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Ascosphaera apis

 

Fig 1: Brood killed by chalkbrood: white and black mummies. (Honey bee diseases and pests: a practical guide -FAO)

Chalkbrood disease (Ascosphaerosis) In Asia, chalkbrood is rarely considered to be a serious honey bee disease, although in Japan the disease has been reported to cause problems to beekeepers. In temperate America and Europe, however, cases have occurred in which chalkbrood has caused serious damage to beekeeping; therefore, Asian beekeepers should be aware of this problem.


Cause

Chalkbrood is a disease caused by the fungus Ascosphaera apis. As its name implies, it affects honey bee brood. This fungus only forms spores during sexual reproduction. Infection by spores of the fungus is usually observed in larvae that is three to four days old. The spores are absorbed either via food or the body surface.


Symptoms

Initially, the dead larvae swell to the size of the cell and are covered with the whitish mycelia of the fungus. Subsequently, the dead larvae mummify, harden, shrink and appear chalklike. The colour of the dead larvae varies with the stage of growth of the mycelia: first white, then grey and finally, when the fruiting bodies are formed, black (see figure 1). When infestation is heavy, much of the sealed brood dies and dries out within their cells. When such combs are shaken the mummified larvae make a rattling sound. In the laboratory the fungus can be identified by its morphology (see OIE Manual of Diagnostics, 2004).


Control

As with other brood diseases, the bees remove the infested brood with their hygiene behaviour (see European foulbrood), which is especially effective for white mummies. Though as soon as the fruit bodies of A. apis have developed, cleaning honey bees spread the spores within the colony by this behaviour. During the white mummy stage the fungus continues to develop at the hive bottom. If the mummies are not removed quickly, the spores may enter the brood cells carried there by circulating air.

The beekeeper can stimulate the hygiene behaviour of the bees by changing the brood- rearing conditions. In this respect, it is most important to adapt the size of the hive to the strength of the bee colony. In this way the bees have a chance to inspect and clean the many brood cells.

Therefore, in most cases, the method of stimulating hygiene behaviour, already described under European foulbrood control, is sufficient for chalkbrood control. The beekeeper should ensure that the colony has a strong worker population, and that the hive is well ventilated and free from accumulated moisture. At early stages of chalkbrood infection, adding young adult workers and hatching brood, combined with sugar-syrup feeding, often proves to be helpful.

Currently there is no known successful chemical control against chalkbrood. It means that chemical treatment shows a little effect to control chalkbrood. In most cases, commercialised substances only show a positive effect because they are sprayed, or fed with sugar water as described above.