Chalkbrood is a fungal disease of honey bee larvae caused by Ascosphaera apis. Most colonies cope with a small amount, but a persistent outbreak can weaken brood production. The best chalkbrood treatment is not one magic chemical: it is a combination of strong colony management, hygienic genetics, clean comb and sensible brood-nest conditions. That broader approach is covered in our Hive Management & Bee Health guide.
TL;DR: If you find chalkbrood mummies, first make sure the colony is strong, well fed and able to maintain its brood nest. Remove heavy accumulations of mummies, replace badly contaminated old comb where practical, manage other stresses such as Varroa, and consider requeening with hygienic or chalkbrood-resistant stock if the problem persists. In Australia, there is no registered chemical specifically used as a simple cure for chalkbrood.
If you are still building your hive-management foundations, the Beginner Beekeeping guide puts brood diseases like chalkbrood into the wider inspection and colony-health picture.
What Is Chalkbrood Disease?
Chalkbrood is caused by the honey bee pathogenic fungus Ascosphaera apis. Larvae become infected after ingesting fungal spores. The spores germinate in the larval gut, fungal growth invades the larva, and the dead brood dries into the hard white or dark “mummies” that give chalkbrood its name.
The disease affects brood rather than adult bees. It often appears as scattered dead larvae rather than wiping out an entire colony, but a serious outbreak can reduce the number of emerging workers and set the colony back. A broader understanding of bee brood and the brood cycle makes that impact easier to judge.
Why Are Some Mummies White and Others Black?
White mummies are dominated by fungal mycelium. When compatible fungal mating types reproduce, fruiting structures develop and mature spores are produced. As those structures form, the mummy can become grey or black.
The darker mummies are especially important because mature spores can spread within the hive and contaminate comb and equipment.
What Does Chalkbrood Look Like in a Hive?
The classic sign is a hard, chalk-like larval mummy. If you are still learning the visual baseline, compare this with what normal capped brood looks like. You may see chalkbrood mummies:
- inside open brood cells;
- on the bottom board;
- around the hive entrance where workers have removed them; or
- among a patchy brood pattern during a more serious outbreak.
White mummies indicate fungal growth, while grey or black mummies contain mature reproductive structures. A colony with good hygienic behaviour often uncaps and removes infected brood before large numbers of mature mummies accumulate.
How Bees Defend Themselves Against Chalkbrood
Depending on their genetics, bees clean infected brood out with a greater or lesser degree of enthusiasm. This hygienic behaviour is one of the most useful colony-level defences against chalkbrood.
The faster infected brood is detected and removed, the less opportunity the fungus has to complete its reproductive cycle and produce large numbers of spores.
Vita Bee Health chalkbrood resource.
Chalkbrood Treatment and Management
There is no single treatment that fixes every chalkbrood outbreak. Government guidance focuses mainly on management because colony strength, genetics, contaminated comb, nutrition and brood-nest conditions all influence how serious the disease becomes.
1. Keep the Colony Strong
Weak colonies have fewer nurse bees available to maintain brood temperature and care for larvae. Poor nutrition, a failing queen, weather stress and excessive hive space can all make brood more vulnerable.
Make sure the colony has adequate food, enough adult bees and a brood nest appropriate for the season.
2. Remove Heavy Sources of Infection
Workers usually remove many chalkbrood mummies themselves, but heavy accumulations on the bottom board or around the entrance can be cleaned away. Badly affected old comb can also act as a long-term reservoir of spores.
Where practical, replacing heavily contaminated comb is more useful than repeatedly leaving the same spore-loaded material in a struggling colony.
3. Requeen Persistent Problem Colonies
Requeening can be one of the most useful tools when a colony repeatedly suffers from chalkbrood. Hygienic behaviour has a genetic component, and colonies selected for rapid removal of diseased brood are generally less susceptible.
The walk-away split guide is useful related reading if you are considering allowing a colony to rear its own queen, although that is not the same as introducing a selected hygienic queen.
Research has identified genetic markers associated with chalkbrood resistance and high royal-jelly production. That is useful evidence that resistance can have a genetic component, but it does not show that resistant colonies protect brood simply by feeding larvae more royal jelly.
For the study itself, see research on chalkbrood resistance-associated genetic markers.
4. Improve Brood-Nest Conditions
Good hive ventilation can help, particularly where excess moisture or poor airflow is contributing to brood stress. But ventilation is not a stand-alone cure.
Temperature stress, colony size, nutrition, queen performance and the amount of empty space around the brood nest also matter. The aim is to help the colony maintain stable brood conditions rather than simply adding more airflow for its own sake.
5. Manage Varroa Properly
Varroa does not cause chalkbrood — Ascosphaera apis does. But a colony under heavy Varroa and virus pressure is less healthy overall, and multiple stressors can make brood disease harder for the colony to manage. The same whole-colony diagnostic approach is useful when assessing other health problems such as Nosema in bees.
So Varroa monitoring and control remain part of good chalkbrood management where Varroa is present.
A Correction About Apiguard
Apiguard is a slow-release gel containing thymol. It is used primarily to control Varroa mites. Vita Bee Health also reports activity against tracheal mites and chalkbrood, but that does not turn it into a direct replacement for colony-level chalkbrood management.
If you use any mite treatment, follow the product label and the legal requirements that apply in your location. Chemical registrations and permitted uses can change.
Varroa and Chalkbrood
I remember when Varroa first appeared in my area. We had hardly ever seen serious chalkbrood before, but after mite pressure increased I began seeing apiaries with brood frames containing large numbers of chalkbrood mummies.
Varroa feeds on developing and adult bees and is an important vector and amplifier of several honey bee viruses. Heavy infestations can reduce colony health and create a broader disease burden. A stressed colony can therefore be less capable of coping with another brood disease such as chalkbrood.
Honey bees have both physical barriers and active immune responses, so the relationship is more complex than simply saying mites puncture the bee and allow every pathogen inside.
Genetics and Chalkbrood Resistance
Genetics still matters. Colonies differ in hygienic behaviour, larval resistance and overall susceptibility to chalkbrood. Selecting queens from colonies that consistently maintain healthy brood is therefore a sensible long-term management strategy.
What I would avoid is assuming that resistance to one problem automatically means resistance to another. Varroa resistance and chalkbrood resistance can involve different traits, even though a generally healthy, hygienic colony may cope better with both.
My Experience With Cape Honey Bees
In the area where I first dealt with severe chalkbrood and Varroa, I was keeping Apis mellifera capensis. When Varroa pressure first became obvious, the impact was terrible.
We would see heaps of dead brood in front of the hive. In the morning it looked like the bottom of a chalkboard in a maths class — there were so many chalky bee mummies lying in front of the hive. The bees would cart them away. Cape honey bees can show strong hygienic behaviour, and that rapid removal was very obvious.
What Changed Over Time?
Over the following years, I saw less chalkbrood and fewer obvious Varroa problems in those colonies. Research on South African honey bee populations supports the broader observation that both Cape honey bees and African savannah honey bees developed substantial population-level tolerance or resistance to Varroa under comparatively low-treatment conditions.
That does not mean genetic diversity alone solved the problem. Natural selection, hygienic behaviour, grooming, mite reproduction, brood biology, wild colony populations and virus interactions all appear to contribute.
So I still believe strongly in maintaining useful genetic diversity, but the lesson is not to mix genetics randomly. A better approach is to keep healthy, locally successful colonies, avoid unnecessarily narrow breeding, select for useful traits, and introduce outside genetics deliberately when it adds value.
Can Chalkbrood Be Prevented?
You cannot guarantee that Ascosphaera apis spores will never enter an apiary. What you can do is make it difficult for the disease to become a serious colony problem.
- Maintain strong colonies with productive queens.
- Select for hygienic and disease-resistant stock.
- Replace badly contaminated old brood comb.
- Avoid transferring suspect comb between healthy colonies.
- Keep nutrition adequate.
- Manage brood-nest temperature, moisture and hive space sensibly.
- Monitor and manage Varroa where it is present.
- Clean tools and equipment appropriately when moving between diseased colonies or apiaries.
The easiest chalkbrood treatment is still the one you do not need because the colony is strong enough to keep the disease under control.
For another common hive problem, read how to get rid of wax moths in beehives.
Chalkbrood FAQs
What is chalkbrood?
Chalkbrood is a fungal disease of honey bee brood caused by Ascosphaera apis. Infected larvae become hard white or grey-black mummies that can often be seen in brood cells, on the bottom board or near the hive entrance.
How do you treat chalkbrood in a beehive?
There is no single registered chemical cure for chalkbrood in Australia. Management focuses on keeping colonies strong, improving hive conditions, removing heavily contaminated comb or mummies where appropriate, and requeening with stock selected for hygienic behaviour or chalkbrood resistance when the problem persists.
Does requeening help with chalkbrood?
It can. Colonies vary in hygienic behaviour and chalkbrood susceptibility, so replacing a poorly performing queen with stock selected for hygienic behaviour or disease resistance can reduce recurring chalkbrood problems.
Does ventilation help prevent chalkbrood?
Good hive conditions can help because chalkbrood outbreaks are associated with brood stress, including temperature and moisture problems. Ventilation can be part of good management, but it should be combined with colony strength, nutrition, hygiene and appropriate brood-nest management.
Is Apiguard a chalkbrood treatment?
Apiguard is a thymol-based veterinary treatment developed primarily for Varroa control. The manufacturer reports that it may also help suppress chalkbrood, but it should not be described as a bacterial biological treatment or used as a substitute for chalkbrood management.
What do black chalkbrood mummies mean?
White mummies are fungal growth before mature spores are produced. As the fungus completes sexual reproduction, fruiting structures and spores can make the mummy appear grey or black. These mature mummies are an important source of infectious spores.
Can Varroa make chalkbrood worse?
Varroa can weaken colonies and increase overall disease stress, so heavy mite infestations may coincide with more brood disease. Varroa control is important colony health management, but chalkbrood is caused by Ascosphaera apis rather than by Varroa itself.
Can chalkbrood be eliminated completely?
Ascosphaera apis spores can persist in hive material, so complete elimination is difficult to guarantee. The practical goal is to reduce disease pressure and maintain a strong colony that removes infected brood quickly.
Dr Garth A. Cambray is a beekeeper, biotechnologist and researcher with nearly three decades of practical beekeeping experience. He holds a PhD in Biotechnology from Rhodes University and has worked extensively in honey production, fermentation biotechnology and value-added bee products. View Dr. Garth Cambray’s credentials, research and experience.
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