Are they fat and huddled together and not doing much? Because I'm running into that same issue with my pennsylvanicus this month. The advice I've been given on here is go ahead and start easing them into hibernation. There's a hibernation guide on the Handy Links post on this forum
PS: This is what my hibernation-ready colonies look like. The abdomens are fat in both workers and queen, signaling that they have built up reserves for the winter they expect to come. The brood is limited to larvae mostly, with no new eggs and few or zero pupae.
Edited by CampoKing, September 16 2018 - 3:30 PM.