Nosema is a microscopic parasite that significantly affects honey bees globally. This internal parasite infects the digestive system of adult bees, leading to various health issues and, in severe cases, colony decline.
Identifying Different Nosema Species
Two primary species of Nosema are known to infect honey bees: Nosema apis and Nosema ceranae. Nosema apis has been recognized as a pathogen of European honey bees for over a century. This species is often associated with symptoms like dysentery and crawling bees, and it tends to be more prevalent in cooler climates.
Nosema ceranae, in contrast, originated in Asian honey bees (Apis cerana) and has only more recently emerged as a significant threat to European honey bees (Apis mellifera). This species was first identified in European honey bees in Taiwan in 2004 and has since spread globally. N. ceranae is considered more virulent, leading to faster bee mortality, and it can be detected year-round, unlike N. apis which is more common in milder seasons.
How Nosema Harms Bee Colonies
Nosema parasites invade and damage the epithelial cells lining the midgut of adult bees. This cellular destruction impairs the bee’s ability to absorb nutrients, leading to a weakened digestive system and overall health decline. Infected bees often experience a reduced lifespan, sometimes living only half as long as uninfected bees.
The physiological damage extends to impaired gland function, as infected nurse bees may lose their ability to produce royal jelly, which is fed to developing brood. This can lead to reduced brood production and poor development of young bees. Infected worker bees may also begin foraging at a younger age than healthy bees, disrupting the normal division of labor within the hive. Queens can also be infected, leading to reduced egg-laying capacity and premature death, which can prompt the colony to supersede their queen. These individual bee health issues collectively weaken the entire colony, contributing to reduced honey production, lower populations, and increased winter losses.
Preventing and Managing Nosema
Nosema transmission primarily occurs through the fecal-oral route, where healthy bees ingest spores from contaminated food, water, or hive components. Spores can persist on combs for many months, serving as a source of infection. Robbing bees, drifting bees between hives, and contaminated beekeeping equipment can also spread the disease.
Diagnosing Nosema typically requires microscopic examination of adult bee gut samples to identify the spores. While some general symptoms like dwindling populations, poor honey production, or dysentery (more commonly with N. apis) may suggest Nosema, these are not specific indicators and can be confused with other issues. A more accurate diagnosis can differentiate between N. apis and N. ceranae using genetic methods if needed.
Management and prevention strategies focus on maintaining strong, healthy colonies and implementing good apiary hygiene. Beekeepers should aim for well-fed colonies with adequate pollen supplies, especially in autumn, and place hives in sunny, well-ventilated locations to promote cleansing flights. Replacing old, dark brood combs can help reduce spore loads within the hive. While fumagillin was historically used for treatment, its efficacy against N. ceranae is debated. Some recent reviews suggest it can still reduce Nosema levels and improve colony health.