When larva ingest a fungus called Ascosphaera apis it penetrates the gut wall to absorb nutrients. The larva eventually dies of starvation as the spores germinate and multiply. After a few days of growth, the larva and fungus swells and will fill the brood cell. After a few more days, this hardens to a distinctive ‘mummified’ appearance, adopting a mottled white and black colour. Each chalkbrood mummy will produce millions of infective spores which stick to the cells, hive components and adult bees thus spreading the diseases.
Symptoms will start to appear in early spring as the colony starts to build up its population; damp and cold weather will promote fungal spores. 

Symptoms of chalkbrood include:

  • Initially the dead larvae will be covered with a white cotton wool-like growth and may swell to fill the cell taking on its shape
  • Chalk-like mummies which turn to a greyish black colour as the fungal fruiting bodies develop
  • Worker bees uncap the cells of dead larvae so the mummies will be clearly visible
  • Shrunken chalk-like mummies in the brood and in and around the hive entrance
  • As the condition worsens, infected hives will also show a pepper pot brood pattern
  • If mummies are still contained in capped cells, when a comb is shaken gently the mummies may be heard rattling in the cells

The fungus is highly infectious and can be easily spread between hives through robbing and drifting of drones and worker bees. In addition the spores can be transferred between apiaries on contaminated equipment and through the intervention of the beekeeper, hence best hygiene practice is always recommended.
Chalkbrood is not normally a serious disease among strong healthy colonies, however, in smaller colonies or those under stress (e.g. those with heavy Varroa infestations) it can become a problem.

As with many diseases, the best method for keeping chalkbrood to a minimum is the maintenance of good strong stock, re-queening from colonies which appear better able to resist the fungus.

Avoiding damp apiary sites will also help to minimise the effect of chalkbrood in colonies.

Images Courtesy of The Animal and Plant Health Agency (APHA), Crown Copyright