This is a photo of a cottonwood sucker leaf.  The main tree sent up new growth in the middle of summer creating this healthy, thick, gargantuan-sized leaf.  (Note the size of a mature fifth instar red spotted purple caterpillar for comparison).  Feeding Limenitis caterpillars healthy host isn't the triggering mechanism to avoid diapause (long day photoperiod is); however, it's always a good idea to feed your caterpillars healthy host plant.  For some species groups, healthy host plant is the triggering mechanism for caterpillars allowing them to avoid hibernation and providing you an opportunity to get a reared adult sooner rather than later.