Stonebrood in Honey Bees: Aspergillus Infection in Brood and Adults
- Stonebrood is a rare fungal disease of honey bees caused by Aspergillus species, most often affecting larvae but sometimes adult bees too.
- Infected brood can develop a pale yellow collar near the head, then harden into very firm mummies coated with yellow-green, gray-green, or black spores.
- There is no labeled hive medication that reliably cures stonebrood. Management usually focuses on confirming the diagnosis, reducing moisture and stress, replacing contaminated comb, and in severe cases destroying heavily affected comb by burning.
- Because Aspergillus spores can also affect mammals, use gloves, avoid breathing dust or spores, and talk with your vet or local bee health professional before handling a badly affected colony.
What Is Stonebrood in Honey Bees?
Stonebrood is a fungal disease of honey bees caused by Aspergillus species. Older USDA honey bee disease guidance describes it as a larval disease usually caused by Aspergillus flavus, with A. fumigatus, A. niger, and other species also associated with cases. More recent research supports that stonebrood is uncommon, often underrecognized, and can involve both brood and adult bees.
The name comes from what happens to infected brood. After the fungus invades the larva, the body hardens and becomes difficult to crush. Later, the surface may become covered with powdery spores that look yellow-green, gray-green, or black depending on the fungal species involved.
Stonebrood can look a little like chalkbrood at first glance, but the mummies are typically harder and more "stone-like." Worker bees may remove these mummies, so pet parents and beekeepers sometimes first notice them on the bottom board or near the hive entrance rather than deep in the brood nest.
Symptoms of Stonebrood in Honey Bees
- Whitish-yellow collar or ring near the head end of an infected larva
- Larvae that become very hard, dry, and difficult to crush
- Mummified brood coated with yellow-green, gray-green, or black powdery spores
- Hard mummies found in brood cells, on the bottom board, or at the hive entrance
- Patchy brood pattern or reduced brood survival
- Weak, stressed, or dwindling colony with occasional adult bee involvement
Worry more when you see hard, spore-covered mummies, repeated brood loss, or a colony that is already weak from moisture, nutrition problems, or other disease pressure. Stonebrood is uncommon, but it can spread spores through the hive as bees remove dead brood.
Because several brood diseases can look similar, a visual guess is not always enough. If you are seeing repeated mummies, unusual spore colors, or a fast decline in brood health, contact your vet, apiary inspector, or bee diagnostic lab for help with identification.
What Causes Stonebrood in Honey Bees?
Stonebrood is caused by fungi in the genus Aspergillus. USDA guidance lists A. flavus as the usual cause, with A. fumigatus, A. niger, and other species also linked to disease. These fungi are common in soil and the wider environment, so exposure alone does not always mean a colony will become sick.
Research suggests Aspergillus behaves as an opportunistic pathogen in bees. In other words, the fungus is often present in apiary environments, but disease is more likely when bees or brood are stressed. Poor nutrition, excess moisture, weak colony strength, old contaminated comb, and other health problems may make infection more likely.
The fungus can grow quickly once brood is infected. A recent experimental study found a short interval between spore germination and visible disease, with rapid larval death after infection. Adult bees can also be susceptible, especially after ingesting spores, although brood disease is what most people notice first.
How Is Stonebrood in Honey Bees Diagnosed?
Diagnosis often starts with the hive exam. USDA honey bee disease guidance notes that stonebrood can often be suspected from gross signs such as the yellow collar near the larval head, hard mummified brood, and later green to black spore growth. Your vet or bee health professional will also consider other brood diseases that can mimic it, especially chalkbrood and some noninfectious brood losses.
A definitive diagnosis usually requires laboratory confirmation. USDA guidance states that positive identification involves culturing the fungus and examining its conidial heads, commonly after growth on potato dextrose or Sabouraud dextrose agar. In practice, that means sending suspect brood or comb samples to a diagnostic lab if the case is unclear or severe.
Handle suspect material carefully. Aspergillus spores can cause respiratory disease in humans and other animals, so avoid creating dust, wear gloves, and consider a mask or respirator if combs are heavily contaminated. If you are unsure what you are seeing, your vet can help decide whether testing, comb replacement, or more aggressive sanitation makes the most sense.
Treatment Options for Stonebrood in Honey Bees
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Hive inspection with your vet or experienced bee health professional
- Removal of obvious mummies and debris
- Improving ventilation and reducing excess moisture
- Nutrition review and correction of obvious stressors
- Basic sanitation of tools and woodenware between colonies
Recommended Standard Treatment
- Veterinary or apiary health consultation
- Diagnostic sample submission when the diagnosis is uncertain
- Replacement of old or visibly contaminated brood comb
- Requeening consideration if colony hygiene is poor or recurrence is a pattern
- Thorough cleaning and sterilization of hive tools and reusable equipment
- Colony support focused on strength, dryness, and nutrition
Advanced / Critical Care
- Full diagnostic workup through a bee disease laboratory
- Aggressive removal and destruction of severely infected combs, including burning when contamination is heavy
- Apiary-wide sanitation plan to limit spore movement between colonies
- Evaluation of neighboring colonies for brood disease and environmental risk factors
- Colony shake-out, major equipment turnover, or colony replacement decisions in severe cases
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Stonebrood in Honey Bees
Bring these questions to your vet appointment to get the most out of your visit.
- Do these brood changes fit stonebrood, or could this be chalkbrood, chilled brood, or another problem?
- Should we send brood or comb samples to a diagnostic lab for fungal culture or confirmation?
- How much of this colony's comb should be replaced versus cleaned and monitored?
- Are there moisture, ventilation, or nutrition issues that may be increasing fungal risk in this hive?
- Do nearby colonies need to be checked or managed differently to reduce spread?
- What personal protective equipment should I use when handling suspect combs or mummies?
- When would you recommend burning combs or other hive materials instead of trying to salvage them?
- What follow-up signs should tell me the colony is improving versus getting worse?
How to Prevent Stonebrood in Honey Bees
Prevention centers on lowering colony stress and limiting fungal growth. University of Arkansas bee health guidance notes that good nutrition and overall colony health help reduce physiological stress on bees and lower the chance of infection from stonebrood and other microbes. Strong colonies also do a better job removing diseased brood before contamination builds up.
Keep hives dry, well ventilated, and elevated away from excess moisture when possible. Replace old brood comb on a regular schedule, clean woodenware, and sterilize hive tools between colonies. Avoid moving suspect comb, honey, or equipment from a colony with brood disease into healthy hives.
If a colony is severely affected, more aggressive sanitation may be needed. Arkansas guidance warns that Aspergillus spores can cause respiratory disease in mammals and advises careful destruction of combs from severely infected colonies by burning. Work with your vet or local bee health authority to choose the safest, most practical prevention plan for your apiary.