How Diseases Spread inside Honey Bee Colonies
Honey bee diseases are numerous, varying from chemically treatable conditions to conditions requiring immediate colony euthanasia. Not all diseases are encountered during foraging; some are transmitted by pests, with Varroa mites being the most prominent disease vector for viral infections.
This article expands on the pests and diseases faced by honey bee colonies, with a focus on the European honey bee Apis mellifera. An overview on the diseases affecting honey bee larvae and adult bees will be covered, as well as their potential treatments. The bottom line in disease management for beekeepers is to maintain healthy hives and practice good beekeeping methods that reduce colony stress.
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Diseases From Varroa Mites
“Varroa mites are the biggest threat to bee populations worldwide” and variations of this have been stated all over the media. Mites feed on the fat bodies of the developing bee larva, weakening and (very likely) infecting it with pathogens throughout the feeding process. The emerging adult bee exhibits stunned growth, body deformation and a shortened lifespan, with severe infestation leading to colony collapse.
Physically, an adult Varroa mite has a flattened, oval-shaped body, 1.5-2 mm wide and 1-1.8 mm long. Their colour varies from a light brown to a darker, reddish-brown tint. Female mites are commonly seen, as male mites spend their entire lifecycle inside brood cells. Despite their small size, mites are mobile, attaching themselves onto the adult bees and moving between honey bee colonies.
Varroa destructor is the parasitic mite of A. mellifera, whereas V. jacobsoni is an endemic pest of the Asian honey bee, A. cerana. Mite lifecycle, associated diseases, identification and treatment methods are covered in bee mites.
Varroa Mites as a Disease Vector
There are no current treatments available for the viral diseases strongly associated with Varroa mite infestations. These diseases are transmitted to the developing larvae during the feeding process, and their symptoms manifesting in the adult bees when the colony is exposed to stressors that allow disease expression (nutrition, stress, pesticides etc…). Examples include:
Acute bee paralysis virus (ABPV): Causes trembling, paralysis and death in the infected bees and can rapidly spread inside the hive
The Israel acute paralysis virus (IAPV) has similar clinical infestations, however the onset and severity of symptoms is greater than ABPV.
IAPV has been linked with colony collapse disorder, being more closely linked to the colony collapse disorder than APBV.
Deformed wing virus (DWV) – the most common virus associated with varroa mite infestations, producing adult bees with visibly deformed wings and shortened lifespans.
Chronic bee paralysis virus (CBPV): Affect strong colonies, simultaneously presenting infections with two different syndromes: paralysis, trembling and crawling bees, as well as hairless, black bees, with shortened abdomens.
The hairless black bees arise due to a combination of progressive abdominal darkening in diseased bees and attacks by other bees. These bees use their mandibles to cut hairs off the afflicted bee in an attempt to get rid of the infection, causing the appearance of hairless black abdomens (De Miranda et al., 2010).
Kashmir Bee Virus (KBV): This virus is similar to ABPV, and affects the motor coordination of honey bees, leading to fatal paralysis. Affected bees show no visible sign of infection before rapidly succumbing to paralysis and death.
Slow Paralysis Virus (SPV): As the name suggests, infected adult bees experience progressive paralysis until death. The flight muscles tend to contract uncontrollably, leading to a “shivering wing” effect that renders them unable to fly.
Black Queen Cell Virus (BQCV): This virus affects the developing queen larvae, darkening and elongating her cell, ultimately resulting in death. This is the most common cause of queen larva death in Australia (Benjeddou et al., 2001)
Parasitic Mite Syndrome in Severe Infestations
Parasitic mite syndrome refers to a collection of symptoms that describe honey bee colonies with high levels of Varroa mite infestation. The symptoms vary between colonies and include irregular brood patterns, deformed or underdeveloped bees, increased disease prevalence, poor overwintering and eventual colony loss. Varroa population control measures are the treatment, with the goal to keep mite populations low to prevent this fatal condition.
Tracheal mites (Acarapis woodi)
Tracheal mites infest the breathing tubes of adult honey bees. Unlike Varroa, which lays its eggs in bee brood, A. woodi mites feed in the tracheal system and damage respiratory tissue. Symptoms include disorientation, weakness, and difficulty flying, progressing to severely weakened bees crawling near the hive, unable to fly.
A tracheal mite infestation causes Acarine disease, and can lead to colony collapse if uncontrolled. A. woodi treatments include miticides, menthol crystal vaporization, grease patties or the stocking of resistant bee strains (eg Russian & Buckfast queen bee strains).
Grease patties, made from sugar and solid vegetable oil (eg Crisco) can disrupt the activity of female mites as they search for a new home, making them an effective, non-chemical method for reducing A. woodi infections during the brood rearing season.
American foulbrood (Paenibacillus larvae)
American foulbrood is a disease affecting brood, with infected larvae dying soon after their cells are capped. Formerly classified as Bacillus larvae, P. larvae is a spore-forming bacterium, which enables it to withstand harsh environmental conditions and remain viable for up to 30 years. It has a complex lifecycle that commences with ingestion by the bee larvae. The bacteria multiplies inside the larval gut, eventually killing it as it continues to grow and reproduce, culminating in the production of billions of new spores when it consumes the now-dead bee’s body.
What remains of the infected larvae is a sticky, gooey mass that dries into a hard scale. Physically, the brood cell appears punctured or sunken, with an irregular pattern across the comb due to a mix of infected and uninfected cells. The infamous “ropiness” appearance is evident when the beekeeper conducts a matchstick test; the sticky mass can be drawn several centimetres outwards with a matchstick. As the number of dead larvae increase, it produces the distinct foul smell that gives the disease its name, “foulbrood”, caused by the production of ketone and sulfur-containing compounds.
American foulbrood may require a laboratory test to confirm its diagnosis since symptoms manifests similarly to European foulbrood. Once identified, is a notifiable disease in many countries, and its outbreaks must be reported agricultural or apiary authorities.
There are no effective treatments1 for AFB due to the tenacity of its spores. In many regions, there is a legal requirement to destroy infected hives. Beekeepers may do this by incinerating the hive and its contents, or by solely killing bees through methods like petrol fumigation. If the beekeeper opts for the latter, they can reuse infected equipment after irradiating it by a specialist vendor.
European Foulbrood Disease (Melissococcus plutonius)
Manifesting with similar symptoms to AFB is is the European Foulbrood disease, EFB. EFB is a brood disease caused by the coccus-shaped bacterium Melissococcus plutonius, whose lifecycle is similar to AFB, as the bacteria also multiply inside the gut of the infected larvae. The bacteria out-competes the larva for food, resulting in starvation.
Infected larvae have the characteristic upward-curved “C” shape, changing colour as the infection progresses. Healthy larvae are white in colour, then turning into a yellowish-white shade, and brown upon death. As the larva drys out, it turns into a brown scale.
EFB infections can be treated using antibiotics like oxytetracycline and as with any bee disease, prevention is the best cure- keeping strong honey bee colonies is the first line of defence against EFB. Other methods include requeening using disease-resistant stock, or by using the shook-swarm method, where bees are shook into a new, clean hive with (ideally) drawn frames.
How does EFB differ from AFB?
There are several distinguishing factors between AFB and EFB:
Unlike P. larvae, M. plutonius does not form spores, which can make EFB easier to treat and manage.
The spores are highly resistant to environmental pressures and chemical treatments, enabling P. larvae to remain viable for decades in beekeeping equipment and hive products.
EFB cause death by starvation whereas AFB consumes infected larva.
Both bacteria multiply in the gut and their effects manifest in sealed brood cells.
EFB can weaken colonies, not necessarily killing them outright, whereas AFB readily leads to total colony collapse.
EFB does not have a particularly foul smell associated with it. Larvae in AFB-infested hives tend to disintegrate into an odorous gooey mass, whereas the larvae in EFB-infected hives dries and hardens into a brown scale.
Treatment for EFB include antibiotics and requeening/rehoming affected hives. AFB has no viable long-term treatment and destruction of infected honey bee hives is often mandatory to prevent disease spread.
Sacbrood Disease (Iflavirus)
Adapted from the parent article on sacbrood virus
Sacbrood is one of the most common viral brood diseases affecting honey bees. “Sacs” form on the dead larvae, which gives the disease its name. Infected larvae die in the prior to pupation, resulting in a stretched-out appearance within their cells as their heads are raised towards the opening. Initially, the larvae are pale and shiny in appearance, progressing to a dark brown mass that resembles water-filled sacs. The outer skin of the larvae hardens, forming a layer that holds the watery fluid containing new viral particles.
No treatment exists, and prevention is the best cure. Good apiary management techniques and keeping strong, hygienic colonies with a balanced diet and minimal stress reduces the chance and impact of Sacbrood infections.
Chalkbrood Disease (Ascosphaera apis)
Another brood disease is chalkbrood, caused by the fungus Ascosphaera apis. Infected honey bee larva become hard and chalk-like with an oily feel after death. A. apis spores completely cover the larval body, leading to a black mass as the fruiting fungal bodies develop. A common disease characteristic is the appearance of “mummies” at the hive entrance; the result of worker bees removing dead brood from the infected cells.
Poor ventilation in damp, weak hives is the leading cause of this disease. Prevention methods include moving hives to a sunny position and adjusting the hive structure to improve airflow (such as using a screened base), or adding insulation to reduce water seepage. This brood disease can be naturally overcome by most colonies, and the beekeeper may assist them with supplemental feeding.
Nosemosis, the Disease Caused by Nosema sp.
Main article: Nosema disease
Nosemosis is a disease that affects the digestive tracts of honey bees caused by the microsporidia Nosema apis and Nosema ceranae. “Microsporidia” refers to a group of spore-forming, single-celled parasites that replicate in the gut of the host; they are considered to be in the same group as fungi.
N. apis is thought to have originated in European honey bees and is less harmful to them than N. ceranae, which appears to kill infected bees faster (possibly due to a recent species jump from its endemic host the Asian honey bee A. cerana). Infected bees have difficulty digesting food and a shorter lifespan.
A worker bee infected at a young age will skip the brood rearing stage of her life as she is unable to produce brood food & royal jelly secretions from her hypopharyngeal glands. Furthermore, the entire colony health is impacted if the queen succumbs to infection, as she will have a reduced lifespan and cease egg laying, ultimately resulting in colony death if uncontrolled.
There are no unique symptoms associated with nosemosis; other pests or pathogens also have similar effects on honey bee health. The symptoms of nosemosis are more apparent during times of poor weather and nutrition, including:
Dysentery, characterised by honey bees defecating on the outside of their hive.
Honey bees lethargically crawling at the hive entrance, with wings held at odd angles (“K wing”).
Bees also appear to tremble during severe infections.
General reductions in honey bee productivity, lifespan & overwintering ability
Interesting, the dysentery and crawling bees is not observed in N. ceranae infections. N. ceranae species are present with higher levels in warm climates and can cause colony losses year-round, unlike the the affinity of N. apis to surge during cooler conditions.
As the weather warms, bees are able to take more cleansing flights and gradually reduce the levels of Nosema in the hive, eventually overcoming the disease. Disease management is achieved through a focus on honey bee health, such as supplemental & non-invasive winter feeding, and reducing colony stress by approaches like minimising winter inspections.
Honey Bee Pests That Do not Cause Disease
Honey bee diseases and the pests that carry them were covered above. There are a few more hive pests that, although they do not cause disease, are worth mentioning as important stressors. Further reading is available within their respective articles.
Greater Wax Moth (Galleria mellonella) & Lesser Wax Moth (Achroia grisella)
Main article: Wax moths
It is the wax moth larvae, not the adult moths, that cause damage to honey bee colonies. Female wax moths enter hives and lay eggs within the dark nooks of the hive, which hatch into larvae. Wax moth larvae emergence depends on the temperature: between 3-5 days at 29-35°C, and up to 35 days at 18°C. The larvae begin to tunnel through the wooden frames and wax comb, causing the uncapping of brood cells, which can impact the adult lifespan of the pupating bee. The tunnels produced during burrowing are lined with silken webs and faeces.
The two wax moths G. mellonella & A. grisella commonly coexist and are differentiated on the basis of physical size. Both species have a similar grey colouration, with the greater wax moth being 13-19 mm long to the lesser’s 10-13 mm. The adult moths do not eat. Females live up to 12 days and males to 21 days, during which time they mate and restart the reproductive cycle.
Small Hive Beetles (Aethina tumida)
Maint article: “Small Hive Beetles“
It is easy to confuse small hive beetle larvae with that of the wax moths; they can be physically distinguished by the distinctive three pairs of legs at the front end of small hive beetle larvae. The burrowing, feeding and excreting actions of small hive beetle larvae cause a “slimeout”, in which the honey becomes fermented, runny, pungent and with a slime-y feel, contrasting the silk webbing trail left by the moth larvae.
Female adult beetles lay eggs in clusters of 10-30 eggs within a capped brood cell or the cracks and crevices inside a hive. Eggs take between 1-6 days to hatch, and are dependent on the temperature and relative humidity within the beehive (eggs are prone to desiccation if exposed to circulating air and relative humidity levels below 50%.)
The emerging larvae burrows through comb and wax, consuming the honey bees’ eggs, pollen and honey. The fermentation of honey is caused by the yeast Kodamaea ohmeri, found in the faeces of small hive beetle larvae. After feeding, the larvae leave the hive in search of a suitable place to pupate (predominately moist soils) and continue the reproductive lifecycle.
Braula Flies, Large Hive Beetles, Hornets and Other Honey Bees
Honourable mention to the Braula flies, which cause cosmetic damage to beeswax as they lay eggs. These eggs develop into larvae that burrow through the wax to feed on the stored honey and pollen. Found all over the honey-producing parts of the world, they are small wingless flies that are relatively harmless, and easily managed by healthy bees.
Large hive beetles are a scarab that infests honey bee colonies in Africa, and are 20-23 mm long (i.e. larger than an individual honey bee). They have an affinity for consuming bee brood, and its primary control measures are to reduce the hive entrance size to restrict its access.
Hornets, such as the Asian giant hornet Vespa Mandarina, are aggressive predators of honey bees whose larvae and adults feed on bees and can impact the productivity of the adult population by causing them to continually focus on defending against attacks.
The Western honey bee Apis mellifera has numerous subspecies with varying characteristics in fields such as aggression, reproduction or honey production. These include the South African Cape honey bees, whose female workers can lay “fertilized” female eggs (a complex manipulation of cell division, meiosis), and the highly aggressive Africanized honey bees. The presence of other honey bee species can lead to competition that results in a zero sum outcome for the colonies involved.
The Importance of a Healthy Honey Bee Colony
A common theme throughout the pests and diseases described above is the importance of colony health. When left unchecked, even apparently healthy colonies can die out when the infestation reaches critical mass, or from excess predation. Maintaining optimal colony conditions, good beekeeping management and minimizing stressors will greatly reduce the risks of contracting brood diseases and the impact of honey bee parasites.
Healthy colonies are not a silver bullet, and dead bees are still found in the strongest of colonies due to lethal diseases such as American foulbrood. In the management of Varroa mite infestations, the beekeeper play a bigger role by performing intensive hive management practices (eg: integrated pest management) in order to keep their colonies strong without relying on chemical treatments.
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***After seven months I spot the headline typo “Manging”… well done me – December 2023
Footnotes
- Antibiotic treatment of honey bee populations is problematic due to emergence of antibiotic-resistant strains and local regulations, which often require beekeepers to discontinue antibiotic treatment for some period of time prior to honey harvests. ↩︎