Brood production continues even through a dearth of nectar--slows down not quite as large of a brood pattern, after all the field bees are not far from the end of their life cycles, and within 2 - 3 weeks will be dead they need replacements. A totally broodless combs the queen probably is dead that's precisely why I personally prefer to purchase the queens of my choosing rather then settle for whatever the bees raise, no risks of queen becoming bird feed on mating flights. Look for more then one egg in cells, not centered in cell, spotty drone brood in worker cells laying workers are then the immediate problem if not enough time has elapsed for laying workers to develop, is still opportunity to introduce new queen, but the clock is ticking, whatever your doing do it today or sooner. remember life cycle of worker bees 21 days from egg to hatching, another 3 -4 weeks for that worker to start foraging, almost 3 months for a substantial increase in population, population you will need to overwinter in MN and feed the heck out of them. consider even pollen substitute, as will need that was well if your encountering a lack of natural sources, fall will start fairly quickly up there, cooler weather even frosts in september the clock is ticking. Do you know what kind of a fall nectar flow you can expect? You do have your work cut out for you. Good luck
Barry