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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.

Rusty

HoneyBeeSuite.com

Is there a way to feed wild bees?

It’s my turn to ask questions, and I have a few of them lined up. This first one just came from a reader in Texas (Mike) and I don’t have an answer for him.

Because Texas is having such a terrible drought, the wild bees are finding little to eat–a situation that doesn’t bode well for the overwintering young (generally, they each need a little pile of nectar and pollen) or the overwintering queens.

Mike put out hummingbird feeders and is attracting nothing but–you guessed it–hummingbirds. This is odd in a way because lots of beekeepers complain about honey bees frequenting hummingbird feeders and even storing pink “honey” in their combs. [Commercial hummingbird food is often colored red.]

Most bees are attracted to food sources by both sight and scent. Sight first, until they get close, and then scent. So if the hummingbird feeder is a color the bees don’t see, it probably wouldn’t attract bees as readily as one they can see. Also, different bees see slightly different parts of the spectrum. Honey bees, for example, don’t see red (it appears black to them) but they do see ultraviolet. I don’t know which colors other bees are sensitive to, although I often see bumble bees on red flowers. Whether the bumble bees found them by color or scent, I don’t know.

As with any other “open” food source a hummingbird feeder may attract predators (wasps) as well as bees, but apparently that is not a problem for Mike who is attracting nothing but hummingbirds.

My “feeding” of wild bees has been limited to planting flowering species they seem to like. I’ve never considered feeding them beyond that, but in such a severe drought, I can certainly understand the desire to lend them a hand. Does anyone have any experience feeding wild bees? Please send me your thoughts.

Rusty
HoneyBeeSuite.com

The secret of bee tea . . . remains a secret

A sidebar in the August 2010 Bee Culture created a lot of buzz about bee tea. The article claimed that bee tea boosts the immune system of honey bees, but it doesn’t say what in the tea helps the bees. It also doesn’t cite any studies that show a beneficial effect from its use.

Bee tea is given to bees as a feeding supplement in late summer or fall if a hive doesn’t have enough honey stores to take it through the winter. It is basically 2:1 sugar syrup made with brewed chamomile or thyme tea with the addition of mineral salt.

Now mineral salt is known to be beneficial to bees, but I’ve never read anything about the health effects of brewed leaves of any sort, so I remain skeptical.

However, the optional formula includes the addition of lemongrass and spearmint oils which have been shown in laboratory experiments to enhance honey bee health. But if you use just the sugar, salt, and tea without the essential oils, I’m not sure you are getting anything you wouldn’t get with just regular 2:1 syrup and a little mineral salt.

Nevertheless, here is the recipe that appeared in Bee Culture:

Ingredients:

  • 16 cups white cane sugar
  • 6 cups hot tap water
  • 2 cups brewed chamomile and/or thyme tea
  • ½ teaspoon mineral salt
  • 4 teaspoons Pro Health or Honey-B-Healthy[1] (optional)

Method:

  • Dissolve sugar and salt in 6 cups hot water and stir until dissolved
  • Boil 2 cups water, pour over tea, and let it steep (covered) for 10-15 minutes
  • Strain tea and add to sugar solution
  • If using, add 4 teaspoons Pro Health or Honey-B-Healthy
  • Mix thoroughly
  • Fill feeders and refrigerate any leftovers

Regardless of any health benefits, the essential oils also help retard mold growth and fermentation in the feeders—a real plus.

Rusty

[1] These products contain lemongrass and spearmint oils with a soy-based emulsifier.

Candy board feeders for honey bees

A candy board feeder is nothing more than a very shallow super with a bottom. A solution of sugar syrup is cooked until it reaches the “soft ball” stage, then it is beaten until stiff and poured into the feeder. When the sugar hardens, the feeder can be inverted over the top of the hive.

Bees seem to like these well enough, but they are not my personal favorite. Here is why:

  • They are very heavy.
  • It is difficult (and dangerous) to make that much boiling syrup all at once.
  • The bees often eat just the center of the candy board and leave you with the sticky, heavy, ant-attracting mess the rest of the year.

I find that making sugar cakes—which is the same stuff poured into paper plates—is easier, quicker, and safer.

  • A minute amount of pan spray will cause the cakes to pop right out of the plates. The plates can be reused many times.
  • If I make a lot of these, I can use them as needed. Some hives take a lot, some none. It is much easier to tailor the amount of sugar to the needs of each hive.
  • I use baggie feeder rims for baggie feeders in the spring and sugar cakes in the fall. This way I get double use from one piece of equipment.
  • I can center the cakes over the cluster, making it easy for them to reach all of it.
  • I can use sugar cakes with equal ease in either a Langstroth or a top-bar hive. In a top-bar hive, I don’t need a rim; I just lay them on the top bars.
  • Left over sugar cakes are easy to store and take up little room.

If you still want to use a candy board, you can either affix a rim to a piece of plywood the size of your hive, cut down a pre-existing super and add a bottom, or alter an inner cover. The rim needs to be about 2 inches (5 cm) deep. The syrup will not run through small cracks, and it hardens almost immediately.

A common recipe for one candy board is as follows:

    • Heat 3 cups (0.7 liter) of water to boiling.
    • Slowly add 15 pounds (6.8 kg) of sugar, stirring constantly.
    • Add 1 tablespoon (15 ml) of apple cider vinegar to retard mold growth.
    • Boil until the temperature reaches 234-240°F, also known as the soft-ball stage (112-115°C). [See note below.]
    • Remove from heat.
    • Stir vigorously until the temperature drops to about 200°F (93°C) and then pour into the candy board. Warning: This stuff is hot and very sticky! Be careful.

Variations:

  • Some people like to start with more water, which is fine. It just will take longer to boil away again.
  • Some people like to add pollen substitute to the mixture after it gets done boiling. This, too, is fine. Add about 2 pounds (0.9 kg) of pollen substitute to the recipe above.
  • If you want to add some Honey-B-Healthy or Pro Health, add about two teaspoons after the mixture stops boiling. Beware: it may splatter.

The same variations can be used to make sugar cakes.

[Note: The soft-ball stage (234-240°F) is the standard temperature given in most beekeeping manuals for candy boards. However, sugar candy will migrate at this stage if it gets warm or sits in a moist environment. I prefer the firm-ball stage (244-248°F 0r 118-120°C) although it will still migrate if it gets too warm. The hard-ball stage works best (250-266°F or 121-130°C). You may have to experiment to find what you like. Candies made from pure sugar are very sensitive to temperature and humidity.]

Rusty

How to prevent moldy syrup in bee feeders

A reader asked this question yesterday and I thought it was worth writing about. Sugar syrup will mold very quickly, especially in the physical conditions of the hive. A white, fluffy growth may be seen in just a few days. A small amount of mold doesn’t seem to bother the bees, but if it gets thick and smelly the syrup should be discarded.

Beekeepers have come up with many methods to combat mold in syrup. One of my favorites is the baggie feeder because it severely limits the amount of syrup exposed to the air, and since the bees drink only from the slits in the bag, the syrup in those areas is quickly consumed.

Honey-B-Healthy contains essential oils which inhibit mold growth. This commercial product is effective because the emulsifier allows the oils to be blended into the syrup. Simply adding essential oils to syrup doesn’t work because the oil floats to the surface and accumulates in puddles, much like an oil spill in the ocean.

If you want to try making your own emulsion, the following recipe is used by some beekeepers for spring syrup:

Lemongrass oil is reputed to have strong antifungal action. You can also use a combination of half lemongrass oil and half spearmint oil.

Other beekeepers prefer to use distilled apple cider vinegar or lemon juice. Both of these decrease the pH (increase the acidity) of the syrup. Honey is relatively acidic with a pH range of about 3.2 to 4.5 so bees tolerate the increased acidity very well while most molds do not. I have not tried this method, but I’ve read that 2 to 4 tablespoons/gallon of syrup is commonly used. Since the pH of water varies tremendously to start with, it is impossible to guess how much will be needed for any one water supply. If I were to try this method, I would start with the lesser amount and see how that worked.

Cream of tartar (related to tartaric acid but not the same) is sometimes used to increase acidity, but most beekeepers today stay away from it because of reports that it can cause bee dysentery.

All these methods can work to slow mold accumulation but none of them will stop it completely. If you have consistent mold problems, you may have to feed less syrup at each feeding so it is used up quickly.

Rusty
HoneyBeeSuite