Honey Bee Viruses

More than twenty viruses have been identified in honey bees, and most colonies that look healthy carry several viruses without symptoms. Viruses turn into disease mainly when the varroa load rises, when the colony weakens, or when crowding and stress increase. There is no medicine for viruses; the beekeeper's most powerful tools are varroa control and good colony management.

How do viruses spread within the colony?

• By mouth: Food sharing, the brood food that nurse bees give to larvae, pollen and honey.
• Through faeces: Contaminated comb and hive surfaces.
• Through the queen and drones: Some viruses pass into the eggs and into drone sperm.
• By contact: Especially with chronic bee paralysis virus, through rubbing and small wounds in a crowded hive.
• Through varroa: While feeding, the mite transmits the virus directly into the bee's haemolymph. This route allows the virus to bypass the gut barriers and raises the virus load from a symptomless level to a disease level.

Between colonies, the infection is carried by robbing, bees entering the wrong hive (drifting), transferring frames and comb, buying colonies, and neglected, dead-out hives.

Viruses that matter to the beekeeper

Besides these, agents such as filamentous virus, the Lake Sinai viruses and Arkansas bee virus have also been identified. They cannot be recognised in the field, and from the beekeeper's point of view they are managed in the same way as those above. The virus known as Kakugo is a variant of deformed wing virus.

When should you suspect a virus problem?

• Young bees with crumpled wings and short abdomens
• Flightless, crawling, trembling bees; increasing numbers of dead bees in front of the hive
• Hairless, shiny black bees
• Perforated cappings, an irregular brood area, unexplained dead brood
• Especially in autumn and winter, a hive left with honey but no bees: a colony that dies out within a short time

The signs overlap. It is usually not possible to tell in the field which virus is involved, and for management purposes it is generally not necessary. The exceptions are chronic bee paralysis (a distinctive appearance, different measures) and sacbrood. A definitive diagnosis can only be made in the laboratory (PCR).

What to do

1. Measure varroa and bring it down in good time. The mite load must be low especially before late summer, when the winter bees are reared. Use authorised products according to their labels and outside the honey harvest; do not use the same active substance continuously.
2. Keep the colony strong and well fed. Sufficient and varied pollen, syrup or fondant at the right time.
3. Limit transmission. Do not transfer frames and bees from a sick colony; prevent robbing; do not leave dead-out hives open.
4. Renew combs. Regularly take old, dark and faecally soiled combs out of circulation.
5. Requeen. In a weakened colony a young, healthy queen speeds up recovery; lines with strong hygienic behaviour may be preferred.
6. Do not buy colonies from unknown sources. Viruses arrive together with the colony.

Common mistakes

• Saying "I don't see any bees with deformed wings, so there is no problem": most virus damage is symptomless.
• Treating varroa by the calendar without measuring.
• Relying on antibiotics or "immune-boosting" products against viruses: there is no medicine effective against viruses.
• Immediately distributing the beeless combs and honey of a dead-out colony to other hives.