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Where this comes from. The temperatures are from the Scottish Beekeepers’ Association’s notes for its Basic Beekeeping Certificate. The disease signs are the National Bee Unit’s. One field study shows what a winter flight looks like on the ground. Sources at the foot of the page.
What a cleansing flight is
Through the flying season, bees defecate outside the hive. 3 In winter they spend most of their time clustered, and on a nice day they leave the cluster for a cleansing flight. 1 It is a normal part of a colony’s winter.
One study followed the first big cleansing flight of a winter in an apiary of four hives. Most of the droppings landed close to the hives, but bees were also found further out on the ground, which suggested some of them were foraging as well as cleansing. 5
How warm it has to be
A colony starts to cluster when the air falls below about 14 °C. On a nice day in winter, bees can leave the cluster for a cleansing flight at 7 to 9 °C. A bee is immobilised at about 7 °C. 1
In February, bees take cleansing flights and bring pollen home on mild days, from snowdrops, crocuses and winter heaths. 2 Planting for bees in autumn covers putting some of those near your hives.
Leave them to it
A cleansing flight is not a reason to open the hive. Disturbing a colony with Nosema makes the disease worse, probably because the bees then defecate inside, and the advice for an infected colony is to leave it alone for a few weeks except to check it has enough food. 3 4
Leave snow in front of the hive where it is, too: it keeps the bees from being tempted out in bright, cold conditions. 1 And do not offer syrup when the bees cannot get out for cleansing flights. Feed fondant instead. 1 Fondant: when and how to feed it has the detail.
When it is dysentery instead
The thing to watch for is soiling of the hive itself. Faecal staining on the tops of the frames, or faeces on the outside of the hive, may be a sign of Nosema. 4 Dysentery is a sign of more than one adult bee disease: chronic bee paralysis virus, Nosema and amoebic disease. 3
- Dysentery with dark, shiny bees and a lot of dead bees inside or outside the entrance: chronic bee paralysis is the likely cause. 3 Dead bees at the entrance covers it.
- Dysentery without those signs, once the feed has been ruled out: Nosema or amoebic disease are possible, and the treatment for both is similar, so the precise cause matters less. 3
- The feed itself. Honey or sugar with too much water in it, fermented or coarsely granulated stores, and acid-inverted sugar can all cause dysentery. 3 Brown and raw sugars cause it too, so feed only white granulated or caster sugar. 6
The short version
- Bees flying on a mild winter day are usually taking a cleansing flight. It is normal.
- They can fly for it at 7 to 9 °C on a nice day. Below about 7 °C a bee cannot move.
- Do not open the hive because of it, and leave the snow in front of it.
- Staining on the frames or the front of the hive is different: that is dysentery, and worth a closer look.
- White sugar only, and fondant rather than syrup while the bees cannot fly.
This guide is general information, not legal advice. Rules change - always check the current guidance on GOV.UK before relying on it.