Nosema apis is a microsporidian parasite affecting the gut of adult bees. It was long thought to be the only nosema agent; today it is considered together with Nosema ceranae, which has become dominant in many regions.

How it develops

The bee takes in the spore with contaminated water, honey or comb. The spore germinates and multiplies in the cells of the midgut. When those cells are damaged the bee cannot digest its food.

The consequences:

• Lifespan is markedly shortened
• The brood food glands of nurse bees weaken; brood care suffers
• Foraging efficiency falls
• If the queen is affected her laying declines and she may be superseded

Signs

The classic picture of Nosema apis appears in winter and early spring:

• Faecal marks: Brown staining around the entrance, on the hive front and on the combs
• Bees unable to fly: Individuals crawling and trembling in front of the hive
• Distended abdomen
• A colony that does not build up in spring: The queen is laying but the colony does not grow
• Early supersedure of the queen

It can, however, run without symptoms. The absence of faecal marks does not mean the absence of nosema.

Differences from N. ceranae

• Season: N. apis peaks in winter and early spring. N. ceranae can be found year-round
• Faecal marks: Typical in N. apis; usually absent in N. ceranae
• Picture: N. ceranae runs a more insidious course; the colony weakens without symptoms

The two can be separated only by microscopy or molecular analysis. Under the microscope the spores resemble grains of rice.

Triggering conditions

• Long periods unable to fly — a delayed cleansing flight increases the gut load
• Damp inside the hive
• Unsuitable winter feed; honeydew honey burdens the gut
• Crowded and stressed colonies
• Old, dirty comb

What to do

• Renew comb. Spores accumulate in the comb; regular renewal is the single most effective measure
• Lower the moisture inside the hive. Ventilation and a forward tilt
• Attend to the winter feed. Replace stores dominated by honeydew honey
• Provide clean water. Have your own water source in the apiary
• Keep the colony strong. Feeding and space management
• Requeen. This can be effective in colonies where it recurs persistently

Medicinal treatment is restricted in many countries, or no licensed product exists. Check the current situation in your own country.