Graduation date: 2008Many non-native weed pests of food, fiber, and nursery crops pose threats to U.S. environment and agriculture. Noxious weed regulations play an important role in preventing the introduction and spread of non-native plants, thereby protecting the local biodiversity, environment and economic activities. However, these regulations could also hamper agricultural trade. The three essays of this dissertation address the causes and consequences of cross-state differences in noxious weed regulations. In the first essay, a political-economic model is developed to derive cross-state regulatory congruence as a function of ecological and agronomic characteristics, and interest-group lobbying through political contributions. Econometric estimation suggests ecological and agronomic dissimilarities are primary causes of cross-state differences in noxious weed regulation. However, evidence of lobbying effects is statistically and economically significant. In particular, agricultural seed producers favor more uniform weed regulations across states, while commodity producers and consumers prefer that their state maintain a distinct weed list to protect its ecosystem and agronomic conditions. The impact of noxious weed regulations on inter-state agricultural trade is measured in the second essay. In particular, the extent of trade distortions arising from interest-groups' lobbying is estimated. For this purpose, each state’s net trade in three industries - agricultural seeds, nursery products and commodities - is derived as a function of weed regulatory congruence. The resulting spatial-autoregressive tobit model of inter-state agricultural trade is estimated using a modified Expectation-Maximization algorithm. The empirical results show that the noxious weed regulatory congruence has a positive and significant effect on inter-state trade flows. Moreover, the inter-state trade distortion from consumer and commodity-producers lobbies is estimated to be as much as $1.1 billion. In the third essay, a leader-follower framework is used to examine when a weed species on one state's (leader) regulation is added to that of another (follower) state. For this purpose, an ordered logit model of noxious weeds' listing time is specified as a function of invader characteristics and ecosystem invasibility. The empirical results suggest that the invader status and attributes are relatively more important than ecosystem characteristics in determining the weed listing time. Together, the three essays suggest greater emphasis on the use of science than interest-group lobbying for listing noxious weeds. Moreover, a better documentation of potential invaders, their characteristics and impacts on non-native environments, is important in controlling the introduction and spread of noxious weeds, and the consequent economic and ecological damages.