Key points
- Dysentery is a symptom, not a disease: bees defecate in or on the hive instead of in flight.
- Common causes: long confinement, honeydew and poorly digestible stores, fermented or overheated syrup, disturbance, damp.
- Nosema apis may be involved, but only a microscope test can tell.
- In spring: let the bees fly, clean up, remove heavily soiled combs.
- Prevention happens in autumn: good-quality winter stores in a dry, undisturbed hive.
At the end of winter, a hive sometimes appears streaked with brown: on the front, the landing board, sometimes the roof, and inside on the combs and crown board. The bees, which normally defecate only in flight, could not hold on. Beekeepers call this dysentery.
The word is misleading: dysentery is not a disease in its own right but a symptom. It can come from an overly long winter, poorly digested food, disturbance or an infection, in particular by Nosema apis.
A question of the rectum
The honey bee is a clean insect: it does not defecate inside the hive. In season, it relieves itself on every trip out. In winter, when the cold stops it flying, it holds its faeces in the hindgut, the rectum, which gradually stretches. In winter bees it can end up filling a large part of the abdomen.
What builds up is the undigested part of the food: the shells of pollen grains, which the bee cannot digest, sugars it cannot absorb, minerals, and water6. The more residue the food leaves, the faster the rectum fills.
As soon as a mild day allows, the bees pour out for a cleansing flight, leaving yellow or brown spots on washing and cars nearby: this is normal. Dysentery begins when the bees can no longer wait for that flight, or are too sick to make it, and relieve themselves in the cluster, on the combs or just at the entrance.
Causes
Usually several factors combine: a rectum that fills too quickly and a long period without flight.
Prolonged confinement
A long, cold winter with no day suitable for a cleansing flight is the basic cause: even on perfect food, the rectum has limited capacity. A hive in the shade gets fewer chances to fly.
Honeydew and residue-rich honeys
Honeydew honeys (fir, oak and other aphid or scale-insect honeydews) contain more minerals and complex sugars than nectar honeys. For more than a century, beekeeping guides have advised against them as winter stores7.
One sugar is particularly implicated: melezitose, a trisaccharide abundant in some honeydews. It crystallises quickly in the cells, giving the “cement honey” that bees struggle to use. In cage trials, bees fed melezitose ate more, had heavier guts, swollen abdomens and higher mortality; the sugar also changed their gut flora. The authors see it as the likely cause of the “honeydew flow disease” described in German-speaking countries and linked to winter losses4.
The link is not automatic: some field studies found no increase in mortality with honeydew stores6. The risk depends mainly on how much honeydew is in the brood nest and how long the confinement lasts.
Fermented or poor-quality syrup
Syrup that is too thin, made in a dirty container or left in the warm can ferment, and fermentation products irritate the gut. Unrefined sugars, which leave more residue, are generally not recommended for winter.
When syrup rich in fructose is heated or stored hot, it forms hydroxymethylfurfural (HMF), which is toxic to bees; levels rise sharply above about 50 °C5. So syrup is never boiled, and old or badly stored syrups are avoided.
Nosema apis disease
Nosema apis (now Vairimorpha apis) is a microsporidian that infects the midgut. It is classically associated with late-winter dysentery1. The relationship runs both ways, however: Bailey’s work showed that infection is often slight and that it is dysentery that spreads the parasite, by contaminating combs and stores, rather than the parasite alone causing dysentery3. Nosema ceranae, the other species, does not usually cause dysentery. See the Nosema page.
Disturbance
A hive that is opened, knocked, or visited by a woodpecker or rodents in midwinter becomes restless: the bees eat more and the rectum fills faster.
Damp
A damp, poorly ventilated hive, or one sited in a cold hollow, forces the colony to consume more. Uncapped stores absorb moisture, become diluted and may ferment.
Consequences for the colony
Mild dysentery followed by a good cleansing flight leaves little trace. Severe dysentery costs bees, which fly out in the cold and do not return or die in the hive, and delays the spring build-up. Soiled combs have to be cleaned by the workers, and the queen is less willing to lay in them. If the colony carries Nosema, the faeces spread spores throughout the hive3. Weak colonies can die.
Dysentery or nosema: how to tell
Stains alone do not reveal the cause. The only reliable method is microscope examination of adult bees, looking for Nosema spores in the gut8.
| Feature | Non-infectious dysentery | Dysentery with Nosema apis |
|---|---|---|
| Context | Long confinement, honeydew stores, doubtful syrup | Can occur even on good stores |
| Colonies affected | Often several, on the same type of stores | Sometimes just one, or already weak colonies |
| After a cleansing flight | Quick improvement | Weakening may continue into spring |
| Microscope | No spores, or very few | Many spores |
The test is done on a sample of bees taken at the entrance and sent to a veterinary laboratory; some beekeepers do a first check themselves. Beware: yeasts can look like spores to an untrained eye6. Only a molecular test (PCR) distinguishes N. apis from N. ceranae.
Regulation
Dysentery as such is not regulated. In France, Nosema apis disease is still on the national list of provisionally regulated diseases, whereas N. ceranae disease is not9. If a laboratory confirms N. apis, check with your animal health organisation or veterinary authority. Rules differ between countries.
Prevention
Dysentery is prevented in autumn, not in spring.
- Good-quality winter stores. The brood nest should hold mainly nectar honey or properly made syrup. When a late honeydew flow has filled the brood box, it is common to remove some of those combs and feed syrup7.
- Clean syrup, never boiled, fed early enough to be capped before the cold.
- A sunny site, sheltered from wind and out of damp hollows, which gives more chances for cleansing flights2.
- A dry hive: adequate ventilation, a watertight roof, a slight forward tilt.
- Peace and quiet: no opening in cold weather, protection from wind and animals.
- Strong colonies with a young queen.
Managing dysentery in spring
On the first mild days the colony flies out and empties itself: often all that is needed is to clear dead bees from the entrance. On the first day warm enough to open up, check population, brood and stores. A weakened colony is reduced to the combs it covers and fed if stores are short. Cleaning and testing are covered below.
In practice: what beekeepers actually do
Dysentery often looks worse than it is. Here is how beekeepers actually handle it, at the end of winter and in autumn.
In spring: hands-on steps
- Wait for the cleansing flight before judging: many stained colonies recover normally. Most beekeepers note the worst-stained hives to check first.
- Clean up: swap or scrape the floor, clean the landing board, fit a clean crown board.
- Remove heavily soiled combs as soon as the colony can spare them, and render them down rather than reuse them.
- Have bees tested when several colonies are affected or one is collapsing: it is the only way to rule nosema in or out.
Before winter: common strategies
- Check stores by weighing or hefting and by looking at the combs: quantity, but also type and capping.
- Keep honeydew out of the brood box where possible: remove honeydew combs after a late flow, then feed syrup.
- Close the hive for winter and leave it shut, except in an emergency.
Equipment and checklist
Spare floors and crown boards (swap rather than clean on site), something to send a bee sample in, and an apiary notebook. The calendar: stores checked in September-October; no opening from November to February; hive fronts watched on the first mild days; cleaning and comb sorting at the first inspection.
Most common mistakes
- Concluding it is nosema without a test, or ruling it out without one.
- Wintering on honeydew because the combs are full.
- Boiling syrup, or feeding syrup that has fermented.
- Opening hives in winter “just to see”.
- Reusing soiled combs from a dead colony without knowing the cause.
Frequently asked questions
Are the yellow spots around the apiary on the first sunny day dysentery? No. They are the normal traces of the cleansing flight. It is dysentery when faeces are inside the hive or in streaks on the front.
Can honey from soiled combs be used? These combs must not be used for human consumption. If they come from a colony carrying Nosema, they can also infect other colonies: better to remove them and render the wax.
Is there a treatment for dysentery? No, because it is a symptom. You act on the cause: cleansing flights, cleaning, stores. Against nosema itself, no medicine is authorised in the European Union: see the Nosema page.
References
- Fries I. (2010). Nosema ceranae in European honey bees (Apis mellifera). Journal of Invertebrate Pathology 103: S73-S79. doi.org
- FAO, IZSLT, Apimondia, CAAS (2021). Good beekeeping practices for sustainable apiculture. FAO Animal Production and Health Guidelines No. 25.
- Bailey L. (1967). Nosema apis and dysentery of the honeybee. Journal of Apicultural Research 6(3): 121-125. doi.org
- Seeburger V.C., D'Alvise P., Shaaban B., Schweikert K., Lohaus G., Schroeder A., Hasselmann M. (2020). The trisaccharide melezitose impacts honey bees and their intestinal microbiota. PLoS ONE 15(4): e0230871. doi.org
- LeBlanc B.W., Eggleston G. et al. (2009). Formation of hydroxymethylfurfural in domestic high-fructose corn syrup and its toxicity to the honey bee (Apis mellifera). Journal of Agricultural and Food Chemistry 57(16): 7369. doi.org
- Oliver R. (2019). The Nosema problem, part 7a: the causes of dysentery in honey bees, part 1. American Bee Journal, December 2019 (online version, scientificbeekeeping.com). scientificbeekeeping.com
- Phillips E.F., Demuth G.S. (1918). The preparation of bees for outdoor wintering. USDA Farmers' Bulletin No. 1012. gutenberg.org
- Fries I., Chauzat M.-P., Chen Y.-P. et al. (2013). Standard methods for Nosema research. The COLOSS BEEBOOK, Vol. II. Journal of Apicultural Research 52(1): 1-28. doi.org
- Anses, Sophia Antipolis Laboratory (2025). Activity report of the national reference laboratory for bee health. anses.fr