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Biology (M.S.)




Herbicides are commonly used to control invasive plants. While agricultural weed scientists have focused on herbicide resistance, little is known about its importance in nonagricultural settings. Hybrid Eurasian watermilfoil (Myriophyllum spicatum x M. sibiricum) is an invasive aquatic plant that is frequently managed with herbicides throughout the United States. In this thesis, I consider the potential for natural populations of this taxon evolving resistance to 2,4-dichlorophenoxyacetic acid (2,4-D), the most commonly used systemic herbicide. Evolutionary change in a trait between generations as a result of selection requires both genetically based variation of the trait within a generation (heritable variation) and differential survival and/or reproduction among members of that generation with different values of the trait (selection differential). Using a laboratory herbicide assay, I tested for differences in vegetative growth and response to 2,4-D among distinct genotypes of hybrid Eurasian watermilfoil collected from natural populations. I found heritable variation in growth and response to 2,4-D, indicating that heritable variation required for 2,4-D resistance evolution is present in natural populations of hybrid Eurasian watermilfoil. I also considered whether herbicide application in a natural lake creates a selection differential. Using a point-intercept survey, I collected plants before and after the lake was treated with herbicides. I then conducted a laboratory 2,4-D assay on the watermilfoil collected during the two time periods. I found increased growth rates and reduced sensitivity to 2,4-D among plants collected post-treatment compared to the pre-treatment plants (all from the same generation), indicating that herbicide exposure creates a selection differential among plants with different growth rates and 2,4-D sensitivities. Taken together, these results indicate that the components necessary for 2,4-D resistance evolution (i.e. heritable variation and a selection differential) are present in natural populations subject to management. 4 These results illustrate the importance of considering evolutionary potential when managing invasive species.

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