Nosema disease of honey bees
Nosema is a serious disease of adult European honey bees including queen bees. In some years, nosema may cause serious losses of adult bees and colonies in autumn and spring.
The disease is caused by the spore forming microsporidian — Nosema apis. Spores of this organism can only be seen using a light microscope.
In recent years, another nosema, Nosema ceranae, has been found to infect European honey bees in a number of countries including Australia.
Life cycle of nosema
When spores of Nosema apis are swallowed by bees, they germinate within 30 minutes inside the stomach. The organism then penetrates cells of the stomach lining. It continues to grow and multiply rapidly, using the cell contents as its food supply.
Large numbers of spores are produced in the host cell in 6 to 10 days. The parasite may also penetrate and infect adjacent healthy cells. This spreads the infection further.
During the normal digestive process of adult bees, healthy cells of the stomach lining are shed into the stomach. They burst open and release digestive enzymes. Infected cells are also shed in this way, but they release nosema spores and not digestive juices. These spores can infect other healthy cells of the stomach lining. Many spores pass through the intestines and are present in the faeces (excreta) of the bee.
Incidence and spread
Infection does not normally pass directly from infected bees to the next generation of adults. Instead, young bees become infected when they ingest spores as they clean contaminated combs.
During the summer months, most honey bee colonies carry a few infected bees with little or no apparent effect on the colony. Spores may also persist on the combs.
As the weather in autumn changes, these spores may initiate an outbreak of nosema. Losses of bees at this time of the year may be very heavy.
Winter losses can also be heavy. Infected bees confined in their hives due to bad weather may defecate inside the hive, soiling the combs and hive interior with excreta and spores. This, together with spores produced in the preceding autumn, causes infection in spring.
Spring outbreaks usually begin in late August or September, when temperatures begin to rise. They may last until late spring or early summer.
When the warm weather comes, the disease begins to decline due to improved flight conditions. The source of infection is largely removed because the bees are able to defecate outside the hive thereby reducing the contamination of combs.
Fortunately, serious nosema outbreaks do not occur every year. Research has indicated that the following conditions appear to be associated with serious autumn outbreaks and epidemics of nosema:
- heavy summer rainfall
- an early autumn break in the fine weather about mid-March to early April
- bees working grey box (Eucalyptus microcarpa), red ironbark (E. sideroxylon) and white box (E. albens)
The exact reasons for these apparent relationships are not known. In these epidemics, strong colonies may be seriously weakened before winter. They can be reduced to the size of a nucleus colony in a matter of days.
Infected colonies that survive the winter may require a long build-up period before the population of adult bees reaches normal numbers.
Losses caused by nosema disease are not confined to areas of Victoria having the field conditions mentioned above.
Spores of Nosema apis may occur in honey or pollen. Research reports indicate that honey bee workers can transmit nosema to queens in:
- queen mailing cages
- queen banks
- queen mating nuclei.
Effect of nosema on bees
- Hypopharyngeal (brood food) glands of infected nurse bees lose the ability to produce royal jelly which is fed to honey bee brood.
- A high proportion of eggs laid by the queen of an infected colony may fail to produce mature larvae.
- Young infected nurse bees cease brood rearing turning to guarding and foraging duties — usually undertaken by older bees.
- Life expectancy of infected bees is reduced. In spring and summer, infected bees live half as long as non-infected bees.
- Infected queens cease egg-laying and die within a few weeks.
- An increase of dysentery in adult bees although nosema is not the prime cause of dysentery.
Symptoms of nosema
Bees infected with nosema either show no symptoms, or none specific to this disease. Many of the so-called symptoms attributed to nosema disease apply to other diseases or conditions of adult bees. Examination of adult bees using a light microscope is the only reliable method of diagnosing the presence of spores of nosema.
Infected colonies can lose adult bees sometimes at an alarming rate. Infected bees often die away from the hive and only a few sick or dead bees may be found near the hive entrance. The term 'spring dwindle' is often used to describe this condition. However, this should not be confused with the normal weakening of colonies caused by the natural dying of old, over-wintered bees in early spring.
Sick or crawling bees outside the hive entrance, dead bees on the ground and excreta (dysentery) on hive components may be associated with nosema infection. However, these conditions can equally be caused by other diseases and abnormal conditions.
Control of nosema
Australian beekeepers use management practices to minimise the incidence of nosema.
Chemical treatments for control of nosema are not registered in Australia for use in honey production beehives. The use of any such treatment is illegal and could result in unacceptable residues in extracted honey.
- Maintain colonies with queens with good egg-laying potential. Colonies prepared for winter should have a good population of young bees.
- Ensure colonies have adequate supplies of high protein pollen in autumn. This will help to ensure good populations of young bees.
- Ensure hives prepared for winter have good supplies of honey. Studies have shown that colonies with generally with half, or more, of honey had lower spore counts compared to colonies wintered with less honey.
- Place the hives in a sunny position in the cooler months of the year. Choose apiary sites that have good air drainage and protection from cold winds. Avoid cool shady and damp sites. Research has shown that the level of nosema infection in a colony can be reduced from about 85% to zero by placing the hive in a sun trap where it obtains maximum sun and maximum shelter from cold winds.
- Maintain winter colonies in a minimum of hive space so they are compact and warm. Remove supers (boxes) of combs not required by the bees.
- Avoid colony stress which can be caused by excessive opening of the hive, manipulation of combs, feeding and relocating colonies.
- Avoid stagnant water sources which may become contaminated by dead bees and bee excreta.
- Minimise the number of squashed bees during normal hive management. Any infection will be spread when their remains are cleared away by hive cleaning bees.
- Replace old, dark brood combs to lower the number of spores in the hive, although this will never totally eliminate the disease. Many beekeepers remove 2 or more old combs from the brood nest each spring, replacing them with sheets of beeswax foundation available from beekeeping supply shops.
Nosema ceranae was first found in Asian honey bees (Apis cerana) in 1994. It was found in European honey bees (Apis mellifera) in Taiwan in 2005 and later in Europe, USA and other countries including Australia. It has been found in New South Wales and Victoria, but appears to be more common in Queensland.
As this is a relatively new parasite in European honey bees, the full effect of this species of nosema on individual bees and colonies is still being researched. It does not appear to cause sudden, quick losses of bees as does Nosema apis.
N. ceranae can be detected all year round, while N. apis occurs mostly in the milder seasons of autumn and spring. N. ceranae infection affects more cells in the honey bee gut than N. apis infection at the same temperature. Researchers have suggested that this difference may explain why there is a higher mortality of bees when they are infected by N. ceranae than when they are infected with N. apis. N. ceranae can kill bees faster than N. apis.
Colonies infected with N. ceranae in summer may gradually lose adult bees resulting in reduced honey production. They may even die. While dysentery may be associated with outbreaks of N. apis, signs of dysentery are markedly reduced in outbreaks of N. ceranae.
N. ceranae has been found in honey and pollen. Recent research has shown that N. ceranae spores lose viability when they are subjected freezing and chilling.
Laboratory diagnosis of nosema
Details on the collection and submission of samples for laboratory diagnosis of nosema can be found at Samples for laboratory diagnosis of bee diseases.