), Crown Copyright
It is usually older queens that become drone layers, but as mentioned above, it can happen with younger queens that did not mate successfully. The best option is to re-queen with a young, prolific, recently mated queen. Colonies with laying workers are very difficult to re-queen. You can try shaking out all the original frames containing laying workers in front of the hive and introduce frames of emerging brood with a new caged queen. If, however, the colony has been left too long or if the colony is in a bad state of decline because of the depleting number of regular worker bees, this method may not be practical or worthwhile.