Open brood produce two pheromones that inhibit worker ovary activation - BEP (brood ester pheromone), produced by old larvae, and E-β-ocimene, produced by young larvae.
This quote is from a research article on the plos.org website
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0013531
"Therefore, by emitting E-β-ocimene and BEP, the young and old larvae signals are involved in enforcing different worker tasks (Fig. 4); nevertheless they also have a common action in the nest: the inhibition of worker ovary activation [12], [17]. This plays a major role in the productivity of the nest because reproductive workers do not work as hard as sterile workers [40], showing a reduction in both tending to larvae and foraging tasks, which decreases the inclusive fitness of the colony. E-β-ocimene and BEP both partially inhibit the worker ovary activation, and a possible synergistic interaction needs to be tested"
So, no queen + no brood = no pheromones = DLW's - can apparently be as much as 75% of workers, which drops dramatically when they are given several frames of brood