Nosema is historically considered the most serious disease of adult bees. It is caused by a single-celled microsporidian Nosema apis (Fig. 1) which exists in two stages – a long-lived spore and a replicating vegetative stage. If an adult bee ingests spores they germinate into the vegetative stage which penetrates the cells lining the bee’s gut. Nosema does not always kill a bee outright, but may trigger associated morbidities, including reduced lifespan, reduced output of brood food and, in the case of queens, increased supersedure rates. These symptoms may also be associated with tracheal mites. The collective result is colonies with low populations and sluggish spring buildup. The disease is regarded as more damaging in cold climates or under conditions that promote a protracted period of confinement in the hive.
A second species, Nosema ceranae, a natural associate of the eastern honey bee Apis cerana, has emerged in recent years as a problem in Europe, mainly Spain, where it is suspected of causing large-scale colony deaths. N. ceranae is known to occur in the US as well, and it probably contributes to similar occurrences of colony morbidity. It is nearly impossible to discriminate N. apis from N. ceranae without molecular techniques. Being a non-natural parasite on A. mellifera, there is concern that N. ceranae may have higher virulence toward the western honey bee – a pattern typical with many non-natural parasite relationships. However, studies from Canada and the US have failed to firmly associate Nosema disease with wide-scale colony deaths in the mid-to-late 2000s, sometimes called Colony Collapse Disorder (CCD).
There are no IPM approaches specifically targeted against Nosema disease, and its management centers on the maintenance of robust colonies and biennial application of the antibiotic Fumagilin® B. The medication is mixed in sugar syrup according to manufacturer’s recommendations and fed to bees in spring and fall. As with any medication, it is applied in the off-season when there is no chance it will contaminate marketable honey.
Resource posters, courtesy of Dr. Marla Spivak, University of Minnesota