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Science Insider

Wintering Colonies Indoors

- March 1, 2022 - Alison McAfee - (excerpt)

Overwinter Beehives Indoors

Some people worry that a lack of defecation flights indoors could cause problems with nosema, but research suggests this is not
a concern

The year 2021 was a record-breaking one on the weather front. We witnessed the “Texas freeze,” where temperatures dropped to 1-10 F (-12 to -17 C), causing a widespread power crisis. California continued to spiral into a deeper drought during the summer, but set heavy precipitation records in December. And, after a severe heat wave in June, British Columbia experienced devastating floods in the fall that wiped out every major highway out of the lower mainland; combined, these events killed an estimated 1.3 million chickens, pigs, and cows.

Honey bees may be adaptable, but they will never be amphibious. Around 150 beehives drowned in the BC flood — a better outcome than most people expected, thanks to the foresight of beekeepers to winter their colonies on stacked pallets and high ground. One month later, a cold snap of sustained Arctic outflow set in, leading me to insulate our colonies for the first time in our normally-mild region.

Colonies wintered outdoors are forced to withstand the elements, and indoor wintering offers tantalizing control over the climate. By keeping colonies in temperature-regulated sheds, extended bouts of extreme cold are not a problem, and colonies are less likely to starve, since the cluster can more easily move within the hive. Even a well-fed colony can starve during the winter if an extended cold snap prevents the cluster from accessing new honey frames. But indoor wintering comes with risk, too, and is most common in exceptionally cold climates like the Canadian prairies, Quebec, parts of the Maritimes, and some northern states.

Indoor wintering reduces opportunities for defecation flights, and some people worry that this could affect the severity of nosemosis. Nosemosis is caused by Nosema apis and N. ceranae, fungal parasites of the honey bee gut epithelium, which are thought to accumulate in bees that aren’t allowed to poop. The pathogens have been formally renamed as Vairimorpha apis and V. ceranae,1 but hereon I refer to them as nosema out of familiarity.

Rosanna Punko, a master’s student at the University of Manitoba, and her colleagues recently published a study evaluating nosema abundance and survival of colonies housed indoors and outdoors over winter in Alberta.2 The researchers performed their experiment at two sites with different latitudes to see if the patterns they observed were also consistent across climes. Their results? Indoor wintering does not appear to promote nosemosis.

Punko and her colleagues found that nosema infection levels were similar in colonies wintered indoors and outdoors. Moreover, indoor wintering significantly increased colony survival rates and population build-up in the spring. This was a relatively small study, with 31 colonies total (16 wintered indoors and 15 wintered outdoors, split between two sites), but it agrees with previous work.3

Nosema ceranae shows a seasonal pattern of high spore abundance in the spring with lower levels in the summer and fall,” Punko says, emphasizing that spring monitoring and treatment is important because nosema abundance in the spring predicted winter mortality at the end of the study, regardless of the wintering method. In this study, most of the colonies were infected with N. ceranae, but occasionally, coinfections of N. apis and N. ceranae were observed.

When to check for nosema

There is much that scientists and beekeepers still don’t understand about N. ceranae, one of which is how its abundance changes during the year. Many longitudinal monitoring efforts have been conducted in different countries and regions, yielding a mixed bag of results. Nosema abundance can either be unpredictable, peak during the spring, peak in the spring and fall, or peak in the spring, summer, or fall of any given year, depending on which sampling method you use and which study you consult.

If you can, it is probably best to refer to a study conducted in your region to learn how N. ceranae may behave. General disease management guidelines often still suggest to monitor for nosema in the spring and fall, but this is based on the more predictable abundance patterns of N. apis, not N. ceranae, despite N. ceranae now being the most prevalent species in most regions.

Punko’s data suggest that monitoring nosema in the spring is the most important time, since spring nosema load was a significant predictor of mortality during the subsequent winter, at least in Alberta. Exactly where in Alberta didn’t seem to matter, despite the fact that the two study sites were in very different climates. One site was near Edmonton and the second site was near Ranier, which is 2.1 C (3.8 F) warmer on annual average. That might not sound like much, but it is a big difference in temperature — about what we might expect to occur in the next fifty years due to climate change.

Despite the cold Alberta climate, typically, only about 14-25% of hives are wintered indoors, depending on the year. In a province with over 300,000 hives (the most in Canada), even a small increase in survival rates would equate to a huge potential for livestock to be saved. In Punko’s study, about 69% of indoor and only 27% of outdoor colonies were alive by the end of their second winter. “Beekeepers should not avoid indoor wintering for fear of disease,” Punko says. “Indoor wintering may help diseased colonies, if anything.”

 Surviving the freeze

Dr. Shelley Hoover, a professor at the University of Lethbridge, Alberta, and Dr. Rob Currie, a professor at the University of Manitoba, who were coauthors on the study, have also participated in other research comparing survival of colonies wintered indoors and outdoors. “The provincial stats don’t always favour indoor wintering in Alberta,” Hoover says. “For example, in 2019-2020, mortality outdoors was 32%, and indoors it was 43%.” However, she points out that these numbers can be misleading, because beekeepers tend to put their weakest, most vulnerable colonies indoors, and leave their more populous hives outside. Even with that caveat, the most recent provincial survival data show that indoor wintered colonies still fared better, enjoying about 7% higher survival than outdoor colonies.

In controlled research studies, where indoor and outdoor colonies go into winter at similar strengths, the survival benefits of indoor wintering are clear. Currie presented data from one of these studies at the 2019 Apimondia conference in Montreal, where he described how he and his colleagues compared survival of 740 colonies which were either wintered indoors or outdoors in Manitoba. The indoor-wintered colonies fared the best, with 92% surviving that winter, compared to just 77% of the outdoor-wintered colonies.

It is no surprise that indoor wintering improves colony outcomes. Research in Belgium4 and Austria5 show that cold snaps in mid to late winter are the single biggest weather predictors of colony mortality over winter, second only to varroa infestation. These findings have been confirmed using mortality data of over a quarter million colonies combined with meteorological records, where, as expected, mild winter weather was associated with improved survival, and longer durations of winter cold spells were associated with poor survival.6 While strong colonies can withstand cold snaps without a problem, colonies with comorbidities may be pushed over the edge.

Improving survival rates may also act as insurance against disruptions of transporting nucs or packages due to COVID, natural disasters, or unforeseen circumstances. As we experienced in BC, just three days of heavy rainfall was enough to cut off land routes from Vancouver to the rest of the province. If survival rates upwards of 90% can be routinely achieved, beekeepers may not need to infuse their operations with exogenous replacement stock each spring.

Since even domestic movement of colonies or packages could be affected by transport disruptions, becoming more self-sufficient would be a prudent move. Indeed, some of Hoover’s ongoing research is directly addressing this issue. She has been experimenting with ways of improving domestic supply, and describes having good success with rearing queens during canola pollination, then wintering them indoors in “replacement units” of as little as three frames in advance of the subsequent production season. “We always get very good, single digit mortality, with these new queens in small colonies that we indoor winter,” she says. 

The cost of climate control

The catch, of course, is that indoor wintering is expensive and labour intensive. An indoor wintering shed is more than just a shelter — it should be capable of maintaining a temperature of around 4 C (39 F) for months at a time. Colonies generate a lot of heat and carbon dioxide, so fans or cooling systems are needed to cycle in cold air. Humidity control also helps — too wet, and mould will grow inside the hive; too dry, and the bees will ….