Stonebrood is a serious fungal infection in honey bee colonies. It can affect both the brood and the adult bees. This condition is caused by fungi from the Aspergillus genus. The disease gets its name from how it infects larvae or adult bees. They lose moisture, mummify, and become hard, much like a piece of “stone.” While it looks like chalkbrood, stonebrood is seen as far more dangerous. The fungi that cause it are more lethal to bees, and some types can also pose risks to other living beings.
Aspergillus species and primary transmission routes
The two most common species causing this issue in bee colonies are Aspergillus flavus (which creates yellow-green spores) and Aspergillus fumigatus (which makes darker, blue-green spores). These fungi are very common in nature. They are easily found in soil, rotting plant material, stored grains, and pollen. The durable parts of the fungus, its spores, can stay alive in the environment for years.
Transmission to the colony happens in two main ways. The first and most common route is the digestive system. Bees get the disease by eating pollen, nectar, or syrup from the beekeeper that is tainted with fungal spores. This is especially true for nurse bees. When they feed this contaminated food (pollen or bee milk) to the larvae, the infection starts right in the brood area. The spores germinate in the larva’s gut and quickly turn into fungal threads (mycelium). They break through the gut wall and invade the larva’s body cavity. This causes the larva to die fast. The second transmission route is through respiration or the outer exoskeleton (cuticle). If the spore levels inside the hive get too high, these spores can also start an infection by settling in the breathing tubes (tracheae) of adult bees or through weak spots on their bodies.
Seasonal/flora conditions and period of occurrence
This fungal infection is usually seen as an “opportunistic” pathogen. This means spores are always in the environment, but the disease only appears when the colony is under stress or when conditions are perfect for the fungus to grow. The disease is most often seen during the cool, damp months of early spring and autumn. These seasonal changes are when the hive’s internal temperature and humidity balance are challenged the most.
The biggest factor triggering fungal growth is high humidity. When the relative humidity inside the hive stays above 80%, it creates a perfect environment for spores to germinate. Several things can cause condensation (sweating) inside the hive: poor ventilation, a weak colony (unable to heat the space), or a hive kept in shade or on wet ground. This damp, warm setting turns comb surfaces and the hive floor into a perfect place for fungal growth. Flora conditions also matter. If the pollen bees collect is damp, or if a pollen patty gets wet, it becomes a main breeding source for Aspergillus.
Symptoms and diagnosis
Diagnosing stonebrood relies on a close look at clear changes in both the brood cells and the adult bees. Infected larvae show a unique hardening. Adult bees show behavioral problems and more dead bees appear at the hive entrance. It is vital to tell these findings apart from other brood diseases, especially chalkbrood. This is necessary to pick the right intervention strategy.
Distinctive findings of stonebrood in brood cells
The clearest proof of the disease is on the brood frames. The infection usually starts within 24 to 48 hours after the larva eats the spores with its food. The fungus quickly invades the larva’s digestive system and kills it. After death, these symptoms are seen:
- Hardening and Mummification: This is the main symptom that gives the disease its name. The dead larva loses water and is fully covered by fungal mycelia. The larva does not shrink like in chalkbrood. Instead, it hardens and takes on the hexagonal shape of the cell. This mummy becomes so hard it is like a “stone” and is hard to crush even with a tool.
- Color Change: The healthy larva’s shiny, pearly-white color fades. The larva first turns dull. It then changes color based on the fungus species causing the infection. If the cause is Aspergillus flavus, the mummies get a yellowish or yellow-green mold coat. If the cause is Aspergillus fumigatus, the color is darker, gray-green, or almost black.
- Adherence to the Comb Cell: This is a major difference from chalkbrood. The petrified larva sticks tightly to the walls and bottom of the comb cell. Worker bees find it very hard to clean these mummies out, or they cannot do it at all.
Symptoms of stonebrood in adult bees (inability to fly, mummification)
This fungal infection also targets adult bees, which is different from chalkbrood. Adult bees get infected when they eat syrup or pollen that has a high spore count. The fungus grows in the bee’s digestive system and spreads into the body cavity. Symptoms usually show up 3 to 5 days after infection. Infected bees move restlessly and tremble. They seem paralyzed and, most clearly, lose their ability to fly.
These sick bees cannot leave the hive. They are seen crawling on the hive’s flight board or right in front of the hive. They die quickly. After death, especially if it is damp, the fungus keeps growing inside the bee’s body. The mold structures (mycelium) grow outward from between the bee’s body segments (like between the head and thorax). Just like the larvae, the dead adult bee’s body also hardens and mummifies. Finding these hard, moldy, and often yellow-green bee bodies in front of the hive is a strong sign of this disease.
Differences between stonebrood and chalkbrood (field + laboratory)
In the field, both diseases are easy to mix up because both show “mummified larva” symptoms. But the differences are very clear. Knowing these differences is key for the right response.
Observational Differences in the Field:
- Affected Individuals: Chalkbrood (caused by Ascosphaera apis) almost only affects larvae. Stonebrood kills both larvae and adult bees.
- Mummy Hardness: This is the most telling difference. Chalkbrood mummies are brittle like chalk. They can be easily crushed into powder between two fingers. Stonebrood mummies are, as the name suggests, extremely hard like stone. They cannot be easily crushed or broken.
- Condition in the Comb Cell: Chalkbrood mummies are usually loose in the cell. If you shake the frame, you might hear a “rattling” sound from the cells. Stonebrood mummies are tightly adhered to the comb cell. Bees cannot clean them out, and they are hard to remove even with tweezers.
- Color: Chalkbrood mummies start white, then turn gray or black as spores mature (sexual reproduction). Stonebrood mummies turn a moldy yellow-green (A. flavus) or dark gray (A. fumigatus), based on the fungus type (asexual reproduction spores).
Laboratory Differentiation:
A precise and clear diagnosis comes from checking the suspect mummies in a lab. Samples from the mummy are viewed under a microscope. The reproductive structures of the two fungi are totally different. Ascosphaera apis (chalkbrood) forms unique spore cysts (cistospores). Aspergillus (stonebrood) is easy to spot with its classic brush-like spore heads (conidiophores). If needed, the fungus can also be grown (cultured) on special media to find the exact species.
Transmission and risk factors
The appearance and spread of this disease in a colony do not just depend on the spore’s presence. They also largely depend on the hive’s internal environment and the beekeeper’s management. High humidity, poor airflow, a weak colony, and bad hygiene set the stage for this opportunistic fungus to grow fast. Controlling these risk factors is the first step in stopping the disease.
Humidity–temperature thresholds and hive hygiene
Aspergillus fungi love damp places. The most critical point for spores to germinate (activate) is when the relative humidity inside the hive stays above 80%. This is often seen in closed, tight hives with poor ventilation. Damp pollen scraps on the hive floor, dead bees, or condensation on frame edges give the fungus a perfect place to breed. For temperature, these fungi can grow in a wide range. But their growth is maximized at temperatures near the brood area, between 25°C and 37°C. This turns the brood area (about 34-35°C) into an incubator for the fungus. Poor hive hygiene, like dirty bottom boards and old moldy frames, are main sources that add to the spore load.
Colony management errors (feeding, consolidation, ventilation)
Some things a beekeeper does can trigger this disease by accident. The main one is improper feeding. Pollen patties or syrup given to bees can mold very easily if they were stored or made in the wrong conditions. When the beekeeper gives this contaminated food, full of Aspergillus spores, to the colony, they are starting the disease themselves. Homemade pollen patties that stay damp are a major risk.
A lack of ventilation is the main cause of moisture buildup. Making the hive entrance too small in winter, covering the hive top with airtight plastic, or letting the hive touch damp ground makes moisture problems worse. Colony strength management is also key. A weak colony cannot heat its large hive and cannot control humidity, which causes condensation. In the same way, packing a very strong colony too tightly also cuts off airflow and causes similar moisture issues. A strong population, balanced consolidation, and constant fresh airflow are key to preventing the disease.
Honey safety and human health
Stonebrood is different from most other bee diseases in one big way: potential human health risks. Some Aspergillus species, especially A. flavus, can make powerful toxins (poisons). These can be dangerous for both humans and other animals. This needs to be handled carefully, for both beekeeper health and the food safety of bee products.
Honey safety and human health risk (aflatoxin)
This fungal infection has two risks for human health. The first is about beekeeper health. When a hive with a lot of disease is opened, millions of Aspergillus spores can fill the air. The beekeeper might have to breathe in this thick spore cloud. This is usually not a big problem for healthy people with normal immune systems. But, for people with weak immune systems (like those in chemotherapy, transplant patients, or severe asthmatics), these spores risk causing a serious lung infection called ‘Aspergillosis.’ Because of this, it is smart to use a high-protection mask when working with hives you think are heavily infected.
The second and bigger risk is about food safety. The Aspergillus flavus species is known to make mycotoxins called aflatoxins. These have strong carcinogenic effects on the liver. The risk of these toxins getting into honey is extremely low. Honey’s natural makeup (high sugar concentration, about 80%, and low water activity) is not a good place for fungi to grow or make toxins. But the risk is not with honey. It is with pollen and bee bread (perga). If pollen traps are not emptied regularly and the collected pollen gets damp, the fungus can grow. The same risk happens if bee bread frames stored in the hive get moist. This is when aflatoxin production can happen. Because of this, honey, pollen, or bee bread from hives diagnosed with this disease must absolutely not be harvested. They should not be used for human consumption.
Control and management
There is no known, approved, or effective chemical (drug) treatment for stonebrood. Because the disease is serious and contagious, control is all about prevention, hygiene, destruction, and strict disinfection rules. Managing this disease needs much stricter steps than chalkbrood. The fungus is more lethal to bees and carries potential health risks.
Proper disinfection and comb/hive destruction for stonebrood
When this fungal infection is confirmed in a hive, the plan must be clear and firm: destruction and sterilization. All combs with heavy disease and mummified larvae must be removed from the hive right away. These combs must be destroyed by burning. This stops them from spreading spores to other apiaries or the environment. Even if spores are buried, they can live for years, so burial is not a good solution. Burning is the only way to ensure the tough spores are completely destroyed.
If the infected hive itself (body, bottom board, cover) is wooden and you want to reuse it, it must go through intense disinfection. First, all propolis and wax inside the hive must be scraped off. Then, all inside surfaces must be sterilized using the scorching (flame torching) method. The wood surfaces should be held in the flame until they turn slightly dark. This high heat kills the fungal spores in the wood’s pores. Chemical disinfectants (like bleach or soda) are less reliable against such tough spores compared to high heat.
Preventing spread: equipment, location, feeding protocols
Preventing this disease from spreading is much more effective than trying to treat it. The basic protective steps to take in the apiary are:
- Equipment Hygiene: The beekeeper must regularly disinfect basic tools (hive tool, smoker, gloves) when working between hives. The hive tool must be sterilized with a flame torch, especially when moving from a sick hive to a healthy one. Gloves should beChanged often or disinfected.
- Apiary Location and Ventilation: Hives should be placed on dry ground (on hive stands) with good air circulation and morning sun. Stay away from damp, shady, and swampy areas. Hives must have good ventilation (from the top or cover, if needed) to stop condensation inside.
- Feeding Protocol: Never give colonies pollen patties, pollen feed, or syrup that is moldy, damp, or seems moldy. Nutritional supplements should be made fresh, given in clean containers, and given in amounts the bees can eat quickly.
- Colony Strength and Hygiene: Always keep strong and healthy colonies in the apiary. Strong colonies are better at balancing internal hive humidity and temperature. They also show good hygienic behavior by finding and cleaning out sick larvae early. Weak colonies are main targets for diseases and should be combined or supported.
