The health of honey bee colonies plays a vital role in ecological sustainability and agricultural productivity. Hives are complex social structures formed by thousands of individuals. When this balance is disturbed, colonies become vulnerable to various pathogens. Honey bee diseases stand out as serious threats that shorten the lifespan of bees, disrupt the colony’s population structure, and reduce overall productivity. These health problems can be bacterial, viral, fungal, or parasitic in origin. Poor hive hygiene, environmental stress, and weak nutrition are key factors that accelerate the spread of these diseases. Early diagnosis and proper management of bee health issues are crucial for preventing colony collapse.
Varroa Disease
The Varroa destructor mite is the most devastating external parasite threat to bee colonies worldwide. This parasite feeds on the hemolyph (blood) and fat bodies of both adult bees and developing brood (pupae and larvae). Beyond directly weakening the colony, Varroa mites are carriers of many dangerous viruses. This puts the bees’ immune system under a two-fold assault. This parasite is one of the most challenging honey bee diseases to combat.
Symptoms and Effects on the Colony
The most obvious physical sign of a Varroa infestation is the presence of bees with deformed wings. These bees have wings that are missing or shriveled. This condition is a result of viruses transmitted by Varroa, particularly the Deformed Wing Virus (DWV). As the mite load increases, dead larvae or pupae may be seen in the brood cells. This is referred to as “parasitic brood syndrome.” A general restlessness and a decline in population below seasonal norms are observed in the colony. The lifespan of the bees is significantly shortened. A heavy infestation can completely destroy the colony’s ability to overwinter.
Causes and Transmission
The direct cause of the disease is the external parasite Varroa destructor. The reproductive cycle of these mites is synchronized with the honey bee brood cycle. The female mite enters the cell just before it is capped. The mites multiply while feeding on the developing bee pupa. Transmission usually occurs through the drifting of bees between hives. Additionally, the robbing of weak and infested hives by strong colonies causes the parasites to spread rapidly. The beekeeper can also unknowingly facilitate transmission by transferring contaminated combs or equipment.
Control Methods and Prevention
Controlling Varroa is based on Integrated Pest Management (IPM) principles. This involves using biotechnical and cultural methods together rather than relying on a single method. Biotechnical methods include the periodic culling and destruction of drone brood. Mites prefer drone cells for reproduction. This practice mechanically reduces the mite population. Regular monitoring of mite levels (using the powdered sugar test or sticky boards) is essential for determining the right time to intervene.
Nosema Disease
Nosema is a disease caused by a microscopic fungus (Microsporidia) that infects the digestive system of adult honey bees. The disease specifically targets the bee’s midgut (stomach). Two common species are known: Nosema apis and Nosema ceranae. This pathogen severely impairs the bee’s ability to absorb nutrients. Infected bees cannot feed themselves properly. Their lifespan shortens, and they become unable to perform essential tasks like brood care.
Symptoms and Diagnosis
The externally observable symptoms of Nosema are often vague. In the presence of Nosema apis, fecal staining (dysentery) may be seen on the hive entrance and frames, especially in spring. However, Nosema ceranae usually does not show this fecal symptom. The most common symptoms are bees crawling, trembling, or unable to fly at the hive entrance. The colony weakens. A definitive diagnosis is only possible in a laboratory setting. It requires microscopic examination of the intestinal contents of adult bees and a spore count.
Causes and Transmission Process
The fungal spores that cause the disease germinate when they enter the bees’ digestive system. They attack the cells of the intestinal wall. Transmission occurs via the fecal-oral route. Cleaner bees in the hive contract the disease while cleaning combs contaminated with spores left by infected bees. Additionally, contaminated water sources or shared feeding stations can also spread the spores between colonies. The spores are highly resistant to environmental conditions.
Treatment and Prevention
Preventive practices are paramount in Nosema management. Hives must be well-ventilated. Reducing moisture inside the hive makes it harder for spores to germinate. Bees should be provided with a clean, fresh water source. This prevents them from taking water from contaminated puddles. Nutritional stress increases the risk for these types of honey bee diseases. It is important that colonies have access to strong pollen and nectar sources or are supported with quality supplemental feed during scarce periods. Disinfecting infected equipment prevents the spores from being carried over to the next season.
American Foulbrood (AFB)
American Foulbrood (AFB) is caused by the spore-forming bacterium Paenibacillus larvae. It is known as the most dangerous and contagious brood disease affecting honey bee larvae. This disease specifically targets larvae in the sealed (capped) brood stage. The bacterial spores are extremely resilient to environmental conditions. They can remain viable in comb, honey, and hive equipment for decades, even more than 40 years. It is one of the most feared honey bee diseases.
Symptoms and Diagnosis
The most typical symptom of AFB is an irregular brood pattern. Normally, the capped brood area appears compact. In a hive with AFB, a “shotgun” or “spotty” appearance dominates. The caps of the sealed brood cells are sunken, darkened, and often perforated. The infected larva melts within the cell. It collapses into a brown, sticky mass at the bottom. If a matchstick is inserted into this mass and withdrawn, it creates a ropy (mucoid) structure that stretches about 2-3 cm. This is known as the “matchstick test.” A very distinct, sharp odor reminiscent of glue is also noticeable in the diseased hive.
The Pathogen and Spread
The cause of the disease is the spores of the Paenibacillus larvae bacterium. Larvae become infected when they consume food (royal jelly or pollen) contaminated with spores. The bacteria multiply rapidly in the larva’s digestive system. After the larva dies, it reverts to its spore form. Spreading occurs through the transfer of beekeeping equipment and the robbing of weak hives. The drifting of bees between hives and the use of contaminated honey also cause it to spread. This is among the fastest-spreading honey bee diseases.
Control and Prevention
American Foulbrood is one of the most difficult diseases to manage due to the extraordinary resilience of its spores. When the disease is diagnosed, it is often necessary to destroy the infected colony and all its materials. The combs and frames must be burned. The hive body must be cleaned by scorching (disinfecting with flame). The reason for this radical approach is to prevent the spores from permanently inhabiting the hive. Prevention is possible by strictly adhering to hygiene rules in the apiary.
European Foulbrood (EFB)
European Foulbrood (EFB) is a brood disease usually caused by the bacterium Melissococcus plutonius, although other secondary bacteria can complicate the picture. EFB is generally less severe than American Foulbrood (AFB). However, it can cause serious brood losses, especially in colonies under stress or during periods of poor nectar flow. This disease affects larvae at an earlier stage than AFB, usually before the comb cells are capped. While less severe, it is still a significant concern among honey bee diseases.
Symptoms and Differences
The main symptom of EFB is the abnormal appearance of young larvae in uncapped cells. Healthy larvae are pearly white and curled in a C-shape, whereas EFB-infected larvae turn a yellowish-brown color. They are found in abnormal positions (twisted, melted) within the cell. The dead larvae are generally not sticky. The ropy consistency (matchstick test) seen in AFB is not present in EFB. The odor varies; sometimes a sour, vinegar-like smell can be detected. The dried larval remains (scales) can be easily removed from the cell. In contrast, AFB scales adhere tightly to the cell.
Causes and Pathogens
The primary pathogen of the disease is Melissococcus plutonius. Larvae become infected when they consume food contaminated with this bacterium. The bacteria multiply in the larva’s midgut. It competes with the larva for food, causing it to die of starvation. The severity of the disease is closely related to the overall health of the colony. The genetic predisposition of the bees and the current nectar flow (nutritional status) are also important. A strong nectar flow can sometimes mask the symptoms of the disease.
Treatment and Prevention
The management of European Foulbrood depends on the strength of the colony. Mild infections may resolve on their own with the start of a strong nectar flow. One effective intervention a beekeeper can perform is the “shook swarm” method. In this method, the bees are transferred from their existing combs to a hive with new, clean foundation. This allows them to get rid of contaminated food and combs. Requeening creates a break in the brood cycle. This gives the cleaner bees time to remove the infected larvae. This method is a valid hygiene protocol for many hive problems.
Viral Diseases
Honey bee viruses are insidious threats that usually emerge in conjunction with other stressors or parasites (especially Varroa). A virus alone may not show clear symptoms in a colony. However, when the bee’s immune system is weakened by parasites like the Varroa mite, these viruses replicate rapidly. They become a serious danger to colony health. These types of honey bee diseases reduce the colony’s overall resilience.
Common Bee Viruses
More than twenty different viruses that affect honey bee colonies have been identified. The most common among these is the Deformed Wing Virus (DWV). DWV is transmitted almost exclusively by Varroa mites. It causes the bees’ wings to develop improperly. Other significant viruses include Sacbrood Virus (SBV), which causes larvae to die within a fluid-filled sac. Also common are Chronic Bee Paralysis Virus (CBPV) and Israeli Acute Paralysis Virus (IAPV), which is associated with sudden colony losses.
Symptoms and Colony Effects
The symptoms of viruses vary greatly depending on the type of causal virus. The most obvious sign of DWV is bees crawling at the hive entrance. These bees have wings that are small, crumpled, or completely undeveloped. They cannot fly and cannot contribute to the colony. Sacbrood virus, on the other hand, affects the brood area. Larvae die and take on the shape of a dark “sac” with the head pointing upward in the cell. Chronic paralysis virus can cause bees to tremble, lose their hair, and take on a shiny, “greasy” appearance.
Control and Prevention
There is no direct treatment for honey bee viruses. The cornerstone of control is the effective management of Varroa mites, the main vector for these viruses. When the mite population is kept low, the spread and impact of viruses in the colony are dramatically reduced. Keeping colonies strong and well-fed increases their resistance. Working with queen bees from genetic lines known for hygienic behavior also raises the overall resilience against the effects of viruses. Managing this parasite is the best defense against these associated honey bee diseases.
Chalkbrood
Chalkbrood is a brood disease caused by the fungus Ascosphaera apis. It specifically affects honey bee larvae. This fungus invades the larvae’s bodies, causing them to harden. The larvae take on a chalk-like appearance. It is usually seen when the colony is under stress. These honey bee diseases often appear during damp and cold spring months when hive temperatures drop.
Symptoms and Appearance
The most distinct sign of the disease is hardened, mummified larvae. These larvae are found inside the cells or at the hive entrance. Their color can be white or grayish-black. These “chalkbrood mummies” are loosely attached in the cells. They may rattle when the comb is shaken. Worker bees recognize these mummified larvae and try to remove them from the hive. Seeing these mummies on the hive landing board is a strong indication of the disease’s presence. The disease usually begins at the outer edges of the brood nest.
The Causal Fungus
The disease is caused by the Ascosphaera apis fungus. Larvae become infected when they consume food containing the fungal spores. The spores germinate in the larva’s gut. They penetrate the gut wall and spread rapidly throughout the larva’s body, causing its death. The mummies formed by the fungus start white. If different mating types come together, reproductive bodies (spore sacs) develop. In this case, the mummies turn a dark gray or black. These dark-colored mummies contain millions of new spores that can spread the disease.
Control Methods
Management of chalkbrood focuses on improving the hive’s microclimate. Good ventilation of the hive is important. Expelling excess moisture from inside inhibits fungal growth. The colony needs to be kept warm. Therefore, uniting weak colonies or moving them to a smaller, more compact space may be beneficial. Genetic predisposition also plays a role. Some bee strains are more resistant to chalkbrood due to their hygienic behavior (quickly detecting and removing diseased larvae). Requeening with such a resistant strain can help solve the problem.
Stonebrood
Stonebrood is a rare fungal infection caused by fungi of the Aspergillus genus, particularly *Aspergillus flavus* and *Aspergillus fumigatus*. The disease can affect both larvae and adult bees. Unlike chalkbrood, stonebrood causes the larvae to turn into a very hard, stone-like structure. These fungal species can also be dangerous for other animals and even humans (especially immunocompromised individuals). While rare, it is one of the more unique honey bee diseases.
Symptoms and Causes
Infected larvae harden and turn a yellowish-green or brown color. They acquire a much harder structure than chalkbrood mummies. The larvae cannot be easily removed from the comb cell. Adult bees can also be infected. In this case, the bees’ bodies harden, and they lose their mobility. The disease usually occurs when bees are exposed to a high concentration of fungal spores. This often results from unsuitable food sources in the hive, such as moldy pollen or damp syrup.
Fungal Development Process
Aspergillus spores begin the infection when they are inhaled or ingested with food by the bees. The fungus grows rapidly (forms mycelium) in the bee’s body tissues. It invades the tissues, causing them to harden. Some species, like *Aspergillus flavus*, can produce potent mycotoxins such as aflatoxin. These toxins increase the severity of the disease and can make contaminated honey dangerous to consume. However, this is a very rare occurrence.
Prevention and Treatment
There is no specific treatment for stonebrood. Prevention relies on apiary hygiene. Hives must be kept dry and well-ventilated. Bees should never be given moldy or spoiled syrup or pollen. As with other hive problems, hygiene is critical here. Combs showing signs of the disease should be immediately removed from the hive and destroyed. Caution should be exercised when working with suspicious materials (e.g.,
wearing a dust mask).
Bee Paralysis Disease
Bee paralysis is a disease usually caused by various viruses, primarily the Chronic Bee Paralysis Virus (CBPV). It affects the nervous system of adult honey bees. This condition causes bees to lose their normal motor control, tremble, and exhibit abnormal behaviors in the hive. It can lead to serious bee losses, causing epidemics, especially in densely populated colonies or during certain seasons. This virus is categorized among the challenging honey bee diseases to manage.
Symptoms and Viral Effect
There are two main symptom forms of bee paralysis. In the condition known as Type 1 (trembling form), bees exhibit uncontrolled trembling of the wings and body. They are unable to fly and cluster at the hive entrance or on the comb. Their abdomens may be swollen. In the Type 2 form, bees lose their hair. This gives them a dark, shiny, and “greasy” appearance. These “black” bees are perceived as strangers by healthy hive workers. They are often attacked at the hive entrance or cast out.
Transmission Routes
The virus is primarily transmitted through direct contact between infected and healthy bees. Increased hive density accelerates the spread of the virus. The risk of transmission is especially high during periods when the nectar flow has stopped, and bees spend more time inside the hive. It is also thought that the virus can be spread through the feces or secretions of infected bees. The role of Varroa mites in the transmission of this virus is not as prominent as it is for some other viruses.
Control and Prevention
As with most viral diseases, there is no direct chemical treatment for bee paralysis. Control relies on preventive measures and colony management techniques. Reducing stress in the colony is critical. This is possible by providing an adequate food supply and optimizing hive density (splitting the hive if necessary). Working with colonies that have genetic lines (queen bees) known to be resistant to the disease can reduce its impact. This improves the colony’s overall ability to cope with many honey bee diseases.
