Lesson 60: Chalkbrood and Sacbrood
Two common brood problems that are usually mild, and how to tell them from foulbrood
Not every dead larva means foulbrood. Chalkbrood (a fungus) and sacbrood (a virus) are common brood diseases that usually do little harm in a strong colony. Learn what they look like so you don't panic, and so you can tell them apart from the serious diseases in lessons 58 and 59.
Chalkbrood
Cause: the fungus Ascosphaera apis. Worker, drone and queen larvae can all get it.
How it happens: nurse bees feed larvae food containing fungus spores. The spores sprout in the larva's gut, but the fungus stays held back until the cell is capped. Then it breaks out and takes over the whole larva in two or three days.
The signs: - The dead larva is covered in fluffy, white, cotton-like fungus that swells to fill the cell. - It dries into a hard, shrunken "mummy" that looks like a piece of chalk: white, or mottled black-and-white or all black when the fungus has formed spores. - Mummies are loose: they can be shaken or tapped out of the cells. Shake a frame and you may hear them rattle. That easy removal also sets chalkbrood apart from other brood diseases. - Mummies pile up at the hive entrance or on the bottom board, where the bees drag them. Seeing them at the entrance is a telltale sign. - It's most common in damp, cool conditions, often in spring.
What to do: there are no chemical treatments for chalkbrood. It's controlled by management and bee stock, and colonies usually recover on their own. - Keep the hive warm and dry. Avoid damp, low, shady spots and heavy vegetation around hives; make sure the hive is ventilated. Tilt the hive slightly forward so rain runs out the entrance (not needed with a screened bottom). - Keep the colony strong, so there are enough bees to keep the brood warm. - Requeen with hygienic stock (lesson 15) if a colony has chalkbrood again and again; it suggests the bees are genetically prone to it. - Replace old brood comb, which holds fungus spores (lesson 53).
How it looks:
Chalkbrood mummies, white and black, dragged out onto the landing board.
Sacbrood
Cause: sacbrood virus, the first honey bee virus ever described (1913).
How serious: usually not serious. It usually affects only a small share of the brood, and adult bees find and remove sick larvae quickly, so it often goes unnoticed. If there's enough to see easily, though, it can be severe enough to reduce the adult population. Outbreaks show up mainly in the first half of the brood-rearing season, during build-up.
The signs: - Scattered uncapped or partly uncapped cells, or capped cells left behind after the bees around them have emerged. - The dead larva lies stretched out on its back, with a darkened head curled upward (it's often compared to a slipper or a canoe). - Its color changes from pearly white to pale yellow, then light brown, then dark brown. - The skin is tough, and filled with watery fluid. Lift the larva out carefully and it's like a small sac of liquid, which gives the disease its name. - It dries into a dark, wrinkled, brittle scale that comes out easily (unlike AFB).
What to do: there are no medicines for any honey bee virus. Requeening and replacing old comb are the best practical steps, along with keeping the colony strong and controlling varroa mites, which spread viruses (lesson 62).
Photo still needed: sacbrood larva. Curtis's own photos are welcome.
Telling brood problems apart
| Chalkbrood | Sacbrood | EFB | AFB | |
|---|---|---|---|---|
| Cause | Fungus | Virus | Bacterium | Bacterium (spores) |
| Remains | Chalky white or black mummy, hard | Fluid-filled sac, head up | Twisted, melted, yellow-brown | Brown, gooey, ropes |
| Comes out of the cell? | Easily (rattles) | Easily | Easily (rubbery) | No (scale stuck tight) |
| Serious? | Usually mild | Usually mild | Moderate | Very |
Colonies can have more than one brood problem at once. If you're not sure, get help (lesson 58).
What to remember
- Chalkbrood: fungus; hard white or black "mummies" that rattle in the cells and pile up at the entrance; worst when damp and cool. No chemical treatment: keep hives dry, ventilated and strong; requeen with hygienic stock.
- Sacbrood: virus; larva stretched out, head curled up, a sac of fluid, pale yellow to dark brown. Usually mild; requeen, replace old comb, control mites.
- Both leave remains that come out easily, unlike AFB.
Quick check
- You find hard, white and black pellets on the landing board. What's the likely cause?
- What does a larva killed by sacbrood look like when lifted out?
- Name two ways to reduce chalkbrood.
Show answers
1. Chalkbrood mummies the bees have dragged out. 2. A stretched-out larva with a dark, upturned head, like a small sac filled with fluid. 3. Any two of: keeping the hive warm, dry and ventilated, away from damp low spots; keeping the colony strong; requeening with hygienic stock; replacing old comb.
Adapted from: Penn State Extension, A Quick Reference Guide to Honey Bee Parasites, Pests, Predators, and Diseases (Underwood, updated 2021) and Viruses in Honey Bees (Grozinger, Underwood & López-Uribe, updated 2026); UGA Bee Program, Honey Bee Disorders: Fungal Diseases and Viral Diseases; Shimanuki & Knox, Diagnosis of Honey Bee Diseases (USDA AH-690, rev. 2000); UF/IFAS Extension, Welcome to the Apiary! (4H446, 2024); Delaplane, Honey Bees and Beekeeping (UGA Bulletin 1045, rev. 2017).
Photo: Chalkbrood mummies, Jeff Pettis (USDA), CC BY 3.0 US, via Wikimedia Commons.
Adapted from university Extension sources. Experienced Georgia beekeepers are reviewing these lessons. See something that doesn't match your experience? Tell us.