Dysentery and Nosema - Winter Threats

Thursday 6 January 2022

In the winter cluster blog of a few weeks ago, we said that bees had nowhere to go during the winter but, actually, they do need to step outside occasionally! Honey bees are tidy creatures and prefer not to defecate inside the hive. Under normal circumstances, honey bees will always leave the hive and defecate in flight. During cold winter months or rainy periods, they will retain waste inside their bodies until proper flying conditions return and this is perfectly healthy. Problems arise when flying days are too far and few between. In this blog, we will discuss the implications for honey bee health if colonies are not able to take valuable cleansing flights. 

Dysentery and Nosema - Winter Threats

If you are checking in on your hives every so often this winter and notice bee feces spotted around the snow, you can rest assure that your bees have gotten out for a cleansing flight. Cleansing flights occur on mild winter days, when temperatures rise above 10°C and there is no precipitation. Until this happens, bees are retaining waste matter within their rectum. This is healthy unless the bees are forced to store waste for too long, at which point they could develop dysentery or, worse, a clinical infestation of Nosema spores1

Dysentery is an infection that causes honey bees to defecate inside their hive and becomes apparent through staining on hive bodies and frames. Serious infections can lead to an early death for colonies. Furthermore, dysentery can also be a symptom of a larger problem such as nosema disease.

Nosema is a disease caused by parasites that live and reproduce within the midgut of adult honey bees. Nosema species reproduce via spores, and when these spores are released during honey bee defecation, they have the potential to be orally transmitted to other members of the colony, thereby multiplying within the hive. 

If bees are forced to defecate inside a hive, the risk of spreading Nosema spores between individuals becomes greater. A study was recently published which explored the relationship between ambient air temperatures and Nosema levels in Swiss apiaries2. Researchers found that when colonies experienced fewer flight days, when midday temperatures were lower than 10°C, they demonstrated higher levels of Nosema infestations. Spore levels were highest in winter and early spring.

Cleansing flights are important for colonies which are overwintered outdoors. When colonies are overwintered indoors, temperature and light conditions are regulated such that bees will remain inside the hive. Researchers in Alberta recently examined how differences in overwintering management might affect Nosema levels within honey bee hives3. Colonies overwintered outdoors have the potential for cleansing flights, but colonies overwintered indoors are subject to less environmental stress which is also known to exacerbate Nosema populations. Results were inconsistent in terms of the level of Nosema spores measured within hives overwintered outdoors or indoors, however, colonies overwintered indoors did have significantly higher survival rates even when suffering from high Nosema levels. This suggests that colonies overwintered indoors, potentially as a consequence of a more consistent and less stressful environment, could have a lower economic threshold level for treatment than those overwintered outdoors. 

Here in Atlantic Canada, the majority of beekeepers overwinter outdoors. This means that cleansing flights play an important role in our honey bee health, in particular when it comes to Nosema control. Testing and potentially treating hives with fumagillin before wrapping up for the winter is a valuable step in integrated pest management. Paying attention to the weather conditions over the winter and considering whether your bees have been able to take cleansing flights can already indicate to you whether Nosema treatment might be necessary in the spring. 

1. Pernal, S.F. and Clay H. (eds). 2013. Honey bee disease and pests, 3rd Edition. Canadian Association Professional Apiculturists, Beaverlodge, AB, Canada 68 pp.

2. Punko, Rosanna N., Robert W. Currie, Medhat E. Nasr, and Shelley E. Hoover. 2021. “Epidemiology of Nosema Spp. and the Effect of Indoor and Outdoor Wintering on Honey Bee Colony Population and Survival in the Canadian Prairies.” Edited by Wolfgang Blenau. PLOS ONE 16 (10): e0258801. https://doi.org/10.1371/journal.pone.0258801.

3. Retschnig, Gina, Geoffrey Williams, Annette Schneeberger, and Peter Neumann. 2017. “Cold Ambient Temperature Promotes Nosema Spp. Intensity in Honey Bees (Apis Mellifera).” Insects 8 (1): 20. https://doi.org/10.3390/insects8010020.

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