hive preparation for winter

 

Refer to Derek’s 2024 paper in the Journal of Thermal Biology, “Are man-made hives valid thermal surrogates for natural honey bee nests?” Derek reports that “bee space above combs increases heat loss by up to ∼70%; hives, compared to tree nests, require at least 150% the density of honey bees to arrest convection across the brood area.”

In a hive, brood heat typically circulates by (1) rising between frames, (2) striking the inner cover, (3) spreading sideways, and (4) descending through the cold bee space.

To improve insulation, I added an external Bee Cozy (R-value 8) and took internal measures to limit brood heat circulation by blocking the bee space above the combs. I used letter-sized plastic sheets (the type made for overhead projectors), placing one spliced and trimmed sheet directly on the top bars beneath each hive’s inner cover.

learning from natural honey bee nests

It’s winter and I’m thinking about my bees in these recent cool temperatures; my curiosity turned to Google Scholar and searching for the beekeeper and engineer - Derek Mitchell. Here I found Derek’s 2024 paper in the Journal of Thermal Biology - Are man-made hives valid thermal surrogates for natural honey bee nests.  This is a highly technical read with an exotic mix of scientific units, but I found take-home nuggets useful to my hobby beekeeping. My goal is not to keep bees in a tree, but to modify my wintering of bees in a thin-walled wood box by learning what bees have, on their own, achieved living in tree cavities.

	Man-made hive	Tree nest
Top ventilation	Beekeeper’s  choice	None
Wall properties	Thin low R-value	Thick high R-value
Comb to wall surface	Continuous bee space separation	Comb attached to the wall with a few small gaps
Comb to roof surface	Continuous bee space separation	Comb maximally attached to tree cavity roof
Comb to bottom surface	Continuous bee space separation made more complex with multiple boxes	One continuous separation
Vertical comb length	Single frame height whether using one or multiple boxes	Continuous

Made with HTML Tables

Using 16 variables adjusted to experimental observation, Derek used computational fluid dynamics (CFD) modeling and compared the convective air flow and metabolic impact to bees in a standard British National Hive (one box of 35 Liters) versus a synthetic tree nest (45 liters).

Derek writes, “bee space above combs increases heat loss by up to ∼70%; hives, compared to tree nests, require at least 150% the density of honey bees to arrest convection across the brood area.”

Derek recommends blocking the bee space above the combs in a man-made hive, which I interpreted as adding a clear plastic sheet resting directly on the top bar frame. I have a box of letter-sized plastic sheets designed for overhead projector presentations - it seems perfect for the task. Stay tuned for how this works out for me as I am over-wintering two hives with a different number of boxes. One hive has two medium boxes, and the other hive has one deep box. I spliced sheets together with tape and trimmed the extended sheet to fit inside my eight-frame box. Each hive received one trimmed plastic sheet beneath its inner cover resting directly on the top bar frame. No doubt the bees will glue the plastic to the top bar frame and this will require some vigorous spring cleanup effort. 🤣

Perhaps you want to give this small modification a try and use different materials (e.g., wood, thick acrylic, or coroplast).

does the cluster provide insulation?

I discovered 2 interesting articles by Derek Mitchell (PhD Candidate in Mechanical Engineering at the University of Leeds), one is intended for a general honey bee audience, the other a hard-core engineering-science article.  This blog post contains 2 images from those articles.

• Honeybees cluster together when it’s cold – but we’ve been completely wrong about why
• Honeybee cluster—not insulation but stressful heat sink 
  

 Facts about bees wintering in thin-wall wood hives:

• bees move from the hive edges towards the center of the hive to form a cluster
• as temperatures fall, the cluster shrinks - see the image from Derek's article
  
• the cluster core is hot compared to the cluster periphery.  
• bees in the core consume honey and keep bees in the periphery warm.
• in extreme winter temperatures, bees on the periphery risk death by hypothermia
• the cluster periphery is denser than the core

Derek writes, "Since the early twentieth century, the outer layer (mantle) of honeybees (Apis mellifera) in the winter cluster has been said to insulate the cluster core."  

A shrinking cluster increases density, reduces porosity, and increases heat loss. Here I'm using porosity to describe the amount of empty (air) space, or porosity = empty space ÷ total space.  Typically a thermal insulator reduces heat loss by trapping air and is therefore low in density.  The higher density cluster periphery is not a thermal insulator - has never been a insulator.  Beekeepers need not stress bees in extremely cold temperatures when insulation is readily available. 

• take a quick look in beekeeping catalogs and you'll discover flexible hive wraps - flexible wraps roll-up for easy storage and reusable next winter
  
• solid foam board insulation for home improvement is available for purchase or free as building waste

Bees have evolved to live in trees (a nest with extremely thick wood walls) that provide real insulation - see the image from Derek's article. To those protecting popular opinion, to the skeptic, to the frugal beekeeper, consider winter insulation using a side-by-side test.

winter ventilation and pocket IR thermometer

Last year as an impulse buy, I purchased an inexpensive RadioShack Waterproof Pocket IR Thermometer.  This budget gadget has a limited range, so get close to your target when taking a temperature reading.  At dawn I record the top vent and concrete paver temperature.  The paver sits on top of the hive and acts as a surrogate for smoothed outdoor air temperature.  The top vent temperature acts as surrogate for bee respiration temperature.  Imagine this - on cool mornings, the bees are tucked inside the hive, cold air flows into the entrance and screened bottom while warm moist bee respiration exits the top vent - sometimes I discover the warm moist air condensed to the outside of the hive cover - see photo.  

What is the relationship between outdoor air and bee respiration temperature?  During a cool morning, the respiration temperature is greater than the air temperature.  In other words, all graph points (and trend lines) lie above the y= x line.  For example, a dead hive plots on the y= x line.  Bee respiration temperature decreases with decreasing outdoor temperature - this is consistent with air exiting the top vent as a mixture of fresh air and bee respiration.  In other words, the warm bee respiration is diluted by cooler fresh air.

How do the hives differ?  Three hives (Kent, Buda and Pest) are located in a row, but each has a unique amount of shade and access to morning sunshine.

• Pest's third order polynomial trend line respiration temperature is warmer than the other hive trend lines. Probably Pest has the largest bee population. Pest is a 2011 swarm, captured with the help of Cassandra and John. Pest is a powerful force of nature - what Cassandra refers to type A behavior.
• Kent's trend line respiration is less warm than the Pest trend, yet Kent usually flys more vigorously than the other hives. FYI, Kent receives more morning sunshine than the other hives. My range of temperature measurements (Nov 18th to Dec 3rd) contains many stretches where the mid-day outdoor temperature is too low for foraging. Perhaps Kent's interesting sigmoid trend line represents 1) winter clustering in the flat lower left region, 2) typical non-clustered state in the flat upper right region and 3) a sloped line which represents a continuum of changes connecting these end-points.
• Buda's trend line respiration is cooler than the other two hives. Buda is a 2012 spring split. Probably Buda has the smallest bee population compared to the other over-wintered hives.

IPM counting board debris

I'm interested in whether the debris on the IPM counting board correlates with the winter cluster location.  I'm using "debris" to describe everything that falls through the screened bottom board--not just varroa mites.  Is there a correlation between the winter cluster location and the debris--how can I use this information? 

In the image above, counting board debris marks the bee space between frames and the absence of debris marks the location of the eight frames.  As the IPM grids does not line-up with the frames, I'll flip-over my counting board so that the white un-marked side faces up. I'm tired of ignoring the counting board grid lines and using my finger to count the frames.

In the image above, the bees on the frames mark the top of the winter cluster (3D sphere)in the first deep hive body.   The counting board debris and winter cluster locations seems to line-up.  To make the debris more obvious, I'm scraping and washing the counting board surface clean after every inspection.

During spring swarm control, a quick look at the counting board debris may help plan which honey frames to remove for "checkboarding" in the second deep hive body or which frames get starter strips to "open the brood nest" in the first deep hive body.