Identification of these two, very different looking, caterpillars had us stumped for a bit …but we believe these are actually the same species. The white / cream-colored caterpillar is simply an earlier instar (younger caterpillar) than the black and rusty-brown caterpillar. Notice they both have similar white hairs sprouting from either end. The caterpillars prefer to feed on leaves of poplar and willow (pictured here), but will also feed on alder, basswood, birch, maple, and oak. Photo by Virginia Heinz on 8/24/19 in Stevensville, MT.