good replies here, i will just add this from randy oliver fat bees:
"bees need to be experiencing a nectar flow of some sort in order to stimulate broodrearing and to properly feed the larvae (with the exception that during early buildup after the winter solstice they will utilize stored honey). Even without a nectar flow, the queen may be laying eggs, but the nurse bees may just as well follow behind and eat them up! When food is short, those larvae that are reared will not be fed to the fullest, and may be capped early. During those times of nectar dearth when you want the bees to continue broodrearing uninterrupted (as in my area between fruit bloom and the main honeyflow, or in September), the feeding of light syrup will keep the broodrearing factory chugging along nicely. And the syrup can be really light! A ratio of 1 part sugar to 2 parts water (33% sugar) will stimulate a colony just fine. In fact, syrup concentrations of greater than 50% are beyond that of most natural nectars, and are not as stimulatory to the bees.
There’s an interesting aspect of bee behavior that has long been noted—colonies are hesitant to fully engage in brood rearing unless they have an adequate reserve of honey (Doolittle 1905). Bees can smell the presence of empty comb (Rinderer 1982), determine nectar flow within the colony (Seeley 1995), but we’re not sure how they sense the amount of honey stores (Seeley, pers. comm). But I’ve sure noticed that when I make up singles with drawn comb, the bees brood up more quickly if they have some combs of honey in contact with the cluster. Otherwise, they seem to hold off until they’ve put away some stores. Same with new colonies on foundation—they like to keep a reserve on either side of the brood nest. This makes perfect sense, since colonies that overextend themselves can easily starve during inclement weather."
