At first glance, these two relatively common plants can, when very small and young, be readily confused. Black locust (Robinia pseudoacacia)is a member of the bean family, however, while prickly ash (Zanthoxylum americanum), is a member of the citrus family. Both are native to southern Ontario. Locust grows into a very tall sturdy tree, while prickly ash is typically considered a shrub or, sometimes, a small tree. They are easiest to differentiate when in flower, for the flowers of the locust are large white clusters, while those of the prickly ash are small and green. Prickly ash also has a small red bud at the paired spines, while black locust does not. In this photo of a giant swallowtail larva on a prickly ash - www.pbase.com/fwg/image/153339662 - you can see a red bud just below the head of the caterpillar.