Hoary cress, Lepidium draba (L.) is an invasive perennial mustard in the United States and Canada. L. draba has been introduced in the late 1800s and is now aggressively invading croplands, rangelands, and particularly thriving in riparian areas. Hypotheses identifying mechanisms that facilitate invasion success include the evolution of increased competitive ability (EICA), the enemy release hypothesis (ERH), and the biotic resistance hypothesis (BRH). While the EICA hypothesis assumes a shift in resource allocation away from defense towards growth and/or reproduction because the invasive has been introduced without its specific natural enemies, the latter two hypotheses assume a release from biotic constraints (natural enemies and/or competition) or more favorable abiotic conditions in the non-indigenous range of the invasive plant. All three hypotheses predict increased vigor, seed output and survival of plant species in their invasive ranges. We compared individual and plant, population parameters and proportion shoot attack by endophagous insect herbivores for L. draba between its native and invaded range over a two year period in 2002/03 to provide data supporting current invasiveness hypotheses. L. draba growth parameters were in both years greater in the invaded range. Plant competition was less in the invaded range. In 2002, endophagous specialist herbivory attack and herbivore diversity were greater in the native range, however, there was no difference for attack rates in 2003. Our field data are in concordance with all three above mentioned invasion hypotheses.