We are not 100% sure, but believe this a sawfly larva, rather than a butterfly or moth larva. Although “fly” is in their name, they are actually in the order Hymenoptera (wasps, bees, and ants). The larvae look very similar to caterpillars, but they’ve got a little extra in the proleg department, the “false” legs along the abdomen that look like suction cups. Butterfly and moth caterpillars have no more than five pairs of prolegs (there’s always an exception — flannel moth caterpillars), while sawfly larvae have more than five pairs. Like caterpillars, most are herbivores.
Size: Around an inch
Photo by: Glenn Marangelo on 5/22/22 near Missoula, MT