Influence of insect herbivory, plant competition and plant defense on the invasion success of hoary cress (Lepidium draba L. (Brassicaceae))

This thesis addressed two goals: to increase knowledge of the basic biological responses of the invasive mustard hoary cress, Lepidium draba, to a variety of biotic stressors, and to study the processes of clonal plant invasions in light of three invasion hypotheses: biotic resistance, enemy release, and evolution of increased competitive ability (EICA).;To test the enemy release hypotheses, I identified four oligophagous insect herbivores from the introduced range to act as surrogates for potential biological control agents to assess the impact of specific feeding niches on the target weed. I applied this novel technique to assess the role of increased generalist herbivory on hoary cress by examining the density-dependent impact of these herbivores individually and in combination. Defoliation by the diamondback moth Plutella xylostella had the strongest, most consistent impact, while the stem-mining weevil Ceutorhynchus americanus had the highest per capita effect. In multiple-species experiments, herbivore impacts were usually additive, and hoary cress tolerated high herbivore densities.;I investigated the competitive ability of hoary cress against ten Festuca and Poa grass species from North America and Europe. In both genera, Eurasian species were more competitive against L. draba than their North American congeners. Regardless of origin, sod-forming Poa species appeared more competitive against L. draba.;I compared vegetative regeneration, herbivore damage, and glucosinolate defense induction of L. draba from European and North American populations in a shared greenhouse EICA experiment. The number of root buds/cm was greater in European populations, potentially allowing better establishment in competitive environments, but no differences were observed between ranges in later stages of plant growth. Feeding rate and survival of P. xylostella also did not differ. Insect herbivory had no effects on the eight identified glucosinolates. Comparing glucosinolate profiles with those previously reported indicated that the plants may have already been in an induced defense state; therefore they were not responsive to herbivory. There were no differences in glucosinolate profiles between ranges.;I conclude that biotic resistance, enemy release, and EICA play a part in the invasion success of L. draba, but that none of these hypotheses completely explain its dominance in North America.