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Chalkbrood

Overview

Ascosphaera apis, commonly referred to as chalkbrood, is a fungal disease which infects honey bees in the larval stage. The fungus reproduces through spores, and larvæ consume these spores in their brood food. The spores germinate in the hindgut of the larvæ, and once the cell is capped, they become vegetative and the active infection begins with visible symptoms.

Drone and worker larvae are infected more often than queens.

How it is Transmitted

Like most fungi, the chalkbrood pathogen thrives in an environment that is cool and humid. Mummies are most often seen along the periphery of the brood nest where temperatures aren't as well-regulated. They are most common in spring, when brood may be exposed to cool temperatures, or in colonies with small populations that are unable to effectively maintain a warm brood nest. Moisture in the hive can also help promote the growth of chalkbrood.

Symptoms

In honey bees, the vegetative fungus resembles very small white threads. As it overcomes the larva's body, the larva becomes covered and eventually comes to resemble a piece of chalk. Hard, white larvae in this form are called chalkbrood "mummies."

After the larva dies, the fungus develops fruiting bodies that eventually produce spores to reproduce. These fruiting bodies first appear as brown or black spots on the chalkbrood mummy and eventually, the mummy turns completely black as it becomes covered in fruiting bodies. Mummies can be seen inside their cells in the brood frame. Depending on the fungus' life stage, the mummies can appear white and chalky in appearance (with or without black spots), or can be completely black and hardened. Mummies can be removed from the brood frame by tapping the frame against a solid surface. This action helps differentiate chalk brood from other brood diseases—tapping the frame does not easily dislodge brood with other types of infections. Chalkbrood mummies are often removed by the bees through hygienic behaviour and left on the bottom board, at the entrance of the hive, and/or on the ground.

Preventing Chalkbrood

The keys to avoiding chalkbrood are

1. Maintain strong colony populations (with sufficient bees to keep the brood nest warm)
2. Eliminate moisture in the hive.
3. Avoid making splits or nucs in early spring without enough bees to warm the brood nest.
4. More generally, ensure that your hives are not in cool, damp places, and provide ventilation in the hive bodies.
5. It is recommended that the two oldest brood combs in each hive body be rotated out each year to remove any existing fungal spores that may be present.

Treating Chalkbrood

Honey bees can create a "social fever" in an effort to kill chalkbrood infections. They accomplish this by increasing the temperature of their brood nests by shivering their wings. Bees elevate the brood nest temperature just enough (around 0.9 °F) to exceed the temperature for chalkbrood to develop, but within the normal developmental temperature window for bees. Social fevers can be initiated after symptoms have begun developing or before symptoms have started. It is still not known how bees identify larvae infected with chalkbrood.

Chalkbrood is not regarded as a detrimental disease to the colony as infections usually clear up in the summer as the temperature rises. It rarely impacts the queen, and nurse bees can usually prevent or control it with hygienic behaviour.

There are no registered treatments for chalkbrood control.

If infections do not clear up on their own, requeening (ideally with hygienic stock) may help a colony clear up the infection. If more than 10–20% of brood cells on a single frame are infected, remove those frames and replace them with frames of drawn comb or foundation.

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