Chalkbrood

Chalkbrood is a brood disease caused by the fungus Ascosphaera apis, in which the larvae turn into chalk-white and later grey-black hard mummies. It increases during cool, damp periods, in weak colonies and in susceptible lines; it rarely kills the colony but noticeably weakens it through brood losses. There is no authorised medicine for it: the solution is to keep the hive warm and dry, strengthen the colony and, if necessary, requeen with a hygienic line.

What is it and how is it transmitted?

The fungal spores are taken in with the food the bees give to the larvae. The spores germinate in the larva's gut, the fungus overgrows the larval tissue and the larva dies, usually shortly after the cell has been capped. The fungus dries out the tissue and turns it into a hard "mummy".

• The spores are very resistant: They survive for years in comb, wax, pollen and hive equipment.
• Routes of spread: Frames taken from a diseased hive, pollen and bee bread, robbing, drifting and shared tools.
• The decisive factor is chilling of the brood: If a larva around the time of capping is chilled, the fungus develops quickly. This is why the disease is often seen at the edges of the brood nest and in drone brood.

How to recognise it

On the comb

• Mummies: White, chalk-hard larval remains in the cells, detached from the cell walls. Once the fungus starts to produce spores, the mummies turn grey or black.
• Loose mummies: The mummies do not stick to the cell; they may rattle when the frame is shaken.
• Scattered (pepper-pot) brood: As the bees remove the mummies, empty cells are left among the capped brood. Some cells may have been uncapped by the bees.

In front of the hive and on the floorboard

Undertaker bees remove the mummies from the cells and throw them out. Seeing white, grey and black pellets on the landing board, in the grass in front of the hive and on the floorboard is the easiest way to spot the disease without opening the hive.

Conditions that increase the disease

• Cool, damp weather: Especially in spring, during cold spells following warm days.
• Early and excessive expansion: If the colony spreads over more space than it can keep warm, the brood at the edges is chilled.
• Weak colony: There are not enough nurse bees to keep the brood warm.
• Damp apiary site: Shady, windy or poorly drained areas; hives placed directly on the ground.
• Poor ventilation: Moisture condensing inside the hive.
• Genetic predisposition: Some lines are markedly susceptible; mummies accumulate in colonies with weak hygienic behaviour.
• Other stress factors: A high varroa load and poor nutrition lower the colony's resistance.

How it differs from similar conditions

How to confirm it at the hive

1. Take a few mummies out of the cells: they should be dry, hard and light, and should not stick to the cell wall.
2. Crush one between your fingers: if it crumbles like chalk, it is chalkbrood; if there is no green-yellow powder on it, it is not stonebrood.
3. If the capped brood has sunken, greasy-looking, perforated cappings and sticky remains, do not settle for chalkbrood; send a sample on suspicion of American foulbrood.

The typical appearance is usually enough for a diagnosis. If you are unsure, send a sample of mummies to a laboratory; a definitive diagnosis is made by seeing the fungus's spore clusters under the microscope.

What to do

There is no authorised, effective medicinal treatment, and none is needed. Correcting management conditions makes the disease recede on its own in most colonies.

1. Reduce the space: Reduce the colony to the number of frames the bees can fully cover; remove empty frames and unnecessary boxes.
2. Reduce humidity: Raise the hive off the ground, tilt it slightly towards the entrance and improve ventilation.
3. Review the site: A dry apiary site that gets the morning sun and is sheltered from the wind makes a marked difference.
4. Strengthen the colony: Giving capped brood about to emerge from a healthy colony, or uniting weak colonies, restores the heat balance. Check the food situation.
5. Comb hygiene: Take frames with many mummies out of circulation and render them down; clean the floorboard. Do not move frames or pollen from a diseased hive to other hives.
6. Requeen: If the disease recurs every year even though conditions have been corrected, a susceptible line is involved. Replace the queen with one from a line with strong hygienic behaviour.

Prevention

• Keep colonies strong; in spring avoid early expansion beyond the strength of the colony.
• Set up the apiary in a dry, sunny place and keep hives on stands.
• Renew old, dark combs regularly.
• Do not use pollen and combs of unknown origin; do not move equipment from a diseased hive to healthy hives.
• When choosing queens, prefer lines with strong hygienic behaviour.