Title: Effects of symptomless infections with Nosema sp. on honey bee colonies
Author: Reinhold Siede1, Stefan Berg2, Marina Meixner1
1) LLH- Bieneninstitut Kirchhain, Erlenstr. 9, 35274 Kirchhain, Germany 2) LWG
Fachzentrum Bienen, An der Steige 15, 97209 Veitshöchheim, Germany
Abstract: The fungal pathogen Nosema sp. has recently been suspected to cause serious
losses of honey bee colonies in Mediterranean regions. Especially the newly detected
species in Europe, N. ceranae, has been linked to cases of colony mortality. The
distribution of N. ceranae in central and northern Europe is well documented, but little is
known about its impact on colony performance under temperate climatic conditions. This
study aims to elucidate the relationship between an infection of Nosema ceranae and
colony mortality and the effects of symptomless infections with Nosema sp. on honey bee
colonies.
Eighteen honey bee colonies in Germany were observed for one year, from July 2005 to
June 2006. Samples of live worker bees were collected and tested for the presence of
Nosema spores in July, August, February and April. The strength of each colony (number
of bees and brood cells) was measured in July, November and May. Seventeen of the
colonies showed high infestation rates with spores of Nosema sp. at the beginning of the
observation period. Subsequently, spore counts decreased in the fall, when spores were
found in only three colonies, but were high again in February, with 10 out of 18 colonies
testing positive for Nosema. By April 2006, however, only 4 of the surviving 16 colonies
contained Nosema spores. None of the test colonies showed typical symptoms of Nosema
disease at any time. Molecular analysis of spore samples revealed that Nosema ceranae
was prevalent in most colonies. Few mixed infections between N. ceranae and N. apis
occurred in July 2005 and April 2006, but single infections with N. apis were never
observed. The amount of Nosema present in each bee sample was determined using
quantitative PCR and related to colony strength and brood area. Although no symptoms of
disease were present at any time during the observation period, an infection with Nosema
spores in February and April had significant negative effects on colony strength and brood
area in the following May. Colonies with Nosema spores present in August also showed a
tendency to be weaker with less brood in the following spring, but the effects were not
significant. We could not differentiate between effects of N. apis and N. ceranae in
symptomless honey bee colonies.
In conclusion, an infection with Nosema ceranae affected colony strength, but did not
result in winter fatalities. With regard to overwintering success of honey bee colonies, N.
ceranae thus appears to be a low-risk pathogen. However, larger field studies covering a
longer observation period are required to correctly assess the risk potential of an
infection with N. ceranae.
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