Typically this problem fixes itself, so wait it out. Not much can be done to prevent the current generation of caterpillars from finishing their life cycle as they should be pupating soon, but plan on tackling things for next year. For fall and winter, this problem can likely be treated like the winter moth as Erranis females are wingless.
See the GOERT pdf which includes options for control using Tree bands to prevent the females from laying eggs at http://www.goert.ca/documents/Operophtera-brumata.pdf and here's another good page on tree banding: http://www.truro.ca/tree-banding.html
While Btk can be used earlier in the year to kill all young Lepidoptera (moths and butterflie) Larvae, think twice before using it in or near a sensitive ecosystem with rare butterflies -- Btk is non-selective and kills all larvae!
Other great predators of these looper larvae are Chestnut-backed Chickadees. Also, be careful not to stomp on any caterpillars that look like they have eggs layed on the outside of their bodies. This could be from a parasitic wasp or fly which will hopefully help out over time.