Black Queen Cell Virus (BQCV)

Black queen cell virus (BQCV) kills larvae and pupae in capped queen cells; the cell wall turns dark brown to black. It is very common in adult worker bees but usually causes no symptoms. It matters most to queen breeders; there is no medicine, and the essentials are controlling nosema and varroa and using healthy cell-raising colonies.

What is it and how does it spread?

BQCV is one of the most frequently detected bee viruses worldwide. It is often found in the adult bees of healthy-looking colonies and generally does no harm. The disease picture appears when the virus passes to queen brood.

The virus reaches the larva orally, through the royal jelly fed by nurse bees. Nosema (especially Nosema apis) plays an important role in nurse bees carrying and spreading the virus: once nosema damages the gut wall, the virus establishes itself more easily in the bee's body. This is why BQCV and nosema are often seen together. Varroa can also carry this virus, but the link is not as strong as with deformed wing virus.

Symptoms are seen mostly in spring and early summer, in starter and finisher colonies where large numbers of queen cells are raised.

How to recognise it

• Darkened queen cell: The wall of the capped queen cell becomes stained dark brown or black. This is where the disease gets its name.
• Dead brood in the cell: When the cell is opened, a dead, darkened prepupa or pupa is found. The skin of the dead larva is at first pale yellow and sac-like (resembling sacbrood), then turns dark.
• Queens that fail to emerge: Fewer queens emerge from the grafted cells than expected; losses are concentrated in particular cell-raising colonies.
• Worker brood: Usually not affected. Deaths of drone and worker pupae have also been reported, but are rare.

No clear disease picture is expected in adult queens. Claims such as an infected queen's laying being impaired have not been proven.

How it differs from similar conditions

How to confirm it in the hive

1. Set aside capped queen cells that fail to emerge or have darkened.
2. Open the cell carefully: a dark, dead prepupa or pupa and a darkened inner wall suggest BQCV.
3. Have the same cell-raising colony tested for nosema (microscopic search for spores in the gut of adult bees) and do a varroa count.
4. A definitive diagnosis is made in the laboratory by PCR. Send suspect cells chilled, in a closed container.

What to do

There is no medicine for the virus. The aim is to reduce the load in the nurse bees that carry it and to raise queens with healthy colonies.

• Choose starter and finisher colonies carefully: Use strong, well-fed colonies with plenty of young bees and no signs of disease.
• Keep nosema under control: Replace old, dark and faeces-soiled combs; keep the hive dry; make up for pollen and food shortages in spring.
• Keep varroa low.
• Take the problem colony out of queen rearing: Stop using a colony with recurring darkened cells as a starter or finisher; consider requeening it.
• Grafting hygiene: Do not move frames or bees from diseased colonies; keep grafting equipment clean.

Prevention

• Raise queens during the nectar and pollen flow, when nutrition is good.
• Check cell-raising colonies for nosema and varroa at the start of each season.
• Record grafting results per colony; if losses are concentrated in certain colonies, that is where the problem lies.