Date of Award
Shannon M. Murphy
Insect herbivores are one of the most diverse groups of multicellular organisms, and the vast majority are specialists, which feed on only a few plant species. The factors that cause some herbivores to be specialists and others to be generalists are still unclear. It is known that the selective forces from natural enemies (top-down) and the host plants (bottom-up) influence an herbivore's diet breadth. In my meta-analysis evaluating the relative important of top-down and bottom-up forces on insect herbivore fitness, I found that herbivores usually have greater performance on better quality plants and in the absence or reduction of enemy pressure. Usually top-down forces were stronger than bottom-up forces, except when considering the diet breadth of herbivores. I found that specialists are more affected by bottom-up forces than top-down forces, whereas generalists had similar performance on different host plants. As specialists and generalists are differently affected by bottom-up and top-down forces, to understand the diet breadth evolution of herbivores, we should consider the impact of these two forces together on herbivore fitness. There is currently no method to measure this combined effect (tri-trophic), so I developed a new approach to calculate the fitness associated with tri-trophic interactions using bi-trophic slopes. I use the relationship between fitness associated with top-down and bottom-up forces as well as the frequency of host-plant use to calculate the top-down and bottom-up fitness slopes, which I then combine to obtain the tri-trophic slopes. I tested my approach using one of the most generalist herbivores as my model organism (fall webworm, Hyphantria cunea, Lepidoptera: Erebidae); I found that generalism is a good strategy for this herbivore, as populations with broader diet feed more frequently on the best hosts available.
One of the mechanisms that can lead to diversification of insect herbivores is local adaptation to their host plants. However, high gene flow between individuals feeding on different plants, as is the case for generalists, can lead to low levels of local adaptation. Using common garden and transplant experiments, I found mixed evidence for local adaptation of fall webworm. The red-headed type of fall webworm that is relatively less generalist had a higher level of local adaptation than the more generalist black-headed type. In addition to local adaptation, other factors such as host plant use, diet breadth, and geographic isolation can influence the diversification of herbivores. I performed genetic analyses of fall webworm using double-digest RADseq to test the relative importance of diet breadth, host plant use, and geographic distance on the genetic divergence of fall webworm populations. Using both red and black types from a broad geographic range, I found the two types to be genetically different, and both host plant use and geographic distance influenced the divergence of the red type, while only geographic distance was influential for the black type. Considering a fine geographic scale of the red type in Colorado, geographic distance was more influential than host plant use and diet breadth on the genetic divergence of individuals.
Cadorin Vidal, Mayra, "Diet Breadth Evolution and Diversification of a Generalist Insect Herbivore" (2018). Electronic Theses and Dissertations. 1463.
Recieved from ProQuest
Mayra Cadorin Vidal
Ecology, Evolution & development