<p> <i>Arundo donax</i> is a rhizomatous perennial grass that invades riparian habitats and replaces native riparian vegetation. The galling wasp, <i> Tetramesa romana</i> (Hymenoptera: Eurytomidae), was selected for evaluation as a biocontrol agent of <i>A. donax</i> in 2009. However, knowledge is scarce on which factors influence host selection and oviposition in <i> T. romana.</i> Through my observations I documented distinct behavioral steps preceding emergence leading to oviposition in <i>T. romana,</i> and elicited an investigatory response in <i>T. romana</i> from plant volatiles extracted from <i>A. donax.</i> My results indicated that <i>T. romana</i> does use a chemical cue as a factor in host selection, but it is likely not the result of a species-specific constituent. Evidence from my study revealed that <i>T. romana</i> may use a physical cue in selecting a host as well. <i>T. romana</i> preferred round glass rods over flat glass slides, indicating that shape is an important factor in host acceptance.</p>

<p> Indirect interactions have increasingly been recognized as important forces influencing population dynamics and structuring communities. Associational susceptibility is a form of indirect effect in which a focal plant experiences greater herbivore damage due to neighboring plant identity or diversity. These interactions remain poorly understood in the context of invasion ecology, though they may be responsible for huge impacts of invasive species on native communities. This dissertation investigates the potential mechanisms and consequences of associational susceptibility of a native perennial shrub, <i> Atriplex canescens,</i> driven by an invasive annual forb, <i>Brassica tournefortii,</i> and an invasive herbivorous stinkbug, <i>Bagrada hilaris.</i> In Chapter 1, a potential associational effect is experimentally demonstrated and a phenologically-driven trait is identified as a potential mechanism for this interaction. In Chapter 2, relative host plant quality is explored for its role in mediating the numerical response of the shared herbivore, and the herbivore&rsquo;s damage impact on <i>A. canescens. </i> In Chapter 3, neighbor density, herbivore presence and herbivore density were manipulated to identify their impacts on spillover timing, extent, and fitness consequences for <i>A. canescens.</i> Overall, potential mechanisms of <i>A. canescens</i> associational susceptibility to <i> Br. tournefortii</i> and <i>Ba. hilaris</i> identified include: <i> Ba. hilaris</i> accumulation on <i>Br. tournefortii</i> followed by <i>Br. tournefortii</i> senescence and depletion, triggering <i> Ba. hilaris</i> alternative host-seeking. Associational susceptibility of <i>A. canescens</i> could not be re-created under experimental conditions, but further study is required to ascertain whether this interaction is due to experimental limitations or ecological implausibility.</p><p>

While pollinators and leaf herbivores have been a focus of research for decades, floral antagonists have been studied significantly less. Since floral antagonists can be as common as leaf herbivores and have strong impacts on plant reproduction, it is important to understand the role of floral antagonists in the ecology and evolution of flowers. I conducted four experiments to better understand the relationship between plants, floral traits, floral antagonists, and other plant-insect interactions. First, I manipulated resources (light and soil nutrients) that are known to have impacts on plants and floral traits to test how they affect floral antagonists and other plant-insect interactions. Plentiful resources increased the proportion of floral antagonists to visit flowers, but also increase tolerance of floral antagonists. Second, I manipulated flower bud gallers, a species-specific floral herbivore that destroys flowers, to test how it affected other plant-insect interactions, floral traits, and plant reproduction. Plants with flower bud gallers tended to have more pollinator visits, but this effect is due to a shared preference by gallers and pollinators for similar plants. Third, I manipulated florivory to examine how it affects subsequent plant-arthropod interactions, floral traits, and plant reproduction. Florivory had systemic effects on other plant-insect interactions, including leaf herbivores, and shifted the plant mating system towards more selfing. Additionally, I tested how several floral antagonists respond to floral attractive and defense traits to understand which floral traits are important in mediating antagonisms. Finally, I manipulated florivory, pollination, and nectar robbing to test for effects of multiple floral interactions on subsequent plant-insect interactions, floral traits, and plant reproduction. There were significant many-way interactions between the three treatments on subsequent plant-insect interactions and reproduction, indicating that the effect of one interaction depends on what other interactions are present. Understanding the role that floral antagonists play in plant ecology can help scientists determine which interactions are most important, and may help determine why some floral traits exist in their current state. Together, this work represents some of the most comprehensive research on the community consequences of floral antagonists, as well as the interplay between floral traits and floral interactions.

<p> Streams along the Copper River Delta, southcentral Alaska, exhibit contrasting thermal and hydrologic variability associated with being primarily groundwater-fed (GWF) or surface water-fed (SWF). Groundwater-fed streams are predictable both thermally and hydrologically year round, whereas SWF streams are unpredictable and exhibit more variable thermal and hydrologic regimes. These differences may strongly influence aquatic insect community structure and secondary production. Four streams, two GWF and two SWF, were sampled twice monthly from late April 2013 through August 2013 and once seasonally in fall (September) and early winter (November). Aquatic insect community structure differed markedly in both hydrologic types. Taxa richness was significantly higher in SWF (43) than in GWF (39) streams and non-metric multidimensional scaling of community structure revealed two distinct groups corresponding to the two hydrologic types. Total secondary production was higher in GWF than in SWF streams with Orthocladiinae (Diptera: Chironomidae) representing 56% of insect secondary production in GWF streams. Results from this study have strong implications for aquatic insect communities in GWF and SWF streams because of differing susceptibilities of these systems to the potential effects of climate change. Due to their thermal stability, groundwater-fed streams are less likely to be impacted by climate change, whereas SWF streams are thermally variable and more likely to be influenced. The effects of altered aquatic insect communities can cascade to higher trophic levels such as salmon and ultimately impact stream ecosystem function and the ecosystem services they provide.</p>