This is a biocontrol agent introduced to Ottawa (and other areas now and later) to try to bring some control to the rapaciously invasive Vincetoxicum (Cynanchum) rossicum, Pale Swallowwort/Dog-strangling Vine (DSV). It is still in the test plots and hasn't been unleashed on the wider area yet. But so far, it is voraciously eating the leaves of DSV in the test plots. however, it will take thousands upon thousands of these caterpillars over many years, to have any sort of impact. Still, it is a beginning and a good one.
This is an earlier instar than the caterpillar in the next photo.