The syrup we feed bees in the fall is generally in the ratio of 2 parts sugar to 1 part water, either by weight or volume. That means the mixture is about 66% sugar and 34% water. But before the bees cap syrup (or honey) they dry it to roughly 17-18% water. Using round numbers, let’s call that 20%.
Now a solution of 80% sugar to 20% water is in the ratio of 4 to 1. So if you were making syrup this thick, you would have to put twice as much sugar into the water as you do for 2 to 1 syrup. That is really hard to do.
And remember we rounded up to 20%. If we wanted 18% moisture we would need 4.56 parts sugar to 1 part water, or 4.56 pounds of sugar to 1 pound of water. The 17% number requires 4.88 pounds of sugar to 1 pound of water—dangerously close to 5:1. As you can see, capped syrup (or honey) has very little water in it.
So the bees take their 2:1 syrup, store it in cells, and fan like crazy to drive off the extra water, of which there is a lot. Trouble is, as the ambient temperature gets colder in the fall, it becomes harder and harder to drive the water from the syrup. Not only is the liquid colder, but the cold air surrounding it can’t hold as much moisture as the warm air of summer. Add to that there are fewer bees doing the work. Everything slows down and capping takes forever if it happens at all.
Eventually, when the syrup itself reaches about 50° F (10° C), the bees give up and the syrup sits unattended in the feeder. You simply cannot feed liquid syrup to bees once the temperature of the syrup dips too low, which is why beekeepers use candy boards, fondant, or dry sugar for colonies that need a feed supplement in winter.