Lime brood is a disease of bee brood caused by a parasitic fungus that leads to larval death.

Lime brood is recorded and widespread on all continents where the Western honeybee occurs

The spores of the fungus Ascosphaera apis are widespread and get onto the combs, into the pollen and honey stores, and onto all surfaces of the hive

The spores are spread by the cleaning behavior and feeding behavior of the bees in the hive, by flying or predation, and by beekeeping activities

Patchy brood nest, lime brood mummies in brood cells. Self-healing of the colony is possible

There is no approved medication available in Austria. If the course of the disease is milder, it helps to remove the heavily infested brood combs; if all colonies of a stand are infested, a change to a drier, warmer location is recommended

Avoid cool and damp locations, use only chalkbrood-free colonies as breeding and maintenance colonies or for filling the mating boxes.

Lime brood occurs frequently in Austria's honey bee colonies. The occurrence of lime brood is not notifiable. Therefore, the frequency of occurrence is only surveyed on a random basis in the course of research projects. In 2015 to 2016, about 200 apiaries were checked for clinical symptoms of lime brood in the course of the project "Zukunft Biene". The frequency varied depending on the season. In summer 2015, lime brood symptoms were seen on 14% of the stands (95% confidence interval: 9.6-19.4%). In fall 2015, lime blight symptoms were seen on 4.2% (95% confidence interval: 1.9-7.7%) and spring 2016 on 10.4% of stands (95% confidence interval: 6.5-15.4%).

The spores of the parasitic fungus Ascosphaera apis germinate in the intestine of the bee larvae and grow through the entire body as mycelium (fungal threads), causing the brood to die. White or, in the case of sporulation, gray-black brood mummies develop. These can be found in the brood combs, on the hive floor or at the flight hole. The occurrence of chalk brood is promoted by hypothermia of the brood: Since the fungus has its growth optimum at about 30 °C, lower brood nest temperatures lead to increased occurrence of lime brood. This can be more pronounced in weak colonies, temperature setbacks or even in unfavorable (micro)climatic conditions.

Ascosphaera apis is a heterothallic fungus that forms fruiting bodies containing spores in large numbers when male and female hyphae meet. The spores get onto the combs, into the pollen and honey stores, and onto all surfaces of the hive. The spores are widely distributed.

Transmission in the colony: the spores are spread in the hive by the cleaning behavior and feeding behavior (trophallaxis) of the bees.

Transmission from colony to colony: transmission occurs through the bees themselves in case of flyaway or predation. However, transmission through beekeeping activities, such as the addition of brood combs with diseased brood, the addition of spore-contaminated combs, the use of contaminated equipment and hives, and the feeding of spore-contaminated honey or pollen, is also particularly important.

Symptomatology

Young brood maggots are particularly susceptible; after germination of the spores in the gut, the passage through the body tissue takes place within a few days. The outbreak of chalkbrood is manifested by a patchy brood nest (by clearing out diseased brood) or by the presence of chalkbrood mummies in uncovered or covered brood cells (see Fig. 1). They appear as white to grayish-black plugs that may initially appear cotton-wool-like, later sit loosely in cells, may be partially pulled out, and rattle when shaken. In chalk brood mummies, the maggot shape is visible - unlike pollen molds where the layered pollen layers can be seen. The mummies discharged by the cleaning bees can be found on the hive floor, on the flight board, or in front of the flight hole.

Initially, isolated larvae are infected by spores. Infected open brood is initially quickly removed by the bees. If the fungus succeeds in forming spores, it infects further bee larvae in increasingly rapid succession. As a result, colony strength decreases, cleaning activity decreases, and severe brood patchiness and the typical chalk brood mummies appear. This can lead to a lasting weakening of the colony, but rarely to its death. If the environmental conditions improve, the colony can heal itself and the symptoms subside.

Therapy

There is no approved drug available in Austria. If the disease is severe, a sweep swarm is indicated in combination with total renewal of the comb structure and transfer to a breeding line with good cleaning drive (lime brood resistance). If the disease is milder, it helps to remove the heavily infested brood combs and to confine the colonies. The combs must be melted down. If all colonies of a stand are infested or if there is repeated occurrence of the disease, it is recommended to change to a drier, warmer location and to transfer the colonies to another breeding line.

Prevention

Cool and damp locations should be avoided. In order to keep strong and vital colonies with good cleaning instincts, strict breeding selection and care in queen rearing is important (use only chalkbrood-free colonies as breeding and maintenance colonies or for filling the mating boxes). In addition, care should be taken to ensure young and efficient queens and a sufficient food supply, and Varroa infestation should be kept as low as possible. Honey or honeycombs from chalkbrood-infested colonies must not be fed. In spring and autumn, all measures that significantly affect the heat balance of the bee colonies should be avoided (e.g. too early setting up, moving brood combs into the poorly occupied honey chamber, too much bee removal for swarm formation, formation of brood trays without a sufficient number of care bees).