Stonebrood

Stonebrood is a rare brood disease caused by moulds of the genus Aspergillus; dead larvae turn into stone-hard mummies covered with yellow-green powder. It is usually a sign of a damp, poorly ventilated hive and a weak colony. Unlike other bee diseases, the causative fungi can also cause respiratory problems in humans, so you need to protect yourself while working.

What is it and how does it develop?

The agent is mostly Aspergillus flavus, and less often A. fumigatus and A. niger. These fungi occur naturally in soil, decaying plants and mouldy feed; they reach the bees via foragers and contaminated feed. The larva takes in spores with its food, or the fungus enters through the body surface; the fungus grows inside the larva, dries it out and hardens it.

Stonebrood can also affect adult bees: sick bees become restless, cannot fly and die in front of the hive. This is one of the points that distinguishes it from chalkbrood.

How to recognise it

• Hardness of the mummy: The dead larva becomes as hard as stone; it cannot be crushed with the fingers.
• Colour: As sporulation progresses, the mummies become covered with a powdery layer of mould that is yellow-green, grey-green or (with A. niger) blackish.
• Cell: Because the fungal threads can attach to the cell wall, the mummy is hard to remove from the cell; the cappings may be covered with mould.
• Inside the hive: A damp hive smelling of mould; mouldy pollen and comb.
• Adult bees: Motionless, dying bees in front of the hive; mould growing on dead bees.

How it differs from chalkbrood

Stonebrood is most often confused with chalkbrood (Ascosphaera apis). Chalkbrood is far more common, so when you see mummified brood think of chalkbrood first, but always check the hardness and colour.

The photographs below do not show stonebrood but chalkbrood, for comparison.

How to confirm it at the hive

• Squeeze a mummy gently with a gloved hand: if it does not crush, is stone-hard and covered with yellow-green powder, think of stonebrood.
• A definitive diagnosis is made in the laboratory by culturing the fungus from the mummy. Send the sample in a closed container.

Triggering conditions

• Humidity and condensation inside the hive, poor ventilation
• A weak colony unable to keep the brood warm
• Mouldy comb, mouldy pollen or spoiled feed
• A shady, damp apiary site exposed to fog or flooding

What to do, step by step

1. Protect yourself: Wear gloves and a high-filtration dust mask; do not shake or brush mouldy comb in a way that raises dust.
2. Remove and destroy affected combs: Do not use combs with mummies and mould in another hive or store them for rendering; destroy them by burning.
3. Reduce humidity: Improve ventilation, keep the hive raised off the ground and tilted slightly forward, and prevent condensation.
4. Reduce the space: Leave only as many frames as the colony can cover and keep warm.
5. Strengthen the colony: Provide sufficient clean feed; if the problem keeps recurring, requeen.
6. Review the apiary site: Move away from permanently damp and shady locations.
7. Clean your tools: Clean and dry the tools and hive parts you have used.

Treatment and medicine information

There is no authorised medicine for stonebrood and it is generally not needed. Once conditions inside the hive are corrected and the affected combs are removed, the disease recedes. Do not use home-made antifungal mixtures in the hive.

Prevention

• Set up hives in a dry, sunny spot sheltered from the wind.
• Renew combs regularly; do not put mouldy combs or pollen frames into the hive.
• Store combs in a dry, airy place.
• Keep colonies strong; do not go into winter with weak colonies.