Stone Brood Disease: A Rare Fungal Threat to Honeybee Colonies

Most beekeepers are familiar with the buzz around common hive threats like Varroa mites or American Foulbrood. But tucked among the list of honeybee diseases is a much rarer, quieter threat – stone brood disease. It doesn’t spread explosively or announce itself with a foul odour. Instead, it quietly turns living larvae into stone-hard mummies inside the comb. Though uncommon, understanding stone brood is essential for any serious beekeeper, because by the time visible symptoms appear, the fungus has already done significant damage.

What is stone brood disease? 🔗

According to the Food and Agriculture Organization (FAO), stone brood – also called aspergillosis – is caused by different species of fungi from the Aspergillus genus. Aspergillus flavus is the most frequently reported species, followed by A. fumigatus, A. niger, and occasionally other Aspergillus species. The disease can affect both honeybee larvae and adult bees.

The name itself is descriptive: infected larvae harden into rigid, stone-like mummies inside their cells – very different from the spongy texture of chalkbrood mummies. As noted in a COLOSS BEEBOOK review on fungal brood diseases, stone brood is a very rare brood disease first described by Massen in 1906 and has since been documented worldwide. Despite its global distribution, its full impact on colony health remains poorly understood due to its infrequent occurrence.

The causative fungi: opportunistic pathogens from the soil 🔗

Aspergillus species are cosmopolitan fungi – they are widespread in soil, decomposing organic matter, and the general environment surrounding beehives. These fungi are facultative pathogens with a broad host range, meaning they do not exclusively depend on honeybees to survive. They are capable of infecting plants, insects, birds, and mammals, making them a concern beyond the apiary as well.

Their role as opportunistic pathogens is key to understanding this disease. Research on the mycoflora associated with stone brood in Egypt found that pathogenicity typically occurs only in colonies already weakened by other stressors – the specific conditions that trigger the shift from harmless saprophyte to active pathogen are not yet fully established. In experimental settings, studies confirmed that A. flavus, Aspergillus nomius, and Aspergillus phoenicis were genuinely pathogenic to honeybee larvae, while some A. fumigatus strains showed lower virulence in larval infection tests.

How stone brood infects honeybee larvae 🔗

The infection pathway begins when larvae ingest fungal spores – typically through contaminated food or exposure to spore-laden hive environments. A 2025 histopathological study published in Veterinary Sciences provides the clearest picture yet of how this disease progresses inside the larval body. The researchers identified three distinct phases:

Phase 1: Germination phase 🔗

Once spores are ingested, they germinate rapidly in the larval gut. The study found the gap between spore germination and the onset of active disease is remarkably short – approximately one day. Infected larvae were measurably smaller than healthy controls even at this early stage.

Phase 2: Spreading phase 🔗

The fungal mycelium penetrates the intestinal wall and spreads throughout the larval body, destroying tissues and organs. This phase was observed only in larvae that had already died. The cuticle (outer layer) remains intact at this stage – the fungus is spreading internally, not yet visible externally.

Phase 3: Distribution phase 🔗

In the final stage, the mycelium breaches the cuticle after the larva’s death and forms conidiophores – spore-bearing structures that erupt through the outer layer. Ring-shaped concentrations of mycelium within the larval body are responsible for the characteristic hardness of the mummies. The researchers confirmed that the mycelium only breaks through the cuticle after death, forming the typical stone brood mummies that give the disease its name.

A critical concern beyond the hive: the airborne conidia produced by Aspergillus in the hive environment can potentially cause serious respiratory illness in humans, including allergic bronchopulmonary aspergillosis and even invasive aspergillosis upon inhalation. This is why beekeepers are advised to wear masks and protective eyewear during any inspection of suspected stone brood cases.

Recognising the symptoms 🔗

Stone brood is difficult to detect in its early stages. The first visible sign in a living larva is a whitish-yellow collar-like ring that forms near the larval head as spores germinate in the gut. At the beginning of infection, larvae may lose their glistening appearance, becoming dull, then turn yellow or greenish-brown.

After death, the changes become more pronounced and distinctive:

• Hard, stone-like mummies: Unlike the crumbly texture of chalkbrood, stone brood mummies are difficult to crush and resist removal from cells even with forceps.
• Powdery spore coating: The dead larvae become covered in powdery fungal spores. The colour varies by species – yellow-green for A. flavus, blue-green for A. fumigatus, and dark brown to black for A. niger.
• Irregular brood pattern: Scattered capped and uncapped cells give the comb an uneven, patchy appearance.
• Difficult removal: Worker bees struggle to remove infected mummies, which can remain lodged in cells and continue releasing spores.

Conditions that trigger the disease 🔗

Stone brood does not thrive under normal hive conditions. It requires specific environmental triggers. High humidity and poor ventilation are the primary drivers. When moisture accumulates inside the hive – due to inadequate airflow, a damp hive location, or prolonged wet weather – Aspergillus spores that are routinely present in the environment get the conditions they need to germinate and become virulent.

A well-ventilated hive, elevated above the ground and free of excess moisture, is a key preventive factor against fungal growth. Colonies that are already weakened – by Varroa infestation, nutritional deficiency, or other stresses – are significantly more susceptible, as their natural hygienic and immune responses are compromised.

Global distribution and status in India 🔗

Stone brood is found worldwide and has been documented across Europe, North America, Australia, parts of Africa, and the Middle East. The disease was first formally described in 1906 and has since been recorded across most major beekeeping regions. It remains rare in all of them, but its cosmopolitan occurrence means no apiary is entirely beyond risk.

Notably, stone brood disease has not yet been officially reported in India, despite the country’s large and active beekeeping sector and diverse climatic zones. Whether this reflects genuine absence, genetic differences in local bee populations, or a gap in systematic disease surveillance remains unclear. With increasing global trade in bees and bee products, and shifting climate patterns altering hive humidity and temperature, continued vigilance is warranted.

Management: no chemical cure, but prevention works 🔗

There is currently no approved chemical treatment specifically targeting stone brood in honeybees. Management is therefore entirely centred on prevention and rapid response.

Improve ventilation and reduce moisture 🔗

Ensure hives have adequate airflow through proper entrance sizing, screened bottom boards, and ventilated inner covers. Position hives in sunny, well-drained spots – avoid low-lying or waterlogged areas. Elevating hives above the ground also helps keep moisture from collecting beneath the hive.

Remove infected brood promptly 🔗

As soon as stone brood mummies are detected, remove them carefully and dispose of them away from the apiary. Replace heavily infected combs with fresh foundation. If hives are severely infected and the disease cannot be controlled, destroying them and avoiding the use of their honey for human consumption is advised.

Maintain colony strength 🔗

Strong colonies have more effective hygienic behaviour – worker bees actively detect and remove infected mummies, slowing the spread. Ensure colonies are well-fed, not overcrowded, and not co-stressed by Varroa or other pathogens. NC State Extension recommends maintaining strong, healthy colonies as the cornerstone of brood disease prevention, and this applies equally to stone brood.

Sterilise and replace equipment 🔗

Old brood combs can harbour Aspergillus spores. Regularly rotating and replacing old comb with fresh foundation reduces the reservoir of fungal inoculum in the hive. Sterilise hive tools thoroughly, especially if moving between apiaries.

Select hygienic bee strains 🔗

Some bee populations exhibit stronger hygienic traits, making them more effective at removing diseased brood before spore formation. Recovery from stone brood depends significantly on colony strength, infection level, and the hygienic habits of the bee strain – with variation observed across different subspecies. Breeding programs that prioritise hygienic behaviour can offer a long-term line of defence.

Stone brood vs chalkbrood: key differences at a glance 🔗

Stone brood is often mentioned alongside chalkbrood, another fungal brood disease, since both produce mummified larvae. However, they are caused by different fungi and produce notably different mummies. Chalkbrood (caused by Ascosphaera apis) produces soft, chalky, sponge-like mummies that are relatively easy to remove. Stone brood mummies, by contrast, are hard, dense, and difficult to dislodge from cells – resembling actual stones. The fungal spore coating on stone brood mummies is also powdery and can be coloured yellow-green, blue-green, or black depending on the Aspergillus species involved, while chalkbrood mummies are typically white or grey.

What do you think? Stone brood disease has been documented worldwide for over a century, yet remains poorly understood due to its rarity – does that rarity make it less of a priority for beekeeping research and disease monitoring, or should rare but serious threats still receive equal scientific attention? And given that the disease has not yet been officially reported in India, what role should national apiculture bodies play in building surveillance systems before a disease establishes itself, rather than after?

References

1. https://www.fao.org/family-farming/detail/en/c/1613398/
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC3816652/
3. https://www.sciencedirect.com/science/article/abs/pii/S0944501311800583
4. https://www.mdpi.com/2306-7381/12/2/124
5. https://en.wikipedia.org/wiki/List_of_diseases_of_the_honey_bee
6. https://ecrotek.com.au/blogs/articles/how-to-treat-stonebrood-and-protect-your-hive
7. https://beehealth.uada.edu/assets/pages/beebroodconditions.html
8. https://content.ces.ncsu.edu/disease-management-and-guidelines-for-the-honey-bee