| This dissertation focuses on fitness trade-offs, which can be defined as variation in the direction of natural selection on a character. My work considers how herbivorous insects affect fitness trade-offs that accompany resistance to the herbicide triazine in Amaranthus hybridus (Amaranthaceae). Triazine-resistant (mutant) plants suffer a fitness cost because their leaf-level photosynthetic rate is reduced compared to triazine-susceptible (wild-type) plants. Using this background information, I focused on how ecological and environmental factors influence fitness costs and why, in general, fitness trade-offs might show a high degree of variability.;I addressed the conditions under which herbivorous insects preferentially feed on mutant plants and display higher fitness on this genotype. Using the generalist folivore Trichoplusia ni (Lepidoptera:Noctuidae) and the specialist folivore Disonycha glabrata (Coleoptera:Chrysomelidae), I tested herbivore preference and fitness between genotypes grown across nutrient and light levels. I found that under higher light and lower nutrient availability, the preference of herbivores for mutant A. hybridus increased, as did their fitness on mutant plants compared to wild-type plants. These results were consistent with other empirical studies that tested for interactive effects among environmental factors affecting the level of carbon availability plants experience.;I also focused on the consequences of feeding damage for mutant plants. I found that mutant A. hybridus has a reduced ability to recover from feeding damage, an effect correlated with its lower investment in below-ground biomass relative to aboveground biomass. Other studies suggest that reallocation of below-ground resources aids in recovery from feeding damage. Although greater investment in aboveground tissue might offset the lower photosynthetic rate of mutant plants, it appears to diminish their recovery from herbivory.;Through a field experiment, I confirmed that the greater preference of herbivores for mutant plants and the decreased ability of mutant plants to recover from feeding increased the cost of triazine resistance. My work illustrates that the magnitude and manifestation of fitness trade-offs can be complex and vary with biotic and abiotic factors. However, by understanding the physiological basis of a resistance trait, the manifestation of its trade-offs can become more transparent. |
