A colony can look busy at the entrance and still be struggling inside. That is why a guide to honey bee diseases starts with regular, calm inspection rather than waiting for obvious losses. Look at brood pattern, larvae, adult bee behaviour, stores and colony strength together. One unusual cell is rarely a diagnosis; a pattern of symptoms is what deserves action.
For UK beekeepers, the first priority is protecting other colonies as well as your own. Some brood diseases are legally notifiable, and moving bees, combs or equipment before you have taken advice can spread a serious problem.
Guide to honey bee diseases: start with the brood
Most significant colony health issues show themselves in the brood nest. Inspect brood on a warm, suitable day, with enough light to see the base of cells. A healthy brood pattern is generally compact for the queen’s laying area, with pearly white larvae sitting in a visible pool of brood food. Cappings should be mostly even and biscuit-coloured.
A patchy brood pattern does not automatically mean disease. It can result from a failing queen, chilled brood, a recent nectar dearth, heavy varroa pressure or a colony recovering after swarming. The question is whether the signs persist, worsen, or occur alongside discoloured larvae, sunken cappings, unusual smells or weak adult numbers.
American foulbrood
American foulbrood, often shortened to AFB, is a serious bacterial disease of sealed brood. It is caused by spores that can remain viable in old comb and contaminated equipment for many years. This persistence is why early reporting matters so much.
Possible signs include irregular brood, sunken or perforated cappings, and brown larvae that have lost their normal segmented appearance. Dead larvae may dry into hard, dark scales that are difficult for bees to remove. A distinctive smell may be present in advanced cases, but smell alone is not a reliable test.
If you suspect AFB, do not move the colony, frames, honey supers, tools or any equipment from the site. Close the hive and contact the National Bee Unit through the appropriate reporting route. Do not attempt to treat, shake out or dismantle the colony yourself. A Bee Inspector can assess the colony and arrange the correct next steps.
European foulbrood
European foulbrood, or EFB, affects unsealed larvae more commonly than sealed brood. Infected larvae may appear twisted in their cells rather than lying in a neat C-shape. Their colour can change from white to yellow, then brown, and they may look melted or uneven. The brood pattern is often scattered because affected larvae die before capping.
EFB is also a notifiable disease in the UK. Its appearance can vary, and colonies may show a mixture of healthy and unhealthy brood. As with suspected AFB, stop moving bees and equipment, and seek an inspection promptly. A diagnosis made early gives the best chance of preventing wider spread.
Common problems that need management
Not every colony health problem is notifiable, but several need active management. The right response depends on the season, colony strength, local forage and the level of infestation. Avoid treating every weak colony in the same way.
Varroa and associated viruses
Varroa destructor is a parasitic mite, not a disease, but it is central to honey bee health because it feeds on developing and adult bees and spreads viruses. A colony can carry a damaging varroa burden while still appearing productive, particularly in late summer when winter bees are being raised.
Deformed wing virus is one of the clearest signs of a heavy viral problem. You may see workers with crumpled wings, shortened abdomens or an inability to fly. However, visible deformed bees often mean the infestation has already become severe.
Monitor mites rather than relying on guesswork. An alcohol wash gives a useful estimate of mites on adult bees, while natural mite drop can help show trends but is less precise. Use authorised treatments according to the label, rotate active ingredients where appropriate, and check that the treatment suits the temperature and time of year. A late or poorly applied treatment can leave a colony unable to raise healthy winter bees.
Nosema
Nosema is a gut disease caused by microscopic parasites. It is often associated with poor spring build-up, crawling bees, shortened adult lifespan and colonies that fail to thrive despite having a laying queen and adequate stores. Faecal staining at the entrance or on frames can occur, although it is not proof of Nosema on its own.
Damp conditions, prolonged confinement and poor nutrition can make gut problems more likely. Good apiary hygiene, dry hives with adequate ventilation, replacement of very old comb and strong forage support are practical foundations. If symptoms are persistent or several colonies are affected, microscopic testing is more useful than making assumptions from staining alone.
Chronic bee paralysis virus
Chronic bee paralysis virus, known as CBPV, can cause clusters of trembling, crawling bees around the entrance. Some bees appear unusually dark, shiny and hairless because they have been groomed or rubbed by nestmates. The colony may look unsettled, with healthy-looking bees rejecting affected workers.
There is no simple medicine for CBPV. Reduce stress where possible: ensure adequate space, avoid excessive disturbance, provide access to forage or appropriate feed when needed, and maintain sensible colony density. Severe cases may need careful advice from an experienced local beekeeper or Bee Inspector, especially if you are unsure whether another condition is involved.
Chalkbrood and sacbrood
Chalkbrood is a fungal disease of larvae. It produces white, grey or black mummified larvae, often called chalkbrood mummies, which may be found on the floor or at the entrance. It is commonly linked with dampness, chilled brood, weak colonies or a genetic tendency in the queen’s offspring.
Sacbrood causes larvae to die in fluid-filled sacs. They may darken and form a small, canoe-shaped scale. Both conditions can improve when the colony becomes stronger, warmer and better ventilated. Replacing a queen can be worthwhile where brood disease continues, particularly if the colony repeatedly shows poor brood viability.
A practical inspection routine
Good disease control is usually quiet, repeated work rather than a dramatic intervention. During active months, inspect often enough to understand what is normal for each colony, without opening hives so frequently that you disrupt them. Keep concise records of queen status, brood pattern, varroa results, treatments, feeding and anything unusual.
When inspecting, check four areas in order:
- adult bee numbers and behaviour at the entrance and on the comb;
- brood pattern, larvae and cappings in the central frames;
- food stores, pollen availability and signs of starvation or robbing;
- floors, comb condition and any dead bees, mummies or unusual debris.
Clean tools between colonies when you see suspect symptoms, and avoid swapping brood frames casually. Do not reuse old, heavily soiled comb indefinitely. Old comb can accumulate spores, pesticide residues and pathogens, so a planned comb replacement programme is sensible, especially in productive colonies that have been in the same boxes for years.
Nutrition is part of disease resilience. Bees cannot make up for a long forage gap with willpower. A colony short of pollen may raise less robust brood, while a colony short of carbohydrate stores can collapse quickly in poor weather. Feed only when there is a clear need, use appropriate bee feed, and avoid spilling syrup that may trigger robbing.
When to ask for help
Ask early when you see discoloured or dead larvae, persistently patchy brood, a sudden unexplained decline, large numbers of crawling bees, or unusually high varroa counts. A clear photograph of the brood frame can be helpful, but it does not replace an in-person assessment.
New beekeepers often worry that requesting help means they have failed. It does not. Disease recognition improves with experience, and experienced keepers still seek a second opinion when symptoms do not fit neatly. A course, local association mentor or professional inspection can help you distinguish a manageable colony issue from a situation that needs formal action.
The most useful habit is simple: inspect with purpose, record what you find, and act before a small warning becomes a colony-wide problem. Healthy bees are not the result of luck alone; they come from observant beekeeping, timely varroa control and a willingness to ask for advice when the brood nest does not look right.