Establishment of plant and insect species in novel habitats provides an opportunity to explore the significance of community composition on colonization and persistence. As a widespread invasive weed, Russian knapweed (Rhaponticum repens: Asteraceae, Asterales) occurs throughout the western US, in both natural and agricultural lands. The gall midge (Jaapiella ivannikovi: Cecidomyiidae, Diptera) is a biological control agent for Russian knapweed. The knapweed gall midge induces gall formation in the apical meristems, altering plant growth. This ability to alter host traits makes galling insects useful organisms to study indirect species interactions with other herbivores. Ramet removal, specifically through grazing by ungulates, may also be useful for knapweed control. We asked: 1) How does manipulating ramet density through simulated grazing impact insect establishment? 2) How do insect presence and ramet removal impact Russian knapweed vegetative traits? In summer 2016, insect establishment and plant traits (height, leaf counts, branch counts, and aboveground mass) were examined in 1x1x1 meter cages placed in a large knapweed infestation. The plants were exposed to a 2x2 factorial design with ramet removal and insect presence manipulated. With 32 total cages, eight cages received each treatment. Ramets and plots were measured prior to treatment and after four weeks.
Insects were over five times more likely to establish in plots with ramet removal, despite the reduction of aboveground plant resources available to insects. This may be due to a change in quality of remaining ramets if they have greater access to clonal root resources. Further, the combination of treatments has a larger impact on plant traits than either treatment alone. There is a clear interaction between treatments of insect exposure and ramet removal in the model describing resulting plant traits, particularly in ramet height. Ramet removal alone increased the height of the tallest plants in the cages, while a combination of ramet removal and insect exposure decreased the height. Clearly, grazing could have a significant impact on both knapweed gall insect populations and Russian knapweed growth in a larger field context. Further research on widespread knapweed populations with variable community members, such as grazing ungulates and biological control agents, will lead to further understanding of this system. Considering the impacts of browsing mammals on invaded communities can inform weed management. Further, determining how and when plants mediate interactions between herbivores can improve competition and facilitation theory by illuminating mechanisms of interaction.