Noserna disease, an obscure killer, is caused by a spore-forming protozoan (Nosema apis) that invades the digestive tracts of honey bee workers, queens, and drones. Spores of the disease are ingested with food or water by the adult bee. The spores germinate and multiply within the lining of the midgut. Millions of spores are shed into the digestive tract and eliminated in the feces. Damage to the digestive tract may produce dysentery and weaken the bees. As a result, the productive life of the worker is shortened and its ability to produce brood food decreases, thus retarding brood production and colony development. When queens become infected, egg production and life span are reduced, leading to supersedure. Infected workers, unlike healthy workers, may defecate in the hive. Diseased colonies usually have increased winter losses and decreased honey production. The loss of queens in colonies just started from package bees is the most serious effect of the disease.

Even though nosema is a common disease of bees, it generally goes unnoticed in the apiary, since it does not produce signs or symptoms that are easily recognized under field conditions. In fact, the presence of the disease is not usually realized until most of the bees in the colony are infected. The only positive way of identifying the disease is through dissection of adult bees. The hind gut and digestive tract of diseased bees are chalky white or milky white. Healthy bees, on the other hand, have amber or translucent digestive tracts. In addition, individual circular constrictions of a healthy bee's gut are visible, whereas the gut of an infected bee may be swollen and the constrictions not clearly visible. Infection is best detected by the microscopic examination of macerated abdominal tissue for the presence of spores.

Nosema disease is most prevalent in the spring. Severity of infection varies among colonies, geographic regions, and from year to year. In over-wintered colonies, confined infected bees may defecate on the combs and infect healthy bees as they clean the combs in the spring. Food stores and soiled shipping cages are other sources of infection. Spores are spread by infected package bees, splits, and contaminated equipment. Combs from weakened colonies that died during the winter often have nosema-contaminated feces. Installation of packages or divisions on this equipment in the spring hampers colony development and often results in queenlessness.

Queens may become infected from various sources after they emerge from the queen cell or are released in the mating nucleus. When the disease is severe, colony populations may become depleted and eventually dwindle to a handful of bees and a queen. This is often referred to as "spring dwindling." In colonies that are only mildly affected, brood rearing eventually allows the colony to recover.

Colony confinement during winter or inclement weather in the spring encourages disease buildup. Winter cleansing flights enable bees to defecate outside, thus preventing spore contamination within the cluster. Nosema-sick bees often fly from the hive at marginal flight temperatures, probably because they are under stress. Since they are weak, they drop to the snow, become chilled, and are unable to return to the colony. Sick bees are thereby eliminated from the colony. The intensity of infection usually subsides in April as field flights begin and brood emergence accelerates.

Brood emergence, the colony's primary natural defense against nosema, replaces infected bees with young healthy bees. If nosema is already present, any break in the brood rearing cycle will likely increase the incidence of the disease, especially in early spring. Newly hived package bees are very susceptible to nosema. During the first three weeks following installation, when the colony has no emerging young bees, the disease spreads rapidly through the old adult population and to the queen.

The best defense against nosema is to winter only strong colonies with plenty of honey in the proper position and with young vigorous queens. Many different chemicals have been tested for control of the disease but only Fumidil-B® or Nosem-X™ (Fumagillin) have proven effective. Fumagillin is especially effective in suppressing nosema in overwintered colonies and newly established packages. Since Fumagillin does not affect spores of the nosema parasite, treatment with this drug will not completely eliminate the disease from the colony. The infection will continue after all the medicated syrup has been consumed.

For optimal nosema control in overwintered colonies, initial infection levels should be reduced in early winter. In late fall, when brood rearing normally declines, colonies should be fed about 1 gallon of heavy sugar syrup (two parts sugar, one part water) containing Fumagillin. The syrup should be stored where the last brood emerges and used as the colony's first winter feed. This procedure delays the initial buildup of any infection from winter confinement, which keeps the disease from reaching the high levels seen in unprotected colonies. Colonies should receive a minimum of 1 gallon of medicated syrup containing 75 to 100 milligrams of Fumagillin (1 1/2 level teaspoons of Fumidil-B or 1 heaping teaspoon Nosem-X per gallon of syrup) in the fall. Packages newly installed in the spring should receive similar treatment. Fumagillin treatments are most effective when fed with sugar syrup. Research has shown Fumagillin's effectiveness is limited when fed with powdered sugar, extender patties, candy, or pollen supplements.

CAUTION: No medication should be fed to colonies when there is danger of contaminating the honey crop. Be sure to stop all drug feeding at least four weeks before the onset of the main surplus honey flow.