The answer of this question is actually – they can’t. Bees can never turn sugar syrup into honey. Syrup is made from granulated sugar (sucrose) dissolved in water. After the bees get done finagling with it, enzyming it, fanning it, and storing it you still have sugar in water—nothing more.
The idea that bees can change syrup into honey comes from the mistaken belief that enzymes in the bee’s honey stomach are responsible for creating honey. But it’s the chemical compounds in nectar—an astounding array of different substances—that gives honey its flavor and aroma.
In spite of its lack of substance, bees treat sugar syrup as if it were honey. They take it into their honey stomachs, pass it around, store it in cells, and dry it to the proper moisture level. This is why honey producers never feed syrup while honey supers are in place. If syrup is readily available to bees, the real honey soon becomes contaminated with the syrup.
I knew a beekeeper who fed sugar syrup to her bees all spring and summer with honey supers in place and then marketed her product as “pure honey.” When I asked her about it, she explained to me that the bees ate the sugar syrup which gave them lots of energy to collect nectar and make honey. She saw nothing wrong with the practice because she thought the bees treated the substances differently. No amount of explanation on my part made an impression on her and, as far as I know, she still does it . . . and teaches a beekeeping class as well.
The important point here is that although syrup cannot be made into honey, bees treat syrup no differently than nectar. If we interfere with the bees’ life processes (by feeding sugar syrup) we must understand the consequences of our actions and take steps to avoid problems.