Chalkbrood in Honey Bees (UK Guide)

Key signs of chalkbrood

The clearest sign of chalkbrood is the appearance of dead larvae that have dried into hard, chalk-like mummies. These are often white or pale grey at first, but they can become darker grey or black as the fungal growth develops.

You may see the mummified larvae inside brood cells, loose on the floor of the hive, or outside the entrance after worker bees have removed them. In stronger colonies the bees may clear many of the affected larvae before you notice much inside the comb.

The brood pattern may also look patchy, especially if a noticeable number of larvae have died. However, chalkbrood usually looks quite different from foulbrood because the dead larvae become dry and chalky rather than sticky, sunken or ropy.

What does chalkbrood look like?

Infected larvae die and dry out into hard, chalk-like bodies known as “mummies”. These are often white at first and may turn grey or black over time. Bees will often remove them, so you may see them on the floor or at the entrance.

What causes chalkbrood?

Chalkbrood is caused by the fungus Ascosphaera apis. The spores can be present in colonies without causing a major problem, but the disease is more likely to show when brood is stressed or chilled.

Damp conditions, poor ventilation, cold weather, weak colonies and brood that is not well covered by nurse bees can all encourage chalkbrood to appear. It is often seen in spring or early summer when colonies are expanding but the weather is still changeable.

Some colonies also seem more prone to chalkbrood than others. If the same colony shows persistent chalkbrood despite improved conditions, the queen line may be part of the problem and requeening may be worth considering.

Is chalkbrood serious?

Chalkbrood is usually less serious than American foulbrood or European foulbrood, and it is not normally a notifiable disease in the UK. Many colonies recover once the weather improves, the colony strengthens and damp or stress factors are reduced.

That said, it should not be ignored if it keeps returning or affects a large amount of brood. Persistent chalkbrood can slow colony build-up, weaken a small colony and make it harder for the bees to reach full strength during the season.

What to do if you see chalkbrood

Start by looking at the condition of the colony and the hive. A small colony in a large box may struggle to keep brood warm, so reducing excess space can help the bees cover the brood more effectively.

Check whether the hive is damp, shaded or poorly ventilated. Improving airflow, keeping the hive dry and making sure the colony has enough food and enough bees to cover the brood can often reduce the problem.

Old or heavily affected comb can be replaced gradually as part of normal brood comb management. If chalkbrood continues despite better conditions, requeening may help because some colonies appear more genetically prone to the problem.

What not to do

Do not assume every patchy brood pattern is chalkbrood. If you see sunken cappings, ropy larval remains, a strong unpleasant smell or suspicious diseased brood, compare the signs with the foulbrood pages and seek advice if you are unsure.

Do not leave a weak colony struggling in too much space, especially during cold or damp weather. Chalkbrood often improves when the colony is stronger, drier and better able to keep the brood nest warm.

Similar problems

Chalkbrood FAQ