Why Bee Health Matters

A honey bee colony is a superorganism — every individual bee's health affects the entire colony. Disease left untreated can wipe out a hive within weeks. Regular monitoring, accurate record-keeping, and prompt action are the foundations of responsible beekeeping.

The diseases covered below are the most commonly encountered by hobbyist and small-scale beekeepers. Some, like American foulbrood, are notifiable — meaning you are legally required to report confirmed cases to the relevant authority in your country.

Varroa Mite (Varroa destructor)

Varroa is the single biggest threat to managed honey bee colonies worldwide. The reddish-brown mite is just large enough to see with the naked eye (1.1–1.8 mm). It reproduces inside capped brood cells and feeds on developing larvae and adult bees, transmitting several harmful viruses in the process.

Signs of Infestation

  • Deformed wing virus (DWV): bees emerging with crumpled, unusable wings.
  • Visible mites on adult bees — look on the abdomen between tergites.
  • High natural mite drop on a sticky board placed under an open-mesh floor.
  • Patchy, sunken, or punctured cappings on brood.

Monitoring

Regular Varroa monitoring is essential. The two most reliable methods for hobbyists are:

  • Alcohol wash (sugar roll): Take a 100 ml sample of nurse bees from a brood frame (approximately 300 bees), place in a jar with 50 ml of alcohol or icing sugar, agitate for 60 seconds, and count the mites that fall through a mesh lid. A count above 3 mites per 100 bees typically indicates treatment is needed.
  • Natural mite drop: Place a greased or sticky board under the hive for 24–72 hours and count fallen mites. Thresholds vary by season and region.

Treatment Options

Several authorised treatments are available. Always check current national regulations before treating, as approved products differ between countries.

  • Oxalic acid (vaporisation or dribble): Highly effective when applied to broodless colonies in winter. Approved for organic beekeeping in most European countries.
  • Formic acid (MAQS / Formic Pro): Can be used during the honey flow on colonies with brood. Penetrates capped cells. Temperature-sensitive — follow label instructions carefully.
  • Thymol-based products (Apiguard, ApiLife Var): Slow-release gel or strips effective at temperatures above 15 °C. Not suitable for use during honey extraction periods.
  • Synthetic acaricides (Apivar / amitraz strips): Highly effective but bees must not have access to supers during treatment due to residue concerns. Follow label instructions precisely.
Integrated Pest Management: Combining biotechnical methods (brood removal, drone comb trapping) with treatments reduces chemical use and slows resistance development. A brood break achieved by caging the queen for 3 weeks in summer, followed by oxalic acid treatment, is highly effective.
Close-up photograph of a Varroa destructor mite

Nosema (Nosema apis / N. ceranae)

Nosema is a microsporidian fungal pathogen that infects the gut cells of adult bees. Both species — N. apis and the more recently spread N. ceranae — reduce bee lifespan and colony performance. N. ceranae is now the dominant species in most of Europe.

Signs

  • Dysentery: brown streaks of faeces on the hive entrance and frames (more characteristic of N. apis).
  • Spring dwindling: colonies fail to build up despite adequate queens and food stores.
  • N. ceranae often shows no obvious external symptoms — definitive diagnosis requires microscopic examination of bee gut contents.

Prevention and Management

Good hygienic practices significantly reduce Nosema risk. Ensure hives are well ventilated and dry; replace old dark combs regularly (comb older than 3–4 years carries a high spore load). Fumagillin, historically the treatment of choice, has been withdrawn or suspended in several European countries. Consult your national beekeeping authority for current approved treatments.

American Foulbrood (AFB)

American foulbrood, caused by the spore-forming bacterium Paenibacillus larvae, is the most destructive bacterial disease of honey bees and is notifiable in most countries. Spores remain viable in hive materials for over 40 years.

Signs

  • Sunken, greasy-looking, discoloured (brown/black) capped brood.
  • Perforated or chewed cappings with a foul smell (rotting fish or glue).
  • "Ropiness" test: insert a twig or matchstick into a sunken cell and withdraw slowly — infected larvae form a brown thread 2–3 cm long before breaking.
  • Severely infected colonies have a characteristic sweetish-rotting odour.

Action Required

If you suspect AFB, do not move equipment between hives. Contact your regional bee inspector or veterinary authority immediately — in most jurisdictions you are legally required to do so. Equipment cannot be sterilised by beekeepers at home; it typically requires incineration or gamma irradiation. There is no approved antibiotic treatment in the EU.

Prevention: Never use second-hand hive equipment without confirmed disease-free certification. AFB spreads most commonly through contaminated second-hand combs and hive bodies.

European Foulbrood (EFB)

Caused by the bacterium Melissococcus plutonius, European foulbrood is less severe than AFB. Infected larvae die before cell capping — they appear twisted and discoloured (yellow to brown) with a sour or vinegary smell. Unlike AFB, EFB does not produce a ropy thread.

Strong colonies often recover without treatment. Requeening with a hygienic or locally bred queen is the first line of response. Oxytetracycline is approved in some countries; check national regulations.

Chalkbrood

A fungal disease caused by Ascosphaera apis. Infected larvae are found mummified — white, chalky, and hard — on the hive floor or at the entrance. The condition is most common in cool, damp conditions and in colonies with poor ventilation.

No chemical treatment is approved. Improving hive ventilation, requeening with hygienic stock, and replacing old combs usually resolves chalkbrood in strong colonies.

Sacbrood

Sacbrood virus (SBV) causes infected larvae to die in their cells before pupation. The larva fills with fluid, forming a fluid-filled sac visible when the capping is removed. Sacbrood is widespread but rarely causes serious colony damage except in combination with other stressors. Requeening and ensuring the colony is strong are the usual responses.

Small Hive Beetle (SHB)

Aethina tumida, native to sub-Saharan Africa, has established in parts of North America, Australia, and southern Europe. It has not yet become established across most of central or northern Europe as of 2025. Beetles breed inside hives, their larvae destroying comb and fermenting honey. Strong colonies in good condition can usually manage small beetle populations; traps placed inside the hive capture adult beetles.

Monitoring Calendar

A regular inspection routine is the foundation of bee health management. The following is a practical overview for temperate northern European conditions.

  • February–March: Check hive weight (hefting) and listen for bee cluster activity. Apply oxalic acid dribble if not done in December.
  • April–May: First full inspection once temperatures reliably exceed 12 °C. Check for queen, brood pattern, disease signs, and Varroa level.
  • May–July: Weekly inspections for swarm preparations; Varroa monitoring every 3–4 weeks.
  • August: Varroa treatment if threshold exceeded. Assess winter stores; begin feeding if needed.
  • September–October: Finalize winter preparations; treat with oxalic acid in broodless period if Varroa treatment not completed.
  • November–January: Minimal disturbance. Oxalic acid vaporisation in broodless colonies (typically December–January).

Record-Keeping

Keeping a written record after every inspection is one of the most valuable habits a beekeeper can develop. Note the date, weather, colony temperament, queen sighting, brood pattern, disease signs, mite counts, stores, and any treatment applied. Over years, patterns emerge that help anticipate problems before they become crises.

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