Chalkbrood Disease and Sacbrood Virus

Chalkbrood Disease and Sacbrood Virus

Be on the lookout for chalkbrood disease and sacbrood virus (SBV) in the springtime when colonies are increasing brood production.

Springtime is filled with many activities for both the beekeeper and the honeybees as everyone prepares for the upcoming nectar flow and swarming season. Colonies increase brood production, comb building begins, and new queens are created while beekeepers prepare for splits and the honey flow. Life is good. However, despite all this springtime excitement, a few potential dangers lurk around the yard that every beekeeper should be aware of, such as chalkbrood and sacbrood.

Chalkbrood

Chalkbrood is an infection caused by the larval ingestion of Ascosphaera apis spores. Considered to be primarily a secondary pest, chalkbrood is found in bee yards throughout the world and is most commonly seen in weak, debilitated colonies during cool, wet, or damp weather. Other contributors include moldy or used equipment and poorly ventilated hives that allow moisture to build up throughout the colony.

beee-diseases-photos
by Rob Snyder The development of white mycelium taking over. Removing a larva to observe this stage is
helpful when other signs aren’t yet visible.

Chalkbrood Disease Symptoms

Chalkbrood spreads readily throughout the colony, targeting primarily 3-to-4-day old worker larvae when nurse bees feed them spore-contaminated bee bread. These fungal spores germinate within the larval gut, sending a network of nutrient-seeking mycelium, or filaments, out from the fungus body and into the cell. This mycelium eventually grows so thick the entire larva becomes wrapped in a white coating resembling a mummy, often before the cells have time to be capped.

When conducting inspections, note that infected larvae may not be wrapped in mycelium before capping and may, instead, appear healthy at first glance. However, infected larvae are often relatively easy to remove from their cell. They’ll often display a whitening or a darkening at the bottom of the larval body as the mycelium work their way from bottom to top.

black-chalkbrood-mummies
by UF/IFAS Honeybee Research and Extension Lab. These black chalkbrood mummies let the beekeeper know the disease has already been present for some time and are indicative of the most highly infectious state of chalkbrood.

In a frame with a lot of capped, mummified larvae, a quick shake will often make the frame rattle — not a pleasing sound to any beekeeper. Left untreated, these spore-infected larvae eventually turn black, indicating the spores are ready to reproduce.

Sacbrood Virus

Sacbrood (SBV), while similar to chalkbrood at first glance, isn’t a fungus but rather a virus caused by the Morator aetatulas virus. Like chalkbrood, this springtime disease is also found worldwide, with most infections being secondary to other stressful conditions within the colony, such as a failing queen and cold, damp hive conditions.

what-is-sacbrood
by University of Georgia This sacbrood infected larvae displays the brown, fluid-filled sac that gave the disease its name.

Adult nurse bees contract the virus via contaminated food and by cleaning infected larvae from cells and passing the virus along to the larvae they feed. Sacbrood is also spread, just like chalkbrood, via robbing, drifting, and contaminated tools and equipment.

Sacbrood Disease Symptoms

The good news about sacbrood is it’s just as easy to identify after a bit of practice as chalkbrood. Sacbrood-infected larvae die under the caps and exhibit a change in color, going from pearly white to yellow and eventually to brown. Spotty brood patterns and sunken or perforated caps appear as the larvae die inside their cells.

Dead larvae will be seen with their heads pointing straight up and stretched out on their backs. The heads are also typically pointy (whereas chalkbrood infected larvae tend to exhibit a more rounded appearance when observed). These larvae fail to pupate as the virus prevents the larvae from shedding their skin. Instead, the larva liquifies inside the skin and remains.

Larvae removed from the cell with tweezers will often hang down in a fluid-filled sac-like manner, hence the name. When punctured, these fluid-filled sacs release their liquid, leaving behind nothing more than a skin.

Brood, however, isn’t the only class affected by SBV. Infected nurse bees stop feeding brood earlier than their healthy counterparts and stop eating pollen. These infected bees begin foraging sooner than normal and only collect nectar rather than both pollen and nectar. When combined with brood that never reaches maturity, a hive incapable of recovering is destined to die.

AFB-rope-test
Photo Credit: PennState Extension. The rope test helps identify another disease, American foulbrood. This AFB infected larvae has congealed into a sticky mass inside its cell that sticks to a match or other probing tools and stretches into a rope-like string when pulled from the cell.

Prevention and Treatment

While there are no chemical treatments labeled for the treatment of sacbrood and chalkbrood, there are many practices beekeepers can use to not only correct these illnesses but prevent them. Keeping hives well fed and ventilated during cool, wet weather while ensuring a strong, thriving queen and high colony numbers are present are key to disease prevention, as stress is often the primary culprit in these infections.

Should a failing queen be suspected, requeening with hygienic stock can be helpful. Not sharing equipment between yards and using new equipment also go a long way toward preventing the spread of disease.

spotty-brood
by Rob Snyder While spotty brood is a clear indicator something is amiss within the hive, don’t be too quick to put a name to it until other tests have been conducted.

However, the coming of warmer weather and drier conditions is often all that’s needed to correct an ailing colony, provided conditions aren’t allowed to exacerbate and grow to the point of colony collapse before interventions.

While springtime activities for the beekeeper and the bees are both fun and rewarding as splits are made and nectar flows begin, the cooler weather of spring also offers the opportune time for diseases like sacbrood and chalkbrood to make their appearance. However, these diseases are easy to identify and usually easy to prevent — in most cases — by keeping colonies heavily populated and well-fed in suitable housing. But when in doubt, never hesitate to contact your local bee inspector.


Kristi Cook lives in Arkansas where every year brings something new to her family’s journey for a more sustainable lifestyle. She keeps a flock of laying hens, dairy goats, a rapidly growing apiary, a large garden, and more. When she’s not busy with the critters and veggies, you can find her sharing sustainable living skills through her workshops and articles.


Originally published in the May/June 2024 issue of Countryside and Small Stock Journal and regularly vetted for accuracy.

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