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Show TRI-TROPHIC INTERACTIONS INVOLVING A PRODUCER, HERBIVORE AND PARASITOID Megan Sinner (Janet Anderson) Department of Biology Many studies have been done on the grass, Festuca arundinacea, and its mutualistic relationship with the fungus, Neotyphodium coenophialum, and the resulting effects on herbivores. However, at present few people have studied the fungal effects on a third trophic level. We are exploring these effects by using a tri-trophic system consisting of a producer (F. arundinacea, grass), an herbivore (Spodoptera frugiperda, fall army worm) and a parasitoid (Euplathryus com-stockii, wasp). While the grass and herbivore have frequently been used as model experimental organisms, the parasitoid has rarely been utilized. Our first step was to identify a source for a viable parasitoid and establish a parasitoid colony at the University of Utah. Within this system, the grass can be infected with an endo-phytic fungus, N. coenophialum, which is mutualistic when limited to the producer-herbivore level. The question we are studying is if the association would still be mutualistic if a third trophic level was added. We were also interested in determining which trophic level had the greatest influence in the tri-trophic system. We addressed these questions through mathematical modeling and original experiments. The mathematical model consisted of three non-linear differential equations representing the interactions between the producer, herbivore and parasitoid. We analyzed the model by looking for points where all three species could co-exist for an indefinite period of time. We also determined mathematically which parameters have the most influence on the existence of such points. We designed our experiments to determine whether these key parameters were statistically different for infected (E+) versus non-infected (E-) grass. The first experiment was designed to determine consumption and assimilation rates of the fall army worm (FAW) fed on both types of grass. We followed the weight of ten FAWs, for each type of grass, from birth to pupation. We weighed non-consumed food, FAWs and fresh food every three days. We found that FAWs eating E+ grass, ate approximately 2.34 times their body weight of grass daily, and FAWs eating E- grass ate approximately 1.24 times their body weight of grams of grass daily. The data also showed that the FAW weight gain rates were quite similar, regardless of which grass they consumed. Since the FAW had to consume more E+ grass than E- grass to gain the same amount of weight, we hypothesized that either (1) the E+ grass contains more water, therefore the |