feeding bees physics for beekeepers

Physics for beekeepers: heat transfer in sugar syrup

Honey bees can elevate their thoracic (core) temperature by exercising their muscles and generating heat. However, below an air temperature of about 57-59°F (14-15°C) an individual bee can soon become immobile if she doesn’t work hard to stay warm. Surely you’ve seen one on a cold landing board—alive but barely able to take a step. This occurs because as her surroundings get colder it becomes more difficult for a lone bee to maintain a temperature that allows her to function. According to Peter G. Kevan in Bees, Biology and Management, a worker bee’s minimum rate of metabolism occurs at about 50°F (10°C).

So a feeder of syrup at or below 50°F—no matter how badly it’s needed—is useless to bees. If they drink it, it will lower their thoracic temperature to a level where they cannot move—cannot even crawl back to the cluster. Since drinking it may mean death, they leave it alone.

But, you argue, the day was sunny and the thermometer on your porch read 65°F (18°C) for over two hours! But they still didn’t drink it. Why not?

The answer to this question lies in the ability of heat to transfer between different substances. Remember, it is the temperature of the syrup, not the air, which is important. In your hive, the air may warm up quickly, but the sugar syrup stays cold. A number of factors affect how fast the heat will move from the air to the syrup, but density is the major one—sugar syrup is denser than air so it takes longer to heat up.

Imagine the process in reverse. Let’s say that in your freezer you have a loaf of bread and a package of ground meat which are roughly the same size. You take them out of the freezer and place them on your kitchen counter. Which one thaws quicker? No contest, the bread will thaw hours before the ground meat. If you let them come to room temperature and then return them to the freezer, the bread will freeze in no time compared with the meat. The difference in the freeze and thaw rates is largely due to the difference in density. Density is defined as mass divided by volume (If it helps, think weight divided by volume.)

Once you start having consistently cold nights—nights in the 40’s or lower—it’s a fairly safe bet that your bees are done taking syrup for the winter. Even on a very warm afternoon, it will take many hours to bring that syrup up into the 50s. Sure, some people will claim that their bees drink later into the winter than others. This occurs because the density of syrup is not the only factor to consider.

For example, if your syrup dispenser is flat and shallow (like a baggy feeder) it will change temperature more quickly than one that is cylindrical (like a pail feeder.) The material the container is made from also makes a difference. Plastic transfers heat at a different rate than metal which transfers heat at a different rate than glass. And like the bread and meat, these differences work in either direction. A flat baggy feeder may get cold faster than a cylindrical pail feeder, but it will warm up faster as well.

About the only way to make your bees drink syrup in cold weather is to heat it. Some folks do this in order to get the bees to finish a batch of syrup, but it is time consuming and the influx of cold air when opening the hive may harm the bees. Far better to start feeding early in the fall and be done before the cold weather sets in for good.




  • The alternative, fondant, relies on the bee liquidising the sugar with saliva, so presumably the substance is then at the correct temperature automatically – is that correct?

  • Very interesting. So . . . does this mean I should stop open feeding? Our nights have been in the 20s but our days in the 50s. I keep shallow dishes filled with rocks and marbles and then pour in 2:1 syrup. Shallow and metal or ceramic, so the liquid is certainly fluid. Under cover and sitting in the sun, most of the time. When it starts getting close to 50 degrees, activity certainly slows. Do you think this is dangerous to them? The feed I had put in the hive wasn’t being touched, so I removed that. I felt this way they would at least get fed.

    • I do not think the open syrup is dangerous to the bees because they know whether to drink it or not. If the outdoor syrup is in the direct sun and in a shallow container it will warm up quickly compared to the syrup inside the hive. It would be interesting to stick a thermometer in it and see how warm it gets compared to the outside temperature.

  • Rusty –

    Going back to my question about temperatures a few days ago, this post plus the questions and replies are exactly what I really needed to know!

    My local club are great people but half are inexperienced and the other half are set in their ways. (e.g. moisture quilts which I am building = newfangled gimmicks that won’t work cause the chips will get soaked.) Just want to say I would be totally lost & panicked without your blog and your community of readers.

    Look forward to many more panicky questions from here, and thanks again!


    • Nan,

      I know this isn’t the point of your comment, but I can’t let it go. 1) To anyone who thinks quilts are “newfangled,” Warre beekeepers have been using them for years with great success. I just modified the idea to fit a Langstroth. 2) The chips get soaked? Do they mean instead of the bees? And??? Isn’t that the idea? My chips never get wet more than about 1/2-inch. But if they did, I would exchange them for dry chips. Dah. If on a budget, you can dry the chips in the oven and put them back. Jeese, people are weird.

  • Rusty, s’ok, thanks for a chuckle. People are just resistant to things they haven’t heard of.

    So, question since we’re still in Beekeeping Physics. Our weather is so erratic, and nights are 35 – 50. They’re still gathering from the asters, but they get out late in the morning. Is there a downside to just getting feeder rims on all of them and starting with sugar cakes now? Since they can’t store sugar, will they eat it and save the honey for serious cold weather?

    Nobody hereabouts uses sugar cakes, so I’m not even gonna ask . . .



    • Nan,

      I usually don’t feed sugar cakes or candy boards until January (or unless the bees are clustered on the top bars), but there’s no rule that says you can’t do it now. Funny thing about hard candy, though—as much as it is maligned by many beekeepers, bees nearly always prefer it to honey. Given a choice between honey and sugar cakes, they go for the cakes.

      I mention this because if you feed it now they may start eating it with relish (as my husband would say). Then, later on in the winter, it’s hard to tell whether they are out of honey (and moving up looking for food) or if they just have this thing for candy cakes (with relish) and are hanging around up there waiting for you to make more.

      To put it another way, I see bees clustered on the top bars as an indicator of low food supplies. But since they like the candy cakes so much—and are attracted to them—it confuses the issue. Do you follow?

  • Rusty,

    Point taken about the candy cakes. Goats surrounded by 30 acres of diverse browse will fight over a flake of hay, probably because it’s concentrated sugar. I’ll just let them keep foraging for now. But if we get one of those diurnal cold fronts, maybe I’ll slip some in to keep them close to the hive. Ask your husband if they prefer sweet or dill relish. He’s the Energy Conservation expert? Clever you, to have married one. The only energy my ex ever conserved was his own.

    Have a great weekend!

  • Hello,

    I live in Wisconsin. I am going to buy some nucs soon to start keeping bees but am reluctant to do so because of the harsh winters we have here. My family has kept bees all my life in Florida, shipping them all over for pollinating but always back down to Florida for the winter. I don’t have that capability so will be keeping them here all year. I’m worried about die offs. I don’t know anyone around here that keeps bees, so I don’t have anyone to run things by. What would be my best course of action for winterizing my bees.

    • Will,

      Much of what I know about keeping bees in harsh environments I’ve learned from Canadian beekeepers. If it were me, I would do the following:

      1. keep hives in a sunny location
      2. wrap hives in tar paper which acts like a solar collector when the hive is in the sun
      3. keep bees in double deeps or triple mediums (not singles)
      4. use ten-frame hives but replace end frames with follower boards (for dead-air space)
      5. put a feeder rim on top of the upper brood box for emergency feeding (remember to leave yourself a way in through tar paper)
      6. place a 4-inch moisture quilt with vents on top of the feeder rim
      7. fill moisture quilt with wood chips
      8. use a telescoping cover with a sheet of styrofoam insulation
      9. use a small lower entrance with mouse guard
      10. remove any frames containing very dark honey and leave the bees with light-colored honey
      11. remember to treat for mites before winter arrives, preferably in late summer/early fall so you can have a colony of virus-free bees going into winter

  • Will,
    This may help you find a local friend you haven’t met yet. http://www.michiganbees.org/links/ For a state with the honey bee as the state insect, finding clubs via Google and the like is a little spotty. I haven’t checked all states, but I have found that if I look for ‘ State beekeepers association’ I get links that never show without “State” in the search.
    The state level associations typically have a list of organizations within the state.

    Those local clubs are a huge help getting started and surviving in that region. They are also great places to talk to people who won’t think you totally stark, raving, mad when you excitedly speak of your bees. 🙂


    Number 10 in your list, is the recommendation for the lighter colored stores because of a smaller ash/solids content in the honey?

  • When winterizing the top of the hive, has anyone ever heard of a board you put on the top that absorbs moisture that starts with the name homo…something or other? Thanks!

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