Chalkbrood: Recognising and Managing This Common Fungal Brood Disease

What chalkbrood is and why it thrives in the UK climate

Chalkbrood is caused by the fungus Ascosphaera apis, which infects honey bee larvae after they ingest fungal spores in their food. Infected larvae die after being sealed in their cells, and their bodies become colonised by fungal mycelium that hardens them into a distinctive chalk-white, brittle mummy - the source of the disease's name. It is rarely fatal to an entire colony on its own, but in a persistent or severe case it noticeably reduces brood output and colony strength, and it tends to recur in the same apiaries and even the same hive locations year after year once established.

The fungus needs cool, damp conditions to flourish, and larvae are more susceptible to infection when the brood nest temperature drops below the bees' preferred range - a chilled brood nest is far more vulnerable than one kept at a steady 34-35°C. This makes chalkbrood particularly relevant to UK beekeeping: a wet spring, an overly large hive body for a small colony (meaning bees can't adequately cover and warm all the brood), or a period of poor weather that keeps foragers confined and stresses the colony can all tip a marginal situation into a visible outbreak.

Recognising the stages of infection

Chalkbrood mummies progress through recognisable colour stages: freshly killed larvae first appear white and slightly fluffy, almost like small cotton balls, as the fungus begins to overgrow the larval body. Within days the mummy takes on a grey, chalky texture as it dries and hardens fully. In the final stage, once fungal fruiting bodies (which produce new spores) develop, mummies turn black or dark grey - these are often the most visible stage, since bees actively work to remove mummies from cells and pile them at the hive entrance, where a beekeeper can spot small black or white pellet-like objects on the landing board or ground in front of the hive.

Inside the hive, look for a spotty, irregular brood pattern with numerous unexplained empty cells among healthy brood, and check both capped and uncapped cells for mummies still in place, since not every mummy gets removed promptly. A frame showing patches of healthy sealed brood interspersed with gaps and the occasional white or grey mummy is the classic chalkbrood picture, distinct from the sunken, perforated cappings and ropey texture of foulbrood.

Management: conditions and colony strength matter more than treatment

There is no approved chemical treatment for chalkbrood in most jurisdictions including the UK, which makes management fundamentally about improving conditions rather than medicating the colony. Improving ventilation so the hive doesn't trap excessive moisture, reducing hive volume to match colony size (so bees can properly cover and warm all brood comb rather than spreading thin over an oversized box), and siting hives with good sun exposure and airflow all reduce the cool-damp conditions the fungus needs. A colony that is otherwise strong, well-fed and queen-right typically clears a mild chalkbrood episode on its own once conditions improve.

Requeening from stock with demonstrated hygienic behaviour is one of the more effective long-term interventions, since colonies vary genetically in how efficiently workers detect and remove infected larvae before the fungus can sporulate and spread further - this is the same hygienic trait relevant to Varroa-sensitive hygiene, and selecting for it addresses multiple disease pressures at once. Replacing old, dark comb periodically also helps, since spores can persist in comb material across seasons and heavily used brood comb accumulates a larger fungal spore load over time than comb rotated out every few years.

When to worry and when to simply wait it out

A handful of mummies at the entrance during a damp spell, in an otherwise thriving colony, is common and rarely worth active intervention beyond noting it and perhaps improving ventilation. Cause for more active management is a persistent, worsening pattern across multiple inspections, a brood pattern that stays consistently patchy rather than recovering as weather warms, or the same colony suffering repeat severe episodes year after year - in that situation, requeening with hygienic stock and rotating out old comb are the most reliable long-term fixes, alongside a hard look at whether the hive is oversized, poorly ventilated, or sited somewhere persistently shaded and damp.