What Is Chalkbrood?
Chalkbrood is a fungal disease of honey bee brood caused by Ascosphaera apis. The fungus infects larvae when they consume spores in their food. Inside the larva's gut, the spores germinate and the fungus eventually kills and mummifies the larva.
The name comes from the appearance of infected larvae — they harden into chalk-like mummies that rattle around in cells or on the bottom board.
How Infection Occurs
- Larvae ingest fungal spores mixed into their food (bee bread or royal jelly)
- Spores germinate in the gut after the cell is capped
- Fungal threads (mycelium) grow throughout the larva's body
- The larva dies and dries into a hard, shrunken mummy
- The mummy produces millions of new spores, continuing the cycle
How to Identify Chalkbrood
Chalkbrood is easy to diagnose — the mummies are unmistakable once you've seen them.
What Mummies Look Like
- Color: White to light gray when young; may turn dark gray or black as spores develop on the surface
- Texture: Hard, dry, and chalky — they feel like small pebbles or chalk pieces
- Shape: Retain the shape of the larva, shrunken and slightly twisted. Often described as looking like tiny rice grains or bits of chalk.
- Location: Found inside cells (often with perforated cappings), on the bottom board, or at the hive entrance where bees have discarded them
🔍 Quick ID Test
Pick up a suspected mummy and roll it between your fingers. Chalkbrood mummies are hard and don't smear. They may crumble slightly but won't leave a gooey residue (unlike European Foulbrood, which is slimy, or American Foulbrood, which is ropy).
Where to Look
- Bottom board: Hygienic bees remove mummies and dump them. Check here first.
- Hive entrance: Mummies often accumulate in front of the hive.
- Brood frames: Look for perforated cappings (bees opening cells) and mummies still in cells.
- Spotty brood pattern: Chalkbrood often causes a scattered pattern of empty cells amid healthy brood.
What Causes Chalkbrood
The fungal spores are nearly everywhere — most hives are exposed. Whether infection develops depends on stress factors that give the fungus an advantage.
Primary Risk Factors
- Cool, damp conditions. Chalkbrood thrives when brood is chilled. Cold snaps in spring, poor hive ventilation, or colonies too weak to cover their brood all increase risk.
- Weak colonies. Small populations can't maintain proper brood nest temperature. The fungus takes hold in the cooler edges.
- Poor ventilation. Moisture buildup in the hive creates ideal conditions for fungal growth.
- Genetic susceptibility. Some bee stocks are more resistant than others. Colonies with poor hygienic behavior don't remove infected larvae quickly enough.
- Nutritional stress. Bees without adequate pollen/protein are less able to fight infection.
Seasonal Pattern
Chalkbrood most commonly appears in early spring when:
- Temperatures fluctuate (warm days, cold nights)
- Colonies are building up but still relatively small
- Brood area may exceed what bees can keep warm
- Spring rains increase humidity
It often disappears on its own as the colony grows stronger, weather warms, and bees can better regulate conditions.
Treatment & Management
There is no approved chemical treatment for chalkbrood. Management focuses on improving conditions so the colony can overcome the infection naturally.
Steps to Help Your Colony
- Improve ventilation. Ensure upper ventilation is adequate. Some beekeepers prop the inner cover or add a ventilation rim. Moisture is the enemy.
- Reduce hive space. If the colony is small, remove empty boxes. Consolidate bees onto fewer frames so they can maintain temperature.
- Ensure a sunny, dry location. Move hives to a spot with good morning sun and airflow if they're in a damp, shaded area.
- Remove heavily infected frames. If frames have severe infections (majority of cells affected), remove and replace them. Burn or freeze frames to kill spores.
- Clean the bottom board. Remove mummies that accumulate — they're full of spores. Replace solid bottom boards with screened ones if possible.
- Boost nutrition. Feed protein supplement (pollen patties) if natural pollen is scarce. Strong, well-nourished bees fight infection better.
Don't Panic
Requeening for Resistance
If a colony has chronic, recurring chalkbrood despite good management, consider requeening from stock bred for hygienic behavior. Some genetic lines are significantly more resistant — their bees quickly detect and remove infected larvae before spores spread.
Prevention Strategies
- Keep colonies strong. Strong populations maintain stable brood temperatures. Combine weak colonies if needed.
- Ensure good ventilation. Upper ventilation helps remove moisture. Screened bottom boards improve airflow.
- Locate hives wisely. Full morning sun, good air drainage, avoid damp low spots.
- Don't spread spores. Avoid moving frames from infected colonies to healthy ones. Sterilize hive tools.
- Select for hygienic behavior. When you requeen, choose stock with good hygienic traits.
- Replace old comb. Old, dark comb accumulates spores and pathogens. Rotate out old frames over time.
When to Be Concerned
Chalkbrood becomes a problem when:
- Infection is severe (many cells affected, not just scattered mummies)
- It persists into summer when it should resolve
- The colony is declining rather than growing
- The same colony has recurring issues year after year
In these cases, more aggressive intervention is warranted — requeening, removing infected frames, or even combining with a stronger colony.