One of the significant fungal (fungus-caused) problems threatening the health of bee colonies, chalkbrood specifically targets honey bee larvae. This condition, which disrupts brood-rearing activities and leads to severe weakening of the colony population, is a major cause of yield loss for beekeepers. The main characteristic of the disease is that affected larvae eventually harden, turning into white “mummies” resembling chalk or lime. These mummies are the most distinct clue for diagnosis. Its management requires an integrated approach involving hive management, hygiene, and genetic factors, rather than a direct chemical treatment. This fungal agent tends to spread rapidly, especially in colonies that are under stress or unable to maintain brood temperature.
What is chalkbrood?
Chalkbrood is a contagious honey bee brood disease caused by a fungus scientifically named Ascosphaera apis. This fungus exclusively infects honey bee larvae, causing them to die within their brood cells and characteristically mummify. The disease reduces the colony’s brood efficiency and, if left uncontrolled, causes serious population decline due to larval loss, weakening the colony and putting it at risk of collapse.
Life cycle of Ascosphaera apis and its spread in the colony
The fungus responsible for the disease, Ascosphaera apis, spreads through highly resilient spores. These spores can remain viable for years in old combs, honey, pollen, and on beekeeping equipment. The infection cycle begins when young larvae (typically during a critical period of 3 to 4 days old) consume food containing these spores. The spores germinate upon reaching the larva’s digestive system, particularly the midgut, and form filamentous structures called hyphae (mycelium). The fungal mycelium rapidly envelops the larva’s body and consumes its internal organs. The larva usually dies from starvation and fungal invasion just before or shortly after the brood cell is capped. The fungus consumes all the larva’s body tissues, turning it into a hard, white structure. When fungal mycelia of different mating types combine (sexual reproduction), dark or black spore sacs (sporocysts) are formed. These sacs produce millions of new spores, ready to continue the cycle. Worker bees, in their attempts to clean these mummified larvae as part of their hygienic behavior, inadvertently spread the spores to other parts of the hive, healthy larvae, and food stores.
Chalkbrood vs. stonebrood: key differences
Although both are fungal diseases affecting bee larvae, there are distinct and important differences between chalkbrood and stonebrood. Chalkbrood is caused by Ascosphaera apis and turns larvae into white, chalky, porous, and easily crushed mummies; if sexual reproduction has occurred, these mummies may be gray or black. Stonebrood, however, is caused by Aspergillus flavus (or sometimes other species like Aspergillus fumigatus). In stonebrood, the larvae take on a much harder, denser, and stone-like structure; their color often turns yellowish, greenish, or brownish, which comes from the fungus’s own spore color. The most critical difference is that Ascosphaera apis only affects larvae, whereas Aspergillus species can infect both larvae and adult bees. Furthermore, Aspergillus fungi can sometimes pose a potential respiratory health risk (zoonosis, aspergillosis) to other mammals, including humans, while Ascosphaera apis is a pathogen specific only to bees.
Chalkbrood symptoms and diagnosis
Field diagnosis of this fungal problem is generally possible thanks to its characteristic symptoms. The most common and clearest finding is the white, chalk-like mummies seen at the hive entrance or on the brood combs. As the disease progresses, the brood pattern becomes clearly disrupted, and this situation directly affects the colony’s overall health, its brood-rearing capacity, and ultimately its population strength. Early diagnosis is crucial for controlling the spread of this fungal problem.
Identifying the white “mummies” at the hive entrance
The most obvious and unmistakable sign of the disease is the dead larvae found on the hive’s landing board or on the ground immediately in front of the hive. These are the “mummies” that give the disease its name, which worker bees have ejected from the hive as part of their hygienic behavior. These mummies are typically white or off-white in color and have a dry, hard texture resembling chalk. However, if the fungus has formed spore sacs (sexual reproduction has occurred), the color of the mummies may darken to gray or completely black. These structures are hardened but brittle. Seeing even a small number (e.g., 5-10 per day) of these mummies in front of the hive is a strong indicator that the infection is active and spreading inside.
Brood pattern disruption and odor: practical field diagnosis
As this fungal infection progresses, the pattern on the brood combs becomes disrupted. Normally, a healthy brood area shows a dense and continuous pattern of brood (eggs, larvae, capped brood) outward from the center, thanks to the queen’s performance. In a diseased comb, this pattern takes on a “spotty,” “scattered,” or “pepperbox” appearance. This look is caused by the empty cells resulting from worker bees cleaning out the dead larvae. The comb will have empty cells or dried, loose mummies mixed among healthy larvae. In heavy infections, gently shaking the comb may produce a rattling sound, like a maraca, from the dried, loose mummies inside the cells. Generally, this disease does not cause a distinct foul, sticky, or sour odor like American or European Foulbrood; however, in some cases, especially if humidity is high, a faint, moldy or mushroom-like odor may be detectable.
First signs in drone brood (peripheral comb focus)
The Ascosphaera apis fungus requires lower temperatures (specifically around 30°C) to develop, rather than the normal brood temperature of about 34-35°C. For this reason, the disease often begins in the coldest parts of the brood nest. These areas are typically the bottom and side edges of the combs; these are also the locations where drone cells (larger comb cells) are concentrated. Drone larvae, being located on the periphery of the brood area, are more susceptible to chilling and temperature fluctuations. Therefore, when beekeepers are performing initial checks for suspected chalkbrood, looking first at the drone brood areas on the peripheral combs is an effective method for catching the first signs and focal points of the disease.
Causes and risk factors for chalkbrood
The emergence of chalkbrood is not solely dependent on the presence of the fungus spores; it also requires a complex combination of environmental and genetic factors that leave the colony vulnerable to this infection. High humidity, temperature fluctuations in the brood area, and general stress factors the colony is exposed to are key elements in triggering and exacerbating the disease. Weak colonies and genetic predisposition significantly increase the risk.
The impact of humidity and temperature fluctuations
The Ascosphaera apis fungus requires high humidity to thrive. Inadequate hive ventilation, placing the hive in a damp location (like a swampy area or near a creek), or prolonged rainy weather conditions can raise the relative humidity inside the hive (e.g., above 80%), creating an ideal environment for fungal development. One of the most critical factors is temperature fluctuation. Sudden temperature drops between night and day, especially in the spring months, lower the temperature of the brood area. Weak or insufficiently populated colonies try to keep the central cluster warm, leaving the larvae on the periphery in the cold (30°C and below), making them vulnerable to fungal infection. This situation, also known as “chilled brood,” is considered a primary trigger for chalkbrood.
Genetic/queen-derived susceptibility
Colonies’ resistance to this fungal problem is largely dependent on their genetic makeup, meaning the quality of the queen. Some bee lines possess a strong cleaning instinct known as hygienic behavior. Hygienic bees detect diseased, dead, or abnormal larvae at a very early stage (often within the first 48 hours, before the fungus produces spores), quickly remove them, dismember them, and eject them from the hive. This rapid and effective intervention prevents the fungus from reaching the spore-production stage and stops the disease from spreading within the colony. Colonies with weak hygienic behavior are slow to clean out diseased larvae or do not clean them at all; this situation allows mummies to form and millions of spores to spread inside the hive. Therefore, in apiaries where this problem persistently appears, the genetics of the queen bees should be the first thing questioned.
Old/dirty comb, antibiotics, and stress factors
Chalkbrood spores can survive for years in the wax structure of old, darkened combs, as well as in the larval debris (caste skins) accumulated at the bottom of cells. These combs act as a continuous infection reservoir and can cause the disease to break out again in a seemingly healthy colony when conditions become favorable (e.g., during a cold spell). Failing to practice regular annual comb rotation leads to the accumulation of this spore load. Furthermore, the unnecessary or incorrect use of antibiotics is also an indirect risk factor. Antibiotics, especially when used for bacterial diseases like European Foulbrood, can kill the beneficial bacteria (microbiota) in the larvae’s gut, thereby weakening their natural protection against opportunistic fungi like Ascosphaera apis. General stress factors (such as heavy Varroa mite infestation, weak colony population, inadequate nutrition, frequent hive transport) also lower the colony’s immune system, inviting this condition.
Chalkbrood control and management
Since there is no direct chemical treatment for chalkbrood, control relies entirely on good beekeeping practices and proactive management strategies. The primary goal is to break the fungus’s life cycle, strengthen the colony, and eliminate the conditions favorable for the disease’s development (especially humidity and cold). Hygiene, disinfection, and the maintenance of a strong colony population are the cornerstones of the management approach.
Disposal of diseased combs and hive replacement
When signs of the disease are observed, the first step is to radically destroy the source of the infection. Combs in the brood area that are heavily contaminated with mummies (e.g., more than 20% of the surface) must be immediately removed from the hive, frames and all. These combs should be promptly destroyed by burning to prevent the spores from spreading into the environment. Wax from lightly infected combs or those containing only honey can be sterilized by melting at a high temperature (115-120°C under pressure), but the frames themselves should still be scorched with a blowtorch. In cases of heavy infection, transferring the colony to a completely new or meticulously disinfected hive, preferably onto new (or clean) foundation (a “shook swarm” or hive transfer), is one of the most radical and effective solutions. This process aims to completely rid the colony of its spore load.
Old comb rotation and disinfection plan
Because fungal spores are harbored in old combs, regular comb rotation is a necessity for preventive beekeeping. As a general rule in beekeeping, at least one-third (approximately 3 frames) of the combs in the brood box should be renewed each year. This means that over a 3-year cycle, all combs in the brood chamber are replaced, which significantly reduces the spore load. Hives that are empty at the end of the season or that came from diseased colonies must be disinfected before being reused. Scorching the hive surfaces with a blowtorch (just enough to lightly brown the wood surface) is effective for killing spores. Alternatively, hives can be washed with a 1% sodium hypochlorite (bleach) solution or with special hive disinfectants and then dried thoroughly in the sun.
Strengthening the colony: feeding and brood supplementation
This fungal problem typically strikes weak and stressed colonies. A strong worker bee population can maintain the brood area at the ideal temperature (34-35°C) and can perform hygienic activities more effectively. Colonies affected by the disease should be fed to increase the population and stimulate brood rearing. This feeding, especially if there is no nectar flow, should be supported with 1:1 sugar syrup (1 part sugar to 1 part water) and protein (pollen, pollen substitute, or bee cake). Another way to quickly strengthen the colony is to provide supplements of frames containing emerging (capped) brood taken from healthy, strong colonies. These newly emerging young bees will help both to maintain the brood temperature (thermal regulation) and to assist with cleaning activities (hygienic behavior).
Requeening and combining colonies
If the disease recurs in the same colony despite environmental factors (ventilation, feeding, comb rotation) being corrected, the problem is most likely the current queen’s genetic predisposition (low hygienic behavior). In this case, the existing queen should be replaced, and a quality, young queen tested for hygienic behaviors should be introduced. If the colony has become very weak due to this disease (e.g., down to 3-4 frames in the spring), it may be difficult for it to recover on its own and it will be unable to maintain brood temperature. In this situation, the best strategy is to combine this weak colony with another healthy weak or medium-strength colony (e.g., using the newspaper method). The resulting strong, unified colony will be better able to cope with the disease and will keep the brood area more stable.
Prevention strategies
The most effective and sustainable approach to managing chalkbrood is to ensure the disease does not appear in the first place. Preventive measures aim to block the negative conditions required for the fungus to develop (especially high humidity and chilling of the brood area) and to keep the colonies’ natural defense mechanisms (hygienic behavior) at their highest level. Apiary placement and queen selection form the foundation of this strategy.
Hive placement and ventilation (stand, orientation, shade)
The location where hives are placed directly affects the microclimate inside the hive. Hives must be raised off the ground (using hive stands or pallets, at least 30-40 cm) and direct contact with the soil must be avoided. This prevents the hive from absorbing moisture from the ground and facilitates ventilation. The apiary should be established in a location that receives good morning sun, but (especially in hot climates) has light shade in the afternoon (like under a tree), and is away from water puddles. Hives left in deep shade all day cannot expel moisture. Hive entrances should face away from prevailing winds but be in a direction with good air circulation (usually south or southeast). Furthermore, ensuring adequate top or bottom ventilation in the hives (e.g., upper ventilation holes, slightly propping the cover, or using ventilated (pollen trap) bottom boards) prevents moisture condensation inside the hive, thereby greatly reducing the risk of Ascosphaera apis development.
Risk reduction through hygienic line/queen selection
Genetic resistance to this fungal problem is the most sustainable and least costly method of prevention. Beekeepers should make their breeding choices or queen purchases in favor of lines that have been proven (tested) for hygienic behavior. Hygienic colonies intervene as soon as the fungal infection begins (before the larva mummifies and produces spores) by cleaning out the infected larvae, naturally preventing the spread of the disease. The queens of colonies that consistently show signs of chalkbrood should be considered genetically susceptible to this problem and should be replaced without delay with a more resistant (hygienic) queen. This genetic selection is one of the most guaranteed long-term ways to protect the apiary from this common problem and lightens the management burden.
Chalkbrood treatment: what is possible, what is not
When faced with chalkbrood, beekeepers often search for a quick chemical solution. However, the nature of this disease is too complex to be resolved with a simple medication. Control is not based on chemicals but relies entirely on management improvements and strengthening the colony’s own defense mechanisms (population strength and hygiene). There is no effective “drug” available for this fungus.
If there is no medication: the effective management protocol
There is no licensed, legal, or proven chemical (fungicide) treatment available for chalkbrood (Ascosphaera apis). Some antifungal agents tried in the past have either been banned or abandoned due to the risk of leaving residues in honey and wax, or because they showed insufficient efficacy. Therefore, the search for a “drug treatment” is misguided and can put honey safety at risk. The effective management protocol that must be implemented instead of medication includes the following steps: reducing internal hive humidity (effective ventilation), keeping the colony strong (balanced feeding and, if necessary, capped brood supplementation), ensuring a sufficient bee population to keep the brood area warm at all times, systematically destroying the source of infection—old and contaminated combs (annual comb rotation)—and working with genetically resistant (hygienic) queen bees. Success depends on the comprehensive and persistent application of these steps as a whole.
The evidence status for “quick fix” claims (vinegar/sprays, etc.)
There are some “natural” or “quick” solutions commonly recommended among beekeepers for this fungal problem (such as adding vinegar to syrup, thyme oil, various herbal sprays, garlic, etc.). However, there is insufficient strong scientific evidence regarding the effectiveness of these methods, or the evidence is anecdotal. For example, while it is thought that vinegar added to syrup (by creating an acidic environment) might temporarily suppress fungal growth, there is no clear finding that this destroys the spores inside the hive or permanently cures the disease. Such applications do not solve the underlying, fundamental management problems (a weak colony, poor ventilation, genetic susceptibility, old combs). The focus should be on proven, integrated beekeeping techniques that address the root causes of the disease, rather than on temporary, unsubstantiated fixes.
