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Hive Medical Spotlight: Chalkbrood

This week on our Hive Medical Spotlight we're discussing the fungal infections of Ascophaera apis, or the dreaded Chalkbrood. This disease infects Honeybee larvae and sometimes the pupae as well, and infects every caste in the hive, from worker to Queen. This is often considered a minor disease in an Apiary, but amongst Hobbyists, it brings dread and fear.

It does have the potential to cause serious economic damage if conditions are right for this fungus to gain a large toe hold in the hive, but it is easy to identify. Infection of this disease begins when the larvae are fed sexually produced spores or ascospores from Nurse Bees who may acquire it from stored pollen. They may also pick it up from cleaning the hive and just stepping in it. After a cell is cleared some spores may remain in the cell to infect the next larvae laid.

Beekeepers must be careful when dealing with Chalkbrood as the spores can be transferred when working hives through bees entering the wrong hive and frames being interchanged. Most commonly larvae as old as 1-5.5 days become infected with 3-4 day old larvae being the most susceptible. After infection the fungus by some yet understood means completely overcomes the larvae and encases it and dries it out leaving what we call "mummies". It takes roughly 2-3 days around the time of the cell capping for the fungus to mummify the larvae.

Chalkbrood by researchers and bee experts is believed to be a stress related disease as the spores may live in the hive without obvious signs until conditions are right for the fungus to emerge. The accepted parameters within the host that show infection are believed to be either an increase in CO2 concentrations, decrease in O2 concentration and a relative temperature of 31-35 degrees celsius or 87.8-95 degrees Fahrenheit. Cooler temperatures in the hive then promote the progression of the disease with temperatures of 25 degrees celsius/ 77 degrees Fahrenheit and high humidity promote the most mummification. Therefore the disease in general is promoted by weak colonies, excessive hive moisture combined with cool, wet weather and poor foraging conditions, other stressors may play a factor owing to a weak colony.

Chalkbrood spores may persist in the hives for years and remain viable for the right conditions. Reproduction of this fungus is sexual and sporulation occurs on the mycelium when the hypae of the opposite sex/mating type (thallus) come together. Both types infect and kill the larvae. Sporulated mummies are covered with many cysts, 100-300 million spores. The wall of cysts is thin and easily ruptured when mature and will open to release spore balls. The spores then break apart from the ball. The whitish mummies represent larvae that have been infected by only one sex type.

Chalkbrood is easily identified by larval mummies littering the hive bottom, the landing board or in front of the hive. You may even find them in the frames in a spotty brood pattern and even some in pinhole opened cells. The mummies can be white, gray or black in color.

The dark color of the cysts gives sporulated mummies their black and grey appearances while white mummies lack such spore producing bodies on the mycelia. In the cell the anterior part of the bee that faces up is dry and pale yellow in appearance. Bees detect dead larvae and chew small holes in the cappings. This may also help the beekeeper identify the disease and the undertaker bees may clean the cells by being able to identify them by walking over them.

A heavily infected hive may have as many as 300-1000 dead larvae on the floor of the hive. If one frame were to be inspected and as many as 10 were found, the diagnosis would be a mild or low severity.

Control of this disease is usually an integrated pest management strategy. Beekeepers should choose good Queen stock for their hives that show a good hygienic behavioral trait. Good housekeeping is one key to helping manage this disease and keep it from never appearing. Colonies should also be well maintained and provided with sufficient resources during dearths in nectar and pollen. While excessive moisture may play a role, a dry hive is not always a guarantee to preventing Chalkbrood. Using pollen collected from other hives may further spread or inflame this disease in a hive unless already irradiated. A pollen substitute is in the Beekeeper's best interest.

Comb should be replaced after the effects break to reduce the spore load in the hive, care should be taken not to inadvertently transfer the spores to other hives.

To date there is no registered product for use against Chalkbrood. Most notably, beekeepers in the US find this disease uncommon, it is most prevalent in countries such as Denmark, Norway, Portugal and Romania.

Treatments often include requeening with a hygienic queen. Replacing combs when applicable and disposing of them accordingly, also being mindful not to transmit this fungus to other hives by working multiple hives without having first cleaned and sterilizing equipment, including hive tools, gloves and suits.

It is further advised that overly infected hives should be neutralized by sterilization procedures. Killing of the infected hive, disposing of the frames and torch scorching the equipment. Increased air flow can help limit the disease and return the hive back to a state of equilibrium, especially when paired with sugar syrup feeding and substitute pollen feeding.

Research in 1989 showed that Winter Savory (Satureja montana) Essential Oil is effective against Chalkbrood at a ratio of 0.01% in a sugar candy board. (Colin et al., 1989)

It must be noted however for the do it yourselfer that the "Banana" theory does not work out as reported by many beekeepers who have tried it and failed, others who have claimed success have not conducted their "experiment" within the parameters of a thorough scientific process to conclude the viability of such a practice. While ideas do have merit in the beginning, they must be tested with strict testing protocols to determine their validity. Beekeepers that have tried it and failed have concluded this on their own that the experiment did not take into account the effects of weather, the addition or removal of food and other parameters they did not seek to control that could have a consequence upon the experiments outcome, please do not trust such advice if you can not determine the validity of anything. Ask 10 beekeepers a question and you'll get 11 different answers, always do your research thoroughly and consult scientific articles and other sources.

This information was compiled from research from two different sources.

Honeybee Veterinary Medicine: Apis mellifera L. Nicolas Vidal-Naquet, 2018

Honeybee Diseases and Pests, Third Edition, Canadian Association of Professional Apiculturists, 2015.

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