Chalkbrood is a brood disease caused by the fungus Ascosphaera apis. It is common but rarely kills a colony; its main effect is to reduce brood numbers and therefore colony strength.

How it develops

The fungal spores are taken in by the larva with its food. They germinate in the gut, and when the larva becomes chilled the fungus develops rapidly. The larva mummifies: first into a white, chalky mass.

The fall in temperature is the determining factor. This is why the disease concentrates at the edges of the brood nest and in cool periods.

Signs

• Mummies: White, chalk-like hardened larval remains
• Colour change: As the fungus begins to sporulate the mummies turn grey, then black
• Mummies in front of the entrance: The bees clean them out; white pellets in front of the hive are the typical sign
• Patchy brood: Mummified cells are left empty
• Sound: A frame containing mummified larvae may rattle faintly when shaken

What triggers it

• Cold and damp: The most important factor. Chilling of the brood nest prepares the ground for the fungus
• Expanding too early or too far: If the colony spreads into a space it cannot warm, the edge brood chills
• A weak colony: It cannot maintain the temperature
• Genetic susceptibility: Some lines are markedly susceptible
• A damp apiary site: Shaded, sheltered, poorly drained ground

What to do

There is usually no licensed medicinal treatment, and none is needed. The approach is to correct the conditions:

• Reduce the space. Bring it down to a volume the colony can warm
• Improve ventilation. Reduce moisture inside the hive; raise it off the ground and tilt it forward
• Review the site. A dry, sheltered position with morning sun makes a marked difference
• Strengthen the colony. Adding sealed brood improves the heat balance
• Renew comb. Take heavily affected frames out of circulation
• Requeen. If it recurs persistently there may be a genetic susceptibility; a line with strong hygienic behaviour solves the problem

Not to be confused with

• Stonebrood: A different fungus; the mummies are greenish-yellow and harder
• Sacbrood: The larva forms a fluid-filled sac and does not mummify
• Chill damage: Edge brood dies but there is no fungal growth or chalky appearance

Summary

Chalkbrood is an indicator of management. If white mummies appear in front of the entrance, the question to ask is not "which treatment" but "why is the colony cold".