Chalkbrood: Why Colonies Get It and What to Do

Chalkbrood is caused by a fungus, Ascosphaera apis, that infects bee larvae. The larva is overgrown, dies, and dries into a hard white or grey "mummy" that the bees drag out of the hive. The spores are present in virtually every apiary. Whether a colony shows symptoms depends almost entirely on conditions inside the hive, which is why chalkbrood is best treated as a management signal rather than a disease to be medicated.

What You Actually See

The first sign for most beekeepers is not on the frames but at the entrance: small chalky lumps on the landing board or the ground in front of the hive, often after a cold snap in spring.

On a frame you see mummies sitting loose in open cells, sometimes rattling when you tilt the comb. They may be white, or grey to black once the fungus has produced spores. The brood pattern looks patchy, with the gaps caused by removed larvae rather than by a failing queen — an important distinction when you are diagnosing.

The Three Conditions That Drive It

Chill. Brood at the edge of the nest that cools below optimum is far more susceptible. Almost all spring chalkbrood follows a period where the colony expanded its brood nest during warm weather, then had to contract when the weather turned. Over-large brood boxes, thin colonies spread over too many frames, and inspections in cold wind all contribute.

Damp. Poor ventilation, a shaded low-lying site, or a hive sat on the ground rather than a stand all raise humidity in the nest and favour the fungus.

Colony stress and genetics. Nutritional stress, heavy Varroa load, or a colony too small for its box weakens hygienic behaviour. Some queen lines are notably more prone than others — this is well documented and it is the reason chalkbrood often follows a particular queen rather than a particular hive.

What to Do — In Order

1. Reduce the space. Bring the colony down to the number of frames the bees genuinely cover. A dummy board or a smaller box concentrates heat where the brood is. This alone resolves most mild spring cases.
2. Improve the site. Raise the hive off the ground, clear vegetation from the entrance, tilt slightly forward so condensation runs out, and if the hive sits in permanent shade, move it somewhere that gets morning sun.
3. Feed if stores are short. A well-fed colony rears brood at a stable temperature; a hungry one does not.
4. Swap out the worst comb. Rotate heavily affected frames out at the next opportunity. Spores survive in comb for years, and old dark comb carries more of them.
5. Requeen if it persists. A colony that still shows chalkbrood after two seasons with good management has a genetic component. Requeening from a clean line is the definitive fix, and hygienic stock clears symptoms remarkably quickly.

What Not to Do

There is no approved chemical treatment for chalkbrood in most jurisdictions, and the folk remedies — cinnamon, garlic, various acids — have no reliable evidence behind them. Do not medicate. Do not panic-destroy comb or equipment; unlike foulbrood, chalkbrood is not a notifiable disease and does not require burning.

Keep the Record

The useful signal is not one sighting, it is the pattern. A colony with mild chalkbrood in April that clears by June was chilled. A colony showing it in July, when there is no thermal excuse, has a genuine problem. Logging severity — none, light, moderate, heavy — at every inspection turns a vague impression into a decision about whether that queen stays for another season.