What is chalkbrood?

Chalkbrood (Ascosphaera apis) is a fungal disease affecting honey bee brood. As in AFB and sacbrood, the disease kills brood in the late larval (prepupal) stage, when the larva has laid out fully along the bottom wall of the cell. Unlike sacbrood, however, the symptoms of chalkbrood are quite distinct and much easier to differentiate from AFB.

The only symptom resembling AFB is the presence of perforated cappings, which are identical in appearance to both sacbrood and AFB, and as with these diseases the bees chew the cappings off when they identify a diseased larva beneath.

Difference to AFB symptoms

Chalkbrood infected larvae turn a vivid white that is much whiter and “chalky” in appearance than the colour of healthy brood (Fig. 46). At this stage they have a furry surface because of the fungal strands growing on the dead larvae.

Figure 46: Early stage chalkbrood infection
Figure 47: A chalkbrood infected larva with a yellow head

If the larvae are removed, they are moist, hexagonal-shaped and swollen. In most cases, the head-end of the larva appears clearly as a dark spot in the remains (Fig. 47). When chalkbrood infected larvae are removed with a stick, they never rope out of their cells.
In the honey bee colony, chalkbrood infected larvae dry out over time and become hard chalky lumps called “mummies” (Fig. 47-50). These remains are mostly white, often with yellow heads. Some mummies turn partially or completely grey or black. The colour change is the result of fruiting bodies, which produce the fungal spores. These spares are the infective stage of chalkbrood disease.

Figure 48: White chalkbrood mummy with an upraised yellow head
Figure 49: Black chalkbrood mummy
Figure 50: Chalkbrood mummies removed from their cells

Chalkbrood mummies are often removed by house bees and are sometimes found either on the bottom board or in front of the entrance of the hive (Fig. 50). Combs with heavy infections of chalkbrood will rattle when the mummies are sufficiently dry. At this stage the mummies can easily be removed just by shaking the comb.

AFB infected larvae do not turn either chalky white or grey, and do not dry down into easily removed mummies. The only condition in the hive that closely resembles chalkbrood is mouldy pollen. However, mouldy pollen is cylindrical or hexagonal in shape and is easily pulverised between the fingers, whereas chalkbrood mummies are more elongated, much flatter, and are not easily crushed.

Chalkbrood infected larvae are white or black, not coffee-coloured like AFB. Chalkbrood does not rope out like AFB and chalkbrood mummies can easily be removed from their cells.

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