Introduction

Nosema apis, which causes nosema disease, is found worldwide. Nosema ceranae, a similar parasite, was found in Asian honey bees (Apis cerana) in 1996. In 2005 it was found in Apis mellifera in Taiwan, and since in Europe, North America and Australia.

The nosema organism belongs to a unique group of spore-forming organisms known as Microspora, many of which are parasites of insects. N. apis is the most common cause of adult bee infection, and is widely regarded as being (in economic terms) the most serious disease of bees in Australia.

N. ceranae and N. apis have similar life cycles. Adult bees ingest nosema spores via contaminated water or food, by food exchange with other bees or in their duties of cleaning contaminated combs. The spores then germinate in the epithelial cells of the mid gut. Here they multiply, producing more spores, to the point at which numbers are so great that they burst the epithelial cells and are excreted in the faeces.

Worker bees, queen bees and drones are all susceptible to infection by spores, which can remain viable for considerable periods of time on hive parts.

The visual difference between Nosema apis and Nosema ceranea spores is only slight. It is not possible to ascertain with any degree of confidence the difference between these two organisms under a normal 400 power microscope. Only DNA tests can differentiate the two species. At this stage, given that both organisms are very closely related, it is very likely that they will respond to the same treatments and management.

Effects of Nosema Apis on the colony

Older field bees die off rapidly when heavily infected – the expected life span of individuals can be reduced by more than half.

Young bees assume field duties in an attempt to maintain the intake of nectar and pollen.

With fewer bees in the hive, difficulty is experienced in maintaining the brood nest temperature.

The hypopharyngeal glands of infected nurse bees do not fully develop, resulting in up to 15% of eggs in severely infected colonies not developing to mature larvae.

Heavily infected queens cease egg laying and die within a few weeks.

Dysentery is aggravated.

In serious cases of nosema disease, the colony may eventually die.

Effects of Nosema ceranae on the colony

As N. ceranae is a newly recognised pathogen of honey bees, the full effects of infection have not been determined. However, the following observations have been made: N. ceranae can kill bees in eight days, which is faster than bees exposed to N. apis

gradual depopulation, higher autumn/winter colony deaths or low honey production can occur

none of the dysentery or crawling bee behaviour usually related to N. apis infection has been reported.

Symptoms

The disease is far more prevalent during winter and early spring, with its lowest levels over summer. Infected bees often show no symptoms, or, if symptoms are present, they are not specific to nosema disease, but may also be attributed to other diseases or conditions of adult bees. Where present, symptoms include: a reduction in the colony population – this may be rapid or subtle, and is often termed 'spring dwindle', occurring in August and September

a serious reduction in honey production in heavily infected colonies, compared with lightly infected colonies

an accumulation of dead bees at the hive entrance. This is not common, as most field bees die some distance from the hive

flightlessness in adult bees. They crawl along the ground, and their hind wings may be unhooked from the front wings and held at unusual angles. This is also a symptom of pesticide poisoning or viral infestations

a sickly look and greasy-looking abdomen in adult bees

greyish-white, dull intestines

dysentery, with hives covered in spots of faecal matter. This symptom is often associated with serious cases of the disease

eath of the colony (occasionally).

Diagnosis

The only accurate means of diagnosing nosema disease is microscopic examination of the gut of infected bees. Samples for diagnosis should be collected by gathering 30 live or freshly dead bees from the hive entrance or from the top bars of the frames. Place live specimens in a cage with a small supply of queen candy, or freeze newly dead samples to keep them fresh. Alternatively, place 30 bees in a jar containing methylated spirits. Deliver or send samples to your nearest veterinary laboratory or apiary officer as soon as possible, along with your name, address and a request to test for nosema. If mailing a sample, post early in the week so that it arrives before the weekend.

NB: It will not be possible to mail samples collected in methylated spirits through the post. These will have to be delivered in person or by courier.

The procedure for measuring the nosema infection of the samples requires a compound microscope with a x400 objective. The most common method used is that published by Cantwell (1970). This method does not differentiate between Nosema apis and Nosema ceranae; to distinguish between these organisms, molecular tests, including a polymerase chain reaction (PCR) test, are necessary, to identify the specific DNA sequences of each organism.

Test results

The results received after submitting samples to the laboratory for analysis or doing your own counts will provide you with an estimated number of spores per bee, ranging from zero to many millions. There is no definite research to indicate what the safe level of infection is, but the rule of thumb is that anything less than one million spores is acceptable, and any reading greater than one million is cause for concern.

This, however, will largely depend on the time of year, as a low level of nosema spores at the beginning of winter may need to be taken more seriously than a similar level at the end of winter. If high readings (many millions) are obtained, it can be expected that the infected bees will be very short-lived, and that the colony will struggle to increase in population. This is typical of the 'spring dwindle' phenomenon. In extreme cases, the colony dies.

In most cases, once the nosema levels have reached very high levels, the colonies will take a number of generations to recover.

Methods of control

Management practices

Climatic conditions play a major role. Protect colonies from cold, wet winds and locate apiaries on the northern side of a hill. Ensure hives have maximum exposure to sunlight during autumn, winter and early spring.

Keep the hive dry – elevate it if possible. In northern New South Wales and Queensland, cane toad stands have been found to reduce nosema levels.

In autumn, compact the colony in preparation for winter. Do not leave excess boxes on each hive; reduce the colony to one or two boxes for the winter period.

Placing beehives on the cooler tablelands will cause the colony to become broodless. This reduces the stress and demands on the colony, and will assist in keeping nosema levels low.

Ensure the hive is headed by a young queen with a strong population – this will greatly reduce any adverse effects from nosema.

From early autumn, avoid moving brood combs around a hive or between hives.

The above article has been reproduced from the website of the AHBIC (Australian Honey Bee Industry Council) for the education and information of readers. :