Ph.D. Dissertation: Native versus exotic Grasses: the interaction between generalist insect herbivores and their host plants. (Alina Avanesyan, 2014; University of Cincinnati)
Advisor: Dr. Theresa Culley, Professor, Department of Biological Sciences, University of Cincinnati
In my dissertation at the University of Cincinnati I explored the interactions between insect herbivores and their host plants within the context of invasion ecology. Specifically, I was interested in the potential impact of generalist insects on the successful spread of exotic plants. Using a grasses-grasshoppers model, I combined behavioral and molecular approaches to explore (1) tolerant and resistant responses of native and exotic grasses to herbivory by grasshoppers, and (2) grasshopper feeding preferences on these plants. I conducted laboratory and field experiments at two research centers (University of Cincinnati and University of Maryland) to explore whether plant responses to different insect populations were similar, and whether the insects acted in the same way.
As part of my dissertation, I developed a new PCR-based method for accurate detection of plant meals from grasshopper guts, which had not been described in experimental studies. This method was published in Applications in Plant Sciences and was featured in Botanical Society of America News, ScienceDaily, ScienceNewsLine, Phys.org, LabRoots, EurekAlert!, as well as Down to Earth (a science magazine of The Society For Environmental Communications in India).
My results from both, behavioral experiments and molecular confirmation of diet, demonstrated that exotic grasses have lower resistance (the ability to reduce damage) to grasshopper feeding than native grasses; whereas plant tolerance (the ability to maintain fitness while sustaining damage) to herbivory does not differ between native and exotic grasses. My results suggested that exotic grasses that do not have a coevolutionary history with native grasshoppers are less adapted to reduce damage from these herbivores, although they tolerate them similar to native plants. These results contributed to our understanding of the trade-off between plant tolerance and plant resistance to herbivory and the possible mechanisms of the success of exotic plans in the introduced range – i.e., mechanisms that facilitate plant invasion. The important applications of this project are: effective control of invasive plants, predictions of plant invasion, and ecological restoration of native communities.
My study grasses species were: native Andropogon gerardii and Bouteloua curtipendula, and exotic, potentially invasive, Miscanthus sinensis and Bothriochloa ischaemum. I conducted choice (many grasses) and no-choice (one grass species) experiments; experiments in the field and in the greenhouse; experiment with intact plants and with their clipped. To estimate plant resistance and tolerance I measured the amount of leaf tissue consumed by grasshoppers, the growth of plants during the feeding and their re-growth after the feeding, the grasshopper’s growth on different plants and many other things. I was conducting most of my experiments in Ohio (UC Center for Field Studies and The Culley Laboratory), but also conducted some at the Western Maryland Research & Education Center – thanks to Dr. William Lamp who was a member of my dissertation committee and provided me with access to this wonderful research facility, and Tim Ellis, the Center’s Agronomy Program Manager, whose help with preparing the plot and growing plants was invaluable.
Also see: How to build a cage
In the same behavioral experiments with plants and, additionally, in the experiments with clipped leaves, I estimated leaf consumption, the proportion of leaves consumed by grasshoppers, as well as their assimilation efficiency and relative consumption rate on native and exotic grasses.
In addition to using a behavioral approach I have developed a PCR-based method of plant DNA detection from insect gut contents to confirm digestion and to accurately determine plant-insect associations. Although previous investigations have focused on a variety of insects, there were no existing protocols for plant DNA detection developed specifically for grasshoppers, which play a significant role in plant communities. Using the developed protocol, fragments (~500 bp) of the non-coding region of the chloroplast trnL (UAA) gene were successfully amplified from grasshopper guts and were found to be detectable up to twelve hours post ingestion (PI) in nymphs and up to 22 h PI in adult grasshoppers.
The proposed protocol was an effective, relatively quick, and low cost method of detecting plant DNA from grasshopper guts, which had important applications: from a better understanding of plant food “movement” during the digestion, to detecting plant-insect associations.