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.