Although chalk brood has a global distribution, little is
Culturing A. apis
known about Australian strains of A. apis and its virulence toward honey bees. We are working to rectify that! Here are some pictures of culturing A. apis from chalk brood mummies last month.
Ascosphaera apis is the fungus that causes chalkbrood disease in honey bees. The spores of this fungus are extremely hardy and can stick around in honey, wax, pollen, bee bread and the hive architecture for up to 15 years! Bee larvae are fed the spores at about 3rd instar and if the conditions are right in the larval gut, the spores dormant and begin to grow hyphae. These hyphae puncture the larval gut lining and eventually the larval cuticle. The hyphae then grow all over the outside of the larva causing death about 3-5 days later during the 5th install- just as the larva is being capped over,
A. apis in culture
elongating in the sell and preparing to pupate.
If a larva succumbs to chalk brood infection the dead body will dry up and become a chalk-like white mummy in the brood cell. This type of fungus needs two mating types to reproduce. When hyphae of opposite mating types come onto contact, ascocarps are formed which is essentially a big spore cyst that contains smaller balls that contain spores. These ascocarps turn the mummy black and when they rupture distribute spores through the colony further spreading disease.
Printed in the Australian Bee Journal December 2014
Chalkbrood Mummies
For those of you just joining in, this column is about on honey bee self defence traits and what beekeepers can do to help optimise honey bee health. The first three articles have focused on the benefits of hygienic behaviour and how beekeepers can incorporate this important genetic bee self defense trait into their operations through testing and breeding for this trait.
Please take note, however, that even the best of genetics cannot entirely prevent the spread of diseases
brought on by environmental stresses, poor nutrition, poor beekeeper management, or a combination of these. In fact, even the most hygienic of colonies can be susceptible disease when stressed.
Transmission
Chalkbrood is caused by the fungal pathogen Ascophaera apis and can be transmitted by spores found on pollen, in honey, and moved about the colony by house bees. Up to about the 3rd instar, larvae are fed royal then worker jelly from the brood food glands located in nurse bees’ heads. After the 3rd instar, the larvae begin to be fed small amounts of pollen and honey potentially containing chalkbrood spores. Adult bees or pupae are not susceptible to infection but can infect larvae with spores. Beekeepers can also spread the disease by moving infected equipment between colonies.
The infection begins in the gut of the larva and slowly grows out until it penetrates the cuticle, which happens during the 5th instar. Larvae are capped over during the 5th instar as they are preparing to spin a cocoon for pupation. A larva dying from chalkbrood becomes mummified and appears white. However,
Pupae that have died of chalk brood infection in the cells
once the transmittable spores are formed, the mummy becomes grey or black. Chalkbrood kills the larva just as it is being capped over, commonly the cells containing chalkbrood mummies will appear perforated having a hole in the center- similar to AFB. Viable spores have been cultured from honey after two years of storage and can remain viable for 15 years. More importantly, spores can remain viable in bee bread for up to 12 months.
Resistance
If chalkbrood spores are so abundant, and can be transmitted so easily, why isn’t the infection more rampant? It turns out that honey bees have a number of ways of resisting infection.
Individual Immune Response
Healthy bees, larvae included have immune systems that can fight pathogens including the fungus A. apis. Similar to our white blood cells haemocytes in honey bees can directly kill the invading pathogen. These systems, however will only function properly when the nutritional needs of the larvae have been met.
Social Immunity
Chalk brood can still exist in a weakened hygienic colony
Hygienic Behaviour
As we have already covered, hygienic behaviour is a trait whereby house bees can smell dead or diseased brood and remove it from the colony before the pathogen becomes infectious.
When a larva is in the early stages of succumbing to a chalkbrood infection prior to mummification and spore formation, it releases three volatile compounds (smells) that are different than the smell of a healthy developing larva.
Of these compounds, one specifically, phenethyl acetate, was shown to illicit a hygienic response from nurse bees. When placed on healthy non-infected brood, hygienic nurse bees removed the brood from the colony. The key message here is that the infected pupa was removed from the colony prior to spore formation.
Fever Induction
A. Apis is quite sensitive to temperature, thriving at lower nest temperatures (30 C) and dying at elevated nest temperatures (35.5 C). It has been observed that colonies newly infected with an A. apis infection can elevate the temperature of the brood nest inducing a colony fever and preventing the spread of infection.
Other Biotic Factors
Recent research conducted on bee gut microflora suggests that several types of beneficial bacterial also have an antagonistic effect on the development and sporation of A. apis.
Predisposing Conditions
However, despite the bee’s best self defense strategies, colonies can still be decimated by chalkbrood and it may be directly related to common beekeeping practices. A. apis is an opportunistic pathogen that kills larvae only when subjected to predisposing conditions.
An infected larva can generally prevent a chalkbrood spore from germinating causing infection and eventually death, but if the larvae is chilled as little as 5 C for 6-12 hours (ideal brood nest temp is approximately 35 C) while in the fifth instar, just capped over and beginning to extend in the cell, the fungal infection can rapidly spread and kill the pupa.
Chalk brood outbreak after making a weak split
Although this type of chilling is unlikely to occur in a healthy, functioning, stationary hives because the bees can regulate the temperature of the nest, common beekeeping practices can influence the otherwise stable nest environment. For example, reducing the number of adult bees by making splits or changing the arrangement of the brood frames within the nest or shifting bees creating drafts through the colony could result in the critical cooling episode necessary for A. apis infection to take hold.
Remembering that spores can remain viable in bee bread for 12 months and are present in wax and honey, subjecting a colony to a cooling event may trigger an outbreak. As more and more larvae succumb to the infection, the spore count in the colony increases hastening the spread of the disease to other larvae.
Management through Prevention
Since no chemical treatments, natural or synthetic have been shown to be an effective treatment against chalkbrood, it is up to beekeepers to be diligent about reducing modes and methods of transmission and keep that brood nest warm.
• Resist moving infected frames between colonies, spreading spores around your apiary
• If equipment is highly infected, get it irradiated before giving it back to your bees
• Keep your bees well fed, warm, and dry
• Be conscientious when making splits or nucs that enough bees are left in both the old and new colonies to keep the brood warm
• Don’t inspect brood on cold or windy days
• Keep just the right amount of space on the colony- not too many boxes
• Requeen with hygienic stock
But remember even the most hygienic of bees will not be able to overcome a severe chalkbrood infestation brought on by poor management.
And the jury is still out about the bananas…..
References
Aronstein, K. A., and K. D. Murray. “Chalkbrood disease in honey bees.” Journal of invertebrate pathology 103 (2010): S20-S29.
Flores, J. M., et al. “Effect of temperature and humidity of sealed brood on chalkbrood development under controlled conditions.” Apidologie 27 (1996): 185-1922.
Omar, Mohamed OM, et al. “Antagonistic Effect of Gut Bacteria in the Hybrid Carniolan Honey Bee, Apis Mellifera Carnica, Against Ascosphaera Apis, the Causal Organism of Chalkbrood Disease.” Journal of Apicultural Science 58.1 (2014): 17-27.
Starks, Philip T., Caroline A. Blackie, and Thomas D. Seeley. “Fever in honeybee colonies.” Naturwissenschaften 87.5 (2000): 229-231.
Swanson, Jodi AI, et al. “Odorants that induce hygienic behavior in honeybees: identification of volatile compounds in chalkbrood-infected honeybee larvae.” Journal of chemical ecology 35.9 (2009): 1108-1116.
